Tyre Presures

Hi
Having tried most combinations over the years, I find that 40 all round works best. Thats on Michelin pilots

40 seems a touch high to me. BMW wants 29 and 32 rear for the stock tires. I* liked to run an extra pound in the fronts. For the wider tires I like about 2 more than that in front and 1-2 more in rear. 32 ft and 33 rear, or 33 and 34.

John, don't your tires wear out in the middle first?

I had 245/40, and 275/35/18 Pilots on my last 850, but have the same sizes in Toyo Proxes T1S on my current one. I havn't used the pyrometer after a hard drive yet to check. Just got the car on the road and the brake pads need 500 miles to break in, (2 months).

40 seems a touch high to me. BMW wants 29 and 32 rear for the stock tires. I* liked to run an extra pound in the fronts. For the wider tires I like about 2 more than that in front and 1-2 more in rear. 32 ft and 33 rear, or 33 and 34.

John, don't your tires wear out in the middle first?

I had 245/40, and 275/35/18 Pilots on my last 850, but have the same sizes in Toyo Proxes T1S on my current one. I havn't used the pyrometer after a hard drive yet to check. Just got the car on the road and the brake pads need 500 miles to break in, (2 months).

I run about 260-270 kPa all round.

I would suggest sticking to BMW's recommended figure of 2.5 bar front and rear, after all, they designed, built and tested the car so will know what they are talking about don't you think? Any other personal preferences are, simply, b*llsh*t.

Actually Beammer, changing the sidewall hight does alter the required air pressure. Ideally the tire has the same force to the ground all the way across it's surface. More air pressure increases the pressure at the middle ralative to the edges and visa versa. This is because the sidewalls prevent the edges of the tire from growing in diameter.

When you change the stiffness of the sidewalls from what the manufacturer intended, the pressures are likely not accurate. When you go from 16" to 18", the sidewalls are much shorter, 1" shorter to be exact. Also as the tires get higher in performance ratings, (Z or ZR to a W) the sidewalls get stiffer.

We don't have the facilities to know what the correct tire pressures are for the non-factory tire and wheel sizes many of us run, so we have to take an educated guess.

A way to be more accurate is to inflate to your best guess and then drive the car hard. Stop and immediatly check the tires surface temp at the edges and the middle. If the tire is warmer at the edges, you had too little air pressure when you started out. Hotter in the middle, you had too much. I just use one of those infared digital temp guages with the laser dot, but big time racers will use a pyrometer, which has a small pointed probe that you push into the tread slightly. There are some that have three probes and measure both edges and the middle at the same time.

In motorcycling, the best way to figure proper tire pressure is to inflate to manufacturer specs, then ride the bike hard down your favorite road. Stop and check the presures. They should have risen 10%. If they rose more than 10%, that tire had too little air pressure when you started off and vica versa. A tire under inflated will work more, (flex more) and get hotter thereby raising the air pressure more. I don't know if this technique can be translated to cars. Personally I don't inflate my tires with air. Air contains moisture and the more moisture a tires air has in it, the more the air expands with heat. You add air to tires on different days, so some have more moisture than others. If you add air at gas stations, some have moisture removers and some don't. You may start off with the correct pressure, but after driving awhile, some of your tires have more pressure than others, (left ft v.s. rt ft, and left rear, v.s. rt rear). I prefer nitrogen from a tank I keep in my garrage.

P.S. BMW does not suggest 2.5 bar ft and rear. At least not on 1991 and 1992 models of the 850. Both of my 850's clearly state on the drivers door jam 29 psi front (cold) and 31 rear.

Oops mistake, that's 29 ft and 32 rear. (2.0 bar ft and 2.2 bar rear) with 2 passangers, and 30 psi(2.1)bar ft and 36 (2.5) rear with 4 passangers and luggage, Ha ha ha. This according to page 20 of my owners manual. So 2.5 bar is only for the rear when you have 4 passangers and luggage, which doesn't happen often enough to ever alter your tire pressures for.

You must have a diferent manual than I do. I was reading straight from it. Actually I have 2 manuals, one for a 1991 and one for a 1992. Both of them as well as the door jam say the same thing.

Yours being different must hace to do with the tires delivered on the car in Europe.

Am attaching a chart which should answer the question. Your tyres are not original sizes so your manual won't be much help.

The original query was for 17" tyres. My '93 850 Ci requires 2.5 bar (~36psi - see J.H's chart) front and rear for the sizes stated and although Deadholy's is a '91 850 which would have had 16's originally, these pressures would be right for him running on 17's as the weight distributions would be virtually identical. To sum up, size matters!

John.

John, is that still the case with staggered wheels? Aren't the stockers the same size all the way around?

Stuart, I've answered the original query and the figure I provided was eventually confirmed by the man himself; the fact that the original wheels/tyres may have been all the same size is completely irrelevant. If you must pursue this trivia then take a look at the tables provided by J.H.