 Figure 1 Steering Axis Tr...  Figure 1
Steering Axis
Trail |
 Figure 2 Rear Trail Steering...  Figure 2
Rear Trail
Steering Axis
Front Trail |
 Different True Trail In this...  Different True Trail
In this example we see that the false trail remains constant for these two bike set ups. However the true trail varies significantly. Thus we should always calculate true trail when determining Motorcycle Dynamics. |
 Increased Rake Increases ...  Increased Rake
Increases Trail |
 Figure 5 Steering Axis 45...  Figure 5
Steering Axis
45 Neck Angle |
 Wheel moves forward due to...  Wheel moves forward due to rocker length |
What is rake and trail, and what does it mean to you? Hit a 100mph on a bike with a 1/8-inch of trail and you will wish you knew about rake and trail before you built your bike. Is your new bike going to wobble, or steer like a tank? With a little planning, your bike can be rock solid, yet able to carve through the canyons.
What is rake and trail?
Rake is the angle of your steering neck. Stock Harleys have about 28 to 34 degrees of rake. Choppers have something more than that; usually 40 to 50 degrees of rake in the neck. The raked neck allows them to have a lengthy and low look with long forks. After all, the difference between ordinary motorcycles and a chopper are those long forks.
So if that is rake, what's trail? The next time you are at the supermarket, take a look at the shopping cart wheels 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. If you don't believe me, head to the supermarket and try it out. Oh, and while you are out, grab me a pizza and a sixxer of cold ones.
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. This is due to the increased leverage the wheel has about the steering axis. It is an easy concept to remember because the longer things are, the harder they are to turn. We can measure the trail on this system as the distance along the ground from the axle to the steering axis.
Now lets look at a motorcycle; both the front wheel and the rear wheel pivot about the steering axis. Both the front and rear wheels trail behind the steering axis. The big difference is that the steering axis is at an angle to the ground. The true trail is still the leverage that the tire has on the steering axis. Since we have tilted the steering axis, the trail tilts along with it.
So, we can measure trail as the perpendicular distance from the point where the tire contacts the ground to the steering axis. We are using the center of the tire contact patch as the tire contact point. In reality, the centroid of the tire contact patch is slightly behind the center of the tire. But that is an entirely different article.
A lot of builders will measure trail from the tire contact point to the point where the steering axis intersects the ground. We call this false trail. To determine vehicle dynamics, you need to measure the leverage that the tire has over the steering axis. The correct measurement is the perpendicular distance from the steering axis to the tire contact point. To clarify things, we often call this measurement true trail.
FIGURE 3 shows that two bikes with the same false trail can have very different measurements for true trail.
Is too much trail bad?
FIGURE 4 shows that as you increase the rake angle of a bike, the trail also increases. Now, is it possible to have too much trail? Yes it is. If you have too much trail, the bike can feel very heavy and sluggish and 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, and by the way, where is my beer and pizza)? 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 big arms to keep the bike from "flopping" over.
What Are The Stability Modes?
Lets talk a little bit about stability. A motorcycle can exhibit three primary modes of instability: the roll mode, the wobble mode, and the weave mode. The roll mode of instability is easy to understand. Just sit on your bike while it is standing still and pick up your feet. Unless your balance is outstanding, you will fall to the ground; a victim of the roll mode of instability.
The wobble mode of instability is where the main portion of the motorcycle stays relatively stable, while the front fork assembly oscillates back and forth. In other words, the main mass of the motorcycle stays steady, while the front fork assembly pivots about the steering axis. Too little trail can cause the bike to wobble. There are also lots of other factors that can make a bike wobble as well. Common culprits are tires, shocks, frame stiffness, stem bearings, and weight distribution, to name a few.
The weave mode of instability is where both the fork assembly and the main portion of the motorcycle oscillate back and forth. The bike will feel as though it weaves. Although there can be a number of causes for this mode of instability, it can sometimes be caused by too much trail. There are also lots of other factors that can make a bike weave as well. Tires, shocks, frame stiffness, stem bearings that are too tight, and weight distribution are often other causes of a weave.
Many riders are unable to tell a wobble from a weave. Either way, your buddies will rocket away from you at high speed no matter how big your motor is.
So what is the right true trail?
Not enough trail will make your bike wobble and too much trail can make it steer like a tank. Too much trail can even make the bike unstable in the weave mode. 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 inches, your bike should be good. Bikes with less trail steer quicker and are more agile, and often more fun to ride. Bikes with longer trail tend to take more muscle to make them change direction. It is really a matter of personal preference.
Now saying that it is a matter of rider preference may make many of you unsettled. I hear people everyday telling me that exactly 4.1 inches is right. This is especially comical because typically they are measuring false trail anyway. In addition, there are errors in determining true trail due to tire deflection, pneumatic trail, and other factors.
I have to admit that I used to think there was a perfect set up myself. But my experiences as a racing suspension tuner changed my mind. While it is not quite the freestyle spirit of anything is possible and everything is OK, there is a lot more flexibility in what the riders want and what we can give them. Now I will tell you a story to illustrate that when someone says that they are exactly right, they are almost certainly wrong.
Years ago, I use to set up race bikes for a living and I was working with two particular riders. The two riders were on nearly identical bikes (except for steering geometry) and were within hundredths of a second of each other during practice. Each rider had dramatically different steering geometries. I was thinking of how to make them faster. I assumed one must have the right set up and the other had the wrong set up. The one rider must just be better and able to ride past the shortcomings of his inferior set up. But how to tell which rider had the best set up? Both riders were former national champions and equals. Just for fun, I swapped settings on them. I figured that one rider was just so good, he's able to ride with the handicap of his terrible set up! Whoever got faster after the swap would now have the better setting. Neither rider was aware of the changes when they went out to practice. Then I sent them out, same time, and same practice. When they came back in, they were both ready to kill me. Both riders said that the new set up was unrideable and just plain terrible. They both hated each others set up. So I changed it back and they each got their own set up. When it came time to race, both riders were within a wheel of each other, and they changed lead about 20 times. It came down to the last turn of the last lap. One rider squeezed past the other at the finish line.
I learned a valuable lesson that day. It changed my thinking about right and wrong.
Personally, I run very little trail. I used to race Superbikes back in the days when a good handling bike was one that did not spit you off immediately. So, I am comfortable trading a little stability for nimble handling.
Female riders are lighter and tend not to muscle the bike around. I will set them up with a very light trail as well. Heavier riders or inexperienced riders often need more trail to keep them in check. A lot of riders can make an otherwise good handling bike wobble. But how your riding can upset the bikes handling is yet another article.
How can we change trail?
Lets take a close look at the bike in FIGURE 2 . The bike has a 45 degree neck and zero degree trees. So it ends up with true trail of almost 7 inches. That bike is going to steer like a tank.
It is 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 is the same bike-but with 6 degree raked trees. The true trail on that bike is 2-1/2 inches; perfect. The bike even looks tougher too. As a good rule of thumb, if your neck rake is less than 38 degrees, use zero degree trees. If your neck rake is more than 36 degrees, but less than 42 degrees, you can use 3 degree trees. If your neck rake is more than 40 degrees, but less than 50 degrees, you can use 6 degree trees. If there is more than 48 degrees in the neck, you can consider 9 degree trees.
We can also change trail with the rocker on a springer. FIGURE 6 shows the same bike, but this time with a springer front end. The rocker kicks out the wheel so that the trail is about 1-3/4-inch. That will give us a quick and nimble bike with the look of an old school chopper. The best of all worlds.
I hope this article helps keep your new bike as fun to ride through the twisted canyons, as it is on the lonesome highway.