Last month we dealt with some of the basics of Tyre science. This time we are going to up our game and look at how ‘reading’ your tyres might give you some big clues about the way your kart is set up and how this might be improved.
Here’s how to translate what your tyres are telling you about how the kart is handling.
Coning is the state where the tyres of a kart exhibit a wear pattern that is greater on their inner edge than their outer. It’s worth visiting because the tyres are giving a really clear visual indication of a chassis set-up problem. In really bad cases the outside edge of the tyre may look like it hasn’t touched the tarmac at all whereas the inner could be badly worn away, the tyre is literally wearing into a cone shape.
Coning on the rear shows that there isn’t enough weight being transferred to the outside rear wheel during cornering. Usual cures include stiffening up the rear end with additional rear bar or seat-stays or narrowing the track.
Coning on the front tyres shows a chassis that is set up with too much caster/camber, reducing the settings will get rid of it
Getting the karts settings about right will quickly remove the visual signs of coning but not being able to see it doesn’t mean that it has gone away completely. A tyre may still be over worked on one edge but not so much that it is immediately visually apparent.
(NOTE. If a tyre exhibits the early signs of coning, change the settings immediately to get rid of the cause. Because if you leave it for too long and chew too much of the inner edge away you’ll irrevocably change the tread profile and lose performance.)
More sophisticated still: Tyre Pyrometers
These are available from £40 to over £350. The best types have a needle like probe that can measure temperature under the rapidly cooling outer layer of rubber. The cheaper versions zap a laser at the tyre surface and read this.
Even the basic £40 tyre pyrometer is an excellent tuning tool as it allows you to check more than how much heat there is in each tyre.
As a primary task taking a reading in three places across the width of the tyre will allow you to determine over and under inflation and adjust tyre pressures accordingly. For karts with adjustable front camber the same three reading technique will help you to know whether you have too little camber or conversely, too much.
Let’s start with adjusting pressure of the rear tyres because it is the easiest thing to do. Take three readings across the tread as soon as the kart comes into the pits, don’t worry about how high or low the actual temperatures are to start with it is the variance we are interested in.
Outside. Middle. Inside.
90. 80. 90 Under-inflated (the middle isn’t pushing into the track hard enough)
80. 90. 80 Over-inflated (the middle is pushing into the track too hard)
85. 85. 85 Just right (even pressure across the face of the tyre)
Because the front end is more adjustable than the rear you will have two things to think about here, the camber setting and the pressure. Unless you get lucky first time you will probably have to do the front end adjustment in several sessions, the first to sort out the camber the second to modify the pressures.
Unfortunately raising or lowering the tyre pressure also has some bearing on the amount of camber needed. If you get the tyre showing even temperatures outside and inside, raising or lowering the pressure to bring the middle into line you will also affect the way the tyre flexes when cornering. The softer the pressure the more the tyre will flex during cornering and the more negative camber you will need to counteract this and vice versa.
Outside. Middle. Inside
70 80 90 Too much negative camber (the inner edge is working too hard)
90 80 70 Too much positive camber (the outer edge is working too hard)
80 90 80 Camber OK but tyre over inflated
90 80 90 Camber OK but tyre under inflated
After a while you will get used to not only the amount of camber adjustment is needed to correct the temperature variance but also what the three temperatures are telling you about the overall pressure of the tyre.
More on coning
Back to the rear tyres, remember we spoke about coning being visible but only to a point? Here is how the temperature reading can help us get closer to the optimum.
Outside. Middle. Inside
75 80 85 Still a weight transfer issue but probably not enough to show on the tyre tread
85 80 75 Too much weight transfer, Kart may be starting to hop in the corners by now
It’s actually very easy provided that you remember that friction (work) generates heat and that by reading that heat you can which part of the tyre is working hardest and do something to compensate.
‘Scrubbing in’ new tyres
After fitting new tyres the general advice is to go out and do a few laps to gradually bring them up to temperature. Once the tyres ‘come on’ do another couple of laps then come in and allow the tyres to cool completely (Some sources even suggest using wet rags to cool them down totally). This procedure is supposed to stabilise the compound and help the tyre to remain consistent thereafter. Does this work? Opinions are divided but most agree that it can’t hurt to go through this procedure. One thing is for certain though, brand new tyres offer more grip than used tyres and the more heat cycles a tyre has endured the less grip it offers.
Those with the budget to fit new tyres between meetings or even between races can still benefit from storing their fresh rubber properly. Remember, tyres don’t like light or extremes of temperature so keep them in a cool, dark place and they’ll be fine. Those running on a limited budget might also wish to experiment with wrapping cling-film around the surface of the tyre when it isn’t being used. This is supposed to help prevent the volatile substances within them from leaching out. I do it, but I’ll also admit that the jury is still out on this one. At the very least helps to keep oil, grease and other tyre-unfriendly substances off them.
Here’s our simplified ten step guide to getting some base-line settings for consistent performance.
1. Buy a tyre pyrometer
2. Do a session on the track and then use the pyrometer on the still hot tyres. Write down the results.
3. On the front… Using the figures you have just obtained as a guide adjust the camber of the kart. Greater negative camber for a hot outer, less for a hot inner.
4. On the rear… Raise or lower the rear tyre pressures to even the temperature across the tread. Down for a hot middle, up if the middle is cooler. Three radically different temperatures? A hot inner edge demands a little more weight transfer a hot outer edge a little less.
5. Do another session and measure with the pyrometer again.
6. This time the rears should be almost spot-on but the fronts might need a pressure tweak to correct a too-hot or too-cold middle. An almost consistent reading across the tread of each tyre tells you that you have the correct camber /caster setting on the front and optimum inflation pressure for all the tyres.
7. Use adjustments to the karts chassis set up to raise or lower the front and rear tyres overall temperature give cool tyres more work to do, hot tyres less. Don’t worry about discrepancy between left and right, all circuits will work one side of the kart harder than the other, it is the difference between front and rear you need to concentrate on, they need to be roughly the same.
(NOTE a big discrepancy in the temperatures of the front and rear tyres left to right can indicate a bent chassis which is forcing one tyre unduly hard into the track surface)
8. Continue to experiment with chassis set up until the tyre temperatures are within their optimum range.
9. Write everything down, chassis settings, tyre temperatures, air temperature and track temperature (tip… Use the pyrometer on the track surface)
10. Once you are proficient with getting these base line settings, experiment with raising or lowering the tyres pressures by two or three psi to bring them on either earlier or later in a race.