The much discussed topic of Steering axis, Rake, Offset and Trail.
With KTM adjustable steering stem soon to be released, we thought we might address an explanation of this subject.
If nothing else it will make you an expert in the pub.
Offset, Rake, Steer Axis and Trail Explained. – All part of your bikes geometry.
Not many aspects of dirt bikes other than suspension has more discussion and confusion about it than steering geometry.
This explanation is not about how the change in offset etc changes the handling of a bike although it touches on it at times, it is about the terms used and the effect that offset changes have on them.
The biggest confusion is about rake and steering axis, in the past on dirt bikes and maybe still on road bikes the rake was not necessarily set the same as the steering axis, however these days it almost certainly is on dirt bikes.
Therefore if the offset is changed on a bike it must be both top and bottom triple clamps offset by the same amount, I have seen some talk about supermoto set ups being tried with different top and bottom offsets, but this is not about supermoto or road so I will ignore that.
The biggest affect that offset changes is trail. Trail is the difference between the steering axis line at ground level and a vertical line down from the center of the front axle, its the trail that causes the shopping trolley effect and can make the bike steer in quicker or slower etc.. and has a bearing on stability at speed.
It is as most things more complicated because it is also dependent on the steering axis, which can be changed by such things as shock or fork preload and spring weight affecting brake dive etc.. but for this explanation with all things being equal in each case.
Reducing offset increases trail, thus moving the front tire contact patch slightly further back towards the engine which loads the front wheel a little more and when turned the contact patch moves slightly less upwards toward the front of the tyre. This change is generally responsible for the bike especially KTM’s steering quicker and pushing the front less, it can also increase the self centering effect slightly.
On KTM’s a lot of riders claim that this also makes the bike feel more stable at speed, I think that suspension setup will play a big role in the stability side of things.
Increasing the offset Reduces Trail, This does the opposite to above and moves the tyre contact patch slightly forward, more “chopperish” this could be good for long distance high speed, and would reduce the effort to steer the bike.
Either way is dependent on other factors like riding style, suspension and personal preference.