Bicycle Mechanics - Fork with lower axle/crown height

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konabunny
03-26-11, 03:10 PM
Hi forum.

I have a 95 Kona Kilauea that has been very faithful over the years and is now used as my all rounder/commuter, dressed in 26x1.5 slicks. The rims have finally reached the point of no return and I need to get a new set of wheels.

I'm playing with the notion of switching to a front cable disk but would have to swap out the stock rigid P2 for something with a disk tab. A cheap, used (disk only) P2 is available to me but the axle/crown length is 395mm while my stock canti P2 is 410mm.

Frame is 16" with a HT angle of 70.5. Does anyone knowledgeable think this would adversely affect handling? I worry that the bike would have oversteer problems.

p.S. I am aware that I need a Disk hub for the front.

thanks in advance.

FBinNY
03-26-11, 03:23 PM
IMO, a 15mm drop in the crown height should barely be noticeable, if at all. You might measure the wheelbase, and two crown heights and use height/wheelbase = tangent angleX. Use a trig function chart (http://www.google.com/imgres?imgurl=http://chestofbooks.com/home-improvement/woodworking/Ira-S-Griffith/Carpentry/images/Table-Of-Natural-Sines-Tangents-Cosines-And-Cotang-204.jpg&imgrefurl=http://chestofbooks.com/home-improvement/woodworking/Ira-S-Griffith/Carpentry/Appendix-II-Directions.html&usg=__hY5flTiuWKpNaJkMD9w5c-KqScU=&h=675&w=498&sz=45&hl=en&start=0&sig2=dYiqhIV0VOevpPkelMPJ2A&zoom=1&tbnid=zExq4SE_zA5KnM:&tbnh=151&tbnw=111&ei=tFiOTdWODOay0QHe9OHGCw&prev=/images%3Fq%3Dsine/cosine%2Btable%26um%3D1%26hl%3Den%26sa%3DX%26biw%3D1280%26bih%3D606%26tbs%3Disch:1&um=1&itbs=1&iact=hc&vpx=127&vpy=67&dur=717&hovh=261&hovw=193&tx=108&ty=145&oei=tFiOTdWODOay0QHe9OHGCw&page=1&ndsp=18&ved=1t:429,r:0,s:0) to find angleX for both heights, and the difference in X will equal the change in the head tube angle.

BTW- you'll also be dropping the handlebar height by the same 15mm, but can compensate with that much in added spacers below the stem.
konabunny
03-26-11, 04:03 PM
...so do I divide the tangent of angle X by the median of angle Y multiplied by ... er...thanks for the first part of your reply...

FBinNY
03-26-11, 05:04 PM
Gee, I didn't mean to scare you with the concept of trig functions or complex math. All the hard part was done by the folks who made the chart so all you needed was simple arithmetic and the ability to find numbers on the chart I linked for you.

Anyway, here it is done for you.

Assuming that you have a 42", (1065mm) wheel base

the original fork 410 / 1065 = .385, from the tan chart, a hair more than a 21° angle (tan = .384)
replacement fork 395 / 1065 = .370, from the "" "" "" slightly more than a 20° angle (tan = .364)

So changing the fork and dropping the head 15mm will change the head and seat tube angles by just slightly less than 1°

Whether 1° is meaningful is your call, but at least you know.