Tuesday, September 11, 2007

Long offset forks and steering "quickness"

I've been reading all about the various new longer-offset suspension forks coming out soon (Fox at 44mm and 51mm, Reba at 51mm, Manitou at 47 or 48mm?) and folks seem pretty psyched. I read a lot of message board posts that go something along the lines of "dude, my bike will steer way quicker! Sweet!".

Let's get something straight - there is no free lunch when it comes to bicycle steering geometry. "Quicker" steering is also "less stable" steering, or "twitchy" steering, depending on what situation you find yourself in. If you want to make the bike react MORE to steering input (ie, make it handle "quicker"), you can do a number of things, but the easiest ways to do accomplish it are to A) add fork offset, or B) steepen the head angle, or C) both. The crucial number here is referred to as "trail", and if you need a basic explanation, you can click here. Higher trail numbers are more "stable" or "sluggish", low numbers are "quick" or "twitchy". Typical trail figures for mountain bikes are in the 65-80mm range. Downhill bikes can get upwards of 100mm, twitchy crit bikes can be in the 40s.

Here's the catch on "quickness", though - if the bike reacts more to you leaning and/or turning the handlebars, it's also going to change course more when the trail surface *causes* you to lean or turn the bars (if, for example, you hit a big ol' rock in the trail, or an unexpected sandpit, or whatever). You can't have your cake and eat it too - a super-responsive bike is going to be a real handful in the rough stuff, and a super-stable bike is going to have to get manhandled through the tight stuff. You've got to look at your local terrain and skills and decide where you want your bike to be on the responsiveness continuum.

You've also got to consider your approach to your riding - do you want a bike that addresses your weaknesses but might hinder you a bit on the terrain that you're more comfortable riding? Or do you just want to suffer on the terrain you don't like anyway, so that you can totally rage on the stuff you're best at riding? For most folks, a middle way is the best option, as boring as it might sound. Just keep in mind that the "quickest" option when it comes to steering geometry may not be the best for you or your situation.

A final note - I am not one of the folks that thinks the longer-offset forks are a great way to get teeny tiny people on 29ers. Honestly, if you have to have an extra 13mm of offset, really long toptube, and slack head angle to fit, you should be looking at a 26" or 650b bike instead. Steering geometry and handling are strongly affected by wheelbase and weight distribution, both of which become problematic regardless of fork offset or toe overlap considerations.


Anonymous said...

Been playing with Anvil BikeWorks' online rake/trail calculator. Its surprising to see how easy it is to screw up a bike's handling with an uninformed fork purchase.

Anonymous said...

You always seem to answer nagging questions to which I've always wanted the answer but was too embarrassed to ask. Thanks

Anonymous said...

right-o walt! i'm glad i'm not the onlt sceptic of all the mtn bike sales pitch b.s. steve, coconino cycles.