Motorcycle Safety Site

Speed Bumps And RR Crossings
Why they can dump a bike

By: James R. Davis

We have all ridden over speed bumps and RR crossings and other than a mild bumpiness it did not seem like such a big deal. On the other hand, most of us have heard of or witnessed a motorcycle lose control and end up on its side doing the same thing. What's going on?

In previous articles I have discussed the important role that trail plays in stabilizing the front-end of your motorcycle. [A quick review ... Trail is the distance between the middle of your contact patch and that point on the ground pointed to by an imaginary line running through your steering stem. It is created as a result of rake angle, offset and tire radius.]

Because of trail your tire develops a restoring force that attempts to keep your front-end pointing straight ahead. The longer the trail is, the stronger this force is. The heavier your front-end is, the stronger this force is (when braking, for example.)

In the diagram below a front tire is about to run over an object on the ground simulating a raised RR track or an unusually harsh speed bump. What is important to note is that at the moment of contact between your tire and that bump your contact patch will be lifted off the ground (to be replaced by the tire riding on the bump) and at that time your tire will have created a NEGATIVE trail. In other words, at that time your steering axis will be behind your contact patch. (ALL bumps shorten your trail or turn it negative.)

So what? If you hit that bump straight on (at 90 degrees) you just ride up and over it and are essentially unaware of any steering control problems caused by doing so. As soon as you are past the bump you are back to normal trail. But please note that while the negative trail lasts for a short time and immediately turns positive, as you are rolling off the back side of that bump you will experience a time where your trail is unusually long (and therefore STRONG.)

But, if your front wheel was pointing off at any meaningful angle when you hit it (you are in a turn) a totally unexpected thing happens - the negative trail, instead of trying to correct an off-center rolling direction, will try to accentuate that off-center direction. In other words, in the blink of an eye you will find that your front tire tries to make the turn TIGHTER! The subsequent reversal of trail to positive again is so strong a change that unless your steering head damper is working properly you can end up in a wild side-to-side tank slapper fight with your handlebars and possibly end up on the ground.

One more thing, if you hit a second bump, as you will in the case of a RR track, and if that happens in almost the same interval of time that it has taken you to recover from the first bump, unless your steering damper is properly adjusted, you will experience a PROFOUND increase in loss of control!!!! This, because you will be causing an 'in phase' reinforcement - that is, you will have happened to 'tune in' to the dynamics of your front-end. {This is the real reason for a steering damper - to disrupt any rhythms ('de-tune') that could cause a pendulum effect on your front-end.}

Clearly the message here is to always attempt to have your front-end pointing dead ahead when you are about to hit an obstacle, and always be sure that your front-end damping system is properly maintained. Further, if possible, stay off your brakes in this situation.

[Note, the magnitudes shown here are slightly too large because trail is actually measured to where the steering axis intersects with the level of the contact patch.]

Motorcycle Tips & Techniques

Motorcycle Safety/Dynamics