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upgrading for, can I change the rake?(10 posts)

upgrading for, can I change the rake?sugarbaker
Sep 20, 2003 8:11 AM
I currently ride a specialized allez sport that has a 45 mm rake on the factory fork. The fork was damaged in a recent accident (I am fine) and needs to be replaced. I am wondering if it would be ok to put a fork with a 43mm rake on instead because there are more options with that measurement. Hope to hear from you all soon. Thanks

Stephen
absolutely, go aheadQubeley
Sep 20, 2003 3:11 PM
43mm rake supposed will steer quicker, I doubt you will ever notice. But yes, you get a fork with different rake.
wrong!C-40
Sep 21, 2003 6:10 AM
Changing from a 45mm rake to a 43mm will produce slightly slower steering, since it increases the amount of trail.

The formula for trail is (R/tanH) - (rake/sinH).
wrong!fallzboater
Sep 22, 2003 9:15 PM
I drew it out, and I'm pretty sure the formula should be:

Trail = (R/tanH)-(rake*sinH) (C40 had rake/sinH)

Where R = the tire radius (use 340mm for a 700C road wheel)
H is the head angle

Note that the head angle has a much larger effect on trail than the commonly available fork rakes.

In addition to the fork rake, you should also consider the dropout-to-crown distance, as there can be quite a bit of variation. A longer fork will decrease the head angle slightly (as well as seat angle), increasing trail.

-David
need a trig lesson??C-40
Sep 23, 2003 6:25 AM
Your formula is not correct. Rake (or offset) is measured PERPENDICULAR to the steering axis. It's the distance between two parallel lines, one through the steering axis and one through the center of the axle. Draw a line perpendicular to the steering axis, beginning at the intersection of the ground and the steering axis to the parallel line through the axle. This line is the offset or rake. The effect on the trail is the horizontal difference between steering axis line and the line through the axle. This line is the hypotenuse of a right triangle with the rake as the side opposite the head tube angle. The hypotenuse of this triangle is obviously rake/sinH.

You are correct that a longer or shorter fork will affect trail, but in general, most brands are within a few millimeters. It's often overlooked, but should be checked before buying a new fork.

Your comment about head tube angle having a greater impact than rake is also incorrect. Changing the rake from 45mm to 40mm for example, has the same effect on trail as reducing the head tube angle by .75 degree. That's a very significant change.

Here's a site which will calculate trail. My formula produces the correct value.

http://www.kreuzotter.de/english/elenk.htm
head angle/rake combinations?fallzboater
Sep 23, 2003 8:13 PM
I'll try it again. I never liked trig, particularly.

What I meant about head tube angle vs. rake is that the range of commonly available rakes (40-45mm) can effect trail by only 5mm, but commonly available head tube angles (72 to 74 deg) can effect trail by over twice that amount. You would have to make a fork quite a bit longer to change the head angle a signficant amount. I wonder what the tolerance of head angles is for most manufacturers? Especially on small frames, a very small change in top tube length, for example, could change head angle by 1/2 a degree or more. It's not that easy to measure with common tools, either.

What I haven't seen is a good description of is the handling effects of different head angle/rake combinations that have equivalent trail. For example, how would a 74deg/40mm bike compare with a 73deg/45mm bike. The trail figure is going to be within about 1mm, but the wheelbase will be longer with the 73deg/45mm. Would you expect to feel a significant difference?

-David
I ride two extremes...C-40
Sep 24, 2003 6:05 AM
The change in head tube angle due to a change in fork length is approximately 1/3 of a degree for every 5mm of length.

I don't understand your statement about a change in the top tube length affecting the head tube angle. Frames are built in jigs that clamp the head tube separately from the top tube and insure the proper head tube angle. The top tube has nothing to do with it. The accuracy of the jig should be better than .25mm over the distance from the head tube to the axle. This would translate to a miniscule difference in head tube angle.

As far as steering response is concerned, it shouldn't make any difference how an amount of trail is produced. The only contention that I've read regarding the head tube angle, it that the more relaxed angle with more rake might produce a ride that is slightly less harsh (typical touring bike setup). Of course, a relaxed head tube angle with lots of rake will produce a longer wheelbase. A longer wheelbase will steer a bit slower than a shorter wheelbase. Most builders increase the head tube angle and reduce the trail on large frames to speed up the steering response on the larger size frames. Colnago for instance, uses the same fork rake on all sizes and relies solely on head tube angle to adjust trail.

To make the bike tend to stay in a straight line, you want more trail. With an existing frame about your only option is less fork rake, unless you happen to find a longer fork.

More trail may help to eliminate a high speed wobble, but often the problem is simply a frame that is too flexible and hits a resonant frequency that makes the frame wobble. There is a discussion of this in Zinn's tech area at the Velonews website right now.

My 54cm Colnago has about 67mm of trail (if the geometry charts are correct), compared to many other brands that use 55mm. It definitely takes a lot more effort to make it turn during a high speed mountain descent. I also ride a Fondriest with a 2cm shorter front-center dimension, steeper head tube angle (unspecified by Fondriest) and 45mm of rake. This bike descends at high speed perfectly, but responds much quicker to steering input.
I ride two extremes...fallzboater
Sep 24, 2003 6:25 AM
I did read Zinn's article, very helpful.

I was planning on specifying a 72 head angle for my custom frame (with 44mm rake), but yours, Zinn's, and others' comments are making me think that 73 would be better. Descending is the main thing I want it to do well, and I don't want the handling too truck-like. Unfortunately, I haven't found _any_ very large frames with less than a 74 deg head angle to test ride.

I'll probably spec a 1-3/8" OX Platinum top tube, and it will be dropped and sloped as Zinn suggested. I'm "only" about 185 lbs, and most of the rest of the tubes will probably be S3 for lighter weight (the S3 downtube is large).

Thanks for your help, C40

-David
want to really make your head hurt?fallzboater
Sep 23, 2003 8:37 PM
Check out this site:
http://www.geocities.com/pganio/english.html

It'd be easier to follow if the translation was better.

What I'm really trying to figure out is if certain head angle/rake/trail combinations are more or less likely to cause either hands-off headshake (the kind you can usually stop by putting a knee on the top tube, or one hand back on the bars), or the death-wobble, which seems to be more common on larger frames, makes it very difficult to control the bike, and can't be stopped (in my experience) until you slow way down.

This is a real issue for me, since I've had a lot of trouble with high-speed descending on variable surfaces with stock 61cm+ frames.

-David
head angle/rake combinations?fallzboater
Sep 23, 2003 8:55 PM
I'll try it again. I never liked trig, particularly.

What I meant about head tube angle vs. rake is that the range of commonly available rakes (40-45mm) can effect trail by only 5mm, but commonly available head tube angles (72 to 74 deg) can effect trail by over twice that amount. You would have to make a fork quite a bit longer to change the head angle a signficant amount. I wonder what the tolerance of head angles is for most manufacturers? Especially on small frames, a very small change in top tube length, for example, could change head angle by 1/2 a degree or more. It's not that easy to measure with common tools, either.

What I haven't seen is a good description of is the handling effects of different head angle/rake combinations that have equivalent trail. For example, how would a 74deg/40mm bike compare with a 73deg/45mm bike. The trail figure is going to be within about 1mm, but the wheelbase will be longer with the 73deg/45mm. Would you expect to feel a significant difference?

-David