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[N55_BBM]

Hey guys, so I thought it would be helpful if some of you knowledgeable people out there could enlighten us ignorant souls with some wisdom..

I was wondering about some of the theory of tire pressure.. general questions I had include:

When do I set the same pressure to all four tires, when do I go with more pressure in the fronts, when do I do so in the rears? And what's the consequence of all of that... For instance, for daily drive BMW recommends 36psi to the fronts and 39psi to the rears. For tracking the car, it's been recommended to me to have 40psi in the fronts and 38psi in the rears. Why is this and why would I not use this pressure for daily driving?

Also, how do we know when to adjust pressure up/down in the front/rears based on how the car feels? What is the effect of increasing/decreasing pressure in both front/rear wheels?

Finally, how can I use theory to help me figure out pressure in these scenarios:
This past fall I had the stock 215/40/18 up front and 245/35/18 in the rears.
1) For the winter I went with 225/40/18 all around. How should I adjust pressure taking the stock recommended 36/39psi as baseline, based on theory?
2) For next summer I'll go with 225/40/18 up front and 255/35/18 in the rears. How would pressure change again taking the stock 36/39psi as the baseline?

Let's consider this Tire Pressure 101 shall we ?

[pixelblue]

^ good read!

[N55_BBM]

hmm good read but i was looking for something a little more applicable to every day driving...

such as describing what to do if the rear feels light/wobbly, if ride is too harsh, etc etc etc... know what i mean? driving on the track is nothing like on the streets. understeer and oversteer are easy to detect when on the track but it's nothing you actually experience in daily driving (unless you are driving way way faster than you should be of course)

[GaryS]

Originally Posted by N55_BBM

what to do if the rear feels light/wobbly, if ride is too harsh, etc etc etc...

Any of the effects you're talking about could also be caused by the type or condition of tires, type or condition of shocks, type of springs, type or condition of various bushings and other suspension parts. In these cases, nothing you do with tire pressure will help. That's why so many of us on this forum have upgraded tires and suspensions.

[jsublime]

What he said.

If the rear feels "light" you can try lowering pressure, but this will reduce gas mileage and probably cause premature wear on the edges of the tires. Ride harshness isn't really something that can be adjusted by tire pressure. It's more the suspension and tire combo.

Personally, I slightly overfill my tires 1-2 psi above recommended for a couple reasons. Most importantly, it just feels better for DD (tire pressure for strip / track is different matter). Convenience - tires slowly leak and if you fill 1-2 psi over, then it'll take longer for them to become underinflated, which means less frequent adjustment. Last, slightly improved mileage.

I'm no tire engineer, these are just personal observations. Just remeber to always fill / measure when the tires are cold.

[Cisitalia]

Trackrat's link and information is very usefull. The main thing to take a way from that reference is:

1. Increasing front tire pressure reduces turn-in understeer.
2. Reducing front tire pressure increases turn-in understeer.
3. Increasing rear tire pressure adds rear cornering grip, increasing understeer.
4. Decreasing rear tire pressure allows car to rotate more on corner entry.

You want 1 and 4 combined. But of course not excessive extremes in these instances.

BMWs recommendation of 36/39 is to induce understeer so challanged drivers don't kill themselves more. If you want handling more towards neutral (less understeer) than you would be better off with 39/36. If you have descent car control skills I don't see any thing wrong with having less pressure in the rears for the street as well as track. Try running say 37/38 fronts and 35/36 in the rears and see how you like it. For your tire/wheel examples that is what you should use except for snow tires. Snows you want lower tire pressures and all four the same.

Good luck.

[N55_BBM]

Originally Posted by Cisitalia

Trackrat's link and information is very usefull. The main thing to take a way from that reference is:

1. Increasing front tire pressure reduces turn-in understeer.
2. Reducing front tire pressure increases turn-in understeer.
3. Increasing rear tire pressure adds rear cornering grip, increasing understeer.
4. Decreasing rear tire pressure allows car to rotate more on corner entry.

You want 1 and 4 combined. But of course not excessive extremes in these instances.

BMWs recommendation of 36/39 is to induce oversteer so challanged drivers don't kill themselves more. If you want handling more towards neutral (less understeer) than you would be better off with 39/36. If you have descent car control skills I don't see any thing wrong with having less pressure in the rears for the street as well as track. Try running say 37/38 fronts and 35/36 in the rears and see how you like it. For your tire/wheel examples that is what you should use except for snow tires. Snows you want lower tire pressures and all four the same.

Good luck.

Good stuff here, thanks.

So currently I'm running on snow tires (Dunlop Winter Sport 3D 225/40/18 all around). I'm at the same pressure as BMW recommends, 36f/39r. You're saying I should go for like 36-37 all around? I thought you should have more pressure in snow in order to have less friction to overcome?

[pixelblue]

"increasing tire pressure up front reduces understeer and gives better traction" this was exactly the opposite of my experience on the track. scratching my head...maybe berk, evolution race or brr can chime in on this?!

[igom3]

Originally Posted by Cisitalia

Trackrat's link and information is very usefull. The main thing to take a way from that reference is:

1. Increasing front tire pressure reduces turn-in understeer.
2. Reducing front tire pressure increases turn-in understeer.
3. Increasing rear tire pressure adds rear cornering grip, increasing understeer.
4. Decreasing rear tire pressure allows car to rotate more on corner entry.

You want 1 and 4 combined. But of course not excessive extremes in these instances.

BMWs recommendation of 36/39 is to induce understeer so challanged drivers don't kill themselves more. If you want handling more towards neutral (less understeer) than you would be better off with 39/36. If you have descent car control skills I don't see any thing wrong with having less pressure in the rears for the street as well as track. Try running say 37/38 fronts and 35/36 in the rears and see how you like it. For your tire/wheel examples that is what you should use except for snow tires. Snows you want lower tire pressures and all four the same.

Good luck.

+1

I would suggest 36psi fronts and 34ps rears.

[Cisitalia]

Originally Posted by N55_BBM

Good stuff here, thanks.

So currently I'm running on snow tires (Dunlop Winter Sport 3D 225/40/18 all around). I'm at the same pressure as BMW recommends, 36f/39r. You're saying I should go for like 36-37 all around? I thought you should have more pressure in snow in order to have less friction to overcome?

This is a conflicting topic. Well I have read that tire pressures should be higher in the winter (see link on winter tire pressures below). My experience in deep snow was that lower pressures worked better. Then I bumped them up for dry or wet conditions.

http://www.tirerack.com/winter/tech/index.jsp

Originally Posted by pixelblue

"increasing tire pressure up front reduces understeer and gives better traction" this was exactly the opposite of my experience on the track. scratching my head...maybe berk, evolution race or brr can chime in on this?!

Yes it's true. BMW decreases fronts and increases rears to induce understeer so people get the warning signs sooner. So if you increase front and decrease rear pressures you can dial out some understeer. It's a pretty well known fact in the track crowd that to stagger the rears slightly less. How much takes experimenting. I typically always had my pressures about 2-3 pounds less in the rears on front engine rear wheel drive cars.

[pixelblue]

^ that's exactly how I have mine set up. The rears are 2 lbs less than front. What I experienced was when I reduced the fronts by a few lbs the understeer was less but I still kept a front biased pressure. What I am trying to say is when I reduced pressure to both front and rear the overall grip improved contrary to what that manual was saying. also I agree with your assertion that bmw designed understeer into our cars for safety reasons hence narrower tires up front.

[Tom K.]

Originally Posted by N55_BBM

Good stuff here, thanks.

So currently I'm running on snow tires (Dunlop Winter Sport 3D 225/40/18 all around). I'm at the same pressure as BMW recommends, 36f/39r. You're saying I should go for like 36-37 all around? I thought you should have more pressure in snow in order to have less friction to overcome?

I would think you would want as much friction as possible in snow. I'd try about 32/35 or so to give a larger contact patch.

One thing which hasn't been mentioned is that while increasing front pressures (relative to rear) will decrease understeer, it will also increase instability in strong crosswinds. I suspect that is one reason my E91 wagon calls for 32/39.

Tom

[Top Gon]

WTF that manual has me confused. Wouldn't lowering front tire pressure give you more grip up front and therefore LESS understeer. And also wouldn't increasing rear tire pressure cause them to lose grip sooner and therefore oversteer/rotate?

I currently have my tires at 35 fronts and 37 rear

[Top Gon]

Originally Posted by TrackRat

1. No

2. No

Tire slip angle determines understeer or oversteer. Lower air pressure allows the tire to reach a higher slip angle sooner thus causing it to understeer.

hmm, ok I guess I'll try this theory and switch up my front/rear psi

[AW128i]

Hmm, I did find it strange the first time I took a look at the tire pressure sticker on my car to note that BMW recommends 32/35 for my car (17" wheels). Especially since the car's weight distribution is just about 50/50. So if I raise up the front tire pressures to match the rear (i.e., 35/35), understeer should be reduced slightly?

[aaron529]

Sorry to jack your thread but i have the same question as #1. What should my tire pressures be for 225s all around? I personally believe if you are getting the best treadwear (strictly using winter tires), your using them how they are meant to be used but i could be wrong. I guess you have to trial and error with the tread wear. if overinflated, one wall get worn down quick and vice versa. Can any1 confirm that?

http://www.1addicts.com/forums/showthread.php?t=479400

[aerobod]

Originally Posted by TrackRat

1. No

2. No

Tire slip angle determines understeer or oversteer. Lower air pressure allows the tire to reach a higher slip angle sooner thus causing it to understeer.

Too high pressure will also cause loss of grip. There is an optimum pressure that changes for each tyre and car combination.

As the tyre pressure increases, the contact patch shrinks and changes shape (shorter front-to-back). The tyre also can't conform to the road surface well, if the pressure is too high and more of the cornering forces are taken by the centre of the contact patch as opposed to being more evenly spread across it (although with too low tyre pressure, the outside edge of the contact patch takes a disproportionate amount of the load).

[pixelblue]

^ this was my experience. That's why I got better grip when I reduced pressure. Thanks for clarifying