Tyre Slip Angle Truth for Racing Drivers

Uncategorized Dec 20, 2019

When you turn the race car into the corner, the forward momentum of the car generates a lateral force at each of the four tyres, forcing what we know as "slip angles" to form at the tyre contact patch. 

There is so much written about tyre slip angles, some from highly credible sources, others less so. As a racing driver, you do need a good understanding, but only at the level required for driving the car and setting up the suspension. There's a lot we can leave to the vehicle dynamics engineers to grapple with. 

As a racing driver, here's what you need to know. 

--------------------------------------------------------

The formation of slip angles is a very fortunate characteristic of the pneumatic tyre, allowing the racing tyre to support very substantial side loads.  

It’s often thought the tyre slip angle is an indication of the tyre losing cornering grip – that the slip angle is formed by the tyres “slipping” or “sliding” on the pavement. 

The truth is, the slip angles indicate the tyres are making grip. In fact, building grip with increasing slip angle (up to the point where grip levels off).

In response to the lateral force being applied, the tyre contact patch does distort significantly.

However, to make grip, the majority of the rubber in the contact patch has to stay keyed in to the road surface.

At racing speeds, racing drivers build grip in the tyre as fast as possible and then hold the tyre at peak grip, until ready to release the car in the corner exit. 

For the tyre represented in this diagram, peak grip is reached at around 6 degrees of slip angle.

If the driver pushes harder and/or friction at the tyre contact patch reduces, the grip area of the contact patch reduces in size, causing grip to fall away.

The tyre is no longer able to support the grip level asked of it. The force of cornering overcomes the tyre grip availability, until the driver takes action to reduce the cornering lateral load, and return the tyre into the grip zone.    
 
This diagram illustrates the tyre slip angle in cornering.

The steering angle achieved ("direction of travel") is always a little less than the steered angle applied by the driver, ("the pointing direction"), the difference between the two angles is the tyre slip angle. 

This tyre is turning right. The lateral force of cornering causes the tyre carcass to flexing sideways towards the centre of the turn, as shown. 

The lines you see drawn in the contact patch represent a particle of rubber passing through the contact patch. You'll see this in a lot of the explanations of slip angle out there. In my view, drivers don't need to know about this. There are some interesting things the engineers can get out of this approach. e.g. Identify the pneumatic trail. The main thing is to recognize the sideways flex of the tyre contact patch and how this works to keep the contact patch rubber keyed into the road.

So, the following is my preferred explanation, where we can clearly see and appreciate the flex in the side wall and how it contributes to the formation of slip angles:

Tyre slip angles are a property of the pneumatic tyre that creates the lateral force and allows the car to turn. (If we had steel treads, for example, there would be no slip angles, and insufficient friction available to create a cornering force.)

In this diagram, the car is turning left. In cornering the steered angle of the wheel is a little greater than the actual direction the wheel is traveling in. The difference between the two is the slip angle, α, as shown in the diagram. The cornering force is applied at the axle, and the reaction force at the contact patch is the lateral grip.

Note the bag of the tyre flexes sideways, as shown above in end view. There is no twisting of the tyre to form the slip angle. The contact patch just lays down successive “footprints” in line with the direction the wheel is pointing. 

 This "walking analogy" comes from page 19 of Race Car Vehicle Dynamics, by Bill and Doug Milliken.

"The ability for the tyre to travel out of it's plane without appreciable slippage is the result of laying down successive prints, each laterally displaced due to the sideways tyre distortion."

Few other authors show this properly. Kyle Engineer on YouTube show does get this right.

 

 Tyres with greater flex in the side wall operate at higher maximum slip angles. Tyres with less flex in the side wall (e.g. low-profile tyres) are more responsive. They build grip quicker. But they are also “less forgiving” than tyres with a bigger bag. There’s less warning for the driver at the limit of grip.

Yes.  If you push past the limit of grip, the rate of change of slip angle increases markedly, and the tyre does start to slide. 

Starting from the straight ahead, and no tyre slip angles, as you turn the steering wheel to initiate cornering, tyre slip angles start building immediately, then build quickly, subject to your speed of turning the steering wheel: 

The diagram above shows Cornering Force (lbs-force), or grip level on the Y axis versus the Tyre Slip Angle (degrees) on the X axis.

In this example, grip builds initially in a fairly linear fashion.  Around 4 degrees the rate of increase reduces, peaking at around 6 degrees of tyre slip angle, and then falls away. 

Note also that vertical load on the tyre increases the grip potential.  If we doubled the vertical load from 900 to 1800 lbs, for sure, we’d increase the grip a lot, but not quite double it. The "tyre (vertical) load sensitivity" is the key concept behind "roll stiffness" distribution, which is our main tool for adjusting the "steady state" balance of the car. 

The situation shown here, where grip level is fairly constant between 5 and 8 degrees, is typical.  The shape of the curve varies with tyre vertical load as shown.  With maximum weight transfer mid corner, the outside tyre delivers most of the cornering force, but inside tyre grip is still important in achieving best overall grip at that axle.

Maximum grip for this tyre is generated around a slip angle of 6 degrees.  Tyres where maximum grip is achieved around 3 or 4 degrees will be peakier at the top of the graph.  Such tyres, making maximum grip over a smaller range of slip angle, can be considered a “less forgiving” tyre. 

Tyre slip angles are basic to your understanding of vehicle dynamics and your ability to tune the suspension of your race car. Get a look at our new E Book here:

Check out our new E Book:

"7 Little-Known Hacks - Your Pathway to Suspension Set-Up Mastery."

The "7 Hacks..." are seven little known insights into race car handling, giving you a unique overview of handling that could transform your understanding of what’s required to do your own suspension set-up.

It's a birds eye view of our latest thinking in vehicle dynamics and our suspension set-up procedures. 

GET THE E BOOK HERE