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Not The Same As 'Slow Steering'

By: James R. Davis

In a previous Tip entitled Counter-steering I spent a lot of time talking about the design of the front-end of your motorcycle in an effort to explain how counter-steering works. One of the concepts I put forward was that of steering 'slowness' caused by having trail. Apparently some of the readers concluded that 'slow' steering was the same as 'under-steer'. Not at all.

Whether the vehicle has two wheels or more, under-steer means that the vehicle's front tire(s) will break away (slide) before the rear tire(s) do during a turn. Over-steer, then, is just the opposite and is a design that tends to result in the vehicle's rear tire(s) breaking away first.

[Despite what I just said above, there is a more formal definition of over- and under-steer that describes how a vehicle behaves in a curve as determined by the slip angles of the front and rear tires. That is, if the slip angle of the rear tire(s) of a vehicle is different than that of the front, the vehicle will rotate from this phenomenon alone. If the rear slip angle is greater than the front, the vehicle is said to over-steer. If less, it under-steers.]

'Under' and 'Over' refers to how the vehicle will track the desired turn when either the front or rear wheel(s) begin to slide. For example, in an OVER-steer situation the rear breaks away first. The vehicle then OVER rotates in the direction of the turn.

You might think that it would be better to have a motorcycle designed and setup so that there is neither over-steer nor under-steer so that you could take a turn as hard as possible without worrying about which tire broke free first. I don't think so. I'm sure you would agree that given a choice you would want your rear tire to slide before the front one. In other words, you WANT a certain 'over-steer' built into the design of your motorcycle. [Over-steering from slip angle differences results in the feeling of a modest drift into a turn. This, to most motorcyclists, is mildly reassuring and preferred to the same modest drift out of a curve that under-steering provides.]

But then you notice that your rear tire has a greater contact patch than does the front (at least when riding vertically.) You might assume that because of this it has a higher Coefficient of Friction with the road than does the front and that this should automatically result in under-steering. Again, not true.

Indeed, the front tire MUST develop more forces than the rear one in order to destabilize the motorcycle and cause it to change direction. (In another article you will find that the rear tire actually STEERS the bike when it is stable. Honest!) Thus, something more is at work than merely the size of the contact patch, the rubber compounds used, tread patterns, and flex of our tires that determine whether a vehicle over- or under-steers. Your front fork system (rake, trail, offset), weight transfer, rear-wheel drive and tire camber also play a part. (There should no longer be a question of why your rear tire has a flatter surface than does the front one - tire camber thrust is not as great.)

In summary:

  • Your bike is designed such that it has over-steer (the rear wheel tends to break away before the front one does in a turn, and the slip angle is greater on the rear tire than on the front tire.)

  • The rear tire contact patch is wider because the surface is flatter than your front tire and this results in reduced tire camber thrust. Further, part of the rear tire's traction is consumed with acceleration. The result, a tendency to over-steer - particularly when accelerating. (Interestingly, unlike with motorcycles, over-throttle in a curve with most cars tends to cause under-steer.)

  • Slow steering is a result of steering geometry and is NOT the same thing as 'under-steer'.

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

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