There's more than one way to fit a bike
26/11/23 01:55 Filed in: Gear
The usual approach for me to fit a customer's bike is to start with the frame that they have, and do what I can to make it fit better. When I purchased a Checkpoint to be my new commuting bike, I selected a stem that would yield my usual saddle to bar reach based on previous (and current) bikes I was riding. And it worked fine.
Then the frame was replaced with the new model Checkpoint. Which has 2 cm more top tube reach than the old model had. I should have swapped to a stem that was 2 cm shorter to achieve the same fit. However, the nearly new stem I had on the Checkpoint was one I found very attractive and both light and stiff. In the meantime, the stem was updated which meant new graphics that I don't like as much. And the price has gone up considerably in a couple of years. Plus they were out of stock everywhere in that size.
I tried the bike with the "too long" stem. I added the maximum permissible (according to the manufacturer's build guide) spacers under the stem to diminish the impact of a longer stem (the stem seems closer when it is higher and as it goes up, it comes slightly back towards the rider thanks to the steering angle).
I ended up with the saddle to bar distance and saddle to pedal distance the same on my racing bike and my commuting bike. The pedal to bar distance was a little longer on the commuter, but this can be "adjusted" by hand position on the bars and elbow bend in the rider's arms. I now think having extra room is mostly good. I definitely adapted over a couple of weeks to the new position. And going to another bike doesn't feel weird. These are good. I have a bit extra weight on my hands. That is potentially bad. If I rode this bike for hours at a stretch regularly, it might not work. I rarely ride it more than 3 hours in a single day, and for that much time it is fine.
Sometimes a bit of a lateral approach will achieve the desired result.
Then the frame was replaced with the new model Checkpoint. Which has 2 cm more top tube reach than the old model had. I should have swapped to a stem that was 2 cm shorter to achieve the same fit. However, the nearly new stem I had on the Checkpoint was one I found very attractive and both light and stiff. In the meantime, the stem was updated which meant new graphics that I don't like as much. And the price has gone up considerably in a couple of years. Plus they were out of stock everywhere in that size.
I tried the bike with the "too long" stem. I added the maximum permissible (according to the manufacturer's build guide) spacers under the stem to diminish the impact of a longer stem (the stem seems closer when it is higher and as it goes up, it comes slightly back towards the rider thanks to the steering angle).
I ended up with the saddle to bar distance and saddle to pedal distance the same on my racing bike and my commuting bike. The pedal to bar distance was a little longer on the commuter, but this can be "adjusted" by hand position on the bars and elbow bend in the rider's arms. I now think having extra room is mostly good. I definitely adapted over a couple of weeks to the new position. And going to another bike doesn't feel weird. These are good. I have a bit extra weight on my hands. That is potentially bad. If I rode this bike for hours at a stretch regularly, it might not work. I rarely ride it more than 3 hours in a single day, and for that much time it is fine.
Sometimes a bit of a lateral approach will achieve the desired result.
Adjusting a Zeb
16/11/23 21:28 Filed in: Gear
After having a couple of Fox 36 forks on past bikes, and really liking the Boxxer forks on my DH bike, I selected a Slash with Flight Attendant; meaning Zeb forks.
My experience with the Boxxer has been ideal from day one. Set the pressure to that recommended for my weight and one or two minor tweaks to make it feel "right". And then nothing since. It was so "right" I didn't bother to install a ShockWhiz for feedback.
The Zeb began in much the same vein. I put in the recommended air pressure and ended up removing a few psi. It was great. Big hits would use up almost the entire travel, riding along it seemed to sit not-too-far into the travel and the Flight Attendant lockout was a very solid lockout. On my final day of a week riding in Tasmania, something changed. The lockout was soft - the fork visibly moved a lot under pedalling and there didn't seem to be the same amount of rebound damping as I'd had up until that point. I thought the Charger cartridge had blown.
The fork went to RockShox and their dyno said it was 100% as intended.
My shop just happened to have a new customer's Slash being built at the same time, so they set both forks up for the mechanic and he rode them both back-to-back and couldn't discern a difference.
I got the bike back and did all my riding over winter on shorter travel bikes. The Slash mostly sat. And with Spring on us and Thredbo opening in sight, I pulled the Slash out and started riding it again. I had to set up the pressures from scratch because of all the experimenting by RockShox and the shop. I put a ShockWhiz on the fork this time. With the same pressure I used in Tassie a few months ago, I found it very stiff and impossible to get more than about 1/2 travel. The ShockWhiz concurred:
Too much air pressure.
Too much compression damping.
Too many tokens (there were none!).
Dropping the air down incrementally until it was about 1/3 less, I was still not getting full travel and while the fork felt soft, it was also not settling into travel with sufficient sag.
The Flight Attendant's function of locking out the shock when pedalling isn't fit for purpose with the ShockWhiz measuring damping and spring rates, so I have had the fork in manual "open" mode the whole time. Thus, I can't comment on the firmness or lack thereof for "locked" mode.
A suspension fork is a pretty simple device, really. I cannot imagine what could have changed that effected such a significant change in the fork's behaviour.
It is going back to the shop for a strip-down. Shop owner has put a Zeb on his own bike recently and went through the fork lubricating seals and checking torque on bolts to get the best possible performance out of his fork - he's going to check everything internally is good with my Zeb.
Fingers crossed he finds something to "fix" because I'm almost out of ideas.
There is a British company that makes negative tokens for forks. Effectively reducing the compression ratio in the air chamber, perhaps it will permit me to run more baseline pressure and still achieve full travel? Check out Trutune.co.uk if you're curious. Obviously, I haven't tried one yet so I can't comment on how much difference it makes. The science seems sound, if a little bit like magic.
I want my "new" Zeb back.
My experience with the Boxxer has been ideal from day one. Set the pressure to that recommended for my weight and one or two minor tweaks to make it feel "right". And then nothing since. It was so "right" I didn't bother to install a ShockWhiz for feedback.
The Zeb began in much the same vein. I put in the recommended air pressure and ended up removing a few psi. It was great. Big hits would use up almost the entire travel, riding along it seemed to sit not-too-far into the travel and the Flight Attendant lockout was a very solid lockout. On my final day of a week riding in Tasmania, something changed. The lockout was soft - the fork visibly moved a lot under pedalling and there didn't seem to be the same amount of rebound damping as I'd had up until that point. I thought the Charger cartridge had blown.
The fork went to RockShox and their dyno said it was 100% as intended.
My shop just happened to have a new customer's Slash being built at the same time, so they set both forks up for the mechanic and he rode them both back-to-back and couldn't discern a difference.
I got the bike back and did all my riding over winter on shorter travel bikes. The Slash mostly sat. And with Spring on us and Thredbo opening in sight, I pulled the Slash out and started riding it again. I had to set up the pressures from scratch because of all the experimenting by RockShox and the shop. I put a ShockWhiz on the fork this time. With the same pressure I used in Tassie a few months ago, I found it very stiff and impossible to get more than about 1/2 travel. The ShockWhiz concurred:
Too much air pressure.
Too much compression damping.
Too many tokens (there were none!).
Dropping the air down incrementally until it was about 1/3 less, I was still not getting full travel and while the fork felt soft, it was also not settling into travel with sufficient sag.
The Flight Attendant's function of locking out the shock when pedalling isn't fit for purpose with the ShockWhiz measuring damping and spring rates, so I have had the fork in manual "open" mode the whole time. Thus, I can't comment on the firmness or lack thereof for "locked" mode.
A suspension fork is a pretty simple device, really. I cannot imagine what could have changed that effected such a significant change in the fork's behaviour.
It is going back to the shop for a strip-down. Shop owner has put a Zeb on his own bike recently and went through the fork lubricating seals and checking torque on bolts to get the best possible performance out of his fork - he's going to check everything internally is good with my Zeb.
Fingers crossed he finds something to "fix" because I'm almost out of ideas.
There is a British company that makes negative tokens for forks. Effectively reducing the compression ratio in the air chamber, perhaps it will permit me to run more baseline pressure and still achieve full travel? Check out Trutune.co.uk if you're curious. Obviously, I haven't tried one yet so I can't comment on how much difference it makes. The science seems sound, if a little bit like magic.
I want my "new" Zeb back.
Dial twirling - how to set those knobs on the suspension
20/10/23 22:42 Filed in: Gear
MTB suspension is very sophisticated these days. If you read my last entry on air pressure in suspension, you'll know I ventured into the setting of the knobs without fully exploring the topic. That is here...
Even on my Slash with Flight Attendant, I still need to set all the electronic knobs properly and get the air pressure right. Flight Attendant really only flicks the platform switch to one of three positions (open, pedal or locked) as I ride based on slope, bumps and pedalling input. On more regular suspensions, it is all manual and the setting up is still the critical thing to an enjoyable ride.
Take the Fox Float X shock or the Grip2 fork. They have 4 adjustments which few other models or brands offer: high and low speed for both rebound and compression. In contrast, entry level forks might have zero adjustments although they usually have a rebound adjuster because that has to be adjusted for air pressure to behave anywhere near properly.
Assuming you read and followed the previous entry to get your air pressure correct, what's next?
First, let's look at what this high speed and low speed stuff is all about. It refers to the shaft speed of the suspension rather than the forward velocity of the bike. When jumping a bike, the face of the jump loads the rear suspension a lot. A big jump face will just about bottom out the shock. As the wheel leaves the lip there is suddenly zero resistance to the shock fully extending. It will do so with full speed - this is high speed rebound. The low speed happens when you go through a big bowl-shaped depression in the trail. As the bike travels down into the bowl, speed of the bike increases. On the other side it goes up the side of the bowl, slowly compressing the shock (this is LSC's territory) and as you roll out of the bowl at the top back onto flat terrain the load comes off smoothly assuming a gradual transition from up to flat - this is low speed rebound. Low speed compression also happens from pedalling but mostly it is the gradual increase in load from gentle terrain changes. High speed compression is running into a rock at high speed. The front wheel instantly wants to move up the height of the rock. There is so much oil flow attempting to go through the low-speed circuit that it is overwhelmed and the bypass IS the high-speed circuit.
Many HSCs are governed by a shim stack. These are merely thin steel washers that are pushed out of the way by the oil flow through the tiny holes they cover. This is why, as I mentioned in the last entry, you require some HSC to get any LSC. If the washers aren't forced down onto the holes by the HSC adjuster, then the oil can just flow through the holes with little to no damping going on. Once the HSC is set the oil will be forced to go through the LSC circuit. I should have mentioned that this circuit is typically governed by a needle in a hole. The needle has a taper (much like a sewing needle) and the adjuster moves the needle up or down, pushing more or less in the hole. More needle in the hole means less flow (more LSC). Less needle in the hole means more flow (less LSC).
In regular riding conditions you'd like the suspension to ride quite high in the travel range. At or just below sag. This leaves more reserve for bumps and preserves the bike's steering geometry which gets really messed up as the suspension compresses. This calls for a decent amount of LSC (but not so much that the bike feels wooden, slow or sluggish to answer bumps in the trail). LSC might be quite different front to rear because rear suspension has a leverage ratio between wheel travel and shock travel usually around 3:1. Forks area always 1:1.
HSC is the last thing you play with. Once air pressure, rebound and LSC are set, HSC can step into control the movement of the suspension when it hits stuff. Too much HSC will feel rough and make hands sore. Too little HSC relies on the air spring to ramp up and can make the suspension not respond to repeated impacts as well as it might.
Finally, once this is all set, go out and ride the bike and pay attention to what is happening. Keeping notes helps. If it feels right most of the time, but bottoms out when you go faster or rougher - the solution is probably a travel token. If it springs too much off jump lips, that is rebound. If it sits too low in the travel, that is LSC or air pressure. Every problem typically has two remedies. But one preferred remedy. Because everything affects everything else in some way, the best solution for your suspension depends on the impact either adjustment will make to the other things.
Which is where ShockWhiz is quite helpful. Because it looks at compression and rebound, for different kinds of events in isolation, it can recommend a single fix for any problem having considered everything else that might be impacted by its recommendations. A rider is doing well if they can look at one thing per run.
When you do make changes, only change one thing at a time and not more than 1 or 2 clicks on the dial. If you change two things and one is an improvement while the other is worse, the end product might be "feels the same". You'll forego the improvement. Keep notes. Bracketing can also help quickly narrow in on what works. If there are ten clicks, first try 3 clicks and then 7 clicks. One should feel better than the other, try either side of the one that is preferred - eg, you prefer 7 clicks so next steps are to try 6 and 8. If you prefer 6, try 5 and 7 (again). One of these three should be your pick. Another great option is to take the manufacturer's suggestion - they usually offer starting points based on air pressures (which are based on body weight). I find them very close.
RockShox has a setting page on their website if you want to play with that. If you have a Trek, they also have a settings page on their website which goes to tyre pressures as well as suspension settings.
You need to experiment a bit, and pay attention to settings in general, in order to get the most out of our suspension and by association, your bike.
Even on my Slash with Flight Attendant, I still need to set all the electronic knobs properly and get the air pressure right. Flight Attendant really only flicks the platform switch to one of three positions (open, pedal or locked) as I ride based on slope, bumps and pedalling input. On more regular suspensions, it is all manual and the setting up is still the critical thing to an enjoyable ride.
Take the Fox Float X shock or the Grip2 fork. They have 4 adjustments which few other models or brands offer: high and low speed for both rebound and compression. In contrast, entry level forks might have zero adjustments although they usually have a rebound adjuster because that has to be adjusted for air pressure to behave anywhere near properly.
Assuming you read and followed the previous entry to get your air pressure correct, what's next?
First, let's look at what this high speed and low speed stuff is all about. It refers to the shaft speed of the suspension rather than the forward velocity of the bike. When jumping a bike, the face of the jump loads the rear suspension a lot. A big jump face will just about bottom out the shock. As the wheel leaves the lip there is suddenly zero resistance to the shock fully extending. It will do so with full speed - this is high speed rebound. The low speed happens when you go through a big bowl-shaped depression in the trail. As the bike travels down into the bowl, speed of the bike increases. On the other side it goes up the side of the bowl, slowly compressing the shock (this is LSC's territory) and as you roll out of the bowl at the top back onto flat terrain the load comes off smoothly assuming a gradual transition from up to flat - this is low speed rebound. Low speed compression also happens from pedalling but mostly it is the gradual increase in load from gentle terrain changes. High speed compression is running into a rock at high speed. The front wheel instantly wants to move up the height of the rock. There is so much oil flow attempting to go through the low-speed circuit that it is overwhelmed and the bypass IS the high-speed circuit.
Many HSCs are governed by a shim stack. These are merely thin steel washers that are pushed out of the way by the oil flow through the tiny holes they cover. This is why, as I mentioned in the last entry, you require some HSC to get any LSC. If the washers aren't forced down onto the holes by the HSC adjuster, then the oil can just flow through the holes with little to no damping going on. Once the HSC is set the oil will be forced to go through the LSC circuit. I should have mentioned that this circuit is typically governed by a needle in a hole. The needle has a taper (much like a sewing needle) and the adjuster moves the needle up or down, pushing more or less in the hole. More needle in the hole means less flow (more LSC). Less needle in the hole means more flow (less LSC).
In regular riding conditions you'd like the suspension to ride quite high in the travel range. At or just below sag. This leaves more reserve for bumps and preserves the bike's steering geometry which gets really messed up as the suspension compresses. This calls for a decent amount of LSC (but not so much that the bike feels wooden, slow or sluggish to answer bumps in the trail). LSC might be quite different front to rear because rear suspension has a leverage ratio between wheel travel and shock travel usually around 3:1. Forks area always 1:1.
HSC is the last thing you play with. Once air pressure, rebound and LSC are set, HSC can step into control the movement of the suspension when it hits stuff. Too much HSC will feel rough and make hands sore. Too little HSC relies on the air spring to ramp up and can make the suspension not respond to repeated impacts as well as it might.
Finally, once this is all set, go out and ride the bike and pay attention to what is happening. Keeping notes helps. If it feels right most of the time, but bottoms out when you go faster or rougher - the solution is probably a travel token. If it springs too much off jump lips, that is rebound. If it sits too low in the travel, that is LSC or air pressure. Every problem typically has two remedies. But one preferred remedy. Because everything affects everything else in some way, the best solution for your suspension depends on the impact either adjustment will make to the other things.
Which is where ShockWhiz is quite helpful. Because it looks at compression and rebound, for different kinds of events in isolation, it can recommend a single fix for any problem having considered everything else that might be impacted by its recommendations. A rider is doing well if they can look at one thing per run.
When you do make changes, only change one thing at a time and not more than 1 or 2 clicks on the dial. If you change two things and one is an improvement while the other is worse, the end product might be "feels the same". You'll forego the improvement. Keep notes. Bracketing can also help quickly narrow in on what works. If there are ten clicks, first try 3 clicks and then 7 clicks. One should feel better than the other, try either side of the one that is preferred - eg, you prefer 7 clicks so next steps are to try 6 and 8. If you prefer 6, try 5 and 7 (again). One of these three should be your pick. Another great option is to take the manufacturer's suggestion - they usually offer starting points based on air pressures (which are based on body weight). I find them very close.
RockShox has a setting page on their website if you want to play with that. If you have a Trek, they also have a settings page on their website which goes to tyre pressures as well as suspension settings.
You need to experiment a bit, and pay attention to settings in general, in order to get the most out of our suspension and by association, your bike.
The importance of air pressure 2
05/10/23 21:45 Filed in: Gear
Part 1 was all about tyre pressures. This bit is very much MTB specific because it is about shock pressures. Very few road bikes have shocks!
I remember having to regularly pump up my shock back in the early days of suspension. They leaked. Regularly. You would start out at 250 psi (as an example) and two weeks later it would be down to 200 psi - soft enough to make a difference. I am quite certain I haven't pumped up the shock on my DH bike since I set it on the first ride! That is 2 years (or maybe it is 3 years - yikes).
One of the issues with checking your shock pressure is that the combination of a very tiny air chamber and a very high pressure means that almost every method of checking results in it no longer being at that pressure any more. A shock pump has a short hose that fills with the pressure from the shock when it is connected - it is long enough and has sufficient volume to take at least 10 psi out of most shocks. You can check this by attaching and removing the shock several times.Each time the pressure will decrease by almost exactly the same amount.
This is different from letting air out when removing the shock. Most shock pumps have that problem covered. My favourite shock pump is a Topeak one I've had for years. The hose threads onto the shock valve, sealing air tight. And only then do you thread in the next "stage" that pushes in the valve to release air into the pump. On removal, the process is reversed. Pump the shock to the desired pressure, move the second stage back so the valve is closed and then release the pump from the shock. The noise of releasing air you hear is only air coming out of the pump. Not air coming out of the shock.
Of course, if you worry about this and reattach the pump, it will fill the hose up with air again thereby diminishing the pressure in the shock by that small amount.
Most of my shock pumps (I must have a dozen from all the ones included with bikes over the years - most of them unused in their original packaging) have a different attachment than the one I described above. Instead they do this whole thing in one step by threading onto the shock a long way - first contact is with the seal making it air tight and keep threading until the pin pushes in the valve and releases the air.
What is the correct air pressure in a shock anyway?
Sag is a good starting point. But it is not the end point. With riding gear on (which includes some water in your backpack if that is how you ride, plus riding shoes and helmet and glasses and tools and whatever else you normally ride with) sit on the bike while someone holds the bike upright. Bounce a little and settle on the seat as you would pedal - which means dropper post in the uppermost position. Assistant should slide the o-ring marker on the shock shaft up to the shock seal and then you very carefully dismount the bike without moving the suspension in the process of getting off - sometimes easier said than done. Check the o-ring. On RockShox you'll usually find a set of sag markers. On Fox and other forks you'll have to break out a ruler and some math. Ideal sag is a bit dependent on the type of suspension and the amount of travel - look to your owner's manual for recommendations here. It is around 30% for a DH bike but less - maybe 20% - for some XC bikes.
For the fork, it is very similar but slightly different. Again with full riding gear on, mount the bike while someone holds it upright for you. Get into standing riding position. Bounce a few times and then hold the position. Move the o-ring marker to the dust seal on the slider. Gently dismount. Again, many RockShox forks have sag markers on the stanchion. Other brands do not. With a fork you require less sag than with a shock - 20% is a good starting point for most forks.
Note that you cannot assume the o-ring will end up fully on the other end of the shock shaft or fork stanchion at full travel. Often there are top out bumpers that prevent what appears to be full travel. My old Slash had a travel reducer inside of what was a longer-throw shock, so it never looked like I achieved full travel even when I did. Same with forks. Whether 100 mm travel or 200 mm travel, there can be several millimetres more space. Check it out by measuring the distance from fork seal to fork crown along the exposed stanchion and compare with fork rated travel. They seldom coincide.
Now go and ride it. There is a rule of thumb that you should achieve full travel at both ends on every ride. This is demonstrably false. If you have a DH bike set up to go fast on rough terrain and then ride it slowly down some smooth trail, it is not going to come close to using full travel. On the flip side, if you set up a short travel bike to use full travel on a smooth trail at low speeds you will dangerously bottom out if you go faster or rougher with the same settings.
I use a ShockWhiz because it gives some guidance to what I might do on my own anyway. I don't rely on it fully but it speeds up the process. Across every fork I've ever put the ShockWhiz on, it has told me to soften it up. I must ride really lightly on the front end because I seldom see full fork travel on any bike and even on my 100 mm travel Fox 32 that I performed surgery on to reduce the ramp-up rate in the air spring (I cut the shaft that holds the air seal piston inside the air spring making it ride lower in the chamber and finish lower in the chamber - the opposite of putting in a travel token - much less increase in air pressure as travel is used up) the ShockWhiz told me the rate was too high. It often wants me to install tokens in the shock - I guess if I'm light on the fork I must be heavy on the shock. I mostly ride flat pedals and that pushes my weight rearwards when I drop my heels for pedal traction.
The best way to dial in the shock/fork pressures is to ride a short loop on a technical and turn-laden trail. Adjust the pressure in one unit by 10 psi and ride it. Repeat. Keep notes. When it feels best, it probably is if you chose the trail correctly. Adjust the pressure in the other unit until it feels right. Now go ride for real. Watch for times the bike feels funny, bottoms out, etc.
If I use my Ibis as an example, it has 120 mm of travel at both ends. I get full travel on the rear end (or within 2 mm of shock travel = more than 110 mm of wheel travel) almost every ride. I get full travel on the fork on any ride I do some jumps or drops, or ride rough tracks fast. It corners well. Whatever air pressure I am using is perfect for me. I don't adjust it to suit the place I'm riding. Or the weather. On my Sender I seldom see more than 180 mm travel up front. But that last 20 mm is there and gets used sometimes. Plus it feels great so I don't mess with it. The ShockWhiz wants me to take another token out of the fork, but they are all removed already. I could try - and have been thinking about it - a TruTune negative token (if you haven't heard of these things then go look it up, amazing chemistry/physics to make a negative volume token - the stuff of science fiction) but it is very good already.
Pay attention to the balance between front and rear. Sometimes if you set them up independently and don't bother to pay attention to the balance, it is possible to end up with an imbalance. A few psi extra in one will usually sort that out. Generally if you want better traction at one end, add air to the other end. That pushes some traction away from the firmer end towards the now relatively softer end. That is without the rider changing anything from their behaviour.
Keep detailed notes. Record your final air pressures. Because when you get a service, they'll probably let all the air out and not restore the same pressure it was at. Or maybe your shock will leak. Or you'll lend your bike to someone who needs to change it. Whatever the reason, being able to restore it to how you like it is important.
I haven't tried it yet, but my new digital pressure gauge has no hose. In fact the volume inside the system is so incredibly small that it doesn't seem to change any air pressures at all on checking. This is great. Given it will register up to 350 psi means it can be used to check shock pressures too. And check them without decreasing them. If I do it this way it means the crappy gauges on cheap shock pumps are irrelevant. I can pump up close to correct and then set with the gauge and know it is same as last time.
Beyond the scope of this article, but just a reminder, once you set air pressures you need to twirl the dials to set the rebound and compression settings to match the air pressure, bike, rider combination. Everyone should run rebound in almost the same setting - for a given air pressure. Not too fast and not too slow. If you have HSC and LSC, most compression circuits require a bit of HSC to get the LSC to work properly. LSC is a small orifice with fairly high resistance to oil flow, but at low speeds the flow is low enough that it can flow through. Unless the HSC is zero and then it can bypass the LSC circuit and just go through the HSC circuit. So a couple of clicks of HSC is a good starting point for everyone. LSC like rebound will scale with air pressure but there is more room for personal preference in LSC than in rebound. HSC can make up for not enough tokens, but too much HSC makes for sore hands (more tokens is a better solution).
Set your air pressure, test your air pressure, record your air pressure and periodically check your air pressure for a happy bike.
I remember having to regularly pump up my shock back in the early days of suspension. They leaked. Regularly. You would start out at 250 psi (as an example) and two weeks later it would be down to 200 psi - soft enough to make a difference. I am quite certain I haven't pumped up the shock on my DH bike since I set it on the first ride! That is 2 years (or maybe it is 3 years - yikes).
One of the issues with checking your shock pressure is that the combination of a very tiny air chamber and a very high pressure means that almost every method of checking results in it no longer being at that pressure any more. A shock pump has a short hose that fills with the pressure from the shock when it is connected - it is long enough and has sufficient volume to take at least 10 psi out of most shocks. You can check this by attaching and removing the shock several times.Each time the pressure will decrease by almost exactly the same amount.
This is different from letting air out when removing the shock. Most shock pumps have that problem covered. My favourite shock pump is a Topeak one I've had for years. The hose threads onto the shock valve, sealing air tight. And only then do you thread in the next "stage" that pushes in the valve to release air into the pump. On removal, the process is reversed. Pump the shock to the desired pressure, move the second stage back so the valve is closed and then release the pump from the shock. The noise of releasing air you hear is only air coming out of the pump. Not air coming out of the shock.
Of course, if you worry about this and reattach the pump, it will fill the hose up with air again thereby diminishing the pressure in the shock by that small amount.
Most of my shock pumps (I must have a dozen from all the ones included with bikes over the years - most of them unused in their original packaging) have a different attachment than the one I described above. Instead they do this whole thing in one step by threading onto the shock a long way - first contact is with the seal making it air tight and keep threading until the pin pushes in the valve and releases the air.
What is the correct air pressure in a shock anyway?
Sag is a good starting point. But it is not the end point. With riding gear on (which includes some water in your backpack if that is how you ride, plus riding shoes and helmet and glasses and tools and whatever else you normally ride with) sit on the bike while someone holds the bike upright. Bounce a little and settle on the seat as you would pedal - which means dropper post in the uppermost position. Assistant should slide the o-ring marker on the shock shaft up to the shock seal and then you very carefully dismount the bike without moving the suspension in the process of getting off - sometimes easier said than done. Check the o-ring. On RockShox you'll usually find a set of sag markers. On Fox and other forks you'll have to break out a ruler and some math. Ideal sag is a bit dependent on the type of suspension and the amount of travel - look to your owner's manual for recommendations here. It is around 30% for a DH bike but less - maybe 20% - for some XC bikes.
For the fork, it is very similar but slightly different. Again with full riding gear on, mount the bike while someone holds it upright for you. Get into standing riding position. Bounce a few times and then hold the position. Move the o-ring marker to the dust seal on the slider. Gently dismount. Again, many RockShox forks have sag markers on the stanchion. Other brands do not. With a fork you require less sag than with a shock - 20% is a good starting point for most forks.
Note that you cannot assume the o-ring will end up fully on the other end of the shock shaft or fork stanchion at full travel. Often there are top out bumpers that prevent what appears to be full travel. My old Slash had a travel reducer inside of what was a longer-throw shock, so it never looked like I achieved full travel even when I did. Same with forks. Whether 100 mm travel or 200 mm travel, there can be several millimetres more space. Check it out by measuring the distance from fork seal to fork crown along the exposed stanchion and compare with fork rated travel. They seldom coincide.
Now go and ride it. There is a rule of thumb that you should achieve full travel at both ends on every ride. This is demonstrably false. If you have a DH bike set up to go fast on rough terrain and then ride it slowly down some smooth trail, it is not going to come close to using full travel. On the flip side, if you set up a short travel bike to use full travel on a smooth trail at low speeds you will dangerously bottom out if you go faster or rougher with the same settings.
I use a ShockWhiz because it gives some guidance to what I might do on my own anyway. I don't rely on it fully but it speeds up the process. Across every fork I've ever put the ShockWhiz on, it has told me to soften it up. I must ride really lightly on the front end because I seldom see full fork travel on any bike and even on my 100 mm travel Fox 32 that I performed surgery on to reduce the ramp-up rate in the air spring (I cut the shaft that holds the air seal piston inside the air spring making it ride lower in the chamber and finish lower in the chamber - the opposite of putting in a travel token - much less increase in air pressure as travel is used up) the ShockWhiz told me the rate was too high. It often wants me to install tokens in the shock - I guess if I'm light on the fork I must be heavy on the shock. I mostly ride flat pedals and that pushes my weight rearwards when I drop my heels for pedal traction.
The best way to dial in the shock/fork pressures is to ride a short loop on a technical and turn-laden trail. Adjust the pressure in one unit by 10 psi and ride it. Repeat. Keep notes. When it feels best, it probably is if you chose the trail correctly. Adjust the pressure in the other unit until it feels right. Now go ride for real. Watch for times the bike feels funny, bottoms out, etc.
If I use my Ibis as an example, it has 120 mm of travel at both ends. I get full travel on the rear end (or within 2 mm of shock travel = more than 110 mm of wheel travel) almost every ride. I get full travel on the fork on any ride I do some jumps or drops, or ride rough tracks fast. It corners well. Whatever air pressure I am using is perfect for me. I don't adjust it to suit the place I'm riding. Or the weather. On my Sender I seldom see more than 180 mm travel up front. But that last 20 mm is there and gets used sometimes. Plus it feels great so I don't mess with it. The ShockWhiz wants me to take another token out of the fork, but they are all removed already. I could try - and have been thinking about it - a TruTune negative token (if you haven't heard of these things then go look it up, amazing chemistry/physics to make a negative volume token - the stuff of science fiction) but it is very good already.
Pay attention to the balance between front and rear. Sometimes if you set them up independently and don't bother to pay attention to the balance, it is possible to end up with an imbalance. A few psi extra in one will usually sort that out. Generally if you want better traction at one end, add air to the other end. That pushes some traction away from the firmer end towards the now relatively softer end. That is without the rider changing anything from their behaviour.
Keep detailed notes. Record your final air pressures. Because when you get a service, they'll probably let all the air out and not restore the same pressure it was at. Or maybe your shock will leak. Or you'll lend your bike to someone who needs to change it. Whatever the reason, being able to restore it to how you like it is important.
I haven't tried it yet, but my new digital pressure gauge has no hose. In fact the volume inside the system is so incredibly small that it doesn't seem to change any air pressures at all on checking. This is great. Given it will register up to 350 psi means it can be used to check shock pressures too. And check them without decreasing them. If I do it this way it means the crappy gauges on cheap shock pumps are irrelevant. I can pump up close to correct and then set with the gauge and know it is same as last time.
Beyond the scope of this article, but just a reminder, once you set air pressures you need to twirl the dials to set the rebound and compression settings to match the air pressure, bike, rider combination. Everyone should run rebound in almost the same setting - for a given air pressure. Not too fast and not too slow. If you have HSC and LSC, most compression circuits require a bit of HSC to get the LSC to work properly. LSC is a small orifice with fairly high resistance to oil flow, but at low speeds the flow is low enough that it can flow through. Unless the HSC is zero and then it can bypass the LSC circuit and just go through the HSC circuit. So a couple of clicks of HSC is a good starting point for everyone. LSC like rebound will scale with air pressure but there is more room for personal preference in LSC than in rebound. HSC can make up for not enough tokens, but too much HSC makes for sore hands (more tokens is a better solution).
Set your air pressure, test your air pressure, record your air pressure and periodically check your air pressure for a happy bike.
The importance of air pressure 1
30/09/23 16:19 Filed in: Gear
Pneumatic tyres were a critical development in the expansion of bike use in the late 19th century. Solid tyres just aren't very nice. Once Dunlop's air-filled tyres were put on bicycles, everyone was better off. Better traction, lower rolling resistance, better handling.
And all these years later, tyres filled with air are still mission-critical on all bikes.
Mountain bike tyres rely on conforming to the surface (where lower pressure is better) and forcing sharp edges into the ground (where higher pressures are better) to enable navigation of unpaved terrain. From MTB commercial origins in the 80s until the late 2000s, we were forced to use a rubber tube to contain that air. The tubes can be heavy - thinner is better from a riding standpoint. But thin tubes are easy to cut. Tubed MTB tyres were always prone to pinch flats necessitating more air than optimal for ultimate traction and handling.
And thanks to Keith Bontrager, we got skinny rims for most of those tubed years too. Keith re-rolled some road rims to 26" diameters, achieving something that the rim companies weren't doing: strong and light double-walled rims. The problem was that road rims were very skinny (around 13 mm inside width) and gave a light globe shape to a 2"+ knobby tyre. That, too, necessitated more air pressure.
Almost all mountain bikes above the very cheapest are now tubeless. To the point that high-end MTB no longer come with tubes inside and leave the tubeless step to the consumer or shop. Now they come with some sealant and valve stems and no tubes.
With tubes and large tyres on relatively skinny 26" rims, I had to run as much as 40 psi to prevent pinch flats. There is a lot of traction loss with that much air pressure in the tyre. In the case of some rim/tyre combinations, I had the tyre rotate on the rim under braking with (not much) lower air pressures - which ultimately rips the valve stem off the tube if it progresses far enough.
My current quiver of mountain bikes all run 30 mm inside width rims. That width supports the sidewall of the wide tyre much better than one half that width can. I run CushCore foam inserts in all of the wheels too. CushCore acts like a fork volume spacer by decreasing the space for air in the tyre (by roughly half). When a bump is hit, the pressure in the tyre increases more quickly due to the lower volume present. Less "travel" is used in the tyre for any given bump than would be the case without the foam. When a big enough impact occurs to bottom out the tyre on the rim the foam intervenes and cushions the impact. Impacts have to be much larger to cause rim damage. On 29" wide wheels with foam inserts I (@100 kg) can get away with around 20 psi - around half of what I used to use with tubes.
On the road side it is only very recently that mainstream tyres and wheels have gone tubeless. And it is by no means (yet, at least) a universal changeover the way it has been in mountain. High pressures and a desire for lightness push more strongly towards tubes. The biggest lever moving us to tubeless tyres has been the realisation that wider tyres are all of: faster, more comfortable, better handling and can be similarly aerodynamic if the wheel is designed for the bigger rubber.
My first new bike in Canberra came with 20 mm wide tyres on it. That wasn't a universal size, but it was very common. Not long after that, 23 mm tyres set in as the main size for a racing oriented bike. For a period of months, until I purchased a floor pump with a gauge, I inadvertently ran upwards of 160 psi in those skinny tyres. I had a frame pump that advertised "up to 150psi" and most of these claims are over-statements of the easy reality, so I pumped up the tyres as firmly as I could with that pump. I assumed it was around 130 or 140 psi. The gauge revealed that it was more like 180 to 200 psi! Good pump. Too much air.
My current race bike doesn't sport tubeless-friendly rims. I run latex tubes and light 25 mm tyres - which is actually considerably lighter than the tubeless alternative. Some 60 grams for a tube plus 200 grams for a tyre and nothing else; adds up to 260 g. I can run these at 100 psi for a solid but comfortable feel.
My commuting bike has tubeless-ready rims. I have used both tubes and sealant over the time I've had the wheels. I could run similar 260 gram tyre and tube options as on the race bike. In tubeless, the similar racy tyre is around 300 grams and the valve stem only weighs a few more grams plus 50 ml of sealant adds up to 330 grams or so. Currently I am running a less racy option (meant to be quite puncture resistant and long wearing) that is only 350 grams. So far they are great.
Sealant does its magic when pushed through a small hole to congeal and seal the hole. Often prior to the rider knowing it ever happened. With 25 psi, there isn't much force behind the sealant and it works effectively. With 125 psi, sealant often sprays under pressure (it has happened to me twice before). Sometimes everywhere (bike, rider, pavement, etc). Currently on 28 mm tyres I only require 75 psi tubeless and it feels fine.
The implied issue here is that as tyre pressures get lower and lower, a single psi difference becomes a higher and higher percentage of the total pressure, and therefore more important. An expensive pump might have an accurate gauge installed, but mechanical gauges are subject to being bumped out of accuracy even if they were good to begin with. An associated problem is scale. Road bike tyres require around 100 psi. The gauge typically goes to 150 psi or higher. That leaves very little resolution for distinguishing between 16 and 18 psi for a MTB tyre (those 2 pressures ride completely differently, by the way). You can get a MTB specific pump with a 50 psi gauge, which improves things a lot (but then you'll require a second pump for the road bike if you have one of those too).
I've just purchased my second digital tyre gauge. The first one recently stopped working after almost 15 years. I can only hope the new one lasts as long. This coincides with the gauge on my main floor pump deciding to become crazy in its readings. Instead of being around 3 psi out at 20 psi (consistently, so I could compensate) it is now something like +15 psi at indicated 20. With no markings below 10, the range I need to in when be filling up MTB tyres is useless to me. It seemed better to buy the gauge than to get a new pump - the pumping part works perfectly still.
The pump explains the difficulties in my household with off-road riding of late. An extra 15 psi in a tyre, when it is only meant to have around 20 psi inside in the first place, means it doesn't ride like you'd expect. Bouncing off of things was common, where normally it would roll over the top. I tried a portable mechanical gauge in the interim, but it really only reads to the nearest 5 psi and even with that I found out using the digital gauge that it was reading low; my road bike tyres had around 5 psi too much inside.
Your tyres are your connection to the terrain, be that the smooth boards of an indoor velodrome, the bare rockslabs in Squamish or anything in between. It really is the first adjustment after bike fit to get correct on your own bike(s) before adjusting anything else. Not the suspension sag or anything else related to suspension should be seen as more important. Without having the tyres right first, the suspension cannot work as it is meant to. On a road bike, go too low and it won't go around corners confidently and safely. Get it too high and you'll be prone to extra punctures, you might bounce off of road bumps and you might damage a rim if you go too high. It also rides poorly with too much air on any surface.
Accurate and consistent air pressure in the tyres is the start of a good experience with any bike.
And all these years later, tyres filled with air are still mission-critical on all bikes.
Mountain bike tyres rely on conforming to the surface (where lower pressure is better) and forcing sharp edges into the ground (where higher pressures are better) to enable navigation of unpaved terrain. From MTB commercial origins in the 80s until the late 2000s, we were forced to use a rubber tube to contain that air. The tubes can be heavy - thinner is better from a riding standpoint. But thin tubes are easy to cut. Tubed MTB tyres were always prone to pinch flats necessitating more air than optimal for ultimate traction and handling.
And thanks to Keith Bontrager, we got skinny rims for most of those tubed years too. Keith re-rolled some road rims to 26" diameters, achieving something that the rim companies weren't doing: strong and light double-walled rims. The problem was that road rims were very skinny (around 13 mm inside width) and gave a light globe shape to a 2"+ knobby tyre. That, too, necessitated more air pressure.
Almost all mountain bikes above the very cheapest are now tubeless. To the point that high-end MTB no longer come with tubes inside and leave the tubeless step to the consumer or shop. Now they come with some sealant and valve stems and no tubes.
With tubes and large tyres on relatively skinny 26" rims, I had to run as much as 40 psi to prevent pinch flats. There is a lot of traction loss with that much air pressure in the tyre. In the case of some rim/tyre combinations, I had the tyre rotate on the rim under braking with (not much) lower air pressures - which ultimately rips the valve stem off the tube if it progresses far enough.
My current quiver of mountain bikes all run 30 mm inside width rims. That width supports the sidewall of the wide tyre much better than one half that width can. I run CushCore foam inserts in all of the wheels too. CushCore acts like a fork volume spacer by decreasing the space for air in the tyre (by roughly half). When a bump is hit, the pressure in the tyre increases more quickly due to the lower volume present. Less "travel" is used in the tyre for any given bump than would be the case without the foam. When a big enough impact occurs to bottom out the tyre on the rim the foam intervenes and cushions the impact. Impacts have to be much larger to cause rim damage. On 29" wide wheels with foam inserts I (@100 kg) can get away with around 20 psi - around half of what I used to use with tubes.
On the road side it is only very recently that mainstream tyres and wheels have gone tubeless. And it is by no means (yet, at least) a universal changeover the way it has been in mountain. High pressures and a desire for lightness push more strongly towards tubes. The biggest lever moving us to tubeless tyres has been the realisation that wider tyres are all of: faster, more comfortable, better handling and can be similarly aerodynamic if the wheel is designed for the bigger rubber.
My first new bike in Canberra came with 20 mm wide tyres on it. That wasn't a universal size, but it was very common. Not long after that, 23 mm tyres set in as the main size for a racing oriented bike. For a period of months, until I purchased a floor pump with a gauge, I inadvertently ran upwards of 160 psi in those skinny tyres. I had a frame pump that advertised "up to 150psi" and most of these claims are over-statements of the easy reality, so I pumped up the tyres as firmly as I could with that pump. I assumed it was around 130 or 140 psi. The gauge revealed that it was more like 180 to 200 psi! Good pump. Too much air.
My current race bike doesn't sport tubeless-friendly rims. I run latex tubes and light 25 mm tyres - which is actually considerably lighter than the tubeless alternative. Some 60 grams for a tube plus 200 grams for a tyre and nothing else; adds up to 260 g. I can run these at 100 psi for a solid but comfortable feel.
My commuting bike has tubeless-ready rims. I have used both tubes and sealant over the time I've had the wheels. I could run similar 260 gram tyre and tube options as on the race bike. In tubeless, the similar racy tyre is around 300 grams and the valve stem only weighs a few more grams plus 50 ml of sealant adds up to 330 grams or so. Currently I am running a less racy option (meant to be quite puncture resistant and long wearing) that is only 350 grams. So far they are great.
Sealant does its magic when pushed through a small hole to congeal and seal the hole. Often prior to the rider knowing it ever happened. With 25 psi, there isn't much force behind the sealant and it works effectively. With 125 psi, sealant often sprays under pressure (it has happened to me twice before). Sometimes everywhere (bike, rider, pavement, etc). Currently on 28 mm tyres I only require 75 psi tubeless and it feels fine.
The implied issue here is that as tyre pressures get lower and lower, a single psi difference becomes a higher and higher percentage of the total pressure, and therefore more important. An expensive pump might have an accurate gauge installed, but mechanical gauges are subject to being bumped out of accuracy even if they were good to begin with. An associated problem is scale. Road bike tyres require around 100 psi. The gauge typically goes to 150 psi or higher. That leaves very little resolution for distinguishing between 16 and 18 psi for a MTB tyre (those 2 pressures ride completely differently, by the way). You can get a MTB specific pump with a 50 psi gauge, which improves things a lot (but then you'll require a second pump for the road bike if you have one of those too).
I've just purchased my second digital tyre gauge. The first one recently stopped working after almost 15 years. I can only hope the new one lasts as long. This coincides with the gauge on my main floor pump deciding to become crazy in its readings. Instead of being around 3 psi out at 20 psi (consistently, so I could compensate) it is now something like +15 psi at indicated 20. With no markings below 10, the range I need to in when be filling up MTB tyres is useless to me. It seemed better to buy the gauge than to get a new pump - the pumping part works perfectly still.
The pump explains the difficulties in my household with off-road riding of late. An extra 15 psi in a tyre, when it is only meant to have around 20 psi inside in the first place, means it doesn't ride like you'd expect. Bouncing off of things was common, where normally it would roll over the top. I tried a portable mechanical gauge in the interim, but it really only reads to the nearest 5 psi and even with that I found out using the digital gauge that it was reading low; my road bike tyres had around 5 psi too much inside.
Your tyres are your connection to the terrain, be that the smooth boards of an indoor velodrome, the bare rockslabs in Squamish or anything in between. It really is the first adjustment after bike fit to get correct on your own bike(s) before adjusting anything else. Not the suspension sag or anything else related to suspension should be seen as more important. Without having the tyres right first, the suspension cannot work as it is meant to. On a road bike, go too low and it won't go around corners confidently and safely. Get it too high and you'll be prone to extra punctures, you might bounce off of road bumps and you might damage a rim if you go too high. It also rides poorly with too much air on any surface.
Accurate and consistent air pressure in the tyres is the start of a good experience with any bike.
