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Trail

Released on: Thu Sep 25, 2008 4:15 pm
from: GearHeadCollege
Viewed: 1595

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Some bikes feel like a crash about to happen while others are rock solid steady, even in the bumpiest of turns. They all have two wheels and a motor. Some bikes may even appear identical. Yet one feels like its on rails and the other runs like a drunken sailor on ice.
I have pondered these questions for decades. First as a professional Superbike racer, then as a suspension engineer, and finally as a manufacturer of forks, that supplies to the best bike builders around the world.
We all share the passion for designing and building the bike of our dreams. I’m writing a series of articles to try to share some of the lessons I have learned. Some of these lessons were obtained by computer simulation. Some by sliding my butt across the pavement after being prematurely ejected from my motorcycle. I hope these lessons help keep your dream bike from becoming a nightmare ride.
So lets start out this discussion by ejecting some myths.
Myth number one: There is one magic number that makes the perfect handling bike - FALSE. – I actually used to believe this one, but it’s not true. Different riders ride differently, and need different set ups. Let me tell you a little story and I think you too will have a more open mind.
Years ago I use to set up race bikes for a living. I was working with two particular riders. They were both former national champions and they were on nearly identical bikes. There lap times 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 both faster. I assumed one set up must be right and the other wrong. The rider with the wrong set up must be able to ride so well that he was overcoming his bad set up.
But how to tell which rider had the best set up? So just for fun I swapped settings on them. I figured one rider to go faster and the other to go way slower. Neither rider was aware of the changes when they went out to practice.
When they came back in they were both ready to kill me! Both riders said that the new setup was unrideable. So I changed it back and they each got there own set up. Evidently they each had they set up that suited their own style. On race day both riders were within a wheel of each other. It was one of the most exciting races I had ever seen. They changed lead about two or three times a lap. It came down to the last turn of the last lap and one rider squeezed past the other at the finish line. I learned a valuable lesson that day: Different riders, like different set ups.

Myth number two) We measure trail as the distance along the ground from the axle to the steering axis. I see how this myth started, and was perpetuated by people who were not thinking about motorcycle dynamics. So lets see what trail really means to us motorcycle designers (that includes you if you are building a bike in your garage)

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. So it was the shopping cart example that may have started this myth.
Now lets look at a motorcycle; (fig 2) both the front wheels and the rear wheels pivot about the steering axis and 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 centriod of the tire contact patch is slightly behind the center of the tire. But that’s an entirely different article.
A lot of builders still 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 how when we measure leverage we have to take the perpendicular distance from the force to the center of rotation.
Fig 4 shows that two bikes with the same false trail can have very different measurements for true trail.

Is too much trail bad?
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, and by the way were is my steak?) 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.

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° Zero degree trees
36° to 42° Three degree Trees
40° to 50° Six degree Trees
Over 48° Nine degree Trees

Next month we are going to talk about springers, how to set them up, and how to set the correct trail for a springer. We are also going to debunk the myth that all springers ride like pogo sticks. Until then get to work on planning your bike. Otherwise you will end up an old man who wishes he got to build that dream bike, but died first.

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