Thread by @fastestlap_f1

In Formula 1, performance is dictated by the tires. But how do they work? How drivers extract their maximum grip? Let's learn about it! #F1Tech

A tire is free-rolling when it rotates at a rate, ω, such that the points at the contact patch move with the same velocity of the tire hub, v. This rate is ω=v/R. Under these assumption, and if we neglect the rolling resistance, the tire produces zero grip and its speed stays...

... unchanged. If we roll a carpet around the tire, we would see that it is left on the road perfectly unwrinkled. The velocity of the contact point, vc = v-ωR, will be zero. Under real conditions, tires need to violate this condition to generate grip.

If the tire rotates faster than v/R, the velocity of the contact point will be negative. Friction, which opposes movement, will originate a forward force. Hence, if ωR>v, grip is positive and it drives the tire forward.

Opposingly, a decelerating tire rotates slower than v/R, the velocity of the contact point vc = v-ωR is positive, which results in a backward friction force. A carpet rolled around the tire will be left in the road stretched, pulling the tire back.

An extreme situation is the case of a locked tire, ω=0, where the velocity of the contact point is the same as the wheel hub, vc=v.

Tire grip can experimentally determined via tire test rigs, where the tire rolls over a surface while friction is measured by gauges as a function of several parameters

Experimental measurements show that the grip generated by friction depend on the magnitude of the contact point velocity and the normal load. Do you remember the high-school physics class when you were told that F=μ·Fz? Well, in real tires it is very non-linear...

Usually, the longitudinal slip, kappa, is used instead of the contact point velocity,which is positive when the tire rolls faster than its hub motion (acceleration) and negative otherwise.

grip depends on the normal load and the longitudinal slip. What you need to understand, is that normal load depends on the car-setup and longitudinal slip depends on the application of throttle/brake by the driver. Hence, to find the maximum grip both car and driver are crucial

First, grip increases with normal load Fz, that is, how much the car presses the tire against the road. It is mostly dictated by the car set-up: downforce, car weight and masses distribution, chassis geometry and suspension settings. In the plot, hotter colors mean higher Fz.

Second, the driver must use the throttle and brakes to find the longitudinal slip that maximises grip. Here is when the best drivers stand, such that they can feel precisely this dangerous sweet spot where tires are squeezed to the maximum while keeping control.

It is important to remark that finding the grip peak is very delicate as grip decreases very abruptly once it is passed. Here is when drivers lose control and the car slides far from ideal.

Considering a Formula 1 car that cruises at 50km/h, we all know that going wild in the throttle at these low speeds without electronic traction control is a really bad idea, but just in case…

Lesson learnt. Why did the car lost control? Well, if the driver uses too much throttle it pushes the tire to longitudinal slip values past the grip peak. With this excessive slippage, the tire loses also its capability to control the car's balance and it spins.

But most importantly, the car fails its mission of accelerating. Due to the crazy slip levels, the poor generated accelerating force is way below those the tire capabilities, hence the car barely accelerates before losing control.

The best drivers are capable of extracting the maximum performance from the tires. In this case, the driver will apply the throttle smooth enough such that the slip levels are within the operating window.

In this simulation we see how the ideal driver finds the maximum grip out of the tires by maintaining the slip as close as possible to the peak value. We see that the grip increases as the velocity increases, thanks to the downforce.

As the grip increases, the car reaches a velocity where the engine cannot provide enough torque to overspin the wheels, and the driver can finally use the full throttle.

In conclusion, tire grip is very non linear which makes it tricky to handle. When we drive our 110cv cars on the highway, we stay on the low slip safe zone and there is never a risk to lose control.

Race drivers, however, need to drive the tires to the limits of handling. In this point, there's the risk of exceeding the peak and the sudden loss of grip. Here is where the skill of the drivers stand, so that they can extract the maximum out of the tires while keeping total...

control of the car.

Writing tech threads in twitter is never easy, so if you have enjoyed this content, you can head to this blog entry where these concepts are explained in depth
link.medium.com/GkpeqGAYusb

This thread only covers straight-line acceleration. If you are interested in tires behaviour while cornering, let me know and I can cover it in the future! Thank you for reading 😊