Tyre pressure effect on balance

[ales]

How does tyre pressure affect the handling of the car? (FWD car in this example, if that matters) So if the pressure of the front tyres is increased/decreased in the rear tyres, what will the effect be? And the other way around - the front tyres' pressure lowered/the rears' increased?

And why?

Thanks,
Alex

P.S. I have a sneaky feeling that the first route would take to understter, and the second - to oversteer, but I'm not sure

[Self]

Honestly don't really know, but I think if you have front decreased and rear increased you will experience some oversteer. Front increased and rear decreased and you will see understeer. Just guessing though, like I said, I have no clue.

[romabimmer]

Little physics:

A tire has 2 parameters in regards to handling balance. First, is the contact friction force with the road and second is the spring rate.

On a soft substance like rubber the amount of surface area does dictate the amount of friction force. Unlike hard surfaces, like steel on steel, where the amount of surface area touching doesn't matter.

So, by using air pressure settings that maximize the contact patch of the tire you are maximizing the grip of the tire on the road.

You can change the grip levels front rear by changing the tire pressure. So if the car understeers a lot you can increase the rear tire pressure so less of the outside of the tread touches the ground. Therefore, less contact patch, less grip and the rear will loss grip sooner...more oversteer. What pressures to use is completely dependent on the car, the climate and your preference. It's all trail and error.


The other aspect of tire pressure is the spring effect of the sidewall. A tire's sidewall has a spring rate that changes with tire pressure. The mechanism is different than the coil spring, but the tire sidewall has the same effect.

Increase tire pressure and you stiffen the sidewall of the tire. This decreases the overall spring rate of the car. It's the same as leaving the tire pressure the same and adding stiffer coil springs. Decreasing the tire pressure lowers the spring rate to a point. If you drop the pressure too much the sidewall will loss all stiffness and the spring rate of the tire will drop to zero, at this point the alloy is the sidewall. Again increasing rear tire pressure stiffens the rear tire sidewall and increases the rear's overall spring rate. This tends to cause the rear to oversteer sooner.

Basically, if your car is understeering you want to maximize the front tire pressure to get the most contact patch from the tire and then set the rear pressures to get a lower then optimal contact patch. With this the rear has less grip and the car oversteers more.

But it's all about optimization. The actual pressures to use depends on the car and the climate. There is no real formula that works, you have to try different pressures and see what you like.

But for a starting point, lower the front's 4 PSI from spec and increase the rear's 4 PSI above spec. Spec being the pressures on the door jamb plate. Then play with it until you like it.

[ales]

Thanks for the explanation! Wow! I'm very glad to have you on AF!

Alex

[romabimmer]

Want to add a few things about tire pressure.

As most know over inflation or under inflation will effect tire wear. Over causes the center to wear first and under causes the outside to wear first.

Under inflation also causes the steering to be sluggish and increases the chances of the tire rolling under. However, it usually helps soften the ride (if your into that).

Over inflation does help sharpen the steering responses, but it also makes tram-lining (following road ruts) worse and makes the car less stable in cross-winds. Of course it stiffens the ride quality too.

Both over and under inflation increases tire temperature when driving. This good be a good thing or bad thing depending on the tire and the road temperature.

For instance a high performance tire likes to be warm. But a winter tire can become very squirrelly when too warm. At extremes the tire can fail due to a combination of high heat and pressure.


My opinion:

Personally, I like to go no more than 7 PSI above and 4 PSI below recommended pressures. And I like to have no more than 7 PSI difference between front and rear. Except for AWD cars, where I think keeping all 4 tires equal is best.

On my WRX I have 33 PSI all around. On my M3 have 35 PSI front and 39 PSI rear.

[race2c]

Great explanation romabimmer!!! He basically said it all, from being in kart racing different tire pressures mean different weight percentages and how the kart/car would handle on the track. Also, if you are using air for filling the tires reacts different in the types of weather you are in because it is a "wet" air meaning humidity can affect the pressure of the tire. Sometimes we would put less air in the right rear tire because it builds pressure which in result would increase the amount of air in the tire. Example of that would be to put a tire in the sun with 4 lbs of air would probably increase several pounds due to being in the heat of the sun.

[MustangRoadRacer]

close, but not entirely accurate.
the mechanical grip a tire has on tarmac(its traction, static friction) is not dependant on tire width, but the compound of the tire, the temperature of the two frictional surfaces(rubber and asphalt), the weight on the tire and that is it. if we have an effective contact patch of 1 square inch with 700 lbs of weight on it, it will have the same traction as 2 square inches of contact patch with that weight on it. why? because now the frictional surface is doubled, yet the weight pushing on the surface is cut in half. however, switching to a stickier tire(and most performance rubbers are) will usually yield better traction obviously, and mostly ppl upgrade when it comes to tires, so they feel more available traction from these upgrades.
threfore, decreasing pressure in an attempt to change the contact patch on the tire on the road is not an effective way of changing grip for more or less under/oversteer.
additionally, if a tire is wider, it might not offer any greater traction to a tire of the same compound and design, but narrower dimensions, but in the dry it WILL however offer a contact patch with a larger face. this allows the contact patch to be more consistent and stable in its traction, as now an uneventy or the like will disturn the larger contact patch less as it would with a smaller contact patch.
So if you want better steering you should adjust your suspension and not your tires

[tricksaturnsc2]

If you have a fwd car w/ little psi front & high psi rear (drag racing setup) it will oversteer more (less understeer). If the front are high psi, rear low psi, it will understeer.

[Alastor187]

Originally Posted by MustangRoadRacer

close, but not entirely accurate.
the mechanical grip a tire has on tarmac(its traction, static friction) is not dependant on tire width, but the compound of the tire, the temperature of the two frictional surfaces(rubber and asphalt), the weight on the tire and that is it. if we have an effective contact patch of 1 square inch with 700 lbs of weight on it, it will have the same traction as 2 square inches of contact patch with that weight on it. why? because now the frictional surface is doubled, yet the weight pushing on the surface is cut in half. however, switching to a stickier tire(and most performance rubbers are) will usually yield better traction obviously, and mostly ppl upgrade when it comes to tires, so they feel more available traction from these upgrades.
threfore, decreasing pressure in an attempt to change the contact patch on the tire on the road is not an effective way of changing grip for more or less under/oversteer.
additionally, if a tire is wider, it might not offer any greater traction to a tire of the same compound and design, but narrower dimensions, but in the dry it WILL however offer a contact patch with a larger face. this allows the contact patch to be more consistent and stable in its traction, as now an uneventy or the like will disturn the larger contact patch less as it would with a smaller contact patch.
So if you want better steering you should adjust your suspension and not your tires

Since this is back from the dead I will add my two cents for s&g’s.

The two primary mechanism for tire friction are adhesion and deformation. Adhesion is the natural “stickiness” of tires that allows the tires to adhere to the pavement on the molecular level. Deformation occurs as the rubber is distorted to follow the rough contour (microscopic) of the road. On dry pavement the total tire friction comes from both adhesion and deformation, while on wet surfaces deformation alone is responsible for tire friction.

The tire compound and temperature are important because they effect the amount of adhesion and/or deformation the tire is capable of generating. A softer tire compound is capable of deforming to the road irregularities better than a harder tire as well as exhibiting a higher net force on the contact path. While a cold tire may not be as sticky as warm a tire, an over-heated tire may be significantly harder than a cold or warm tire.

As load is placed on a tire the rubber is pushed onto the road causing it to fill in the microscopic irregularities that make up the asphalt. For some initial load the rubber will only partially fill the irregularities. As the load is increased the rubber will continue to fill the irregularities until the cavity is completely filled with rubber. After this point additional load results in mostly compression of the rubber and localized distortion of the contact patch.

As a result there is a particular load at which the effective coefficient of friction is maximum. At all other loads the effective coefficient of friction will be less. This phenomenon is referred to as tire load sensitivity. In most vehicle application (on asphalt) the tire is operating at a load above that of which gives the best coefficient of friction. So any decrease in load that can be achieve will result in better coefficient of friction performance.

Adjusting the tire pressure for a given load will change the contact patch area. An increase in tire pressure will reduce the contact patch area, and a reduction in internal tire pressure will increase the contact patch area. If the load on the tire is fixed and the contact patch size is varied the coefficient of friction is also a varied. If the contact patch area increases the load per unit area decreases and the effective coefficient of friction increases. If the contact patch area decreases in size then the load per unit area increases and the coefficient of friction decreases.

With the load on the tire fixed any change in coefficient of fiction results in a change in lateral force generated by the tire. Increase in coefficient of friction equates to an increase in lateral force (for a given slip angle) and vice versa. Adjusting the front and rear lateral forces with respect to each other allows the vehicles balance to be tuned via tire pressure.

Both under-inflating and over-inflating will resulting in uneven tire wear. Under-inflating will also increase tire temperatures. Heating of the tire occurs as the sidewall deforms above the contact patch. As the tire rotates a new section of side wall deforms and then is restored to its original shape and the cycle is repeated. Under-inflating the tire cause additional deformation in sidewall and therefore an increase in heat generation.

Excessive heating will greatly shorten the life of any tire, and can result in catastrophic failure of the tire. Over-inflating the tires reduces the tires resilience against bumps and curbs which also could cause premature failure of the tire. When tuning the tire pressures it is critical to adjust for the tires operating temperature. The internal pressure of the tire will increase as the tires temperature increases.