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Track Testing: Tyres, Compound, Performance & Wear

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Carlo out on track

One of the most impactful elements on a kart or any racing car are the tyres. The behavior and performance of our kart are mostly influenced by tyre compound, structure and wear. Even though rubber thread hardness is not the only parameter of the tyres it is extremely important both from a performance and a durability point of view.

The tyre thread compound hardness is measured in IRHD (International Rubber Hardness Degree) or Shore value: The higher the value the hardest the compound. Values go from around 35 to over 70 IRHD, with the lowest values for rain tyres to the highest values for indoor karting tyres. Shore and IRHD have different scales and differ from type test machines, but are fairly equivalent in results.

The Rok class uses Bridgestone YJB tyres which are indicated as medium Shore hardness. Unfortunately the Bridgestone web site does not indicate the IRHD equivalent value.

Position Size Casing Rim Width Overall Diameter Section Width Tread Width Shore Serv. Pressure
Front 4.5/10.0-5 DR9 4.5″ 263mm 129mm 102mm Medium 0.8 bar (+/-0.3)
Rear 7.1/11.0-5 DR9 8.0″ 273mm 209mm 150mm Medium 0.8 bar (+/-0.3)

We started our test day with completely new tyres. Mounting them on wheels is quite a quick procedure with some expertise. First of all we cover the diameter of the wheels with soapy liquid, and there is specific soap used for this at car part and car tyre shops, which will help the internal part of the tyre to slide onto the wheel. Always insert the wheel into the tyre starting from the internal side of the wheel and checking that the direction of rotation of the tyre is correct. An arrow on the sidewall indicates the direction. This helps both because tyres are produced with the rubber laid in a certain direction so resistance and best performance are guaranteed when direction of rotation is correct. Also, when removing and remounting tyres and wheels on the kart, it is best to keep the same direction of rotation.

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To avoid ruining the wheels when pushing on the tyres to mount them, it is best to do it on a soft surface and avoid direct contact of the wheel with the tarmac. If you have nothing with you to cover the asphalt it is always possible to use the floor mats from your car! Use two or three steel bars to lever the internal circumference of the tyre (the bead) onto the internal circumference of the wheel.

Also be sure that the three screws on the outside of the wheels are sufficiently unscrewed so that the tyre slides easily and is not damaged when it is mounted on the wheel. These screws are to make the tyre does not lose its position on the wheel, especially when tyre pressure is low and the tyres are have a soft compound and structure. Damage by these screws can happen, especially when inflating the tyres for the first time to position them on the wheels. In fact, the first time you have to check that the tyre sidewalls and bead bundles are in contact with the external diameter of the wheels. Finally, remember to reduce the pressure of the tyres that have just been inflated before measuring their pressure. Otherwise you will risk bringing the measure of the instrument to maximum scale and losing calibration and consequently precision in future measurements.

Using new tyres is always important and can be complicated. Performance is best in the first laps but tyres often need to warm up a little more when the rubber on the thread is thinner, especially if it is not a soft compound. Also reading tyres is a great help and is even more possible with new tyres. Chassis setup can be understood from the tyre tread surface, which of course also changes depending on tarmac type and condition, with more or less rubber down and consequent grip levels.

What is important is to consider that the best setup and highest grip is shown when tread surface is quite even, but not perfectly smooth. If the grip is too high the tread surface will be uneven and bulky. If grip is too low on one or more tyres then the surface of the tread will look perfectly smooth, showing the tyre is insufficiently sticking to the tarmac.

We went out on the track for a few laps. The balance of the chassis was good, but as predicted a couple of laps were needed to warm up the tyres. The grip on the track was pretty low on a mid week day, so the effect of the new tyres was only partially there. In detail, the front tyres worked well in the entrances to the corners, finding the right grip to take the right line without too much front and rear slide. Speed was high in the middle of corners and there was strong acceleration exiting them, although there were some rear grip and traction problems, since the low quantity of rubber on the tarmac made it difficult for the rear tyres to stick sufficiently to the ground.

The problem was felt even more around fast corners where generally with high grip on the tarmac average speed was much higher, while in these conditions the rear of the kart could be felt to slide more and more, making it impossible to keep accelerating and also slowing down the kart because of the need to correct the trajectory by counter-steering.

Lap times were quite good even though great loss was, as said, on fast bends that can be run at full throttle when grip is high, for example during race weekends.

After a couple of sessions we verified the tyre situation with a look at the thread surface. While the fronts showed that the grip was not 100%, it was acceptable, and rear internal tyre, despite suffering strong sliding, was still resisting wear and tear! What was quite astonishing was the strong wear of the rear external tyre, the one working on a long fast curve just after the main straight. The rubber on the tread almost gone with the holes that show the quantity of rubber almost gone. While Bridgestone YJB tyres have quite good performance, they cost a lot (around 160 Euros in Italy) and wear too much and too fast. Maybe this is one of the few weak points of a good and professional category such as the ROK. Surely Tony will work to improve the class further.

In more detail, the front tyres were quite evenly worn, more internally then externally. The tread surface showed a good equilibrium between bulkiness and smoothness. The rear internal tyre had more or less the same aspect, even though the tread was smoother showing less grip and greater lateral sliding of the rear of the kart. Finally, the rear external tyre was greatly worn, with the internal part of the tread showing that there was a fast lateral sliding of the tyre working with strong vertical pressure.

Lap times were quite competitive initially on the first seven laps and then began to get higher by around 0.2-0.3s. After the first 20 laps, lap times increased once more by another 0.2s, but then became constant until around lap 40. At this point the rubber on the tread was very thin and lap times grew to over 1s more compared to the first laps and the kart became extremely difficult to handle both in the braking phase, and entering corners with great understeer and exiting with strong oversteer.

Next issue we will test different types of tyres with different rubber hardness and will verify the performance, durability and wear.

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