Tyres

Tyres

At top speed, an F1 tyre will be making 50 rotations a second. An F1 tyre will rotate approximately 150,000 turns over a full racing distance 

Modern high performance tyre construction is a closely guarded secret maintained by a privileged few. However, there are similariteis in construction and performance between all the tyres that are a useful model for car performance optmisation.

The tyres is constructed around something call the ‘bead’. This is a steel ring which runs around the inside of the wheel rim. There are two beads per wheel – one fits the inside rim and the other, on the outside rim. These two beads are connected together by incredibly strong man made fibres – typically a material such as kevlar, nylon or rayon. These fibres are not set at 90 degrees to the rim – they are usually set at angles of about 2-9 percent from the true radial.

Grooving

If you have a smooth surface, the more rubber you have against the surface, the more traction you will have. By cutting grooves in the tyres, you lose some surface area meaning you lose some traction. An interview with Bridgestone’s Hisao Suganuma a while back quoted him as saying that a slick tyre may well be 2 seconds a lap quicker than a grooved tyre under the same set of conditions. The tarmac has to be smooth – not like our roads – so as to get the most traction. If you have conditions such as mud, obviously tread will give you an advantage as it will ‘bite’ into the road giving you better traction.

However, that isn’t the full story. The longer answer is, it depends on the track and the car. 

Anytime you put a groove or a sipe into a tyre, the wear on the tyre will accelerate. The trick is to find a balance between grip and wear. You can sipe a tyre to prevent the tread surface from glazing over and becoming a slick tyre.

Soft and hard tyres will obviously also make a difference. Grooved soft tyres are normally best for qualifying or in short stints in the wet. They are best used when the track is moist and there isn’t a lot of friction. This is because they wear at a much faster rate than hard tyres, so you want to minimise the wear on them. Grooves in soft tyres help to clear away dirt and rain (hence also why wet tyres have deeper grooves in them than dry tyres). The softness of a tyre is determined by the amount of carbon, sulphur and oil in it. The more oil there is, the softer the tyre.

Hard tyres withstand grooving much better than soft tyres. On asphalt and similar surfaces, it is easy to run with no grooves at all. However, you can groove the shoulders of a tyre to help clean away loose dirt and moisture. You can also sipe the shoulders on hard tyres to help cars running lower tyre pressures on slick tracks. The sipes help to stop glazing and help reduce traction loss.

There are three main groove shapes which are used – square, V-shaped and sipe. Square grooves are the same width and depth all the way around. V-shaped grooves start out wide at the top and taper down to nothing. Sipes are thin slips cut by installing the blade upside down and using ends of the blade to cut slices in the tyre.

V-shaped grooves are often used where the track is expected to need more tread contact later in the race because as the tyre wears down, the grooves begin to shrink and then to disappear. Square tyres are almost the same, except that they are better on a more abrasive track. Sipes are used to make the tread more pliable. They also help to maintain a more consistent wear that helps keep the tyre uniform.

If a track gets hot, tyres may start to blister, or even melt. Grooving can help to control this by moving air across the tyres to keep the tread temps down. Grooving also increases the surface area by which heat can transfer away from the tyre. However, there is a fine balance between heat dissipation, and wear and tear on the tyre. The best balance is to groove in the centre of the tyre instead of cutting all the way across it to limit the weakening effect of the grooves. This is best because it means you have as few grooves as possible (for speed purposes) but gives a good cooling effect.

Slip Angles

Tyres do not go exactly where the driver points them unless they are going in a straight line. Slip angles are the degree of angular distortion which occurs from the twisting of the bead relative to the footprint ie the angle between the wheel’s actual direction of travel and the direction in which it is pointing. The slip angle results in a force which is perpendicular to the direction of the wheel’s travel. This is known as the cornering force. The cornering force increases approxmiately in a linear fashion for the first couple of slip angle degrees. As the slip angle increases more, the force increases in a non-linear fashion before starting to decrease again.

A tyre which has a non-zero slip angle will become deformed. As the tire rotates, the friction between the contact patch and the road result in individual tread ‘elements’ (infinitely small sections of tread) remaining stationary with respect to the road. If a side-slip velocity u is introduced, the contact patch will be deformed. As a tread element enters the contact patch the friction between road and tire means that the tread element remains stationary, yet the tire continues to move laterally. This means that the tread element will be ‘deflected sideways. In reality it is the tyre/wheel that is being deflected away from the stationary tread element, but convention is for the co-ordinate system to be fixed around the wheel mid-plane. As the tread element moves through the contact patch it will be deflected further from the wheel mid-plane. This deflection creates the slip angle, and hence the cornering force.

F1 Specifics

 F1 typres must have four longitudinal grooves on them. Each groove must be 2.5mm deep and spaced 50mm apart. The are constructed with very soft rubber compounds to give optimum grip. Because of this, they wear very quickly so tyre choice is a critical part of an F1 teams’ decision making process.

A car with a heavy fuel load will wear the tyres a lot faster. There will always be a compromise between achieving the best grip/wear rate from the tyres and the speed, fuel consumption and handling of the car. F1 cars shed a lot of rubber when cornering. These collect in little ‘balls’ at the side of the track and are commonly referred to as ‘marbles’. When a car goes over these marbles, the tyres lose contact with the track surface which can cause the car to slide due to the lack of grip.

Optimum tyre temperatures are generally between 90 and 110 degrees centigrade. Tyre warmers are used to heat the tyres up in the pit lane or on the grid to get them up to a about 90 degrees centigrade to provide the car with a reasonable level of grip. Within a few corners at racing speed, the tyres will be up to full racing temperature. Tyres at this temperature will in fact be hot enough to fry an egg on!

Pace cars can cause a number of problems for F1 cars – the tyre temperatures can fall below their optimum causing a lack of grip, and the suspension can bottom out.  Tyre pressure is a fundamental element of the suspension system – it affects both the spring rate and the ride height of the car. Gas increases in volume when it heats up. As the temperature in the tyres decreases, the volume of gas in the tyre will also decrease which causes a lower ride height. This can cause the car to bottom out and go off the road. Drivers will often weave from side to side when behind the pace car to keep the tyres warm and decrease their chances of damaging their cars.