Tyre Tread Compounds
There’s a great deal of confusion amongst car enthusiasts, particularly the “rubber burners”, on tyre tread compounds and their make-up. One enthusiast on a car blog announced that tyres weren’t made from rubber at all, but from oil. You know what- he was mostly right!
Oils ain’t oils, and rubber ain’t rubber any more.
Tyres contain 3 or 4 different rubbery materials. One is natural rubber; which is the juice of a tree, which is coagulated using acetic acid, smoked and dried. The others are all made from oil, and are called “polymers”- another term is “long chain macromolecules” but don’t worry too much about that. It is now possible to make “natural rubber’ from oil too, but it’s cheaper to let the tree do it.
These various rubbers can be mixed together in different ratios in giant blenders to make a compound. At the same time, other important ingredients are added to make the resultant product tougher and stronger. A typical tyre compound may contain 10- 14 ingredients, all added for a specific reason. The most important of these is carbon black, of which there are many types.
A tyre typically has 7 to 11 compounds, each doing a specific job, be it encasing the bead wires, keeping the air in a tubeless tyre, flexing the sidewall, sticking the layers of nylon or polyester together, and so on. But there is only one compound that hits the road where it all happens, and that’s the tread compound.
This is basically the only criterion on which the motorist can judge the performance of the tyre, so it receives the most comment from car enthusiasts. The tyre engineer and chemist can vary the compound formula to maximise/minimise any tyre characteristic that he requires.
A typical passenger tyre tread compound contains as the base polymer styrene-butadiene copolymer, around 35% carbon black to reinforce it, and maybe some silica. These increase the abrasion resistance, tear strength, and cut resistance. Without them, the tyre would go gooey, and wear out very rapidly. Remember those old crepe rubber soled shoes?
Vulcanisation chemicals such as sulphur, zinc oxide, stearic acid, and accelerators make up 3-5%; antioxidants and antiozidants to stop it perishing or cracking, processing aids such as oil, resins, tackifiers to aid in the lay-up of the assembly are all incorporated.
Some tyres have two tread compounds- either side by side (very rare), or a cooler running undertread compound under a harder, hotter running cap stock. Most, however, only have one compound in the tread area.
The rubber used in tyres is normally a copolymer ( mixed and then polymerised together) of 23% styrene, and 77% butadiene. However, this ratio is not set in concrete, and specialty rubbers of different proportions of these two refinery products can be made. For example, “cling rubber”, which was widely touted for its improved wet grip, is 40% styrene, 60% butadiene. The resulting rubber ran hotter, and wore out quickly under Australian conditions. A 90% styrene, 10% butadiene rubber is used to make floor tiles, not car tyres.
Another rubber developed for use in tyres is polybutadiene. Butadiene is the most common feedstock from a refinery. However the resultant polymer suffers one big disadvantage- its wet grip is poor. Its big advantages are however, that it stands up to extreme abrasion much better than other rubbers, and runs cooler. Back in the days when the Armstrong 500 Miler was run at Phillip Island on standard tyres and rims, and the track was not a smooth hot mix like it is today, Harry Firth won the race by changing only one tyre, whilst everyone else changed at least twelve on the very abrasive track. But he lost 3 seconds a lap, because down on the ocean side, the tyres wouldn’t grip to racing levels. The tyres were made from a high proportion of polybutadiene in the tread, specially airfreighted out for the race.
We’re not as skilful as Harry Firth was then, and the Australian motorist puts “grip in the wet” as the top desirable characteristic from his tyres, so its use is now mainly confined to truck tyres in blends with natural rubber, where heat is the main enemy of tyre performance.
So ultimately, it’s the “grip” of the tread compound that drives, steers, and brakes the car, through the contact patch, around the area of a size 12 shoe. It does this by slipping! Sliding generates friction, and this causes things to happen. All tyres slip, particularly driven and steering tyres, which is why they wear out. No friction- no progress. Try driving and steering on black ice sometime to see what I mean.
“Rubber burners” overlay this with “sticky friction” by heating the tread surface till it starts to revert- goes gooey. Lots of smoke! On top of this, tyres generate heat internally from the stresses generated by flexing (the hysteresis loop). As the rubber warms up, the rubber changes its grip characteristics, provided that the compound hasn’t degraded to the gooey stage (“goes off”). This occurs generally in the thickest part of the tyre under the greatest load, like the outside shoulder of a tyre being driven on a banked circuit. That’s why you see tyre technicians who are evaluating tyres, drive a thermocouple needle into the shoulder of the tyre tread- the thickest part. The electric blankets on the wheels ready for a change onto a race car are there for the same reason- so that the car will handle similarly to the old, warmed up tyres.
The position of the white stripes in the tread grooves of the Formula 1 cars indicates the type of compound used in the tread. The tread grooves are there to slow the cars down, even when the road is dry. Race team managers under F1 Rules have to use at least two types of tyre during the course of the race. This adds another source of tactical variation for managers to consider, as though they haven’t got enough on their plate. But the race result may have been decided in a tyre development laboratory in Kobe or Luxemburg or wherever, since so much data has been accumulated on the vagaries of each circuit, and the tyre compound that performs best on that circuit.
It’s almost time for the Melbourne Grand Prix- so enjoy your viewing
Entry filed under: Automotive industry, Our Experts, Sports & Racing, Tyre Industry, Tyre safety & maintenance, Tyre Technology. Tags: Auto Racing, auto service, butadiene, car service, Grand Prix, Melbourne Grand Prix, Racing, rubber, Tyre tread, tyres.