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 motorsport. Normally it is when someone is trying to explain how a car corners. However, it is not immediately obvious what a “slip angle” is, nor how this relates to tyres. This article will help you make sense of what is going on.
There is a lot to tyres – as you will see if you explore some of the other race car articles on the site – so the aim here is just to give you a “starter-for-ten”.
Armed with this new insight you can then read my recommendations for more in-depth tyre information, should that be of interest to you.
Tyre Slip Angle – What It Is NOT…
The first thing to know about tyre slip angle is that there is no actual slipping going on (don’t you love engineering descriptions!)
When the tyre touches the road (at the contact patch – see below) the tyre and road are actually briefly stuck together:
This sticking together is a good thing – it is what generates your grip.
So Why “Slip” Angle?
When you turn the steering wheel to drive round a corner, the contact patch remains stuck to the ground, so your wheel rim ends up twisting the rest of the tyre.
I have found helpful is if you imagine this. Hold a rubber eraser on its end and press it down into a desk:
While pressing your eraser down, imagine twisting it. Just like the tyre, the rubber in contact with the desk will not move and the rest of the rubber will twist around it.
Rolling The Twisted Tyre
If you now imagine rolling the this twisted tyre what is going to happen?
Well what happens is that the next piece of tyre comes down slightly offset from the current contact patch.
As the tyre continues to roll this newly presented piece of tyre grips the road, as the previous piece of tyre is losing grip.
This effectively creates a lateral force that pulls you across the road and around the corner:
Given the twist, the steered angle of the wheel is going to be different to rolling direction of the tyre.
The difference between these two angles is called the slip angle.
It is a kind of dynamic slipping I suppose. This is because the path you follow is not inline with the direction the wheel is pointed. Therefore it’s “slipping”.
The whole thing is a bit of a difficult concept to get your head around in my humble opinion. I guess this is because it’s kind of a 3d problem.
Here is another analogy which might help you thinking about this:
The Hand-Over-Hand Analogy For Tyre Slip Angle
Another way I’ve found helpful to explain this to people is to use a hand over hand analogy.
Imagine the contact patch is like your hand:
Place it flat on a desk.
Now get your other hand and line them up one above the other like this:
Imagine that your second is the next piece of tyre that isn’t yet in contact with the road. You can curl it up a little if that helps.
I’ve mentioned already that when you turn the steering wheel, the tyre twists. Try this with your hands.
With your second hand, move it slightly offset from your first hand and slightly twist it in the direction of the corner.
This is the equivalent of the wheel twisting the tyre.
As the tyre rolls, your first hand comes up off the desk and your second hand starts to press into the desk. Your second hand becomes the new contact patch. And look, that contact patch is now offset slightly.
As your tyre rolls, new tyre, or hands!, get constantly get presented to the road slightly offset from the previous ones.
As the new hands become the contact patch, so you move laterally, and go around the corner.
A bit like in this image:
As I say, this is not a straight forward concept to get clear in your mind – largely because it is a 3D model you’ve got to imagine.
Hopefully you are clearer now though.
Why Understanding Tyre Slip Angle Is Important
Understanding “tyre slip angle” is fundamental to understanding how to go faster in your racing car.
When you are driving in a straight line and then turn, the lateral force created 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 faster you can turn.
In the case of your track 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.
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 too 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 driver when you try to corner too fast.
Luckily, in some racecars, 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, 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 Racecar
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. 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.
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.
Further Slip Angle Explantions… Tyre Contact Patch Whilst Cornering
This diagram below shows what happens at the contact patch when you are driving along, turn the steering wheel and start cornering:
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 doesn’t twist and so, just like twisting an eraser on the desk.
This second diagram 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.