Front fork rake/offset???

[jalabert]

Hi

I'm thinking of buying a Reynolds front fork. It comes with an optional 40, 43 or 45mm rake. It will be as an replacement for the fork that came on my '01 Quantum Race, which I thin might have a rake somewhere around 35mm, but I can't measure it exactly.

My questionis whether or not I will be able to feel any difference between the differet options and if yes, then what to choose?

Thx

 

[ant evans]

There are two effects you want to watch for: self-centering, and what I would call gearing, both more noticeable at low speeds. Self-centering is a function of the distance between the tyre contact patch and the steering axis, which increases with more rake. At zero rake, there is no self-centering. Not very important if you use your hands.

More important is the the distance from your hands to the contact patch (not the steering axis). The closer they are, the slower the steering will feel... that's why a longer stem (or wider bars) slows the steering and vice versa. More rake, quicker steering.

I've changed to a much shorter rake and the steering feels almost dead compared to what I'm used to. The only benefit is a shorter wheelbase (I carry my bike in narrow spaces a lot).

You also have to think about weight distribution. It's now a bit easier to go over the front of my bike under braking.

I'd go for more rake if I could, but if you go for less, and there is room in the cockpit, get a shorter stem to compensate for the effect on the steering.

 

[jalabert]

Okay thx, that clears up a few things for me. However I'm still not sure why a shorter wheelbase is generally to prefer? And I know it's not because people like that fact that the bike is easier to carry around narrow spaces

 

[rv]

there's a lot of info about rake, offset, trail, etc. out there...I don't think you want to mess with changing your rake. your bike manufacturer chose the original rake for a reason and you probably want to stick with it because you could introduce unwanted shimmy or instability. that said...

Lennard Zinn, a frame builder, posted this answer to a question on VeloNews.com --- hope it helps.


"Very briefly, "fork rake" is the offset distance of the front hub from the center of the steering axis, measured perpendicular to said axis.

"Fork trail" is the distance between the wheel contact point on the road and the intersection of the steering axis with the road. Generally, greater trail means greater stability, and it is dependent on head angle, fork rake and wheel diameter.

In answer to your question, increasing the head angle from 73 to 75 degrees with the same fork does not change the rake. It does, however, decrease the trail and hence the bike's stability.
Trail is also reduced by increasing the fork rake with a given head angle.

Please see the Tech Report article in the July 1, 2002 issue of VeloNews. I explain all of this in great detail there, along with illustrations to further clarify it."
--Lennard Zinn

 

[rv]

there's a lot of info about rake, offset, trail, etc. out there...I don't think you want to mess with changing your rake. your bike manufacturer chose the original rake for a reason and you probably want to stick with it. That said...

Lennard Zinn, a frame builder, posted this answer to a question on VeloNews.com --- hope it helps.


"Very briefly, "fork rake" is the offset distance of the front hub from the center of the steering axis, measured perpendicular to said axis.

"Fork trail" is the distance between the wheel contact point on the road and the intersection of the steering axis with the road. Generally, greater trail means greater stability, and it is dependent on head angle, fork rake and wheel diameter.

In answer to your question, increasing the head angle from 73 to 75 degrees with the same fork does not change the rake. It does, however, decrease the trail and hence the bike's stability.
Trail is also reduced by increasing the fork rake with a given head angle.

Please see the Tech Report article in the July 1, 2002 issue of VeloNews. I explain all of this in great detail there, along with illustrations to further clarify it."
--Lennard Zinn

 

[ant evans]

To get an idea of what effect wheelbase has, just ride a 10-foot long bike. A longer wheelbase means it takes longer to change direction, so it feels less responsive, more stable, and takes up less road (both lengthwise, and widthwise when cornering).

You can have a wheelbase of zero: a unicycle, which has no inherent straight-line stability.

This is a different effect from the stability you get from steering geometry.