Atlas F1   Taking the Lid Off F1

Formula One Technical Analysis

  by Will Gray, England

Atlas F1 presents a series of articles by certified engineer Will Gray, that investigates in greater depth all the technical areas involved in design, development, and construction of a Formula One car.

Part 5A: Tyre function

The four round, black objects in each corner of a Formula One car are equivalent to a runner’s feet or a swimmer’s arms and legs: The power comes from within, but it meets the outside here.

The tyres are important for two reasons - they must deliver the power from the engine to the racetrack, and they must allow the driver to steer. Teams have dabbled with four wheel steering, four wheel drive, and even used six wheels, but in general four wheels is the norm, with the rear wheels to drive, and the fronts to steer.

To do this they require grip, and this is only available in the area where they meet the road - called the contact patch. In creating grip, vertical loads (the loads pushing the car onto the track) are important. These are the sum of the weight of the car, driver and fuel, and the aerodynamic downforce. The tyre takes these loads, and creates a force on the racetrack by gripping to the unsmooth surface. The more the tyre is pressed to the surface, the more grip is created. To help with this, the rubber, when at operating temperature, is so sticky it picks up stones and debris on the circuit, which also means that it will stick to the racetrack. Without the grip between the racetrack and the tyre, the car could neither move nor turn - you can see this by driving on ice… no grip: no control. The better the grip, the more effective the car is at moving around the circuit, and the lower the lap time will be.

The grip, or friction, between the racetrack and tyre comes in two forms - elastic and sliding. With a small force, all the friction is elastic (sticky), and the tyre grips to the racetrack completely. As the force on the tyre from, say acceleration, increases, some of the friction becomes sliding friction, but with more elastic friction than sliding, the tyre will continue to grip. This explains how tyre marks are left on acceleration: When too much force is input for the elastic grip of the tyre to fully cope with, some of the tyre will slide, but there is still enough elastic friction to create the grip to move the car forward, and the car moves away leaving lines behind. If the force becomes too great, the sliding friction becomes dominant, the wheel no longer has good adhesion with the track, and you get wheelspin - the wheels spin and you go nowhere!

In simple straight line terms, a wheel being pushed down and rotating forward will effectively push the road backwards underneath it - try it by pushing a can down in a fixed position relative to the table, on top of a piece of paper, and rotating it. When the wheel is connected to the car, the road will be pushed behind the car - but as the road is in fact fixed to the earth and the car is not, the car will move forwards.

As explained above, sometimes a lead-footed driver may accelerate to a level where the force is more than the grip can cope with, and will get wheelspin. The same may be applied to braking, but in this case rather than the track being stationary and the wheel accelerating, it is the opposite. The driver tries to slow the wheel whilst the track is still moving. If the amount he tries to slow by is more than the grip allows, the wheel will slide, the rubber will be worn away by the abrasive track surface - leaving plumes of smoke - and the driver will be left with Eddie Irvine locking up during the 1998 European GPa ‘flat spot’. This is exactly as is says - a flat part on the normally round tyre, and will cause the car to have vibrational and handling problems. To prevent this occurring, systems such as traction control (now banned) and anti-lock braking electronically control the amount of force input by the driver, and will not allow any more than that which will take the tyre to the limit of adhesion.

Now for cornering. When the driver turns the steering wheel to take a corner, he requires grip. Without such adhesion the tyre turned would simply slide in the direction the car is currently traveling rather than change its direction. When the steering wheel is turned, the road wheel is turned either right or left. Most of the wheel turns as expected, but at the contact patch the tyre resists this movement because of its grip with the road - you can feel this resistance when trying to turn the non power assisted wheels of a stationary car. You therefore have the contact patch pointing in the direction of the road, and the rest of the tyre pointing in the direction it wants to go - a situation which is possible because the tyre is elastic and can twist.

The next bit is easy to understand if you imagine a vertically held rectangular pencil eraser. If the top is rotated with the bottom held, this represents the steering input by the driver, and the rubber, like the tyre, will be twisted. Now, if the top is held in the rotated position and the bottom released, the rubber will snap round to re-align itself with the rotated part of the rubber. This elastic energy, stored in the tyres, is released onto the track surface and is the cornering force which takes the car around the bend.

When there is too much side force for the tyre to cope with, and it can no longer deform, it begins to slide on the surface, and the car begins to slide outwards from the apex of the corner. Once this happens, all is not lost - for the good driver that is! The tyre still has significant cornering force, and the driver can re-claim the adhesion and stop the slide by reducing the side force with opposite lock (rotating the wheel in the opposite direction to that in which the car is traveling). Although this is spectacular to watch, it ain’t the right thing to do! A car should be driven on the limit of adhesion and not over it.

Previous Parts in this Series: Parts 1 & 2 | Part 3 | Part 4A | Part 4B | Part 4C

Will Gray© 1999 Kaizar.Com, Incorporated.
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