Fork – trail, offset and rake – please explain

I’ve read the piece on Sam at Singular website about trail of forks and it’s effect on handling. I still don’t get it! Can somebody explain it to me??

Say if you have a 420mm rigid fork on a bike with a 71 degree head angle.

How’s does changing the trail, offset and rake of a fork effect the handling of the bike? and how do the 3 different elements interact or be affected by each of the other ones?

Anybody?

Have a read
http://twentynineinches.com/2008/07/27/an-experiment-in-front-end-geometry/

http://www.tonyfoale.com/

Excellent bed time reading.

Cheers

So from any given starting point….

Increasing fork length slackens HA, increases trail, slows steering.

Increasing offset (rake – same thing) reduces trail, quickens steering.

That’s the basics, but then two bikes with the same trail number and even everything else the same but a different combination of offset and head angle will still feel different to ride.

Thanks Sam but:

First point – understood

Second point – understood

Third point – my mind has been blown yet again

rake and offset are not really the same thing. In the bicycle world its often confused but rake is the steering head angle – offset is how far ahead of the steerer the spindle is held which controls how large the trail is

Rik,

Taking the pic below, if you took the top left set up and increase offset (by quite a bit admittedly), you could get the same trail as the top right set up. They’d feel quite different though.

So on the example:

Say if you have a 420mm rigid fork on a bike with a 71 degree head angle.

If the fork has:

Rake/offset: 48mm compared to 35mm – you would get an decrease in trail, so therefore a quickening of the steering without a change in head angle even though you are effectively lengthening the wheelbase.

Is that right??

Oh all good stuff those drawings – cheers

Rik – yes you’d ‘quicken’ the feel of the steering.

Wheelbase is a whole different thing…

An area to cause great confusion as well is that things can alter the feel or alter the actual effect. Long stems for example cannot have any effect of steering geometry so cannot slow the steering – however they alter the weight distribution and the feel of the steering.

Slowing the steering with rake or with trail feel differenct

Latest issue of privateer has a good article on this very subject, aswell as other basics about geometry.

Clubber – I know wheel base is different but in this example if you increase the offset – the wheelbase of the bike will get slightly larger. That’s what it seems to look like in the 4th drawing.

So you should have a slightly more stable wheelbase even though the steering is slightly quicker (if all the numbers/values stay the same). Or am I missing something?

Yes, you’re right – the steering will feel quicker (easier/lighter, really) but because the bike is longer, the actual effect of steering is less (to simplify!)

Well at least I seem to be getting it – even if it is just the basics.

Experimentation is useful once you understand the basics and those articles and diagrams explain the basics pretty well, you seem to have got it.

The truth is there’s few rules and no ‘right’ answers to all this, it’s a case of getting a feel for it. Try a couple of different rigid forks on your bike and see how it feels, honestly it’s the best answer to your question.

I think it’s all something best visualised once understood, ie where is my weight, where are my wheels, how do I want it to feel / respond – and the numbers then come together. Maybe that’s a crap answer, but after years of playing around with all this that’s as good as I can give. Theories are ok but practical experimentation is worth a lot more.

There’s also a lot of waffle out there about ‘light’ or ‘fast’ steering, ‘responsive’ etc. I think it takes so little effort to turn a bike that many bikes are too short in trail as a result of someone thinking ‘responsive’ numbers are good.. or that ‘fast’ handling is better for going fast, or twitchy feels ‘light’. That’s where having some practical experience is valuable – ie knowing roughly what +/-10mm of trail is worth.

steering is also complicated by leaning action on bikes, weight distribution, tread design etc

if you want some real fun reading of an old school in the have a look at this old article – long but an interesting read,

The Stability of the Bicycle

– also useful for all those supermakrt bso with backward forks

my suspicion is that that slacker angle have a a bigger effect in handling due to the changes in rider weigh distribution (rearwards) which is of course beneficial to riding downhill than the actual effect on steering

rake and offset are not really the same thing. In the bicycle world its often confused but rake is the steering head angle – offset is how far ahead of the steerer the spindle is held which controls how large the trail is

I think there is a standard accepted nomenclature in the bicycle industry that rake and offset are interchangeable. Rake is usually used when the ‘offset’ is achieved though bending (raking) the fork legs, offset where is is done by off-setting the fork crown or dropouts. To then say ‘rake’ equates to head angle (as true as that may be in the motorcycle world) serves only to confuse the matter, especially so where someone is trying to get their head around the concepts.