Know your Tyres



Inconsistency between batches of tyres causes much consternation and muttering with the most recent victim being Michael Simpson at the Larkhall Super One. “We were going alright until we put the race tyres on,” said Andy Cox as he and Michael consoled themselves with a Scottish breakfast. He isn’t the only one it’s happened to and it’s by no means limited to the Rotax classes and Vega tyres, as European KF3 Champion Alex Albon oon Dunlops thought he was at a big disadvantage in his heats. Carlo Forni demonstrates the handling effects of different compound tyres with the same carcass on page 82.

So I asked Jason Parrott, team manager of Max team Tim Parrott Motorsport, whether the situation is really as bad as all that.

“The junior tyres, the SL8s, are OK, they don’t change much, although some people do complain. The SL6s (Senior) are the worst problem, the balance of the tyres changes dramatically, there’s a big slide then a big understeer. The kart won’t stop, and at the middle of the corner the kart drifts. Once we tested four sets at PF with Tom Armour, and there was half a second between the best and worst set.”

“At Larkhall for the Super One the tyres were mostly OK, then at PF for Kartmasters the following weekend we did some testing on the tyres from Larkhall and then the new tyres were only as quick as the 120-lap old ones! It shouldn’t be possible and there is definitely something not right. With all three drivers saying the same thing there must be something going on.”

Jason believes there needs to be some sort of code on the Vegas as there is on Bridgestones so people know what they are getting.

Inconsistent tyres aren’t a new issue though. Alex Zanardi was an early test driver for Vega and in his autobiography he recounts a batch that were particularly good. This was a batch that [Mr Vega’s] severed finger had fallen in to! So in my house a decent set of tyres are referred to as having a juicy bit of finger in them. Who knows what might have fallen in to the bad batches! Although I would imagine that the processes are a big more controlled now…

However, tyres and compound mixing is chemistry and a lot of work goes into making sure things don’t interact with each other when they shouldn’t. The make up of race tyres is a closely guarded secret but in general tyres are made from:

  • Natural rubber, or polyisoprene is the basic elastomer used in tyre making
  • Styrene-butadiene co-polymer is a synthetic rubber which is often substituted in part for natural rubber based on the comparative raw materials cost
  • Polybutadiene is used in combination with other rubbers because of its low heat-buildup properties
  • Halobutyl rubber is used for the tubeless inner liner compounds, because of its low air permeability. The halogen atoms provide a bond with the carcass compounds which are mainly natural rubber. Bromobutyl is superior to chlorobutyl, but is more expensive
  • Carbon Black forms a high percentage of the rubber compound. This gives reinforcement and abrasion resistance
  • Silica is used together with carbon black in high performance tyres, as a low heat build up reinforcement
  • Sulphur crosslinks the rubber molecules in the vulcanization process
  • Vulcanizing Accelerators are complex organic compounds that speed up the vulcanization
  • Activators assist the vulcanization. The main one is zinc oxide
  • Antioxidants and antiozonants prevent sidewall cracking due to the action of sunlight and ozone

Tyres are changed from a gloopy mess to something that stays in shape by vulcanization where sulphur or other curatives are added. These additives modify the polymer by forming crosslinks (bridges) between individual polymer chains. So as you can see there are several stages where the manufacturing can go slightly wrong and the resulting tyres won’t work as expected.

However, a lot of the investment into pharmaceuticals goes into making a consistent formula, and medicine is 1) life or death, and 2) often very expensive. It might be that the tyre manufacturers have found the best balance they can between price on the one hand and consistency and quality on the other. However, if a company can offer an improved product we need to go with that.

Next year of course, the Max classes will all change to the Mojo tyres. They would have been changed for this year, but PCA-free tyres are being encouraged for all applications by the EU, and the 2011 Mojos will be PCA-free so the ABkC and MSA decided to avoid having two changes in two years.

In Jason’s experience in Euromax, these seem to be a lot more consistent so hopefully Max racers will have better experiences once the Mojo tyres are phased in. In testing by the ABkC, performance has been found to be similar.

“The tyres are so important that if they aren’t right, changing the chassis, the axle and all of that makes absolutely no difference. It does give people an excuse though!” was Jason’s final point.