Tire Pressure Advice - Take 245

Ok, I know this is a common subject. I have searched the forum for posts on tire pressure and read everyone's advice. What I've taken is that the only way to know for sure is to experiment and see what you like.

I have a 2002 330ci with Sports Package (Conti Sports tires). 225/45R-17 front and 245/40R-17 rear.

I commute 22 miles thru some twisty roads daily to work and about 150miles of highway driving on the weekends.

What I wanted to know is more of a safety concern. I have a bad habit of speeding on open areas of the highway, not uncommon to be over 110-120mph. And I like to really push it thru the twisties on the way to work. Any concerns over the tire pressures below that I'm going to experiment with? With high highway speeds what tire pressure is best. Thanks all for the advice.

Read about having significantly higher pressures in the front than the back to eliminate understeer and may try that as well after I test the ones below.

Going to experiment with:
34f/35r
35f/38r
36f/38r
37 all around

The pressures look a bit low.
If you're REALLY pushing hard, your sidewalls will likely roll.

Personally, I wouldn't go below 40 up front. 35 in the rears would allow for some rolling and increase oversteer. At the moment, I'm running 44PSI all around simply because it gives the least buick-like ride. It makes the car FEEL better, although it handled better at 42F/37R.

Try 44 front and 40 rear, just see how you like it.

There is going to be no perfect PSI, it would change with temp, road camber, your personal likes, ... etc.

How's the tire wear at that 44 psi all around?

Can't really say... I've only been running it for a few weeks. Frankly, I don't really care about tire wear, though. My next set of tires may very well be S-03s simply for the stiffer sidewalls.

I will say, though, that for long distance highway cruising, 42F/37R is simply awesome on my tires. Almost no wander, smooth, quiet ride... When I'm going almost 500 miles in a day, I DO appreciate that.

So I was reading this post by "Eddie" at tire rack which was in the Tire Rack forum over at E46Fanatics. Based on the advice he gives here I readjusted my pressures this morning before I went anywhere.

I started with the recommended pressures per the plate on the door jam.

30psi front 35 rear.

I tried it and the "floaty" feeling I was having the past couple of weeks when I was running at 35f and 40r was all but gone.

Now the question is how much will the sidewalls roll under hard cornering.

Here is Eddie's advice if anyone hasn't read it.

Okay gang- seeing some questions on best pressures, here is a quick rundown that
is worth a read to make sure you are getting the best handling & ride & most even wear.. read up & go check your tires!

Your tires support the weight of your vehicle, right? Well, they
don't! It's the air pressure inside them that actually supports
the weight. Maintaining sufficient air pressure is required if your
tires are to provide all of the handling, traction and durability of
which they are capable.

However, you can't set tire pressure...and then forget about it!
Tire pressure has to be checked periodically to assure that the
influences of time, changes in ambient temperatures or that a
small tread puncture has caused it to change.

The tire pressure recommended in your vehicle's owner's
manual or tire information placard is the vehicle's recommended "cold"
tire inflation pressure. This means that it should be checked in the
morning before you drive more than a few miles, or before rising
ambient temperatures or the sun's radiant heat affects it.

Since air is a gas, it expands when heated and contracts when cooled.
In most parts of North America, this makes fall and early winter
months the most critical times to check inflation pressures...days are
getting shorter...ambient temperatures are getting colder...and your
tires' inflation pressure is going down!

The rule of thumb is for every 10° Fahrenheit change in air
temperature, your tire's inflation pressure will change by about 1 psi
(up with higher temperatures and down with lower).

In most parts of North America, the difference between average
summer and winter temperatures is about -50° Fahrenheit...which
results in a potential "loss" of about 5 psi as winter's temperatures set
in. And a 5 psi "loss" is enough to sacrifice handling, traction, and
durability!

Additionally, the difference between cold nighttime temperatures and
hot daytime temperatures in most parts of the country is about 20°
Fahrenheit. This means that after setting tire pressures first thing in
the morning, the vehicle's tire pressures will be almost 2 psi higher
when measured in the afternoon (if the vehicle was parked in the
shade). While that is expected, the problem is when you set your
vehicle's tire pressures in the heat of the day, their cold pressures will
probably be 2 psi low the following morning.

And finally, if the vehicle is parked in the sun, the sun's radiant heat
will artificially and temporarily increase tire pressures.

We put some of these theories to the test at The Tire Rack. First, we
mounted two tires on wheels. We let them sit overnight to equalize
and stabilize their temperatures and pressures. The following morning
we set them both to 35 psi. One tire and wheel was placed in the
shade while the other was placed directly in the sun. We then
monitored the ambient temperatures, tire temperatures and tire
pressures through the day. As the day's temperatures went from 67°
to 85° Fahrenheit, the tire that was kept in the shade went from our
starting pressure of 35 psi to a high of 36.5 psi. The tire that was
placed in the sun and subject to the increase in ambient temperature
plus the sun's radiant heat went from our starting pressure of 35 psi to
a high of 40 psi. In both cases, if we had set our tire pressures in the
afternoon under the conditions of our evaluation, they would have
been between 2 and 5 psi low the following morning.

Next we evaluated the effects of heat generated by the tire's flexing
during use. We tried to eliminate the variable conditions we might
encounter on the road by conducting this test using our "competition
tire heat cycling service" that rolls the tires under load against the
machine's rollers to simulate real world driving. We monitored the
changes in tire pressure in 5-minute intervals. The test tires were
inflated to 15 psi, 20 psi, 25 psi and 30 psi. Running them all under the
same load, the air pressure in all of the tires went up about 1 psi
during every 5 minutes of use for the first 20 minutes of operation.
Then the air pressures stabilized, typically gaining no more than 1 psi
of additional pressure during the next 20 minutes. This means that
even a short drive to inflate your tires will result in tires that will
probably be "underinflated" by a few psi the following morning.

Add all of these together, and you can understand why the conditions
in which you set your vehicle's tire pressures are almost as important
as the fact that you do set it.

It's important to remember that your vehicle's recommended tire
pressure is its "cold" tire inflation pressure. It should be checked in the
morning before you drive more than a few miles, or rising ambient
temperatures or sun's radiant heat affects it.

And by the way, if you live in the North and park in an attached or
heated garage you will "lose" pressure when you leave its warmth and
venture into the real world outside during winter. Add 1 psi "cold"
pressure tire pressure to compensate for each 10° Fahrenheit
temperature difference between the temperature in the garage and
outside

Originally posted by 02330ci
Ok, I know this is a common subject. I have searched the forum for posts on tire pressure and read everyone's advice. What I've taken is that the only way to know for sure is to experiment and see what you like.

I have a 2002 330ci with Sports Package (Conti Sports tires). 225/45R-17 front and 245/40R-17 rear.

I commute 22 miles thru some twisty roads daily to work and about 150miles of highway driving on the weekends.

What I wanted to know is more of a safety concern. I have a bad habit of speeding on open areas of the highway, not uncommon to be over 110-120mph. And I like to really push it thru the twisties on the way to work. Any concerns over the tire pressures below that I'm going to experiment with? With high highway speeds what tire pressure is best. Thanks all for the advice.

Read about having significantly higher pressures in the front than the back to eliminate understeer and may try that as well after I test the ones below.

Going to experiment with:
34f/35r
35f/38r
36f/38r
37 all around

A. Those are all REALLY low, IMO.

B. Your front pressure needs to be HIGHER than your rear pressure if you want to offset the understeer inherent in the staggered tire setup.

Personally, I'd run 40F 38R or 42F 39R. CD-55's advice might be a bit radical for most. But I've run the pressures I listed here on my daily driver 330i w/SP and on my M3 sedan (same tire sizes as the 330s w/SP).

I'm very happy with 40psi all the way around... over/understeer seems perfectly balanced that way, with the upgraded UUC sways (full stiff front and back).

Maybe 42/40 with stock sways would yield the same effect.

I think you're getting a lot of advice from track junkies here...

I run 38psi all around for street use on my 18" 225/40/18 tires, and 41-44 psi all around for track use. I find that 38psi gives better comfort and has better wear characteristics, gives ample protection to the larger rims and enough sidewall support for some fairly aggressive driving.

If you don't drive at or near the max all the time, you might want to take their advice and back off ~5 psi for best all around comfort and wear.

Originally posted by The HACK
41-44 psi all around for track use.

HACK, I think 41-44 on the track is way too high, ESPECIALLY if those are cold pressures. Even if those are hot pressures, I think you need to lower them to about 38 on your car. I ran 38 hot all around at Buttonwillow and Willow Springs and found them to be the best combo for my car (same size wheels and tires as yours).

By the way, when I was at Willow Springs last Friday, I initially set the pressures at 35, figuring they would rise to 39 hot after a session - WRONG! They were at 44 psi after 30 minutes on the track!!! It was, after all, 100 in the shade!!!

Soooo, check your pressures after each session.

Have you signed up for the Buttonwillow school? Did you convince the missus?

Later.

Thanks all for the advice. When I went to put air in my tires to try out new pressures they were at the recommended 30f and 35r. I changed them to 35f and 37r. I love the difference this makes for local driving but at very high highway speeds 80+ the car doesnt seem as solid as it did before. Does this make sense?

Also, I would like to experiment with the higher pressures you recommend but dont these exceed the manufacturers recommended tire pressures?

My door jam pressure for 2002 330ci sp package states max pressure:
light load: 30F/35R
heavy load: 35F/42R

That is the reason I didnt inflate the front over 35. Are you saying it is ok to inflate over the maximum pressure BMW recommends. Thanks all for your advice.

Originally posted by Raffi


...snip...

Soooo, check your pressures after each session.

Have you signed up for the Buttonwillow school? Did you convince the missus?

Later.

I'm waiting for a freelance project to commence that will pay for the next few track schools. It's a lot easier to convince her if it doesn't impact our cash flow at all.

I've also been through this debate with my own 2001 330CI with SP. The dealer (Center BMW) recommends 36 psi in all four tires. Their "M" specialist says he sets all M3s at that for maximum balance, and even wear. An indepedent BMW speciality garage also told me to set my tires at 36 all around. I have about 6800 miles on my car and have been experimenting with differerent settings as well. For street driving I seem to have settled on 36R, 33-32F cold. Though most of my driving is stop 'n go city stuff, I do occasional canyon carving and some freeway (70-80 MPH) driving. I've found that this setup maintains a comfortable ride, (apparently) good even wear, not much tire noise, and crisp handling.

Originally posted by 02330ci
Thanks all for the advice. When I went to put air in my tires to try out new pressures they were at the recommended 30f and 35r. I changed them to 35f and 37r. I love the difference this makes for local driving but at very high highway speeds 80+ the car doesnt seem as solid as it did before. Does this make sense?

Also, I would like to experiment with the higher pressures you recommend but dont these exceed the manufacturers recommended tire pressures?

My door jam pressure for 2002 330ci sp package states max pressure:
light load: 30F/35R
heavy load: 35F/42R

That is the reason I didnt inflate the front over 35. Are you saying it is ok to inflate over the maximum pressure BMW recommends. Thanks all for your advice.

I've found that dropping the rear tire pressures seems to affect high speed stability a lot more than the fronts. I've been running 44F, 37R for the last two days and I'm rather happy with it, both at cruising speeds and in the corners.

Edit: I keep the fronts at 44F because it contributes to an overall feel of precision. I drive a WRX very frequently, and it makes the E46 feel like an absolute pig in comparison. I suggest that you try 40F, 35R and see how you like it.37 is the absolute minimum I can go without really rolling the sidewalls, but your driving habits and tire pressures are likely different. I DID go through my first set of tires in under 11K.

The ONLY maximum pressure you need to observe is the one written on the tire's sidewall. BMW's recommendations are guidelines only, and intended to generate lots of understeer which makes the car "easier" to drive.

hth

keep in mind that Nick is giving advice based on his xi... wouldn't pressure settings generally be different for a ci?

kurt

Kurt: Yes and they would also vary depending on the tires, too. However, lower pressures up front contribute greatly to the sloppy, imprecise feel of current generation bimmers. (Certainly not the cause. It's really a fundamental design issue, however...)

Nonetheless, my recommendation to try 40F/35R still stands.

Nick: I see.