Tyres and Formula 1

We have seen many press articles being published lately, commenting, assessing, analysing the impact of the new Formula 1 rule about tyres.

To explain briefly the concept of the new rule; Bridgestone will be the sole tyre supplier this year, providing each teams with identical specifications and quantities of tyres. All drivers will be required to use two different compounds during each race; making the tyres an even more important component for success. (more about the new F1 tyre rule)

The Australian Grand Prix in Albert Park, Melbourne, is always the big test for all teams, and the extreme conditions there make the track really demanding for the tyres. Bridgestone was having trouble to choose between the hard/medium and medium/soft compounds for the race.

To make any sense at all of this treatise, we recommend that first you go to our “all about tyres” section on CarbonBlack, then click on the section “Tyre construction Q & A”, and finally browse down to “Let’s get back to where this heat comes from”, and “what’s rubber”. Having done your homework, you will then be prepared for this rational discussion on what makes a tyre tick under the extreme conditions of car racing.

Rubber compounds are comprised generally of around 12 chemicals, mainly carbon black, rubber, and the vulcanisation system of sulphur and other chemicals. Others are added to protect the rubber, aid its processing, or improve it resistance to a perceived hazard.

Rubbers used in tyres are generally styrene-butadiene co-polymers, natural rubber, or polybutadiene. These can be blended in whatever ratio is desired. The styrene content can be varied to give a hard wearing rubber, or a high styrene cling rubber to maximise wet grip at the expense of heat build up. Everything is a compromise aimed at giving the best performance for a particular application. For example, when a new circuit is laid down the surface is very abrasive, and polybutadiene has a superb abrasion resistance to sliding on sharp surfaces. However, it is hopeless in the wet, since grip suffers appallingly. In car racing of course, grip is everything. However, there is one famous case where a car won one of the last Phillip Island 500 mile races, by changing only one tyre, whereas everyone else changed 12! The tyres had polybutadiene treads, but were 3 seconds a lap slower on the newly laid circuit, particularly so at the ocean side of the circuit. But they didn’t call the now 89 years old Harry Firth “the FOX” for nothing.

The second most important variable is the carbon black type that is used. There is a huge range of carbon blacks, derived from burning oil in a specially designed furnace in insufficient air for complete combustion. The “soot” that is gathered is carbon black, which may vary from the ultra-fine types used in printing ink, to the ‘soft’ blacks used in the casing compounds of a tyre. As a general rule, the finer the carbon black, the higher the abrasion resistance, and the higher the heat build-up in the rubber compound in service. There is another variable, the “structure’ of the carbon black, a measure of the agglomeration of the particles, but no further correspondence will be entered into on this, as it’s getting a bit too technical. Most tyre compounds will contain 25% to 35% carbon. It’s the carbon that reinforces the rubber, improves abrasion and tear resistance, tensile strength, and lots of other characteristics.

The third variable is the “state of cure”. In other words, you can make “hard rubbers” merely by making adjustments to the level of “curatives” – sulphur and accelerators added to the mix. Add all these variables together, and the usual range of tread rubber hardness can vary quite considerably. It is measured using a simple penetrating tool called a Shore A Durometer. What is significant is that this hardness varies with the temperature of the rubber. This affects other properties as well. You are probably all familiar with the range of squash balls and their different colored dots, signifying different resilience or rebound. However, all change their rebound characteristics with temperature, and they get hot when you bash them up against a wall. Race car drivers are quite used to this, and adopt techniques to “load their tyres up” during slow periods of the race under the pace car discipline, for example.

Formula 1 go even further by wrapping their tyres in the pits in electric blankets to warm them, so that the driver won’t be caught out by an abrupt transition from race hot tyres, to cold newly fitted tyres, which don’t grip as well.

As the temperature of the tyre changes, so does the dynamic qualities of the tyre. If it gets TOO hot, the amount of heat being generated cannot escape to the cooling airstream quickly enough, rubber being a very good insulator, the heat builds up at either the thickest part, or the part under most stress, for example the outside front tyre on a banked circuit, and the tyre “goes off’ in race car speak. It loses some of its rubbery characteristics, reverts to a puggy dough almost, and will probably blow apart. Tyre technicians monitor the temperature at the thickest part of the tread by inserting a needle pyrometer, which gives a digital readout of the temperature at the point of the needle inside. The tyres have a dimple on the buttress to indicate where the needle should be inserted to a calibrated depth. Being race tyres, the tread is not very thick, so this has to be done carefully, and quickly.

An interesting development in Formula 1 has been the compulsory adoption of grooved tread patterns. The tyres are grooved circumferentially, rather similarly to the patterns used on aircraft tyres. They have one purpose- to slow the cars down! There is less rubber on the road than a slick tyre. A high proportion of the drag generated by an F1 car comes from the tyres. The downforce is controlled by the spoiler or wing on the rear of the car, plus other “aerodynamic aids” placed at critical points around the car, following wind tunnel testing, designed to hold the car on the ground.

These tread grooves cannot be placed over the edges of the reinforcing belts in the casing, or they will quickly cause the tread to crack circumferentially, which will “throw the tread”- a catastrophic failure. Recently, one of the presenters on the BBC show “Top Gear’ crashed a jet powered car at high speed due to a tyre failure on the front wheel. Wait for the full show, so that you can freeze frame the blowout, but it looked very much like that sort of failure on the film clip seen so far.

However, even though all tyres now have a tread pattern, there are still “wet weather” and “dry weather” compound tyres fitted during the course of a race, as circumstances dictate. This is a measure of the absolute ‘coefficient of friction’ developed by the rubber compound on the road, particularly after it has removed the film of water from under the tyre.

When to change from wet to dry, or the reverse, is dictated by experience, the lap times being recorded, the weather forecast, and the ‘lap of the gods’. That’s why all the gurus in the pit lane all seem to have grey hair, or none at all. The tyre technicians can advise, test to their hearts content on indoor test wheels at impossible speeds, and at outdoor circuits, have access to all the computer aided programs they desire to optimise any particular property of the rubber compounds they may be seeking, but in the long run, someone has to make a decision on what is the most suitable tyre for the race on the day. What works at Albert Park in Melbourne, will be different indeed to the new circuit at Bahrein, which is very abrasive on tyres.

The difference can be the difference between finishing first, or tenth, or worse.