Front fork 'Rake' Question

dru

Hi there,

In a nut shell can anyone tell me the affects of changing the ‘rake’ on my set of forks

For instance, if we take the 2 sets of fork which have either a 18mm rake or 43mm rake.

If my current bike has a fork of 18mm rake, what would the affects be to put on a set with 43mm rake and also visa versa?

Cheers and thanks,

Dru

Ben6899

Decreased rake = more positive, 'twitchy' handling.

rake

longer rake should give a more stable ride. its also connected the the head angle. so dont change it too dramatically or you could screw up the handling.

dru

cheers, so this would explain why my road bike has a large rake on it, bt my TT bike has forks with very little rake on it

Monty Dog

Fork rake or offset is one of the combination of the factors that influence trail on a bike - the following will help explain:
http://www.phred.org/~josh/bike/trail.html

John.T

Ben6899 wrote:

Decreased rake = more positive, 'twitchy' handling.

No. Less offset will increase trail by moving the wheel further behind the steerer centre line. This increases the castor effect and makes for slower or more stable handling. All other things being equal.
This is why many TT frames have slack head angles and small offset.

AidanR

The trail is the distance between the intersection of an imaginary line through the head tube and the ground, and the contact patch of the wheel. The fact that the contact patch of the wheel is behind the intersection creates a self-centering effect, increasing stability.

If you increase the rake of the fork, the wheel is effectively pushed forward. This decreases the trail, making the steering faster.

If you slacken the head tube angle, you push the intersection point forward more than you push the wheel forward, increasing the trail and making the steering slower.

If you slacken the head tube angle and increase the fork rake at the same time you can keep the same steering sensitivity whilst pushing the wheel forward (though you also have to factor in the affect of increased wheelbase), which is useful if you have a small frame, fat tyres, mudguards, or a combination of the above and wish to avoid toe-overlap. The downside is wheel flop, which can negatively affect handling. In practice manufacturers do not want to incur extra costs by having different forks with different rakes, which is why many smaller frames are geometrically compromised affairs with toe overlap and unreasonably steep seat tube angles.

Ben6899

John.T wrote:

Ben6899 wrote:

Decreased rake = more positive, 'twitchy' handling.

No. Less offset will increase trail by moving the wheel further behind the steerer centre line. This increases the castor effect and makes for slower or more stable handling. All other things being equal.
This is why many TT frames have slack head angles and small offset.

But for a given frame and head tube angle, a straight fork will offer more positive steering than a raked one? Won't it?

John.T

Ben6899 wrote:

John.T wrote:

Ben6899 wrote:

Decreased rake = more positive, 'twitchy' handling.

No. Less offset will increase trail by moving the wheel further behind the steerer centre line. This increases the castor effect and makes for slower or more stable handling. All other things being equal.
This is why many TT frames have slack head angles and small offset.

But for a given frame and head tube angle, a straight fork will offer more positive steering than a raked one? Won't it?

If by straight you mean one with the hub on the centre line of the steerer (no offset) then this will give very stable and rather slow steering. You will not find forks like this on any road bike.
If you mean a fork with straight blades but the same offset as one with curved ones then it is no different. There may be a little more 'give' in the fork but that is all.
If by positive you mean sharper, quicker responsive steering you need less trail so more offset, again assuming the same head angle.
It is better to use the term 'offset' as while in cycling rake and offset are really the same thing our motorcycling friends have a slightly different deffinition of rake. This difference cropped up on here some time ago so for clarity I only use offset now.

Ben6899

Thanks John.

Peddle Up!

I was asked this question during a job interview once, in the days when it was fashionable for HR wonks to ask "clever" questions unrelated to the job under discussion.

I had no idea then, and neither, I suspect, did the b*ll*ck heads on the other side of the table.

However, I am enlightened by the knowledgable replies above, so thanks for that.