Thursday, April 19, 2007

Fork Confusion

I've been getting messages and seeing posts on the web forums that indicate to me that there still are alot of you confused by this fork geometry thing. I thought I'd post a quick primer for you all to help remind you that this isn't as difficult as it seems at first glance. Here's the simple take on fork geometry.

It's all about fork trail. Yes, head angle does play a part, but so does wheel diameter and fork offset. The thing is, you could go nuts trying to wrap your mind around this concept. You could go on for days calculating your theoretical fork trail numbers for any given combination. Or, you could just look at it like this. Check it out.

A steeper head angle or a slacker head angle we all get. One makes your bike handle stable, slow, and floppy; like a chopper. The other makes it handle all nervous and twitchy, like a crank addict! Ha ha! No, really.......the head angle thing is easy to grasp. Now lets say you have a 29"er that was for the most part your standard issue 29"er up to now. Usually we're talking about a bike with a 71-72 degree head angle. Typically these handle somewhat sluggishly in comparison to alot of XC oriented 26"ers. The reason is that the fork offset is the same as on a 26"er and the wheel diameter, being larger, has caused the trail figure to increase over that of a 26"er XC bike.

Fork offset to the rescue! The newer crop of 29"er forks hitting the market this year have markedly longer offsets. Instead of 38-40mm, as in the past, which came from 26"er forks, we have offsets in the range of 43-50 plus millimeters. What this increase in offset does is decrease the final trail figure on your 29"er, all other things remaining the same. A smaller trail figure translates into quicker handling. Handling that closely resembles 26"er XC bikes. This happens because the less trail that you have, the twitchier the handling, and less sluggish the handling becomes. Just like increasing head angle does. The reason 29"ers don't have steeper head angles is because of toe overlap, which would be a bad, bad thing. So, offset had to be the solution to slower handling 29"ers.

Okay, got that? Any questions?

Fire away, ya'all.

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