Tubeless and valve cores
20/06/23 19:31 Filed in: Gear
Those pesky presta valve cores. How many times have they caused a problem on the trail?
My most vivid memory of a problem was once I was riding on the backside of the Stromlo. A group of young men were out there with zero tools amongst the group. One guy had burped a tyre and needed some air to keep pedalling. Reluctantly (this should be a blog entry in its own right) I leant him my pump. Those who know, will know - but the rest of you will not. I use a Lezyne pump. It is great for pumping but because it threads onto the valve it also tends to unthread the valve cores once pumping is finished. Especially if the full pressure is still in the rubber hose part of the pump. My solution to this is to really snug up the cores when I install them. So, guy pumps up his tyre until it feels full and when he unscrews the pump hose it brings the valve core with it. All the air departs his tyre. I explain what has happened. He reinstalls the valve core and refills the tyre. And again the hose removes the valve core. At this point I departed with my pump and he started walking back to his car.
Check those valve core people. Even with the pressure release button on the hose (a direct reaction by Lezyne to this problem) it tends to unthread loose valve cores.
The other problem with tubeless and presta valve cores is that they love to clog up with sealant. A pump usually pushes the air with enough force that it will go in, but the release of air is much more gentle and can be completely blocked by a blob of dried sealant in the valve shaft. I also find the exit hole into the rim cavity gets blocked. These can be pushed out with a tiny Allen key pushed up the core-free shaft of the tubeless valve. Individual cores can usually be cleaned of the offending sealant but they can also be purchased for very little money - one way or the other, hard to pump tubeless tyres can be fixed easily.
Some tubeless valves are T-shaped, where others are Y-shaped. Some rims do better with one than the other. If the hole is snug around the valve stem, then the Y-shaped ones often don't seal well - in these cases the T-shaped ones are better. With larger holes, the T-shape can leak, where the Y-shaped ones will snug down into the hole and create a good seal. Bontrager valves are T-shaped but have an o-ring on the underside of the T. They're specifically designed to work with Bontrager plastic rim strips (which have a smooth flat surface at the valve hole) and are probably the best option for Bontrager wheels with the rim strips. They might not work so well with other rims.
In other words, it pays to match the valves and the rims.
Insert-friendly valves are ones that have the air exit port in a location that cannot be blocked by the foam of the insert. Cush Core valves have a large base with a large hole on the side of the base - free of interference by the foam insert. Early Cush Core inserts (before they had their own branded model) were Bontrager-looking inserts with a hand cut slot in the T-piece to allow air to escape if the foam rests on the top of the T.
Santa Cruz has spun off their carbon rims to its own sub-brand: Reserve. They now have a high-flow tubeless valve that is kinda like presta, but flows several times more air because the entire valve stem is the valve - not some miniature piece that threads into the stem. I've heard of, but never seen, people using Schraeder valves in tubeless too - they're more robust than presta.
Whatever valves you might use on your tubeless rims, when a new tyre goes on, or even a top-up of sealant, that is a great time to pay a little attention to the valve.
My most vivid memory of a problem was once I was riding on the backside of the Stromlo. A group of young men were out there with zero tools amongst the group. One guy had burped a tyre and needed some air to keep pedalling. Reluctantly (this should be a blog entry in its own right) I leant him my pump. Those who know, will know - but the rest of you will not. I use a Lezyne pump. It is great for pumping but because it threads onto the valve it also tends to unthread the valve cores once pumping is finished. Especially if the full pressure is still in the rubber hose part of the pump. My solution to this is to really snug up the cores when I install them. So, guy pumps up his tyre until it feels full and when he unscrews the pump hose it brings the valve core with it. All the air departs his tyre. I explain what has happened. He reinstalls the valve core and refills the tyre. And again the hose removes the valve core. At this point I departed with my pump and he started walking back to his car.
Check those valve core people. Even with the pressure release button on the hose (a direct reaction by Lezyne to this problem) it tends to unthread loose valve cores.
The other problem with tubeless and presta valve cores is that they love to clog up with sealant. A pump usually pushes the air with enough force that it will go in, but the release of air is much more gentle and can be completely blocked by a blob of dried sealant in the valve shaft. I also find the exit hole into the rim cavity gets blocked. These can be pushed out with a tiny Allen key pushed up the core-free shaft of the tubeless valve. Individual cores can usually be cleaned of the offending sealant but they can also be purchased for very little money - one way or the other, hard to pump tubeless tyres can be fixed easily.
Some tubeless valves are T-shaped, where others are Y-shaped. Some rims do better with one than the other. If the hole is snug around the valve stem, then the Y-shaped ones often don't seal well - in these cases the T-shaped ones are better. With larger holes, the T-shape can leak, where the Y-shaped ones will snug down into the hole and create a good seal. Bontrager valves are T-shaped but have an o-ring on the underside of the T. They're specifically designed to work with Bontrager plastic rim strips (which have a smooth flat surface at the valve hole) and are probably the best option for Bontrager wheels with the rim strips. They might not work so well with other rims.
In other words, it pays to match the valves and the rims.
Insert-friendly valves are ones that have the air exit port in a location that cannot be blocked by the foam of the insert. Cush Core valves have a large base with a large hole on the side of the base - free of interference by the foam insert. Early Cush Core inserts (before they had their own branded model) were Bontrager-looking inserts with a hand cut slot in the T-piece to allow air to escape if the foam rests on the top of the T.
Santa Cruz has spun off their carbon rims to its own sub-brand: Reserve. They now have a high-flow tubeless valve that is kinda like presta, but flows several times more air because the entire valve stem is the valve - not some miniature piece that threads into the stem. I've heard of, but never seen, people using Schraeder valves in tubeless too - they're more robust than presta.
Whatever valves you might use on your tubeless rims, when a new tyre goes on, or even a top-up of sealant, that is a great time to pay a little attention to the valve.