Set Up : Tyre Pressures

[RobM]

Can someone tell me why every set up book tells you to decrease pressures at the front and increase at the rear to cure oversteer and vice versa to cure understeer?

 

The reason I'm confused is that surely the lower the pressure, the greater the contact patch that the tyre has with the road, therefore the greater the grip? If that were so though, increasing pressures at the front would reduce grip and therefore increase understeer, but we're told the opposite!

 

thanks

 

- How can such a cute looking car sound so ferocious! -

[anthony1956]

my guess is that their advice effectively alters the weight of the car at each corner (front and rear in your case) and so alters the weight being applied to each tyre.... lifting the rear (more pressure) would add weight to the front and vice versa

 

Anthony

[RobM]

The effect would surely be minimal though? Lightening the rear gives oversteer though, such as when you back off mid corner or trailbrake.

[Mike C.]

Some of the guidance notes relating to factors that can increase/reduce understeer and oversteer then, unhelpfully, state, " Please note that, in some instances the opposite applies".

 

 

 

Mad Manx

[Chris W]

Rob

 

You'll probably find this hard to believe but if you do the maths, a larger contact area does NOT increase grip.

 

When driving straight, the weight of the car pushing down on each wheel is roughly a quarter of the total weight of the car per wheel. Let's say roughly 150kg/wheel (ie: a 600kg seven, fully loaded).

 

So for my tyres which are 205/45/16's running at 16psi (1.13kg/sq cm) the weight of the car will squash the tyre until the internal pressure of the tyre just balances the down force. Therefore the tyre will squash until the contact area is 150kg/1.13 = 133sq cm of contact patch. So for my tyres with a width of 205mm, the contact patch will be an approximate rectangle of size 20.5cm x 6.5cm. If I change the width of the tyre, the shape of the contact patch will change (eg: a narrower tyre will have a narrower contact patch) BUT the AREA of the contact patch will remain the same. ie: the rectangle will increase in "length" to compensate.

 

Lowering the tyre pressure will of course increase the size of the contact patch, BUT.... then, the same downward force (150kg) is simply spread over a larger area, so the pressure/unit area pressing on the road lowers too. ie: the frictional force does not increase. The increase in contact area exactly cancels out the decrease in downward force/sq cm.

 

The way to increase grip is to increase the downward force, either by increasing the actual weight of the car OR by increasing the effective weight of the car by adding downforce wings as they do on F1 cars. That's why those wings produce such effective grip and why they are so necessary on F1 cars.

 

Another way to increase grip is to use a softer tyre compound and fatter (ie: wide) tyres are generally made of a softer rubber which has a higher coefficient of friction, ie: more grip. Of course the payback here (nature is cruel) is that a softer rubber will wear out faster = greater expense.

 

In a cornering or braking situation the weight distribution changes dramatically from the steady-state case above and when, say, braking hard the effective weight on the front tyres increases. Since the internal pressure of the tyres remains the same but the actual downforce on the front increases when braking, the contact patch at the front will increase AND this time WILL provide more grip because the downforce per unit area is increased. At the rear, the opposite happens and the downforce will decrease, reducing the size of the contact patch AND lowering the grip because of the reduction in weight/unit area. That's why under heavy braking, a car is very prone to oversteer.

 

So by decreasing the pressure at the front and increasing the pressure at the rear you make the force/unit area higher at the back under increased load and vice versa at the front thereby tending to reduce the effect of oversteer.

 

Chris

 

2003 1.8K SV 140hp see it here

 

Edited by - Chris W on 4 Jul 2003 00:46:46

[scooby dooby doo]

slight spanner in the works here - increased force of rubber on road doesn't quite give you a linear increase in grip. ie the coefficient of friction tails off as force increases.

 

Its too early to apply this to this situation and work out the consequences though ❗

 

Damn - I really need to buy Milliken and Milliken ❗

 

HOOPY

R706KGU Hoopylight R

[barry.h]

it also depends on whether the starting pressures were at, above or below the optimum - i.e. whether the contact patch was working evenly across its full width. If the pressures were below optimum and were increased to the optimum then grip would be improved. The further they moved away from the optimum the worse the grip would get.

 

It is a pretty crude way of removing understeer by worsening the grip at the opposite end of the car, and vice versa.

 

Ideally, all wheels should be running their optimum pressures and focus should then be on the true cause of the problem.

 

That said, ultimately the front or rear will break away; the objective is to delay that point as long as possible, and, when it does occur, to make it as controllable as possible. Tyre pressures are only one factor in the equation.

 

Barry

 

 

[Matthew Wheatley]

I was thinking about this recently, but I couldn't remember which way round it is (ie does less pressure at the front reduce or increase understeer) - as there's no search facility etc. etc....

 

Reading RobM and anthonym it sounds like they disagree?

 

RobM suggests that increasing front pressures increases understeer but anthonym suggests the opposite?

 

Intuitively I would have thought that reducing the pressures at the front will reduce understeer - as per anthonym's argument? (I tried reducing my front pressure to reduce understeer and couldn't really tell much difference anyway.......)

 

 

can any shed any more light on this

[barry.h]

Matthew,

 

Read the first paragraph of my earlier posting.

 

Reducing the front pressures will only reduce understeer if they were too high to begin with. If they were too low then reducing them will increase understeer.

 

Secondly, there is NO weight transfer from rear to front if both rear tyre pressures are increased. The weight at the rear of the car remains the same. All that changes is the size and shape of the contact patch - depending on whether it was above of below the optimum.

 

Just as with adjustable spring platforms, weight is never transferred from end to end or from side to side. All that can be adjusted is the sum of the weight between the two diagonals (NSF + OSR and OSF + NSR).

 

Barry

 

 

[Paul Ranson]

The assertion was that decreasing front pressures and increasing rear would reduce oversteer and vice versa.

 

FWIW it works with Hillman Imps...

 

I suspect that this is a potential fine tuning tool, and that what happens on radial tyres is that the reduced pressure leads to higher slip angles, and vice versa. I doubt the contact patch changes shape or even if ultimate grip changes, it'll be to do with the sidewall and the camber thrust.

 

Now off to read Milliken and see what he says. I may be some time.

 

Paul

[RobM]

I think Chris explains the theory very well. There's a few things in there that I hadn't thought of. I just have a few comments if Chris doesn't mind:

 

"The way to increase grip is to increase the downward force, either by increasing the actual weight of the car OR by increasing the effective weight of the car by adding downforce wings as they do on F1 cars. That's why those wings produce such effective grip and why they are so necessary on F1 cars."

 

Bear in mind that increasing the weight of the car may theoretically increase the grip with the road, but you've also increased the centripetal force needed to keep the car on the track! The two cancel out and the net result is that weight has very little effect on steady state cornering speed. Of course, aerodynamic downforce pushes the car down into the track, but doesn't affect the car's actual weight with respect to cornering, ie centripetal force needed to hold the car in a corner; so you get the increase in grip without the comensurate need to increase centripetal force, ie higher cornering speed. A clever bit of Physics!

 

Another thing I've never quite understood is that Physics actually says that the purchase on the road is nothing to do with how wide your tyres are! i.e. Lateral Force = mu*m*g (IIRC!?). Maybe there's something I'm missing, such as in these two quotes from Chris' excellent post, which I see as contradictory, but maybe I'm missing something:

 

"Lowering the tyre pressure will of course increase the size of the contact patch, BUT.... then, the same downward force (150kg) is simply spread over a larger area, so the pressure/unit area pressing on the road lowers too. ie: the frictional force does not increase. The increase in contact area exactly cancels out the decrease in downward force/sq cm."

 

However, surely this is the same as:

 

"Another way to increase grip is to use a softer tyre compound and fatter (ie: wide) tyres"

 

Thanks for your replies guys, especially Chris. I'm off to decrease my toe in!!

😬 😬

 

- How can such a cute looking car sound so ferocious! -