Fork University, that is.
When I’m talking to people about designing a new bike, I always stress the importance of choosing a fork (and tires, for that matter) early in the process. Although it may not seem like a big deal, subtle differences in fork geometry can turn a stable ride into a twitchy nightmare. To help understand why this is the case, a quick review of important terms is needed.
Rake (a.k.a. Offset)
Fork rake is the perpendicular measurement from the centreline (drawn through the steerer tube) of the fork to the wheel axle. Although most modern forks typically have rakes within a ‘narrow’ range (between about 40mm and 50mm), a few mm difference can be enough to noticeably affect handling.
Head Angle
Head angle is simply the angular measure that a projection along the head tube makes with the ground. Expect this to be a number between about 71 and 75 degrees.
Tire Width/Diameter
Tire width is something that we often don’t think much about. Want a more comfortable ride, slap some 28mm tires. Although that may be the case, changing tire size too much can have an undesirable effect.
Trail
On its own, though, rake/offset does not tell us enough tell us how a bike is going to handle. For that, we’ll need to also discuss trail (as well as wheelbase, BB drop, and weight distribution if we want to be anal about it).
Trail is measured as the distance between the tire contact patch (with the road) and the projection of the steerer tube centreline to the road surface in front of the bike. For most bikes, trail will normally be between 50mm and about 65mm with a number around 57mm being associated relatively neutral steering characteristics.
Geometrically, trail is dependent on not only rake but also on head angle and tire diameter. Think of it as the length of the base of a triangle defined by the projection along the centre of the steerer/head tube and a vertical line from the wheel axle to the ground. It follows, then, that increasing wheel diameter (but keeping other geometry parameters constant) will increase trail while increasing rake will decrease trail. This should be fairly obvious,. But what might not be apparent, is that increasing head angle will also decrease trail.
As an example, think of a gravel grinder that has been designed to run with 35mm tires on 700c/29’er rims. The diameter of these wheels will be 692mm. Throw a set of 25mm tires (although not recommended) on these same rims and you’ll have a wheel diameter of 672mm. The effect of putting these new wheels on your gravel grinder will be to move the wheel axle closer to the ground which will reduce trail. In this example, our original trail of 60mm will become 57mm. So we’ll move from a stable handling bike with potential for understeer to one that is more agile and neutral in feel.
Although this might accomplish the goal of converting your bike from a gravel grinder to a road bike, running narrow tires with a cyclocross or gravel grinder fork may look a bit ‘goofy’ due to the extra space that was designed into them to accommodate wide tires. A better solution would be to decrease trail by using a shorter (A-C) fork with a different rake. The shorter A-C will increase the effective head angle (which decreases trail) while the new rake is chosen to bring the final trail closer to what you’re looking for. The following figures show what happens when we first reduce the A-C length and then when we also reduce the rake.
What this shows is that we’re able to swap out the Wound Up Composites 45mm gravel fork with an Enve 2.0 43mm rake road fork and end up with a skinny tire bike that handles like it was made for the road.
Axle-Crown Length (A-C)
A-C length can vary a lot between different makers and styles. For the most part, we can expect that road forks will have A-C between about 355mm and 380mm. This is a pretty large range so it’s a good idea to know what the original frame design called for. If you started with a 367mm A-C 45mm Enve road fork and replaced it with a 380mm A-C 45mm Wound Up Composites road fork, the net effect would be to raise the front end of the bike which reduces the effective head angle. Which then increases trail, giving a more stable (more understeer) ride.
Hopefully it’s now apparent that choosing the right fork is important. Putting a fork with the ‘wrong’ rake on your bike can affect handling. This may or may not be a good thing (depending on what you’re looking for), but with an understanding of fork and frame geometries, it is possible to select the best fork for your bike.
Useful Links…
Bicycle tire diameter chart – http://www.bikecalc.com/wheel_size_math
Wound Up Composites fork specs – https://woundupcomposites.com/wp-content/uploads/2016/11/Wound-Up-Fork-Specifications.pdf
ENVE road fork 2.0 specs – http://enve.com/products/road-forks-2-0/
