Fork Offset?

 
06 December 2017

What's all the fuss about fork offset?

When we talk about bike setup, it is often suspension that comes to mind. ‘Hard and fast’, ‘slow and soft’ are phrases thrown about by many, but there's way more to it than that. What are we actually talking about? 

Really, anything that can be altered on your bike accounts for bike setup. One bike might be set up with a certain handlebar sweep; another rider may use the same bars but roll them further forwards or backwards altering how they feel. At this level of detail, bike set-up as a whole now looks like a lot to take in… Well, there’s now a whole new variable in the mix. Something a bit more fundamental. 

For 2018 Transition Bikes and Orbea have chosen to use custom forks from suspension manufacturers with a fork crown which shortens the offset between the front wheel's axle and the head tube. But why?   

This may be considered as a way of shortening the wheelbase whilst keeping the reach of the bike unchanged. It only seems like yesterday when companies were throwing 40-50mm onto their DH wheelbases to help with the ‘man tracks’ that the elite riders are asking for. So why would any of us want to make it shorter?

Well, it’s not wheelbase we should be looking at. It’s a dimension called ‘trail’. Trail is the distance on the ground between the front wheel steering axis and the contact point between ground and tyre. It can be defined as “the amount the contact patch ‘trails’ the steering axis.” This can be seen below.

The trail of a bicycle makes it easier to ride because it links the lean angle of the frame with the turning angle of the fork. Leaning the bike causes the fork to turn in that direction, because the frame is lower after the fork has turned. So when the front wheel is knocked out of line by a bump or camber in a track for example, the front wheel will turn in to the lean of the bike and act to stand the bike back up.

If the front wheel of bike was directly beneath the head tube, i.e. head angle of 90 degrees and fork offset of zero, the bike would have zero trail. As shown in the diagram below, this would cause a high tendency for the wheel to rotate about the steering axis, and this can become ‘high speed wobble’ or a ‘tank slapper’ at speed which can be uncontrollable and dangerous.

So the more trail a bike has, the greater the self-centring effect of the front wheel. This gives the bike more directional stability, but it’s harder to turn the bars.

So by shortening the offset between the head tube and the fork stantions, Transition Bikes will be increasing their trail value, thus improving directional stability. For big tracks, this will help riders through rough sections of tracks, by dampening out feedback from the terrain and giving the bike a ‘point and shoot’ feel.

Looking past the change in trail, shortening the fork offset will also move the combined centre of gravity of the bike and rider forwards. This will put more weight on the front end and will help to increase grip.

So what’s the final verdict? Although changing fork offset has been around for a long time in motorsport, this new adjustment to mountain bike set-up is a welcomed addition. Despite a vague feeling at the front end being expected at slow speeds, riders who want to ride fast, steep and aggressive descents will benefit from the increased directional stability and front end grip that a shortened fork offset provides. We look forward to putting it to test!

Written by Jono Jones