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A fork rake question...(13 posts)

A fork rake question...Psychler
Sep 1, 2003 6:47 PM
A slightly new development on the handling issue outlined below (I am going nuts thread)...
I, in an attempt to get crazier, was going to put my old frame back together to see 1. if the toe overlap is as great an issue as I think it is on the new frame (I just don't recall it on the old frame, but on the new one, it was immediatly noticed) and 2. the handling.
Upon taking the fork off, I see it is a 43mm rake...the new fork (which I didn't install, used the old fork instead) is a 45mm ... both Ouzo Pros.
What will going from a 45mm to a 45mm have??? any distance change from the crank center to the front skewer (I did a prelim measurement and found it went from 77.5 to 78)? Handling? The blueprint that came with the frame states it was designed for 45, not the 43 I have been using.
Don't use the 45mm rake....C-40
Sep 2, 2003 5:11 AM
I've got both a 54cm Colnago (which has a lot of trail) and a quicker steering Fondriest. The Fondriest front-center is 2cm shorter than the Colnago due to a 1.5cm shorter TT and a steeper (but unspecified) HTA. The Fondiest has a hint of toe overlap, while the Colnago has lots of clearance. The Fondriest is quicker, but scarcely noticeable, except on high speed mountain descents, where the quicker steering requires a lighter touch than the Colnago (that seems to steer itself).

A 73 HTA is very standard. The 43mm rake produces a 57.7mm trail. If you want fairly slow steering, you should not use a 45mm rake. This would further reduce the trail to 55.7mm and speed up the steering even more. I would agree with the others who suggested a change to a 40mm rake. This will increase the trail 60.9mm. More trail will slow the steering.

Some builders, like Tom Kellog of Spectrum, are very insistent that 55-56mm is the perfect "neutral" amount of trail. For comparison, a Colnago with a 55cm TT would have a trail of 65.2mm. Obviously builders like Colnago do not agree with Kellog.
There's something wrong...divve
Sep 2, 2003 5:40 AM
....with my understanding of the terms rake and trail or possibly you have it reversed.

To establish what's what, in the image below F is the rake and I is the trail, correct? If so then increasing F (rake) would decrease steering speed, even though it shortens I (trail). To elaborate, increased rake will slacken the caster angle so to speak and result in better self-centering.
No, he's correct.Chen2
Sep 2, 2003 6:09 AM
See the rake and trail examples given below in the "I am going nuts" thread. Increasing rake does not change any critical angles. Increasing rake decreased trail and decreases the self centering effect of trail.
No, he's correct.divve
Sep 2, 2003 6:32 AM
Yes, I looked at that thread and almost went nuts myself therefore the new post with an illustration:) My understanding is that rake itself is responsible for a degree of self-centering as well? If you use an extreme example for instance by increasing the rake (F) to 100mm and leave the rest the same, wouldn't that cause the bike to lift at increasingly higher levels as you continue to steer sharper, thus increasing the self-centering effect?
My experience (long)Chen2
Sep 2, 2003 8:35 AM
I've actually seen published contradicting explainations of how trail effects steering or handling. I'm no expert on the subject, but I have done quite a bit of net research due to my general interest. Most experts would seem to agree that less rake and more trail result in greater stability and more of a self-centering effect. Where there seems to be some disagreement is on whether this is "slower steering" or "quicker steering". In my own experience I recently changed the fork on my primary bike from a 43cm Ouzo Pro to an identical 40cm Ouzo Pro (came off of my wife's old bike). Immediately I could feel a difference, not necessarily an improvement. I would have to say that the 40 is more stable and feels more secure when riding hands off. I think I would prefer the 43 if I were a crit racer intent on moving laterally quickly. My bike is a 56cm 5500 Trek with a 73.8 degree HTA, pretty steep so the trail is fairly low with either fork. My commuter bike has a more laid back HTA and I think I could walk up and down the top tube without dropping the bike.
Sep 2, 2003 9:07 AM
I've seen a lot of contradictory info out there, too. In my experience a bike with less trail (like my Merckx, which has a lot of fork rake--around 50mm) will be quicker handling at lower speeds but then firm up at higher speeds. It's also easier to ride no hands than say a mountain bike with a lot more trail, I think because it responds more quickly to just a tad of input (weight shifts, nudges, etc.). It also corners very well, allowing more input or even a change of line in the middle of a turn. The Merckx in particular manages not to be too sketchy, though, because it has a longer wheelbase than your typical "crit" style road bike. I still think more neutral trail (like I have on my custom frame) is better for all around riding. With all that said I don't think 2mm in rake is gonna make a huge difference.
Sep 2, 2003 10:52 AM
I understand what both of you are saying it just seems counter intuitive to me. Perhaps I'm thinking about it in the wrong way and should compare it more to the little wheels on some shopping carts, i.e., by adding rake the front has a greater tendency to twist from side to side and in an extreme geometry would want to reverse. Thanks for the explanations.
Sep 2, 2003 12:22 PM
At firs it seemed counter intuitive to me also. I was too concerned with wheelbase which has nothing to do with steering. Wheelbase has a lot to do with how big an area we need to reverse direction, but does not effect how the front end steers.
With most cars the castor angle can be adjusted to change the trail or lead. We can't change the castor angle (HTA) on a bike without bending the frame. So with bikes it's the fork rake that may be changed to make a steering adjustment. Rake and trail are linear distances, not angles.
Sep 2, 2003 11:07 AM
The formula for trail tells the whole story:

Trail = (R/tanH) -(rake-sinH)

R is the tire radius and H is the head tube angle. The first half of the equation is the amount of trail with no rake. As you can see, all rake REDUCES the the amoutn of trail.

A bike with a lot of trail "self centers" at normal riding speed. In other words it takes more input to make it change direction. If you want more trail with a given HTA, the rake must be reduced.
Sep 2, 2003 11:23 AM
....I think the correct image of how it works is finally starting to sink in:)
Big trail = good, small trail=bad!! (nm)Alexx
Sep 2, 2003 3:31 PM
Big trail = good, small trail=baddivve
Sep 2, 2003 5:44 PM
...I understood the effects of trail. The problem was that I wasn't able to correctly visualize how rake effected it.