Science Behind DONUTS

 In the entire automobile machine, the only part that is in contact with the road is the tires. Tires have a significant role in the performance of vehicles. In fact, the most limiting factor in the performance of most sports cars is tires. In this blog, We are going to look into how tire affects the performance and the role of grip in tires.

DEFINITELY, THEY HELP US DO DONUTS. LET US SEE HOW.

 

The grip is basically a bonding force that binds the tire compound with the road.No surface in the earth is perfectly smooth. So when this imperfection in the surfaces locks together, there comes a phenomenon called surface adhesion. More the surface adhesion more the grip. Elements like rubber, which is elastic in nature come in handy with creating a good amount of grip with the road. Because the elastic nature of the rubber tends the material to flex with the surface( road ) and lock even better with the surface, creating more grip.

The term grip can be expressed with the alternative word called "Traction", So in this blog, we will talk about grip in the name of traction. In simple physics terms, Traction is nothing but static friction. 

Assume a block with a weight (mass*gravity) is set on a surface with a static friction coefficient (f). So the minimum force that is required to move the block should be greater than f times weight (mass*gravity). After that particular force that block is going to slide the table and move. 

Similarly, now assume a tire instead of a block. The torque from the engine is multiplied with the help of a gearbox and transmitted to wheels, which is transferred as a tractive force to the road by the tires. Assuming the weight of the car is "mg"(mass*gravity) and Traction coefficient ( peak coefficient of static coefficient) as"f". The amount of force required for the wheel to slide is f times mg. Which will cause the wheel to slide and burn out ( this is how donuts are done ). This is an undesirable effect in tires when it comes to speed and acceleration because it will reduce the acceleration performance of the car. Thus the acceleration of the car is always limited by traction. To increase traction the vehicle must possess either maximum weight or maximum traction coefficient. Maximum weight is not considered because it will cause maximum energy losses by rolling resistance and grade resistance, so the ultimate option left to us is just to increase the traction coefficient.

The number of ways to increase peak coefficient is not our area of scope, it is left to tire engineers. But there are some physical qualities that are said to increase the Traction Coefficient.

1. Softer and more elastic compound.

2. Less pressure on the compound ( More contact area )

3. More uneven surface ( Reason for  Tarmac road over Cement road )

We will look into all these aspects in the next blog.

So when the tractive force exceeds the traction limit of the car, the rubber slides and affects the performance of the car. It is a very important duty of engineers to design a powertrain that is more suited for the tire and vice versa.

The traction property of tires is often expressed in terms of g-forces. For example, a tire with f = 1.5 is said to have 1.5g traction, because the traction force is directly proportional to the weight which is a unit g force. so the traction force is 1.5 (f) times g. This also ultimately means, the vehicle can't accelerate more than 1.5g  that is about 15 m/s/s or 48 km/hr/s. This also works for the breaking condition. The car can't decelerate more than 1.5g if the braking force is more, than the wheel slides ( lock-up ), Which is again an undesirable effect reducing the stability and cornering properties of the vehicle.

Anti-Locking braking come in handy at these case to know more about it click here.