The Angle: Fork Rake, Part I

So far we’ve hit bottom bracket drop and head tube angle in the Angle. It’s a good start on understanding why a particular bike handles the way it does, but as I mentioned in my last installment, head tube angle doesn’t mean much until fork rake is considered.

The combined effect of head tube angle and fork rake is called trail and while that’s easily the most complicated dimension in a bike’s geometry, we don’t have to worry about it just yet. This week we look at fork rake, which is one of the most straightforward elements of a bike’s geometry.

What it is
Fork rake is the distance between the steering axis (blue line) and the fork blades (purple line) at the front wheel’s axle. The measurement is taken in millimeters on a line extending from a right angle from the steering axis and bisecting the center of the axle (the orange line).

How it works
The effect fork rake has on a bike’s handling is straightforward. The more rake a fork has, the quicker its handling. Now here’s the funny thing about that: You’d think that rake would be a number between zero and infinity, right? Well, rake can be negative as well. This is best illustrated with the front wheels of a shopping cart; they trail behind the steering axis, which is why a shopping cart can be steered from the rear.

The way rake affects the movement of the front wheel is pretty easy to visualize. Think of it this way: As you ride, you move through time, right? When you roll over a pavement seam that sends up a thunk-thunk to your body, you feel the thunk from your front wheel before the one from your rear wheel. Now, imagine that you’re on a bike whose fork has a rake of one meter (1000mm) and you have a friend riding next to you riding a bike with a rake of 0mm. Not only will you run across a bump before your friend does, but when you both initiate a turn, your bike will begin turning sooner than theirs will.

What that means is that your bike will be more reactive, quicker-handling, as we like to say, than your friend’s.

What it means
So a bicycle with more fork rake, comparatively, will give a bicycle a racier feel, while one with less rake will feel more reassuring and relaxed. In a turn, a bike with less fork rake (say, 40mm) will carve a line like it’s on rails. The same bike with a 45mm will give a rider the ability to change lines mid-turn.

This is why rake is a significant part of a bicycle’s steering geometry, no less so than head tube angle, and that is why talking about just a head tube’s angle tells little about a bicycle’s handling. Next up, we’ll look more at the factors that affect fork rake.

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