Tech Talk - Dec 12, 2012

Getting the best from your tyres

Getting the best out of your tyres is a vital element in racing and often one that drivers fail to fully comprehend leading to a variety of inaccurate conclusions.
 
Interestingly if you watched that last action packed grand prix from Brazil you’ll have seen just about every possible complication to tyre performance, from overheating with a track temperature of 50 degrees to struggling for grip in low temperature damp conditions.
 
There are a few golden rules but aside from that then it very much comes down to you the driver (or Dad sorting son) to make the best of the conditions.
 
So the golden rules are:

  • Pressure makes a huge difference to tyre temperature. So if you have 10psi in your tyres they will heat up far less and more slowly than say 30psi. As a tyre gets warmer the pressure will also increase by a few pounds.
     
  • Softer rubber gives more grip but wears quicker. But there’s not much you can do about that.
     
  • The wider/larger/flatter the tyre the longer it takes to warm up.
     
  • Many tyres – especially wets – are directional and it is vital they are fitted the right way round. Check and double check because mistakes can happen even at the highest levels.
     
  • Our tyres (unlike F1) do not give their best performance straight out of the box. They can need some use to be at their best.
     
  • Rubber doesn’t like to be too hot or too cold to give its best performance.
     
  • Track surface can make a big difference to grip levels and also to how much the tyres heat up.
     
  • When switching tyres you must take into account gearing which will need significant change to get the best performance.

Of all those points there are really three areas that stick out as being of prime significance and in our control as drivers. That’s the pressure/temperature element, the wheel rim width size which affects tyre shape, and the gearing required.
 
For most of the year the temperature in this country falls into a mid range of around say 10 – 17 degrees C. And broadly within that tyres will work fine. So with the Maxxis tyres we use generally the slicks should be pressured at around the 12 – 15 psi area and they will work fine. And similar for the new wets.
 
However at extremes of temperature you need to be much more thoughtful in your settings. So if it is a red hot day at say 25 – 30 degrees C ambient and a lot more on track, then you will need to lower the pressures or else you will cook the rubber which will give less grip and maybe even damage the tyre for further use. So as low as 8psi might be needed for slicks.
 
And at the other end of the scale with low temperatures you’ll find the grip suddenly drops away like falling off a cliff. And tyre temperature is more important than whether the track is wet or dry. A hot tyre will work on a wet track, but a cold tyre will not.
 
Maxxis themselves do a great deal of research to develop tyres and their insight tells us that for the slicks somewhere between 70-80C is an optimum temperature to work in. Above 90C they lose grip. For the wets you need to be somewhere around 40 – 50C for best results. Much above that and again you lose grip.
 
It is also interesting to note that road tyre manufacturers and car makers suggest 7degrees C as the point at which you should change to winter tyres because the grip will have fallen off so much. Winter road tyres are designed to work at lower temperatures in terms of compound and tread. We don’t have different tyres for the cold but we can do things very differently to make our race tyres work.
 
So for both wet and dry tyres you need to use a lot more pressure. It is impossible to be exact because there are so many parameters such as the weight of the kart (Junior or Senior), the nature of the track (some have grippier surfaces then others) and nature of the kart.
 
But the simple rule is that you need more air in the tyres to make them work. How much is the bit you need to experiment on. With wets on a cold wet horrible day you may well need 40psi to get them turned on and working effectively. And with slicks maybe as much as 30psi. 
 
Especially with wets the wheel rim width can make a big difference too. The new wets have a flat profile and need probably a 130mm front rim and a 180mm rear rim for best performance in normal conditions. But when it is very cold maybe a narrower 120mm front rim with a 140 or 160 mm rear rim will help them work harder so they get some heat build up.
 
Gearing for the wet is complex because on the one hand you are going slower round the bends and the straights are shorter because you have to brake earlier, but on the other hand sometimes slightly lower gearing can help give you more traction instead of wheelspin.
 
It is vitally important to note that whereas the old Maxxis wets were smaller diameter than the slicks, the new ones are a larger diameter. That means you have got to put a lot of extra teeth on the axle sprocket to get the same gearing increase over the dry tyres.
 
Typically on the old wets drivers were putting on say 3 or 4 extra teeth. With the new tyres maybe 8 will be needed on the axle sprocket. Or of course you can switch from the standard 10 tooth to a 9 tooth engine sprocket to get roughly the same increase.
 
That’s going to be an area where testing and experience are required to fully understand the maximum that can be achieved with the tyres.
 
To get the maximum from your tyres and kart at all times I’d suggest keeping a very careful log of track temperature, gearing, tyre pressure, and the tyre temperature when you come off track. Actual track temperature is more important than ambient temperature so bear that in mind.
 
To do the job properly you need a heat measuring gun which gives an instant read out of temperature on anything you point it at. It is not as accurate as a proper tyre temp gauge which measures inside the rubber but it does give you a very clear picture of what is happening.
 
Use the heat measuring gun to check the track temp. So if you see that with a track temp of 10 degrees you got good grip on 17psi then you know where to start from.
 
Remember match your tyre pressure to the track/ambient temperature. Don’t expect tyres to work instantly at their best whether new or old. And make notes on what you do to achieve best results.
 
Finally do remember that new tyres will always have a certain amount of releasing agent on them which gives zero grip. It is so embarrassing to spin coming out of parc ferme onto the track so get them warmed up and bedded in before you start really motoring!

Grahame Butterworth
Formula TKM Co-ordinator



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