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Rubber wear and the art of performance.
How tyre choice and suspension set up can work together to make you quicker.

Are you could be missing out on a couple of seconds per lap because your car isn’t set up to suit its tyres? Should you copy the leaders tyre choice or will it kill you? Will filling your tyres with Helium make your car lighter?’
The world of tyre lore is full of myths and legends, but all we want to know is the simple truth; the answer to the eternal question; ‘How can I go faster?’.
Starting with basics, the tyre has best grip when its contact patch is flat on the road, if its angled over then the contact patch and grip are reduced.
Now here is the important bit. You need this situation most when you are cornering hard, breaking or accelerating. Not when the car is parked in the garage on the set up stand.

Section 1: Acceleration, braking and cornering

First, hard acceleration.
Weight transfer can cause the back end to dip and with independent suspension the wheels tip inwards at the top a bit. If you use really stiff tyres then the tread area will tip too, the contact patch reduces and you have less traction. If the side walls have a bit more give then the tread stays flat on the ground. Of course, if you have a Dedion or beam axle then you won’t have this problem.

Case two, hard braking.
Now front wishbones move and tip the tops of the front wheels in slightly (or if you have struts then the inverse may occur), thus reducing the contact patch and leading to a slight weave at the front and a greater chance of locking up.
Another problem comes if you start to turn in whilst still breaking, which some corners need. This means that at the back the camber has changed again plus there is not much weight pushing the tyres into the ground and we find the phenomenon of lift off over steer. If the back looses traction too much then it will swing round and overtake you shortly before you visit the scene of an accident.
So again, the tyre needs some flexibility in the side walls to cope. So far, flexible sides seem quite a good idea.

Now the third case; cornering.
As the car turns in, the weight shifts to the outside wheels and hopefully the suspension cleverly compensates for the car leaning over and adapts the outside wheel’s attitude to the ground, keeping the wheel upright, as on beam axles.
However, during hard cornering the tread is being pulled over to the inside edge and so peeling itself off the ground.
So here a very stiff side wall seems the best instead.

Section 2: Tyre choice vs car set-up: the compromise

Now, you could compensate for the car rolling by putting more negative camber on, thus getting better grip mid corner. But this means that the wheels tip in more when braking, thus loosing grip going into and out of corners. So a compromise has to be made. You can see how the suspension settings and the tyre choice are heavily interdependent.
Obviously if the side walls of the tyres are stiffer then the peeling effect is reduced in cornering, but then the tyres ability to adapt during breaking and accelerating is reduced. So no matter what is changed, it effects everything else in some way.
Another way round the geometry changes is to stop the suspension from moving! And indeed pure bread racing cars like F1 do have very little suspension movement. But this requires the road to be dead flat, any slight groove or rut could cause a wheel to loose contact with the ground and thus traction goes out the window.
Profile has a big effect on sidewall flex and thus stability, the stiffer the side wall the better the tread is held relative to the wheel rim. Car weight also influences profile choice, a heavy car will force the tread on a low profile tyre into the road, but on a half ton special needs a much higher profile to allow the tread to flex over the lumps and bumps in the road. That's one of the reasons that F1 tyres have such a high profile.
So generally a lower profile is better but not too low so as to stop the tread adapting to road irregularities. But if you find the car lets go too violently then a higher profile tyre will make the break away more progressive, this may give you more confidence and thus improve lap times.
The design of the tyre will effect how quickly it lets go, the more progressive it is then the easier it is to feel it going and drive on the limit but it will have less ultimate grip. So generally a more experienced driver can use the less progressive tyre to greater effect than a novice, and a novice should not necessarily copy the class leaders tyre choice because you'll spend all your time spinning off.
Wheel width has an effect on how the tread is held, a narrow wheel will let the tread move side to side more easily, too wide a wheel and you risk the tyre bead coming off. The best size varies depending on the design of the tyre so its always best to check the tyre manufacturers’ recommendations.

Section 3: So what can I change?

In club level motorsport the problem is always constrained by what you are physically able to adjust and by the regs.

Adjusting the tyres.
Tyre grip changes with inflation pressure, the exact relationship varies from one type of tyre to another but basically there is an optimum pressure, lower or higher than this and grip reduces. Lower pressures have a more dramatic effect than higher pressures.
Believe it or not, you might not want the ultimate grip from all four tyres. You can change how the car under/over steers by changing the relative grip front and back.
To fix understeer; reduce front tyre pressure, if minimum tyre pressure is reached increase rear pressure.
To fix oversteer; reduce rear tyre pressure, if minimum tyre pressure is reached increase front pressure.
As a bit of an aside, filling the tyre with nitrogen can give a slight advantage, the oxygen in air permeates through rubber but nitrogen doesn’t. Air is about 20% oxygen. Also nitrogen doesn’t expand with temperature as much as air does, so the pressure in the tyre stays more constant through a race. This is a small difference though, so don’t be surprised if you don’t notice any difference in performance. Don’t try helium though, it leaks through the rubber and goes flat in less than a lap, and the weight saving is about 52 grams.
Contact patch area, and thus grip, depends on the inflation pressure and the weight on that wheel. A tyre inflated to 30psi which is supporting 300lb will take up 10 square inches, 30 pounds per square inch times 10 square inches equals 300 pounds, easy. Only it isn't quite that easy because the side walls also contribute a little to this, just think how much weight can a flat tyre support, it might be as much as 100kg but the basic principal holds.
Grip also depends on temperature; as the tyre gets hotter the rubber becomes more flexible and can be pushed into the road surface more easily, but above the optimum temperature it starts getting too soft and the tread moves slightly over the surface of the tyre.
The best temperature varies with type of tyre so again, consult the manufacturers’ data. My Toyo T1-R tyres are best at 80-90 and go vague at 120C. So if the car is driven too cautiously the tyre will be below its best temperature and grip reduced, drive it harder and paradoxically it grip more as the temp optimises.
Measuring the tyre temperature can be quite useful to a club racer can do, it needs to be done immediately after several hot laps, the temp will fall by several degrees per second once you slow down so its easy to get false readings. Even temperature across the tyre shows that the full width of the tyre is being used. Anything more than 4C variation needs fixing.
If the middle is hotter than the outer then its over inflated. If the inside edge is hotter than the outside edge then the suspension needs adjustment (either camber or tracking).

Adjusting suspension.
As stated earlier, huge amounts of static camber on the front wheels will cause instability when braking.
If you need lots of camber in mid corner to combat tyre distortion then one other way is to increase the castor angle. Then as the wheel turns in it sort of falls over a bit thus inclining the wheel. The down side is increased steering effort.
On suspension with compliant bushes at the front the wheels toe out under breaking which makes the front weave a bit. To fix this dial in some static toe in such that the wheels end up nearer parallel under hard braking.
To help keep the tyre tread flat on the ground whilst cornering, roll must be minimised. This is best done by lowering the centre of gravity or changing the springing, but can also be done with an anti roll bar. The down side to a bar is that bumps at one wheel get transmitted to the other, so clipping the rumble strip causes the outside wheel to lift a little too and you can end up with less grip and a twitchier car.

Section 4: Still got tyre wear? Adjust the driver.

At a recent test day, two top-level drivers took turns to drive one of the class-leading cars in our championship. They both put in similar lap times but were using the tyres in totally different ways. One was braking hard into the corner then using hard acceleration thus kicking the back out through the corner. The other was going in to the corner faster and almost pushing the front all the way through the corner. The first was wearing the tyres heavily, mostly the rears and the second was wearing the tyres a lot less but the front was getting it more than the rears.
In each of these cases the suspension and tyre pressures could be adapted to suit their driving style, or the driving style could be adapted to suit the car. It all depends on which is easier to change.
If you are roasting the outside edges of the tyre through hard cornering, and all possible adjustments to the set up have been made, then it may be worth braking harder in to the corner then lining up for a straighter exit and coming on the power earlier. Use the tyres to their best advantage.
Remember that there are many good lines round a corner, the best one depends on the car, the tyres and the driver, all of which can be adjusted.


Grip.
Tyres really do grip, the rubber wraps round macro and micro deformities in the road. Hysteresis, which is the rubbers ability to rebound over a short period of time rather than instantaneously, allows the tread time to grab hold of the road as the tyre rotates. The bigger the contact patch area and the greater the downward force the more grip you get. But doubling the area does not double the grip. Increasing the contact area spreads the load over a greater area thus pushing the rubber less far into the road, the extra grip you get gets smaller as you go up in size - diminishing returns.
Also when increasing down force, there is a point where the rubber gets pushed as far into the road as it is possible to get, all the crevices are full.
It turns out that friction between the tread and road only accounts for a small part of the grip, and then only when there is no dust or oil film on the road, which almost never happens. Oddly, the friction part of grip is not affected by contact patch size, but the main part of grip from cohesion and a bit of adhesion does depend on contact patch area. So that should end some heated pub debates.

Scrubbing in new tyres.
Once past the release agent, new rubber is very grippy. But the heat of the first few laps causes a permanent molecular change and grip drops a bit. Once the tyre has gone through two or three heat cycles then is conditioned for the rest of its life and it becomes more consistent and predictable.

Buffing.
On a circuit racer excessive tread depth causes the tyres to over heat due to the tread blocks moving back and forth. So it is normal to cut treaded tyres down to about 3mm or less, as a budget racer I have always gone for 4mm so I get a little more use out of the tyre. But at the end of the day the effect of tread depth depends on your driving style and the type of car, if you are just starting off and drive relatively slowly then you wont get the tyres so hot and can carry a bit more tread, heavier cars work the tyres harder so need less tread depth.
Buffing must only be done by qualified companies, ask other racers in your chosen series who they use.
If the tread depth is too much you will find that the tyres work well for the first few laps but then the car starts sliding much more, the tyres get hot on the long straight so you don’t notice until you brake hard for the corner and fall off the track!

My tyres being almost pulled off the rims at Croft!
The extra blobs of rubber are of other cars tyres!
Still quite a lot of daylight under the inside set of tyres.
Well set up racer, outside tyres are flat to track.
Tyre grip vs contact patch area
Tyre grip vs pressure
Tyre grip vs slip angle
Tyre grip vs teperature
Braking on the front IRS
Cornering
Tyre peel
Different wheel widths on the same tyre when cornering.
 
 
 
 

©Ralph Hosier