The flipside of offset

While I was in Whistler riding, PMBIA founder and Zep training owner Paul Howard published ( his take on fun bike geometry and why the new reduced offset forks are a good idea. The gist of it is the reduced offset fork decreases the front centre (the length of the bike between bottom bracket and front hub axle) to the benefit of riding.
I told him then, and I'll write about it now; I disagree.

For my "big" bike (Trek Slash) the front centre is 814mm in length. The high offset fork I use has 51mm of offset (the hub axle is 51mm forwards of the centreline of the steerer tube). The low offset forks Paul is advocating typically have 44mm of offset. That is 7mm less. That 7mm is not much, but it has a direct impact on two aspects of the bike's geometry: front centre and trail.

Seven millimetres is less than 1% of the front centre dimension (0.86%).
The geometry chart for my bike tells me it has 114mm of trail (the distance that the contact point of the front tyre "trails" behind the imaginary contact point of the centre of the steering axis - ie, where the steerer tube points at the ground. See image below.). The low offset fork increases that by 7mm to 121mm - a change of 6.1%.

There is almost a 10x proportional increase in trail compared to the decrease in front centre. I know which one I think is going to be felt more.
Whitt and Wilson's book Bicycling Science (I recommend you read it if you are interested in the Physics of bike riding) has an entire section devoted to trail, and its impacts on bike handling. Suffice to say that trail is critical, but what is appropriate depends on both the wheel size and the head angle. [Their formulae show that the extra trail from a low offset 29r fork move the bike quite close to the “unstable” region.]

I have a hardtail that has a pretty steep head angle and unfashionably short reach but it is the only mountain bike I've ridden with two different offset forks. Originally it had a Manitou Minute fork with 46 (I think) millimetres of offset. Now it has a Fox 32 with 51mm of offset. Five millimetres more front centre and 5mm less trail. It is impossible to separate the two without building a pair of experimental bikes (one with the extra offset in the frame to preserve front centre while changing trail) but I much prefer the bike with the lower trail number. Several discussions with a former professional product development rider revolved around his opinion that all mountain bikes having too much trail.
I also had a road bike with three different fork offsets on it during its life. It was only a couple of millimetres but it made a big difference in the riding. All down to trail.

The reality is that five or seven millimetres is not much, really, for either trail or front centre. One of Pinkbike's testers has swapped the two offset forks in and out of one bike in an attempt to learn all about the effects of the change. He concluded that one quickly adapted to either and forgot about it. That is true enough - there was no night & day difference in my hardtail when I put the Fox fork on. But I did like it better and I wouldn't want to revert.

borrowed from