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Fork Swap Geometry Calculator

I've made a calculator for anyone that's interested in the effect that changing a fork would have on some of the important geometry numbers of their frame.

You have to know/measure the following:
Existing fork axle to crown and rake
Head tube angle
Wheelbase
Wheel radius (including tire)
New fork axle to crown and rake

Knowing all of these you can easily see the effect the change would have on headtube angle, wheelbase, and trail.

This has all been part of my obsessive search for a new disc fork for an old 26" mountain bike that would either be neutral to or improve (tighten) my handling. The current fork is 398 ac and 35.5mm rake (not perfectly confident in that rake measurement but it's the best I could come up with, seems odd).

Any suggestions for a replacement with a lower a2c that will stand up to polo abuse?

I have my sights on the Kona P2 Disc fork but it looks like I might just have to get a custom. Anyone using the P2?

Nice. I was looking to change the fork on my Surly Steamroller and used these:
http://yojimg.net/bike/web_tools/trailcalc.php
http://www.kreuzotter.de/english/elenk.htm

Those calculators are interesting to see the trail for a given fork, but I wanted to know for my specific bike, how does changing the fork affect the front end geometry.

That is where the wheelbase comes in.

Somewhat nerdy explanation follows:

Imagine changing the fork but holding the bike in the same position as it was with the original fork. Now, assuming the fork is shorter than the original, your front wheel will be above the ground (vertical change) and in front of or behind (horizontal change), depending on rake, its original position. When you stop holding the bike, the front end will drop by pivoting from the back wheel until it touches the ground. This rotation or difference corresponds to the change in head tube angle, as the whole bike has rotated through that angle, including the head tube, when the front end moved towards the ground. You can see from the thought experiment that it is dependent upon the wheelbase since the front end rotates into its new position from a pivot at the rear wheel. With a shorter wheelbase, changing the fork will more dramatically effect the front end geometry such as headtube angle and therefore trail.

Probably bikecad is the tool for you, assuming you have a good idea about your current frame dimensions.

with respect to forks- i've got the black market dirt jump fork on a similar bike- 390 a-c, 30mm rake. Only problem is that it doesn't come with brake mounts, so you'd have to get someone to weld on a disc tab. You might also want to look at forks designed for 700c cross or 24" trials forks-
the kona p2 is ~400 a-c and 45 mm rake,
or this item:
http://www.ebay.com/itm/BLOODLINE-24-chrome-aluminum-fork-disc-brake-rig....