Tire Pressure - Front and Rear Different???

[mattski]

Ok, I am sure this has been discussed before, but I couldn't find it using search.

What benefit (other than ride comfort) is there to having rear tires at 30 PSI versus 32 PSI for front? I have always set mine to 33 PSI all around. Is the difference in pressure advantageous in terms of grip, handling etc.?

[CNY_Dave]

The fronts see more weight, so in order to keep the rolling circumference the same and to have all 4 have the same give, the fronts need more air pressure.

Dave

[mattski]

Originally posted by CNY_Dave
The fronts see more weight, so in order to keep the rolling circumference the same and to have all 4 have the same give, the fronts need more air pressure.

Dave

Ok, so if rolling circumferences are slightly different front to rear, what is the adverse effect?

[Axess Denyd]

I would imagine that different rolling circumference front and back could cause the car to interpret one set of wheels as slipping and wreak all kinds of weirdness (possibly damaging) to the AWD system.

I'm not sure whether the small difference in tire pressure would cause anything like that, though.

[CNY_Dave]

Originally posted by mattski


Ok, so if rolling circumferences are slightly different front to rear, what is the adverse effect?

Could lead to torque bind, but not likely.

Handling is the important reason. Fine-tuning of the relative tire pressures could help tune for understeer or oversteer (slight effect, but real on loose surfaces like snow).

Dave

[Bimmer]

Originally posted by CNY_Dave

Handling is the important reason. Fine-tuning of the relative tire pressures could help tune for understeer or oversteer (slight effect, but real on loose surfaces like snow).

+1

I usually run all four tires the same pressure, or even do the opposite of what Subaru recommends and put more air in the rears to try to make the handling more neutral (less understeer).

[bikemobile]

Originally posted by Bimmer


+1

I usually run all four tires the same pressure, or even do the opposite of what Subaru recommends and put more air in the rears to try to make the handling more neutral (less understeer).

V-rated tires 36psi all around.

[number 41]

My thinking is this. The 2008 manual calls for an additonal 2 psi in the rear if you tow a trailer. I don't but I'm thinking they wanted the additional psi because of tongue weight. So I always carry extra weight in the back of a larger spare, two gear bags, sleeping bags, etc. The etc. can be extra winter gear or two German Shepherds (100# each). So after picking my psi of 36 lbs. (after many recommendations on this thread and others) for my Goodyear Triple Treads I decided to keep all four tires at the same psi. My rear cargo area is almost never empty.

[mattski]

Thanks for the input. I saw this posted over on an STI forum and it seems to make sense:

A tire has an optimal pressure at which it has the most grip. If the tire pressure is below this optimum, the tire will have more rubber in contact with the road, but not much sidewall stiffness. As a result, the tire is prone to rolling over, and loosing grip. Conversely, if you have higher pressure than optimum in the tire, the tire will not roll over as much, but will not have very much rubber in contact with the road.

For example, let's take a fairly well balanced car, and say the optimum tire pressure for our tires is 35 psi. We put 35 psi in all four corners and the car should be relatively neutral, neither understeering or oversteering. Now lets up our front pressure to 38 psi. We will find the car to now understeer, as the rear has more grip than the front. Now lets change the front pressure to 32 psi. Same thing will happen, understeer, as the rear has more grip than the front. The above is an idealized example, and holds for cornering. In acceleration, you aren't concerned about the sidewall flex as much, so lower pressures are generally better.

Another thing to consider is the effect on response. High pressure also means that you have increased your spring rate. This can be good for response, but might be bad over bumps. It's a compromise.

[TeamFCAR]

mattski,

That is good advice as far as it goes. As the last sentence says, it is all a compromise.

Based on decades of working on racing teams as everything from Tire Squid to Chief Wrench, I have worked with the Tire Engineers from Goodyear, Bridgestone, Yokohama and others. In racing, the tire temps will "tell" you how to set up the suspension to point the wheels in the right direction and what pressures to run but every track is different. Each company representative will tell you the temp at which the tire operates best.

For street cars temps will work but it is much harder. The easier way is to "play" with the pressures over time and see how the car responds. Keeping good gas mileage records will show you the effect on economy. Your backside will tell you the rest.

The general trends are pretty simple:

Higher pressure = stiffer sidewall, lower rolling resistance, greater load capacity and harsher ride.

Lower pressure = more tread compliance (better traction), softer ride and tends to generate heat in the side walls.

Don't go below the Pressures on the door sill. They are generally on the low side to give a plush ride.

Don't go over the max pressure on the sidewall of the tire. Auto-crossers will often go above the max pressure for better traction but it is very hard on the tires and the grip goes away all at once instead of predictably.

The "optimal" pressure is a balance of these factors and what you like. Many folks will argue to blindly follow what is written on the door sill. Others, such as the previous posters offer other settings that "work" for them.

In my case, I have the General Altimax HP's. For my normal driving, spirited, I run 40 psi in the front and 38 psi in the rear depending on the load in back as number 41 mentioned. I strive for even tread wear (across the tire), long wear and firm footing. Again, this works for me, your results will vary depending on driving style, specific tires mounted, vehicle load and comfort needs.

Good Luck,

Mike