ACC | How to balance tire wear

(taken from the   Aris videoguide)

The front and rear tires wear differently depending on various situations: circuit layout, type of car (front, rear or mid-engine), different set-up, driving style. Especially over the course of long stints, this can negatively affect the balance of the car. What can we do then to mitigate the problem or even eliminate it?

Let’s try an extreme situation, choosing as a car a Lamborghini which is one of those very heavy cars due to the ballast provided by the BoP (concentrated at the rear). Due to this ballast distribution, the car tends to wear the rear tires more than the front ones. As the race progresses, the balance of the car will suffer and consequently the handling as well. For our test we chose the Brands Hatch circuit which is a circuit in which you run clockwise, with many fast right turns and few left turns. This leads to greater wear of the tires on the left side.

Perhaps it would have been better to do this test at Suzuka where the asphalt is very abrasive and there is no difference in consumption between the right and left side and therefore it is easier to compare the wear of the front tires compared to the rear ones but the time on the ride is higher than in Brands Hatch and it is easier to make mistakes; therefore, doing this type of test in Suzuka is more complicated and takes too much time. At Brands Hatch we chose to try at noon and in the heat. Let’s start with the aggressive setup and do a few laps (four for example) with the aggressive setup, new tires and at a constant pace (1:24 high is not bad for race pace).

Let’s see now what was the wear of the tread.

In ACC, unlike other simulators that express tire wear as a percentage, wear is evaluated on the basis of the reduction in tread thickness. The tread of the never used Pirelli tires has a thickness of 3 mm. After we have covered 4 laps, the inner side of the left front tire has worn out and in fact the tread has a thickness of 2.87 mm. Although it is interesting to know the wear of the inner side (thickness 2.89 mm), for the type of analysis we are conducting it will not be taken into consideration. As for the left rear tire, wear has reduced the tread depth on the inside to 2.83 mm. Therefore, the tread wear on the inside was 0.13 mm at the front and 0.17 mm at the rear. Already with just 4 laps we have worn the rear tires significantly more than the front ones. Now let’s try to understand, through a graph (which relates the grip to the distance traveled), what happens to the tires.

When the tires are new (3 mm tread thickness) the grip is slightly lower than the maximum grip available as there is a sort of protective patina on the tread surface which makes it slightly slippery. After you have traveled a few km (generally one lap of the track but can also be two, depending on the circuit), the patina that surrounds the tire is removed by the abrasive action of the asphalt and the grip reaches the optimal value (this degree of adherence of the tire on the asphalt is called qualifying grip). As you do more laps of the track, two or three for example, at some point the grip will begin to decrease relatively significantly (this does not mean that you start to slip but rather that the lap times start to get a little higher). Therefore, after 3 or 4 laps of the track the tires no longer have the qualifying grip; there was a decrease in grip which tends to drop very gradually until it almost stabilizes for an entire race stint (60 minutes). The slow and progressive reduction of grip is accompanied by the lightening of the car due to fuel consumption and therefore the lap times remain almost unchanged. Remember that as the tread wears out, the car’s ground clearance will decrease; therefore, if at the end of a race stint you have consumed 2 mm of tread, your car will be 1 mm lower and this will also affect the aerodynamics. 

Remember: with a tread depth of between 2.9 and 2.86 mm we are still in qualifying grip conditions.

So let’s see what this entails in our specific case: the front tire is still in conditions of maximum grip, being the thickness of the tread equal to 2.87 mm; behind, on the other hand, we have 2.83 mm of tread depth.

If we go to report everything in our graph, after 4 laps we are in the situation in which the front tires (green dot) still have the maximum grip (qualifying grip) while the rear ones (blue dot) are a step below (i.e. in conditions of race grip); this situation determines greater adherence of the front wheels compared to the rear ones. In such a situation, after 4 laps your car will oversteer and this behavior will still be accentuated when braking at corner entry, where you will have that the rear tends to lighten and maximum grip to the front (the aerodynamic load moves in fact forward) and you are without the support of the rear wheels which are more worn. Ultimately, you find yourself in a situation of imbalance in the mechanical grip. As the laps of the track increase, for example after 10 laps, also the front tires, after the rear ones, will be out of the qualifying grip condition (i.e. in race grip condition). From the graph you can still see that the front tires continue to have more grip than the rear ones, but this difference is not very marked. That is why, once you have covered more than 5 or 6 laps of the track, during which you think your car is completely undriveable, you will be able to regain control of the situation. 

How can we improve the condition of the rear tires so that they wear less?

There are two things to keep in mind:

• nothing replaces the driving style; you can make any changes, adjustments, even with the help of telemetry, but the benefits will be minimal since the thing that has the greatest impact on tire wear is driving style. The first lap of the race is decisive on the behavior of the tires for the entire duration of the race. Consider our previous four laps, during which we pushed hard looking for performance, from a graphic point of view we are in a situation where the distance in terms of tire consumption between front and rear was like that shown in the previous figure; that is, the front tires were in qualifying grip conditions and the rear tires in race grip conditions and this situation was created in just 4 laps. Now, if you were able to cover the first three laps in such a way as to preserve your tires, without exaggerating with the search for performance, you could reach the end of the fourth lap in the condition shown in this other figure (see image below) . That is, the front and rear tires would be closer in terms of wear. This is what we should aim for; if you manage to do it for the first three laps, it means that even in the rest of the race the wear between the front and rear tires will be comparable and the car will always maintain its balance. If, on the other hand, you push too hard at the beginning, you will have balance problems after about the fourth round; you will have to wait at least 7 or 8 laps to regain the balance which however will not be optimal. 

• if after 4 laps you have consumed 0.13 mm of tread at the front and 0.17 mm at the rear, let’s see what the tire wear will be after 40 laps of the track at Brands Hatch (i.e. at the end of a 1h stint). At the front, if after 4 laps we have a consumption of 0.13 mm, after 40 laps (or after a distance 10 times greater) we will have more or less: 0.13 mm * 10 = 1.3 mm of tread reduction; similarly, at the rear, after 40 laps we will have a tread wear of 0.17 * 10 = 1.7 mm. Between the front and rear tires there is therefore a big difference in terms of wear which at the end of the race translates into a big difference in grip between the front and the rear that will alter the balance of your car. Therefore, even apparently minimal differences at the beginning of the test (4 laps), at the end of a stint of 1h (equal to 40 laps), or by increasing the mileage by 10 times, substantial differences in terms of consumption become between the front and rear tires. Instead of trying to improve the wear of the rear tire, focus attention on the front tire: the wear of that tire is not enough because there is not enough load, enough grip, enough strength to wear out the front tire. Paradoxically, it is easier to intervene to make the tire work harder than to protect it from wear. When you intervene to protect the tire from wear, you will lose performance; when instead you ask the tire for more wear, it means that you will probably have a better performance or at least you are not requesting a reduction in performance (which is a great thing in our case because we want to go faster and have a well balanced car). So, if you know that at the end of the race you will have a rear tire wear of 1.7 mm, you will reach the finish line with a tread depth of 3 – 1.7 = 1.3 mm which, however, is a limit value but still possible. So why ask the rear tire for less performance? Instead, make sure that the front tire also has a greater wear, such as to obtain a tread depth of 1.4 or 1.5 mm at the end of the race stint. In this way the level of wear between the front and rear tires will be similar, the car will be more balanced and you will not have a drop in performance.

How can we wear the front tire more?

To make more use of the front tires, we generally increase the camber but, with this type of intervention, in the straights the tires rest on the asphalt only with the inside more marked and only in the corners, where the tire flexes, it would increase the surface resting on the asphalt. For our purpose it is therefore important to intervene not only on the camber but also and above all on the toe; in particular, considering that we are working on the front tires, we need to increase the toe-out. This, in addition to increasing tire consumption even on the straight (and not only on corners as generally happens), will simultaneously facilitate the turning. On the other hand, this setting will result in greater rolling resistance and therefore a slight reduction in the maximum speed (about 1 km / h or less in circuits with short straights). So let’s go back to the pits, mount new tires and bring the toe-out on the front tires to -0.25 degrees; at the same time, we reduce the toe-in at the rear as much as possible (you could also bring it to 0 but having some stability at the rear is an important aspect and therefore we set the toe-in at the rear to 0.05 degrees). Another thing you can do is to slightly reduce the camber to the rear wheels (for example from 3.5 to 3.2 degrees), without however having a reduction in performance. At this point we do not modify anything else and we go to the track to make another 4 laps and evaluate.

We do another 4 laps of the track consistently, without interruptions and the first thing we observe while we are turning is more oversteer.

At the end we go back to the pits to check the difference in tire consumption between front and rear. The important thing is not that we have consumed less tires; the important thing, rather, is that we have a tread depth of 2.90 mm at the front and 2.88 at the rear and therefore a difference in consumption between front and rear tires of only 0.02 mm. So this is a clear improvement when compared to the previous result of 0.04 mm.

To achieve this progress we have done three things:

• change in driving style especially in the first laps (less aggressive and more fluid);
• worked on the front tires to make them work better (more toe-out) and wear the tread a little more;
• worked on the rear tires (less toe-in and camber) to reduce tread wear.

The result is that we have reduced the difference in tire wear between the front and rear.

If I wanted to balance the consumption between front and rear tires even more, I could intervene by reducing the opening of the front brake duct to warm them up more. In fact, a greater overheating of the tire corresponds to a greater consumption of the same. Attention, here we are not trying to reach the perfect setup but, rather, to make people understand, even with extremes, what are the interventions that can balance tire wear.

The oversteer problem we encountered is easily solved: for example, we can intervene on the aerodynamics and reduce the ground clearance at the rear by 2 mm.

Another intervention you can do on circuits like Brands Hatch and Barcelona, ​​where the left (outer) tires wear out more than the right (inner) ones, is to reduce the differential preload.

Why do this?

Because when you have the most stuck differential, the moment you press the accelerator, the inner tire will tend to slip and the same thing will happen to the outer tire. If, on the other hand, you lower the preload to 50 this will mean that, before the differential locks, the inner wheel will slip a little more and then the outer tire will also engage and therefore slip less with consequent less wear.

We do another 4 laps: we see that the car has a more neutral behavior and that the lap times are in line with those achieved previously. You can also see that the left front tire has started to overheat and the pressure has increased accordingly. This is a good thing and exactly what we wanted to achieve. Let’s go back to the pits to see the results of the changes. We note that we have a tread depth of 2.90 mm at the front and 2.88 at the rear; tire wear has therefore not changed.

Let’s do some calculations:

Front left consumption after 4 laps is equal to 0.1 mm and therefore after forty laps the consumption will be equal to 1.0 mm (remaining tread: 2.0 mm).

Rear left consumption after 4 laps is equal to 0.12 mm and therefore after forty laps the consumption will be equal to 1.2 mm (tread left: 1.8 mm).

At the end of a race stint I will have a more than acceptable tire wear situation.

In conclusion, the points on which to intervene are the following:

• Less aggressive driving style.
• Ask more from less stressed (front) tires.
• Reduce the wear of the rear tires.

If you participate in a competition in which the tire sets available are limited in number, you can think of leaving for the race with a set of used tires for about 5 or 6 laps in order to have overcome the qualifying grip conditions on both rear tires than on the front ones and therefore already balanced between in order to maintain this situation for the entire duration of the race stint, especially if the stint is less than 60 minutes (in order not to run the risk of arriving with finished tires).

If you are using a front engined car, it will wear the front tires more and therefore on these tires you will probably need less toe-in and, if possible, even a little less camber. The advantage of driving a front-engined car is that even if you lose grip with the front wheels, the car remains more controllable as it understeers (managing oversteer is more difficult).

Remember that in the race, the aspect to focus on is consistency (which will make you gain track positions, improve your CN and SA ratings). It is useless to have a car that allows you a great fast lap alternating with slower laps due to, for example, a terrible oversteer.