Tire Pressure Question

[BMWe34CMM]

i have a 98 328i vert with M3 contours on. What would be the correct tire pressure since the car is not an M3 and the weight of the a 328i and an M3 are different i presume. I usually have me(230lbs) and sometimes my fiance in the passenger seat 120lbs. or if not its just her driving with no one else and no major load...

[jonlcraig]

Owner's manual should tell you

[328iFun]

It says it on the inside of one of your door jambs.

[BMWe34CMM]

im aware. just wondering since these wheels are not for the car they are on ...i just want to know if that plays a role on tire pressure.

[JDStrickland]

Your door jamb will give you the correct pressure. It lists several tire sizes and load profiles.

As a general rule, 30 psi is sufficient. Some might prefer 29 front and 32 rear, but the difference, as a practical matter, is inconsequential.

When you go to a wide tire and low profile, then the rule can jump to 35psi, but you will see this requirement because the shoulders of the tires wear faster than the centers.

[BMWe34CMM]

I see, thanks!

[JDStrickland]

UNDER INFLATION will cause the shoulders of the tires to wear faster than the center.

OVER INFLATION will cause the center of the tires to wear faster than the shoulders.

ALIGNMENT ISSUES will cause the inside or outside shoulder to wear faster than the other shoulder on the same tire, or faster than the shoulders on the other tires.

Two shoulders wearing at the same rate indicates under inflation. One shoulder on a tire wearing faster than the other, and/or faster than the other tires indicates an alignment problem. The center wearing faster than the shoulder indicates over inflation.

The inside shoulders of the rear tires wearing faster than the other parts of the tires, or faster than the other tires, indicates a heavy gas foot. If you drive with a heavy gas foot, then the back of the car squats down and this puts wear on the inside shoulders of the rear tires. This can be countered with alignment changes, but less agressive application of the go-pedal is free.

[doclees]

A pet peeve tire pressure. I went round and round with my car and motorcycle tires on this issue. How can one tire pressure be good for all makes of tire with different compounds. Didn't make sense so I went to the manufacture. As an example my Pirelli tires for my bike have a max pressure of 42. Manual says 31 front and 33 rear. Pirelli says 33 is the absolute minimum. Gave Pirelli my bike weight and my weight and they gave me 36 front 38 rear. Major difference. Same problem with car tires. Recommended tire pressure caused major cupping and poor wet road hold. I use 10% less than max on rear and another 2 lbs less in the front.

[jadatis]

Tire-presure is calculated with use of the load on the tire , and the maximum load and reference pressure of the tire. Camber-angle ( part of the alignment) of above 2 degrees influences it to, bu.t that is seldomly the case.
So if you know the exact weight on the tires you can add a reserve and calculate the pressure,with spreadsheet I made .
But if the loadindex of the tires are the same as the original ones, and also the tires did not chanche from standard-load to Extra load, you can use the same pressures.

[fun2drive]

Assuming you want even wear across the tire the one most reliable method to determine that is what the racing community does to dial their tires in for even wear then adjust for under/over steer as desired.

This method is to put a line using chalk across the tire. Have a place you can drive straight for a good distance say 1000 ft. Mark all the tires and drive straight, stop get out and look at the wear pattern. What rubbed off the chalk? Middle means too high tire pressue, reduce pressure. Wear on the sides, too low adjust up and retest.

This has worked to dial in the correct pressure for your car or truck everytime.

Manufacturers compromise on comfortable ride and acceptable tire wear but if you want the purist answer this is the only way I know how to get that. My limited experience doing this has held true for different makes of tires...

FYI there is very little difference in weight between your 328 and the M3, both vert run about 2780 pounds give or take 20 pounds so 33/ 35 is what the door frame placard states on mine..

[JDStrickland]

A pet peeve tire pressure. I went round and round with my car and motorcycle tires on this issue. How can one tire pressure be good for all makes of tire with different compounds. Didn't make sense so I went to the manufacture. As an example my Pirelli tires for my bike have a max pressure of 42. Manual says 31 front and 33 rear. Pirelli says 33 is the absolute minimum. Gave Pirelli my bike weight and my weight and they gave me 36 front 38 rear. Major difference. Same problem with car tires. Recommended tire pressure caused major cupping and poor wet road hold. I use 10% less than max on rear and another 2 lbs less in the front.

The max pressure printed on the tire is the max pressure the tire can hold, not the recommended pressure for normal operation.

NEVER put the max pressure in a tire unless the car is overloaded, and never overload a car.

[jadatis]

Assuming you want even wear across the tire the one most reliable method to determine that is what the racing community does to dial their tires in for even wear then adjust for under/over steer as desired.

This method is to put a line using chalk across the tire. Have a place you can drive straight for a good distance say 1000 ft. Mark all the tires and drive straight, stop get out and look at the wear pattern. What rubbed off the chalk? Middle means too high tire pressue, reduce pressure. Wear on the sides, too low adjust up and retest.

This has worked to dial in the correct pressure for your car or truck everytime.

Manufacturers compromise on comfortable ride and acceptable tire wear but if you want the purist answer this is the only way I know how to get that. My limited experience doing this has held true for different makes of tires...

.

This chalk test is good for diagonal tires , and they are seldomly used anymore. Radial tires stay with their wide on the ground within a large range. So only much to high or to low pressure gives uneven wear.
The goal of the official formula is to calculate the lowest pressure for the weight and speed at wicht the tires wont get damaged, and then especially the side-walls. Have you ever tried the chalk test on "modern"radial tires? I didn't because I dont think it is accurate enaugh.

The official formula is introduced in 1928 for diagonal tires, and has a power in it. It was meant for the tire specialist to calculate pressure for a lower then maximum load, at a symple way. The power in the formula though , makes it hard to understand for most people, so now it is only used by the tire-makers to make lists of maximum load for pressure.
In about 1970 the power was chanched for radial tires to 0,5, and there it went wrong. Shortly after that the European ETRTO chanched it to 0,8 for every kind of radial tire , from standard load to even truck-tires.
The American TRA thoug kept using power 0,5 and for tires with aspect ratio of less then 50% 0,65, From C load 0,7.
As late as 2005 the TRA harmonised the power to the European one of 0,8
, with reason of harmonising to a worldwide system. To my opinion the real reason is the problems that occured with the Ford/Firestone-affaire, in wich to low advice-pressure was mentioned, but by the TRA wiped away because according to the calculation it was right.
The lower the power, the lower pressures for a sertain load, and so more indentation of the tire. That is why TRA stopped at 1,8 bar/26psi, under that the calcuation gave to much indentation and so tire-damage.
European ETRTO dares to go down to 1,5 bar/21psi and because of the higher power it stil gave less indentation and so no tire-damage.
If you want to read this from an American, J.C Daws you should read next pdf. click the 5th wheel on the next page for it, and mayby the other articles are interesting too. But take a cup of coffee or two , they are long story's. You can scroll over it to the conclusions and the graphics.
http://www.dawsengineering.com/dewebsite2_003.htm

Greatings from Holland

[doclees]

UNDER INFLATION will cause the shoulders of the tires to wear faster than the center.

OVER INFLATION will cause the center of the tires to wear faster than the shoulders.

ALIGNMENT ISSUES will cause the inside or outside shoulder to wear faster than the other shoulder on the same tire, or faster than the shoulders on the other tires.

Two shoulders wearing at the same rate indicates under inflation. One shoulder on a tire wearing faster than the other, and/or faster than the other tires indicates an alignment problem. The center wearing faster than the shoulder indicates over inflation.

The inside shoulders of the rear tires wearing faster than the other parts of the tires, or faster than the other tires, indicates a heavy gas foot. If you drive with a heavy gas foot, then the back of the car squats down and this puts wear on the inside shoulders of the rear tires. This can be countered with alignment changes, but less agressive application of the go-pedal is free.

Good common information but by the time you have noticeable tire wear you got problems. Although combining the tire chalking method with this on new tires could be useful. I may try this just to see where it falls on my car tires.

[JDStrickland]

Good common information but by the time you have noticeable tire wear you got problems. Although combining the tire chalking method with this on new tires could be useful. I may try this just to see where it falls on my car tires.

Most people will underinflate because they simply wait too long to look at tires. If one notices shoulder wear, they can counter it with overinflation of about 5 pounds. So, instead of filling to 30, fill to 35 until the wear evens out.

Or, get a degree in rocket science and calculate the weight and load rating...

Sorry, I shouldn't have gotten that jab in, but it was fun.