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Trail
The Righting Effect

By: James R. Davis


There is no aspect of motorcycle design that is more misunderstood nor more important, to my mind, than trail and what it does for you, and how/why.

It is because your bike's front-end is designed to normally have a positive amount of trail that when it is moving it tries, on its own, to stand vertical and ride in a straight line. In other words, trail provides the automatic correction to your steering that keeps the bike stable and manageable from a steering point of view. It is trail that allows you to 'go along for the ride' contributing ZERO balance information or input into steering in order to ride a straight line. It is why you can fall off the bike when it is moving and the bike will continue in a straight line and not fall over until it is moving so slowly that it cannot use centrifugal force to overcome gravity.

So, we need to know what trail is.


Point A is the point on the ground that is directly below the pivot of your steering stem, B is the center of the front tire's contact patch (which is what is actually touching the ground), C is the place on the ground pointed to by your steering stem axis and C' is the place on the ground pointed to by your offset steering stem (that is, where your forks point.) The horizontal distance between points B and C (NOT C'!) is known as trail.

When I said that trail is normally positive I meant that there are conditions that can turn it negative (that is, pointing to the ground BEHIND where the contact patch (B) actually touches the ground. Whenever you hit a bump your trail is diminished in size and if the bump is severe enough it can turn NEGATIVE. Note, too, that coming off a bump turns your trail into a much larger positive length until your front tire is back on flat ground.



When you turn your handlebars you are turning the steering stem axis and, ignoring lean angle information, that means you are trying to turn the front wheel relative to point C, not B!.

Now let's see what happens when you try to turn your handlebars to the left while you are moving.



It is absolutely necessary that you understand this - that you realize that this is not some kind of abstract idea that in the real world is not relevant. You are trying to pivot the tire around point C, NOT POINT B!!!

In other words, you are trying to SLIDE the contact patch from being at point B1 to the position marked B2. When you are moving that is relatively easy to do, but when you are stopped it is almost impossible for a mere human to accomplish.

That is, if you held the body of your motorcycle rigid and tried to turn the handlebars you could not do it without a major amount of effort because there is TRACTION that you are trying to overcome.

Lest you think this is somehow unique to motorcycles, think again. Your car works the same way. When it is stopped and you try to turn the steering wheel it is almost impossible to do and, thus, the invention of power-steering was required!

Any student of physics will tell you that when you slide something there is an opposing frictional force from the ground. It is THAT FORCE that is known as the righting effect, the automatic effort of your bike's front-end to straighten out the steering in response to any effort to change the direction of travel of that front-end.

Now let's consider just how powerful that righting effect is and what it's composed of.

We have already seen that TRACTION (that is, the coefficient of friction between the tire and the ground) is a major component of it. The two other components, other than trail, are weight on the front tire and speed of travel.

But I said 'other than trail' in the above paragraph. The other component that makes up the righting effect is trail. The longer your trail is, the stronger is the righting effect. And now you should understand why.

So we know that if we increase the weight on the front tire or if we increase our speed there will be a stronger righting effect. It should no longer be a mystery as to why when you are panic stopping (thus increasing the weight on the front tire via weight shift) you are almost incapable of changing the direction of travel - the righting effect becomes very large and you are merely mortal.

Similarly, it can now be seen just how important weight distribution on your bike is. If you are used to how your bike handles and then change the amount of weight carried by the front tire you must relearn how to handle your bike. Attach a trailer to the rear of your bike and that will remove considerable weight from the front tire. That, as we have just seen, means that the righting effect will become much smaller than it was without that trailer and the front-end will seem to become a bit 'too easy' (a sense of riding on something slippery like ice) in its steering instead of being rock solid.

Now let's recall that your trail is not always positive - that when you encounter a bump it can become negative. So? Well, when you turn your handlebars to the left the righting effect tries to turn it back to the right - that is, it does so as long as trail is positive. But if trail is negative then the righting effect tries to help you turn it to the left and that can have disastrous results!!! When you are going to surmount an obstacle in the road you should, by God, try to hit it with the front tire pointing straight ahead or you will, with high odds, soon be tasting asphalt!!!

Copyright © 1992 - 2019 by The Master Strategy Group, all rights reserved.
http://www.msgroup.org

(James R. Davis is a recognized expert witness in the fields of Motorcycle Safety/Dynamics.)

     
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