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how to measure fork rake?(14 posts)

how to measure fork rake?hayaku
Aug 6, 2003 11:37 PM
I have a time millenium carbon fork that I'm thinking of using to replace the steel fork currently on my bike. it'll save a couple of hundred grams and is a little taller than my current fork so I'll be able to remove my last spacer.

But first I want to measure the rake. if it is different, will 2mm or so make much of a difference?

So what's a good way to measure the difference? Thanks.
Look on the steerer tube!the bull
Aug 7, 2003 3:35 AM
If I am not mistaken Time always has a label on there forks.
it's on the time steerer but not the other...hayaku
Aug 7, 2003 7:07 AM
That's why I need to measure.
What's taller?...TFerguson
Aug 7, 2003 4:19 AM
"and is a little taller than my current fork so I'll be able to remove my last spacer"

If taller from ends to top of crown, it will change the geometry of the bike. If taller stearer, will require more spacers. Did I miss something?
ends to crown, apart from a BB that is about...hayaku
Aug 7, 2003 7:17 AM
one mm higher, I can adjust the saddle and bars to roughly the same position. even if I noticed the difference I think I'd get accustomed to it pretty quickly, I wonder if the same be said for the fork rake?
HUH?C-40
Aug 7, 2003 5:55 AM
A taller fork, from the center of the axle to the top of the crown, would have no effect on the number of spacers that you need and it would change the head tube angle, if the length is very much different.

Measuring the fork rake can be done at home, with only a precision scale, a smooth board and a flat table, but it's not real easy, and accuracy is questionable.

If you know the brand and model of frame or the head tube angle, it would be easier to recommend a rake rather than try to measure the old fork.
Actually...Nessism
Aug 7, 2003 6:32 AM
A longer fork will result in a higher head tube, and to a lesser extent, a higher bottom bracket. Assuming the rider doesn't want to reduce the step height difference between the saddle and handlebars, the stem must be dropped down to a lower position - thus the spacer removal answer.

Ed
Exactly.hayaku
Aug 7, 2003 7:04 AM
that's using yer noggin.
of course...C-40
Aug 7, 2003 12:16 PM
You're right that the whole front of the bike would be higher, pivoting on the rear axle, but as I noted, if there's too much difference in the fork length, the HTA will be affected. Unless you calculate the orginal amount of trail, then recalculate the the new trail with the new HTA and rake, the changes to the steering may not be as desired.

Most forks vary by only a few millimeters. The saddle would also be raised by about 1/3 of the increase in fork length, reducing the spacer change to about 2/3 of the increase in fork length. Pesonally, I've never measured bar height down to the 1-2mm range.
re: how to measure fork rake?Chen2
Aug 7, 2003 8:11 AM
I don't see a practical way to measure the rake. You would need to define the axial center of the steer tube and then measure the perpendicular offset to the center of the drops.
Take a look at this:

www.kreuzotter.de/english/elenk.htm

~Al
re: how to measure fork rake?Derf
Aug 7, 2003 10:31 AM
So how does fork rake change the steering. On my bike I am thinking of going from a 44mm rake to a 41mm rake. Will this make my bike excessively twithy at high speeds?
more rake = less trail = faster steering (nm)Nessism
Aug 7, 2003 12:14 PM
slower steering....C-40
Aug 7, 2003 12:22 PM
Reducing the rake will increase trail and slow the steering a tad. It will be more stable at high speeds.

Trail = (R/tanH )- (rake/sinH). Where R is tire radius and H is the head tube angle.

From the equation, you can see that all rake reduces the amount of trail. Increase the rake, reduce the trail and speed up the steering.
fork rakeChen2
Aug 7, 2003 12:46 PM
I went from a 43 to a 40 rake, identical forks, and I could feel the difference immediately, but it was not a big difference and I could be happy with either fork. The 40 tends to continue in a straight line, the 43 turns quicker. My head tube angle is 73.8, pretty steep (Trek 5500), so the trail was low to begin with. Reducing the rake and increasing the trail slowed the steering slightly, but it's still quick compared to other bikes.