Tire Pressure 101

[Level8Drummer]

I read a few of your posts on tire pressures earlier, and I thought I'd write a little explanatory article on tire pressures...Enjoy!

Level8Drummer's Tire Pressure 101



The greater your tire contact patch is, the more traction you have. Also the more rolling resistance your tire gives. The more your rolling resistance, the worse your gas mileage. If you decrease your tire pressure below optimal pressure, you have less contact patch, but more rolling resistance because the tires are bending and flexing more with every wheel revolution. Your tires will wear on the outside edges more, and less in the middle. If you increase your tire pressure above optimal pressure, then your contact patch narrows, and you have less traction. You also have less rolling resistance, since less tire is touching the ground. Less rolling resistance equates to better gas mileage, but your tires will wear more in the middle.

Tire pressures for optimal gas mileage do not necessarily equate to tire pressures for optimal traction. Think about it, the more a tire grips the ground the more it will resist rolling forward.

Also keep in mind that a tire with less pressure turns more revolutions per mile than a tire with more pressure does. So tire pressures do affect your gearing.


As for handling and tire pressures for the front and the rear, If you have more pressure up front than the back, your car will tend to oversteer. If you have more pressure in the back than the front, your car will tend to understeer. Since E34's (like most normal cars) tend to understeer as they are, for an experienced driver who knows how to steer with the throttle and properly rotate a car, not to mention reign in a strong oversteer, I would reccomend more pressure up front (and more negative camber up front), and less pressure in the back tires. For the average driver, I'd reccomend keeping the pressures closer to even.

Why does adding more pressure to a tire than the optimal amount for a maximized tire contact patch make that tire stick more to the ground? Our cars don't have the negative camber that track cars do. When we take a corner at full speed, our outside wheels and tires do not remain perfectly flat to the ground, the tops are leaning out too far for an optimal tire contact patch. At optimal contact patch pressure (for driving in a straight line) when taking hard turns, the outside of the tire has more contact with the road than the middle of the tire does. So if you increase the pressure and make the middle balloon out, then more of the middle of the tire comes into contact with the road, therefore increasing your traction. Also more pressure does tend to make your tire more stable under harsher conditions, and for those of us with higher profile tires, the tire will "roll over" less when it has more pressure in it.

So when you hit the track, knock your tire pressures up. For the normal gas-mileage conscious driver, keep your tire pressures wherever they have to be for your tires to wear evenly on the outside and the middle of the tread. For the performance-minded driver, take the pressures up to whatever level feels the best, and makes your car handle neutrally.

Also when you check and adjust your tire pressures, do it when the tires are warmed up. The front tires will not always be same temperature as the rears when you're driving on them, and that's when tire pressures matter the most...not when your car is sitting in the driveway. Most of the braking that you do is done with your front brakes, and that generates more heat (and more pressure since pv=nrt) in the front tires than the rears, especially if you have fancy alloy wheels (like come stock on the E34) that like to conduct heat.

One final note: The "MAXIMUM INFLATION PRESSURE" pressure is just what it says, a "maximum" inflation pressure when the tires are hot. Don't inflate your tires above that number.


ps. A Bimmer is a car, a Beemer is a motorcycle :p

[TheGeak]

Might also want to add the MAXIMUM PRESSURE is determined by the tire manufacturer. NOT BMW. So when you change tires, make sure you pay attention to any changes in max pressure. Don't wanna blow a wheel.

[bahnstormer]

also u should never inflate to max psi - u have to factor in on a hot day of hard driving the tire pressure will rise. keep it 10psi below max to be safe. although i saw an m3 who after doign donuts had 53psi in the rear =[

[Dan in NZ]

"Our cars don't have the negative camber that track cars do."
I thought bmw put in a little negative camber on the rears, which is why they always wear out the insides first... Several posts saying things like "check your inside rears" back this up.

"I would reccomend more pressure up front"
Almost everyone has said the rears should have a couple of psi more than the front, even the manual and sticker inside the door says so... And that's what the bmw dealers do.

[Level8Drummer]

We do have a little bit of negative camber in the front and the rear. We don't have as much as track cars do. Here are some pics of track cars with some real negative camber:

http://www.bmw-motorsport.com/03-8FE68A99C899E0E399860A7C40BD09F5/ms/en/4_serien/47_etcc/471_news/4715_bildergalerie/index.html



And they have much tighter suspensions (and therefore much less body roll) than we do, so they need even less negative camber than we would for maximum performance. And yes, more negative camber will wear the inside of your tires if you drive your car in a more relaxed fashion. If you wear your tires out from taking corners at speed, rather than wearing them out driving in a straight line or on the highway, then extra negative camber will make your tires wear more evenly. They won't chew on the outside edge (as much).


Yes BMW does reccomend that you put more pressure in your rear tires than your fronts, which perpetuates even more understeer than the car would naturally give with even pressures. If you join up with the BMWCCA and take some driving schools, the instructors will reccomend more pressure in the front tires and less in the rear tires . They'll even get more specific than that, taking into account whether the track has more left hand or right hand turns, and reccomending more pressures in the outside tires as well (so that they'll wear more evenly and won't chew on the outside).

[Warren N.CA]

also u should never inflate to max psi - u have to factor in on a hot day of hard driving the tire pressure will rise. keep it 10psi below max to be safe. although i saw an m3 who after doign donuts had 53psi in the rear =[

[MikeV]

If you decrease your tire pressure below optimal pressure, you have less contact patch,

Um, you mean "more contact patch," don't you? Less psi means more square inches of tire in contact with the ground per pound of car...

[Level8Drummer]

No, I mean exactly what I said. On a flat road surface, a properly inflated tire has a greater (or more even) tire patch than a deflated one does.

I see what you're getting at, but you're going to have more traction with a properly inflated tire, than a deflated one. A deflated tire might have more of the sidewall on the ground, but the middle of the tire's tread is making less contact with the road surface. And the tread of a tire is where your traction comes from.

I know that deflating your tires a little when offroading can be helpful, because it makes your tires more flexible to the uneven surface. And when people have really soft rear tires for drag racing, sometimes they deflate them a little, which with a really soft tire can increase the contact patch and give you a little more traction in a straight line...But speaking generally, which is the point of my write-up:

With normal tires on paved roads where you turn left or right, an inflation range that makes your tires wear evenly (or an optimal inflation range) gives you the greatest contact patch.