Ackerman steering geometry and why race cars don't always use it

Acker... what?

Once you've wrapped your head around oversteer and understeer, tyre distortion, the road surface, the lack of linearity, and especially load transfer and tyre slip angle first, we can begin to explore steering geometry.

Tyre slip angle determines whether pro- or anti-Ackerman steering is best. Photo: Shutterstock.

Tyre slip angle determines whether pro- or anti-Ackerman steering is best. Photo: Shutterstock.

If you'd like to read about those concepts, and others, here are the links to make it easy:

You may have noticed on some open wheel race cars that the outside front turns just a little more than the inside front.

But that makes no sense, right? The inside tyre is taking a smaller circle, so surely it would need to turn a bit sharper.

Well, at low cornering loads that is true. The inside steered wheel should turn to a tighter angle than the outside one.

When the angle of the inside and outside steering tyres completely agree with one another about how far the vehicle is turning (so the math has been done, and the resulting design means that neither is forced to slip or slide at all), this is called Ackerman steering.

It dates back all the way to the horse and cart, which often had the front wheels rigidly fixed to one another at the same angle (that's called parallel steering).

It was actually invented by German carriage builder Georg Lankensperger in Munich in 1817. Rudolf Ackerman (who was his agent in England) then patented it in 1818.

This design was beneficial for the tall, wooden spoke, metal-rimmed wheels of the carriages they made and sold, and the principle still works nicely on ordinary road cars.

Steering geometry that turns the outside wheel sharper is referred to as anti-Ackerman, and the reason some race cars will, at some tracks, choose to use this arrangement is a combination of load transfer and the ideal slip angle of the tyres they're using.

When a car accelerates, brakes or turns, the load (downward pressure) on each tyre changes. So, when braking and turning, the outside front has the most vertical load, and the inside front has much less.

Tyre slip angle, which is the difference between where it's pointing and where it's actually going, also changes with load transfer (if it's pointing 25 degrees left, but the change in direction is 20 degrees left, its slip angle at that moment is 5 degrees).

Up to a point, the more downward pressure on the tyre, the greater the slip angle can be, so (assuming the road angle is flat or favourable) the outside front is going to work with a larger slip angle than the inside front.

The ideal amount of anti-Ackerman is unique to each situation though. Different tyres have different ideal slip angles. Different corners induce different amounts of load transfer to each other, thus varying that ideal slip angle further. Different suspension settings and designs also have an effect on the amount of load transfer.

Different race events, or just different strategies, mean tyres have to last different lengths of time, so that may mean getting less aggressive with the slip angle for longer stints or longer races. Then wet weather and other slippery conditions greatly reduce the slip angle, so that also reduces how aggressive you can be with the anti-Ackerman angle.

As for what you can adjust on any car without changing the design, there is the toe setting. Toe-in means the front of the tyres (viewed from above, and the steering wheel centred) are closer together than the backs of them. Toe-out means the fronts of the tires are further apart than the rear of them.

Setting the front wheels to toe-in achieves a bit more straight-line stability, but at the cost of a duller turn-in response. Conversely, setting the fronts with some toe-out sharpens the steering response a little, the nose reacting more eagerly to the initial steering input.

You can, on some vehicles, also adjust the rear toe setting. Rear toe-in adds understeer and stability, whereas rear toe-out can cause oversteer at the corner exit.

This is, of course, complicated further by suspension bushes in road vehicles offering plenty of flex (and therefore deflection under various loads) for the sake of ride comfort, whereas race cars like joints such as spherical bearings to almost eliminate this flex.

Sam Hollier is an ACM journalist and a motoring fanatic who builds cars in his shed in his spare time.