Ackermann Angle

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Ackermann angle steering geometry solves the problem of wheels on the inside and outside of a turn needing to scribe circles of different radii. Ackermann is often mis-spelt Ackerman.

Rudolf Ackermann (1764 – 1834) was the patent agent for German horse drawn coach builders “Lankensperger”. They submitted a patent in 1818, but Eramus Darwin was said to have had prior claim to the invention in 1758.

Passenger cars have a steering geometry somewhere between True / Pure Ackermann steering and Parallel steering while it’s common among race cars to use Reverse Ackermann. By using True / Pure Ackermann steering on passenger cars, or other vehicles only exposed to low lateral accelerations, it is ensured that all wheels roll freely with no slip angles because the wheels are steered to track a common turn centre. Race cars are often operated at high lateral accelerations and therefore all tyres operate at significant slip angles and the loads on the curve inner wheels are much less than the curve outer wheels due to the lateral load transfer. Tyres under low loads require less slip angle to reach the peak of the cornering force. Using a low speed steering geometry on a race car would cause the curve inner tyre to be dragged along at much higher slip angles than needed and this would only result in raises in tyre temperature and slowing down the car due to the slip angle induced drag. Therefore race cars often use parallel steer or even reverse Ackermann.

The inside wheel on a bend will need to turn more the outside wheel. In turning a tighter circle, the inside wheel will complete less revolutions.

image: Ackermann turning circle
True / Pure Ackermann Steering

Too much Ackermann angle from True, will make the car loose on turn exit or will cause premature tyre wear. Too much Ackermann can over−heat the left front tyre so that it will not perform on a long journey.

Excessive Ackerman can sometimes be seen by the wear pattern on the left front tyre. If you see a grain pattern in the tyre surface or the left front tyre seem very hot you may want to consider reducing the amount of Ackermann.

The right amount of Ackermann will help through the middle of a turn. Fine tuning car handling is possible through changes in Ackermann. A car that is overly tight through the middle of a bend can be cured by more Ackermann. Too much can slow the car down as your Engine power is used to overcome tyre drag. Tyre wear may also be an issue.

It is important to remember that adjusting Toe angles will affect Ackermann Geometry. It is also important to understand the difference between increased Toe Out and More Ackermann as many reference sources often confuse the two.

Before taking any Ackermann measurements you must insure that ride height is set, weight percentages correct, driver weight accounted for, bump steer set, camber and caster set, air pressure set etc…. All of these factors will have an effect on Ackermann measurements and getting a stable ‘Base Point’ as reference is very important.

True / Pure Ackermann − Zero Toe On Turn In

True Ackermann steering geometry is defined by angling the steering arms so that a line drawn between both the king pin and steering arm pivot points intersects with the centre line of the rear axle.
As this gives true Ackermann steering geometry, there is no Toe Angle change on the inside wheel (the wheel is aligned with the circumference of the circle.

image: True Ackermann AngleTrue Ackermann and Zero Toe In:
Modern cars do not use ‘Pure’ Ackermann angles as there are many other factors to take into account.
image: True Ackermann and Zero Toeimage: True Ackermann and Zero Toe In
Zero toe (wheels pointing straight ahead) and True/Pure Ackermann will result with both tyres being aligned with the circumference of the circle or arch of corner.

 

image: True Ackermann and Toe Inimage: True Ackermann and Toe InTrue Ackermann and Toe In:
Both of the wheels are being toed−in relative to the circumference of the circular path they are actually following.
The inner wheel is trying to scribe a larger circumference and the outer wheel a smaller one.
This situation is known as ‘Parallel Ackermann’. If Less Ackermann angle was introduced the situation could be pushed into ‘reversed ackermann’.

image: True Ackermann and Toe Outimage: True Ackermann and Toe OutTrue Ackermann and Toe Out:
Both of the wheels are being toed−out relative to the circumference of the circular path they are actually following.
The inner wheel is trying to scribe a smaller circumference and the outer wheel a larger one.

More Ackermann Angle − Toe Out On Turn In

More Ackermann is achieved by angling the steering arms towards the central axis, so that the point of intersection is forward of the centre line of the rear axle.
This steering geometry achieves greater angular inequality of the turned wheels, which generates Toe OUT on the front inside wheel. The inside wheel is trying to turn tighter circle than it needs to.

image: More Ackermann and Zero Toeimage: More Ackermann and Zero ToeMore Ackermann and Zero Toe :
Zero Toe and More Ackerman will result in the inside wheel trying to follow a smaller diameter circle than it actually does. The inside wheel is toed-in relative to the actually circular path.

 

image: More Ackermann and Toe Inimage: More Ackermann and Toe InMore Ackermann and Toe In:
By precisely adjusting the toe in angle, the inside wheel could scribe a circumference true to the path it actually follows. The outside wheel is toed−in and is trying to follow a smaller circle than it’s actual path. The inside wheel has less toe-in than ‘True Ackermann and Toe In’.

image: More Ackermann and Toe Outimage: More Ackermann and Toe OutMore Ackermann and Toe Out:
With toe angle set to out and More Ackermann will result with the a larger inequality between the turned front tyres. The outside wheel is toed-out and trying to follow a larger circumference than the circular path. The inside wheel also toed-out and is trying to follow a smaller circle than it actually is. The outside wheel has increased toe-out over ‘True Ackermann and Toe-Out’.

Less Ackermann Angle − Toe In On Turn In

Less Ackermann angle can be set on a steering set−up, which involves adjusting the angle of the pivot points on the steering arms so that the point of intersection is behind the centre line of the rear axle.
Some race cars use Reverse Ackermann geometry to compensate for the large difference in slip angle between the inner and outer front tyres while cornering at high speed. The use of such geometry helps reduce tyre temperatures during high-speed cornering but compromises manoeuvring in low speed manoeuvres.

image: Less Ackermann and Zero Toeimage: Less Ackermann and Zero ToeLess Ackermann and Zero Toe :
Less Ackermann and Zero Toe In will result in progressive Toe In on the inside wheel. The Inside wheel will try to follow a greater circle than it actually does. This situation can approach Parallel Ackermann.

 

image: Less-Ackermann and Toe Inimage: Less Ackermann and Toe InLess Ackermann and Toe In:
Both of the wheels are being toed−in relative to the circumference of the circular path they are actually following.
The inner wheel is trying to scribe a larger circumference and the outer wheel a smaller one. The inner wheel has increase toe-in over ‘True Ackermann and Toe In’ above. This situation is known as ‘Reverse Ackermann’.

image: Less Ackermann and Toe Outimage: Less Ackermann and Toe OutLess Ackermann and Toe Out:
By precisely adjusting Toe Out the inside wheel could scribe a path true to circumference of the circular path it is actually following. The outside wheel is toed−out relative to the circumference of the circular path it is actually following. The inside wheel has less toe-out than ‘True Ackermann and Toe-Out’.

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