Akerman Steering

[cmathews]

Yeasterday, during a discusion about bump steer, "bbp" requested a description on caster and Akerman steering. We described caster under the other heading. Today I am going to cover Akerman steering under its own heading.

Back in the days of horse drawn carriages, someone (I am guessing his name was Akerman) discovered that the inside wheel on the carriage had to turn a tighter radius than the outside wheel. Because of this he developed a way to make the inside wheel turn in at a sharper angle then the outside wheel. It was easy to determine the amount by having someone look at the wheels (from a point that is both in line with the rear wheels and at the center of the circle in which you are trying to navigate) and make sure that the wheel was tangent to the circle it was  trying to navigate. This worked great on carriages. Now, enter pnumatic tires (air fille tires).

This wonderfull thing happened with pnumatic tires, this scenario was no longer valid. As speeds go higher the scenario becomes less valid (to a point). Suddenly we have to take into account slip angles (the angle at which the tire is pointed vs. the direction of travel). If the fronts were the only tires to have a slip angle, the scenario would still be true, but as we all know, the rear tires also have a slip angle. Now you have a "steer from the rear" effect because of slip angles. Akerman steering suddenly becomes a controversial subject. The idea, now, is to make the slip angle on all four tires the same. How to accomplish this is widely debated. Generally, though, the consenus is that there should still be Akerman steering, but, less. This is accomplished by making the same adjustments, but moving the line up point forward from the rear axle.

I think the method of adjusting this is to move the steering rack or tie rod ends. It is ussually adjustable on race cars. Street cars may also be adjustable, but most of the people who know how to do it will want you to pay them to do it.

P.S. - I don't even have a clue how this would be affected  by HICAS. This might actually be a case of what Carrol Smith calls, "complicating things beyond our capacity to comprehend them."


Chuck