Get the Right Fork
Determine Fork Length
Refer to this sheet to determine your correct length fork.
Fork length is measured from the bottom of the top tree to the center of the axle. Stock length is 32-1/4 long plus +12 would be 32-1/4 + 12= 44-1/4 long
Next time you are at the supermarket take a look at the shopping cartwheels as you race around the aisles. As shown in Figure 1, the wheels “trail” behind the steering axis. As you push the cart the wheels are stable because they “trail” behind the steering axis. If the wheels flip in front of the steering axis they will wobble and flop back behind the steering axis, until they again trail the steering axis. Oh and while you are there, bring back some steaks; I am getting a little hungry. So we call it trail because the wheels “trail” behind the steering axis. As a general rule, when the wheels trail behind the steering axis they are stable. When the wheels are in front of the steering axis, they are going to wobble and flop back. It is kind of like putting the cart in front of the horse. The cart needs to trail the horse. The amount of stability is proportional to the leverage that the tire contact patch has on it. Conversely, the longer the trail, the harder it is to turn the wheels, due to the increased leverage the wheel has about the steering axis. This is easy to remember because the longer things are the harder they are to turn, and the shorter, the easier. We can measure the trail on this system as the distance along the ground from the axle to the steering axis. Perhaps the shopping cart example started this myth.
How can we change trail?
Lets take a close look at the bike of figure 2. The bike has a 45° neck and zero degree trees. So it ends up with has true trail of almost 7”. That bike is going to steer like a tank. Raked trees to the rescue. Oddly enough if adding rake in the neck increases trail, adding rake to the trees reduces trail. Take a look at figure 5, it’s the same bike – but with 6° raked trees. The true trail on that bike is 2-1/2 inches, perfect. The bike even looks cooler too. As a good rule of thumb if your neck rake is less than 38°, use zero degree trees. If your neck rake is more than 36° but less than 42°, you can use three-degree trees. If your neck rake is more than 40° but less than 50°, you can use Six-degree trees. More than 48° in the neck and you can consider 9° trees.
Neck Rake Up to 38 degrees Zero degree trees
36 degree to 42 degree Three Degree Trees
40 degree to 50 degree Six Degree Trees
Over 48 degree. Nine Degree Trees
Same False Trail
Different True Trail
Figure 3 shows how when we measure leverage we have to take the perpendicular distance from the force to the center of rotation.
Also you can see that two bikes with the same false trail can have very different measurements for true trail.
Same False Trail
Different True Trail
Now, is it possible to have too much trail?
Yes it is. If you have too much trail the bike will feel very heavy and sluggish. The bike will also tend to weave, because the front trail will start to tune into the rear trail and the front and rear wheels will oscillate back and forth. So a bike can be unstable with too much or too little trail. Another common complaint about too much trail is that the bike will flop. This is because the steering axis is inclined (unlike the shopping cart) The inclination causes the bike to be raised and lowered as the wheel pivots about the steering axis. The long leverage that the wheel has on the steering axis requires muscle just to keep the bike from “flopping” over.
So what is the right true trail?
How do we make sure our trail is going to work for our bike? While there are a number of variables involved we have seen that as a general rule if the true trail is between 1-1/2” to 5” your bike should be good. Bikes with trail towards the low end steer quicker and are more agile. I prefer bikes with a trail closer to 1-1/2 inches and have been happy with less. (I also used to race superbikes in the days when a really good handling bike was one that didn’t spit you off on the straightaway. So a little wobble, if controllable is an acceptable trade off for the gain in agility. However I never recommend this to anyone) Bikes with longer trail (closer to 5”) tend to take more muscle to make them change direction. However they tend to be very confidence inspiring in sweeping corners. They also are more fun to slide in a corner because the lazy geometry makes them easy to control. It’s really a mater of personal preference. However you need to consider true trail, as false trail can give false results.