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
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;
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
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
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’
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
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
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).
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.
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
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
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
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!