Ackermann steering geometry seems like a complex setup tool at first, and many beginners might never have even heard of it before. However, if you break it down and fully understand it, you can easily see how Ackermann steering geometry will benefit your race car.
Ackermann steering geometry helps to prevent tires from slipping. This is done by angling all four tires to one central point around a corner. All four tires need to travel different distances in the same amount of time and having them at the correct angles will help them provide enough grip.
In this article we are going to explain everything there is to know about Ackermann steering geometry, when to use it and how to adjust it. It can be a useful setup tool if it is used in the right conditions, but you need to have a full understanding of it.
The Origins Of Ackermann Steering Geometry
The Ackermann theory was first invented by a carriage builder by the name of Georg Lankensperger in Munich, Germany, in 1817. It was patented by Rudolph Ackermann in England in 1818, after whom the theory was then named.
This was put into practice with horse drawn carriages in order to help the wheels to turn and prevent them from tipping over. It is still relevant today and used in modern day vehicles, from your average daily driver to Formula 1 cars.
What Is Ackermann Steering Geometry?
In order to understand the Ackermann steering theory, we need to understand how slip angles work. If you can grasp the turning angles of wheels and why they need to be different, then you can understand why Ackermann steering geometry is important.
When a car turns its steering wheel, the front wheels angle themselves in the same direction. However, if both front wheels point at the same angle, it will cause them to lose grip and slide along the tarmac.
Different Distances
That is because the outside wheels have further to travel than the inside wheels. In other words, they need to cover more ground than the inside wheels. This is essentially why a four-wheel drive car uses a differential. Differentials allow varying amounts of power to be sent to different wheels, so they all maintain the maximum grip, and none spin faster than they should.
The ability to control the amount of ‘slip’ on each wheel means that all four tires will always have grip. Ackermann steering helps to prevent the tires from slipping by pointing the front wheels at different angles. The inside wheel needs to be at a greater angle, i.e. more open relative to the back wheels, than the outside wheel in order for them both to point towards a central point.
Center Point
When cornering, there is a theoretical ‘center point’ in the corner, and both front wheels need to point at the same center point in order to turn the car with maximum grip. It’s a theoretical center point because it is constantly moving as the car is moving, and although it’s not fixed in space, it’s a reference point relative to the car.
This means that the turning angle of the inside wheel will be greater than the angle of the outside wheel. If you were to look at them from a front view, the inside wheel would be pointing more to the inside than the outside wheel. You can see this in the diagram above, with the front outside wheel pointing at a less aggressive/sharp angle to the center point denoted by the blue circle.
If all four wheels are pointing at the same central point, they will be able to work equally, and all provide enough grip and traction. However, if they were to all point in at different angles they would be fighting for grip and losing traction through the turn.
Because cars just have one steering wheel to control the front axle, and not the individual front wheels, the tires naturally point in the same direction as each other. This leads to the different angles to the center point as seen above, which means they need to spin at different rates to maintain maximum grip.
Travelling At Different Speeds
Each tire has its own path or radius when going around a corner. However, they are all traveling at different speeds because they have different amounts of ground they need to cover. For example, the outside front tire must cover more distance than the inside rear tire in the same amount of time.
Think of it like running round a track, with those running on the outside lanes starting further forward than those on the inside. This is because the inside lanes have a shorter distance to travel to do a lap. The outside tire is therefore traveling at the fastest rate and working the hardest in terms of providing grip.
This is called angular velocity. The angular velocity of each tire is different because of the distance that they need to travel. The outside front tire will have the highest angular velocity because it is traveling the furthest, and therefore has the highest chance of losing grip. If the outside front loses grip, it will cause understeer.
The outside rear will be the second most likely to lose grip, with inside front third, and the inside rear will be the slowest since it has the shortest distance to travel. This means that the outside tires rotate faster than the inside tires, and the front tires are always rotating faster than the rear tires. This only applies when they are providing enough grip.
Spinning The Wheels
For example, if you are driving without traction control and you accelerate too hard coming out of a corner, the rear wheels will spin and slip. Because the rear wheels are now rotating faster than the front wheels, they will try to ‘overtake’ the front wheels, causing an upset in the balance of the car which results in a spin.
When it comes to four-wheel drive, a differential is put in which controls different areas of the car. Firstly, a differential in the middle of the vehicle controls the amount of rotation between the front and rear wheels. This ensures that the rear wheels do not rotate more than the front wheels, and the front end will always have more grip than the rear, reducing understeer.
Secondly, differentials will be put in between the wheels on the front and on the rear axles to ensure that the outside wheels are always rotating faster than the inside wheels. Effectively, these differentials work to counteract the effect of having a rigid axle that doesn’t allow the tires to turn at different angles relative to the axle.
In Summary
In short, the Ackermann steering geometry helps to solve the problem of the wheels turning around a different radius. Applying Ackermann steering geometry will help to get all the wheels pointing in the same direction of the center point (but at different angles) when cornering, reducing tire slip and loss of traction.
If the wheels were to point at the same angle, there would be no rotation around the corner! The tires would both be pulling the car in different directions causing a loss of grip on the front end of the car.
What Is Anti-Ackermann?
Anti-Ackermann is the opposite of Ackermann, as the name suggests. This is the when the outside wheel is turning at a greater angle than the inside wheel. After understanding Ackermann, it might seem a bit illogical to apply Anti-Ackermann.
However, Anti-Ackermann is used in extreme circumstances with extremely high speeds. This includes series such as Formula 1, IndyCar and Endurance Prototypes. Anti-Ackermann helps with the high-speed cornering ability and provides more grip and stability around faster corners.
Use In F1 Cars
You can also clearly see Anti-Ackermann from an onboard shot of a Formula 1 car. While the car is cornering, specifically during slower corners, you can see one of the wheels pointing at a much greater angle than the other. If the outside wheel has a greater angle, it is running Anti-Ackermann, and if the inside wheel has a greater angle, it is running normal Ackermann.
Usually, car designers won’t create a 100% Ackermann/Anti-Ackermann setup. It will often change depending on the track, taking into account things like speed, corner radii and how many of each type of corner there are. Finding the balance is what separates the best cars on track from those in second place, as cornering is where they can make up the most time.
How Does Ackermann Steering Geometry Help You?
Most racing teams will adjust their Ackermann based on the racetrack. Different levels of Ackermann are useful at different types of corners. For example, a car will run completely different Ackermann settings at Monza compared to Monaco.
Generally, higher amounts of Ackermann is more useful for slower corners. More Ackermann means a tighter turning radius since the inside tires will be pointing at a greater angle to take more of the load.
On the other hand, if a circuit is comprised mostly of high speed, long radius corners, such as Monza, the car will most likely be running an Anti-Ackermann setup, where the outside tire is taking most of the load and provides the most grip.
Need To Balance It
It takes a careful analysis though because Ackermann needs to be balanced for all corners in order to provide the best benefit to the car. For example, if you go full Anti-Ackermann on a circuit that only has one long fast corner, you will be sacrificing every other corner on the lap to gain a slight amount of extra grip and speed around one corner.
Having the correct Ackermann settings can also help to reduce tire wear. If the tires are working in harmony and sharing the lateral loads correctly it can significantly reduce the amount of overall wear on the tires.
Tire Wear
In racing terms, this means the car can push harder for longer without wearing the tread on the tires too much. In addition, less sliding and scrubbing on the tires means that they will keep their temperature better and not overheat as quickly.
More Ackermann will make the car more stable and prevent the tires from slipping. If you are struggling with the tires and losing traction in the middle of a corner, you need to add more Ackermann.
How Do You Adjust Ackermann Steering Geometry?
In most cases Ackermann will not be adjusted on your average car. It is normally in the optimum position. Ackerman is adjusted in order to fine tune the set up, and it might make a small difference to a car that isn’t a single seater race car.
You can adjust the Ackerman angles by moving the front steering rod end in a slotted spindle arm. Moving the steering rod end closer to the ball joint will create more Ackerman. Moving the steering rod end further away will create less Ackermann, or more Anti-Ackermann.
Very Precise
When adjusting your Ackermann angles, it is crucial that you have a laser wheel alignment kit ready. Adjusting the Ackermann can often offset your camber and toe settings, which will mean you need to reset them to your preferred settings.
In most cases, it’s not worth playing around with the Ackermann settings. If you get it wrong it can completely throw off the balance of the car, and it can destroy your tires in the long run. However, if you are looking to squeeze some extra tenths out of your lap time and you have done thorough research about Ackermann, it can definitely be useful to you.
Final Thoughts
Ackermann steering geometry is used to prevent the tires from slipping on the tarmac due to having to rotate at different rates and angles. Using the correct Ackermann settings can help give your car more grip and help to conserve your tires.
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