Tyre slip angle = tire slip angle
Introducing “Tyre Slip Angle”
Tyre Slip Angle – What is tire slip angle? How does it influence your racing car and how can you use it improve your racecar’s ability to corner?
You might hear tyre slip angle mentioned a lot racing. Normally it is when someone is trying to explain how a car corners or turns. It is not immediately obvious however, what a “slip angle” is. It is also not clear how this relates to tyres!
What you’re after is to know what it means, why that is important and how it impacts your racing cars performance.
There is a lot to tyres and much more than just slip angles – as you will see if you explore some of the other race car articles on the site.
The aim here is just to give you a “starter-for-ten.” Armed with this new insight – and a video below that you will never be able to un-see! – you can then read my recommendations for more in-depth tyre information, should that be of interest to you.
What Is Tyre Slip Angle?
Tyre slip angle describes the mechanism tyres use to grip the road surface.
Tyres gripping the road surface are what enables your racing car to corner.
Understanding “tyre slip angle” is therefore fundamental to understanding how to go faster in your racing car.
When you are driving in a straight line and then turn, your tyre creates a force that is known as “cornering force“.
It is cornering force that makes a vehicle able to turn a corner. The more cornering force you have available the sharper you can turn.
In the case of your racing car this is really interesting. Why? Because the more cornering force your tyres can generate, the less you have to slow down for corners …
The less you have to slow down for corners, the quicker you can do a lap/complete the stage/get up the hill etc…
Cornering force is really important for your cornering performance.
A Simple Analogy For Tyre Side Slip Angle – An Eraser!
When thinking about slip angle, one way I have found helpful is if you imagine holding a rubber eraser on its end and pressing down into a desk:
While pressing your eraser down, imagine twisting it. The rubber will resist both you pushing it down and you twisting it.
The reason is that rubber has elastic properties. It wants to retain its original shape so resists your attempts to force it otherwise. In practise, this means it will resist your twisting force until it slips on the desk.
Interestingly and importantly, the harder you push the rubber down on the desk, the more twisting force you have to apply before it slips on the desk.
This is an important concept to have in your mind when trying to understand the subject of tyre slip angle.
Driving Straight Ahead Is Like …
Have a look at the diagram below:
This shows a picture of a wheel and tyre on the left. On the right is a graphic of a steering wheel.
In this picture, we are driving straight ahead. There are no forces on our car that are trying to turn us or deviate us from our desire to drive in a straight line.
Thinking back to our rubber eraser example, this is the same as holding the rubber on the desk and pressing down without twisting it.
Cornering Is Like …
This second diagram below shows (kind of) what happens when you turn the steering wheel.
The diagram aims to show that when you turn the steering wheel, the tyre twists on the wheel.
The part of the tyre not touching the road (i.e. most of it) is twisted away from the wheel when the steering wheel is turned. The part of the tyre touching the road (white bit) doesn’t twist and so, just like twisting an eraser on the desk, the twisting tension this creates generates the cornering force.
This alternative diagram from wikipedia aims to explain the same thing:
Both diagrams aim to show you that turning the steering wheel creates a similar resistance to the twisting the rubber eraser on the desk.
In practice, a tyre is slightly different from a rubber eraser. Also, my analogy is a little flawed as the rubber eraser is not rotating like the tyre, but, I hope, the concept holds good enough for people to understand that the twisting of the tyre creates the tyre slip angle that creates cornering force.
See A Tyre Generate A Lot Of Cornering Force
Watch the video below and you can see the twisting for yourself. It is quite an extreme example but at around 30 seconds you can see the twist in the tyre quite clearly. This is generating a lot of cornering force …
Tyres live in a pretty harsh environment. If you are ever temped to see this for yourself, try rigging up a GoPro to point at your tyre and then go out for a track session – it is eye-opening!
Your tyres resistance to twisting is what creates cornering force. The cornering force is what enables your racing car to change direction.
Cornering Force Increases With Slip Angle – For A Bit
All other things being equal, the more slip angle you apply, the more cornering force you can generate and the faster you can go.
The amount of cornering force is initially proportional to your tyres slip angle. This means that for each increasing degree of slip angle, you get an increasing amount of cornering force.
Unfortunately, this relationship doesn’t last forever. You will be all to aware that you can run out of grip in a racing car. Eventually, what happens is that your tyre can give no more cornering force for the extra slip angle.
When a tyre can’t generate any more cornering force it is at the limit of grip. It is what you’ll experience as a racing driver when you try to corner too fast.
Luckily, in some race cars, you can do something to increase your tyres cornering force – by pressing down harder on it…
Tyre Side Slip Angle Increases With Vertical Load
The diagram below shows you this quite nicely
In the diagram on the right, you can see three different lines. These are for three different cornering forces that are available for each slip angle. There is one curve for each of three different vertical loads.
Hopefully, you can see that the cornering force generated for each degree of slip angle increases with more downward pressure on the tyre?
Great news but how do you increase the downward pressure on the tyre?
Why Aerodynamics Is So Powerful On A Racing Car.
To increase the downward pressure you can broadly do two things; either make the vehicle heavier or use aerodynamics.
No One Wants A Heavy Racing Car
Heavy racing cars are slower because they require more energy to accelerate to the same speeds at the same rate, as lighter vehicles. Just think about how much easier it is to quickly lift a light dumbbell compared to a heavy one.
If you only have a fixed amount of energy available – i.e. power from the engine – then you want to have as light a vehicle as possible. There are some caveats to this but a lighter vehicle will help you to accelerate quicker and go faster.
Aerodynamics Give You Free Grip
Aerodynamics is in many ways a “free” opportunity to create downwards load on your racing car. This is because you are travelling through the air anyway. If you can control the air to your advantage, then you might be able to go faster. In this case, you have the opportunity to create downward load on the tyres without adding weight. Again nothing is completely straight forward here, for example a trade-off you might have to make is in increased drag.
Being able to generate more vertical load on the tyre without increasing vehicle mass is the reason that aerodynamics is so important in motorsports – especially in Formula 1 or on other “aero” cars.
Hopefully this has given you a little more insight into what tyre slip angle is all about and why it is so important. The subject is complex and I have deliberately tried to keep things at a conceptual level.
At any one moment in time, your tyres will be operating on a load verse lateral grip chart similar to what you have seen above in Figure 4. The general shape of that chart is conceptually consistent across most tyres.
What influences the precise shape of your tyre curves however is a huge range of things – such as tyre pressure, tyre temperature, road surface, spring and damper forces, the rate of change of load on the tyre … and many other factors.
You might be comforted to know though, that tyres are so complex that even the tyre manufacturers do not always know what they are doing!
If you are keen to explore more on tyres, have a look at some of the articles below.
There is a lot to know about tyres on racing cars. Try these articles to learn more – be sure to sign-up to the newsletter below for the latest race engineering insights from Your Data Driven.