Slip angle is the angle formed between the direction your car is actually travelling and the direction where the wheel is pointing..
Simply we can say that there will be some friction between the tyres and road and the tyres will steer less than the amount you have commanded because tyres will stretch by some amount because of that friction.
As tyre is elastic there will be some deformation in the part of the tyre that is in contact with road(well it is called contact patch) due to the forces acting on the tyres during cornering.
So when tyre is static conditions means there is no forces acting on it
That means that Contact patch will be pointing forward
But during cornering because of the forces Contact patch distorts and it points in other direction
So this difference in angle is called slip angle
Tyre friction
You have definitely heard about the friction in your schools.. So let's understand how this works in case of a car...
Let's take a example of pushing a block over the table
As you applied the force the block will not move until you overcome the static friction
After you have applied the peak force, the block will starts sliding.
Similar is the case with tyres
As the forces in tyres are below it's elastic capabilities tyres will not slide and as these forces goes beyond the tyres elastic properties sliding will occur.
What causes slip angle
As you turn your car in the corner..
Your tyres will face a force that is generally called as centripetal force.
The tyres will try to resist this force and in doing so tyres will develop a resisting force termed as lateral force. This force is due to the friction which is present between the tyres and road.
Fy =uW
You may have seen in the race cars that sometimes tyres release smoke and that is because there is a huge lateral force in the tyres due to high speed and friction and this will make tyres heat up very much.
If the centripetal force acting in the tyres becomes larger than the lateral force then the tyres will slip.
Fc>Fy
So it is upto driver to maintain the slip angle while driving to avoid skidding.
Slip angle relationship with friction coefficient and lateral force
Slip angle varies with the amount of coefficient of friction between road surface and the tyres and with the friction lateral force or cornering force also varies with coefficient of friction.
So we can say the lateral force tyre will genrate will depends on the slip angle for the tyre and the road surface.
As you can see in the figure. The lateral force genrated on the tyre varies linearly with the increase in the slip angle. This linear region is known as elastic region in which tyre is will face only an elastic friction.
The maximum value of coefficient of friction or lateral force (as Fy=uW) will provide maximum grip in tyres for cornering.
After this linear region, if the slip angle region, tyre will face combination of friction that is static as well as sliding friction.
Increasing slip angel after this region, more portion of the contact patch starts sliding and less of the portion remain in contact with the road surface.
Beyond this region this will also lead to decrease in coefficient of friction and tyre grip.
And the tyre also start sliding which will make the vehicle unstable and difficult for the driver to handle.
Why slip angle is important
The terms slip angle describes the gripping ability of the tyre during the cornering of the vehicle. For the proper turning , slip angles needs to be adjusted to a optimum value in order to get best possible grip during Cornering.
Slip angle depends on the various factors like :-
- Tyre properties
- Load on tyre
- Road surface or race track
- Acceleration
- Braking
While driving, slip angle depends on how much load is acting on a tyre. This load can vary due to
- weight transfer because of cornering
- the speed of the vehicle on cornering.
- Acceleration and braking
Acceleration and braking transfers weight backwards and forwards respectively leading to the variations of load on the tyres which affects slip angle of the tyres.
So driver can maintain the proper slip angle by varying it's speed to get maximum possible grip during cornering.
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