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View Full Version : (2009) Tire Pressure Questions For e90 325xi



forrealb50
04-28-2009, 08:45 PM
What do you guys recommend I set my tire pressures to? What does everyone else set there tire pressures to? I do have an XI with OEM runflat tires.

pseto
04-28-2009, 09:24 PM
for everyday driving use what BMW put on the inside of the door jam. if you're talking autox, i'd go 5-8psi over recommended

Jim Dunlap
04-28-2009, 09:49 PM
The factory spec pressures favor understeer; much safer - all around - of course. Some of us tend to more towards equal F and R tire pressures for driving events. Chalk your tires and see how much rollover you're getting...and respond accordingly. But as for the street, going with BMW's numbers has to be a good thing to do.

Pete
04-29-2009, 09:17 AM
Start out with 6-8 lbs over manufacturers spec. Then chalk your tires to see if you're rolling over or not getting to the edge of your tires (in terms of wear). Adjust accordingly until you get all the way to the edge of your tread. Most of the time, there is a little triangle that indicates the edge of the tread. That's where you should be.

Once you've got your baseline pressures, you can adjust them a bit to suite your cars behavior to make it more or less neutral. But first get to your baseline. Depending on what your car is doing, increasing/decreasing pressure or varying the pressure bias (front vs rear) will help dial out understeer/oversteer.

In a wet course, I leave stock pressures as there is usually not enough grip to roll your tires.

noelleslie
04-29-2009, 04:44 PM
I am on new tires this season: Dunlop Direzza Star Specs. With last Sunday's hot weather, I finally had a chance to fiddle with my Z4's tire pressures.

In addition to what Pete stated, I would keep a log of what pressures you are running and how the car feels. After each run, note the hot tire pressure and your comments on vehicle handling. There is a lot of information available on the internet too. Basically what I have learned is there is an optimum tire pressure which, both above and below will yield less grip.

What I do is attempt to determine this optimum grip for both the front and rear tires, then keep the front tire pressure, and fiddle with the rear to get the desired handling changes.

The BMW recommendation is 33psi front and 36 psi rear on my car, which is a staggered 255 rear and 225 tire setup on 18" wheels. I found 36 front and 35 rear worked excellently for me. I would suggest making small changes to one end of the car only at a time and going from there. The internet and chalk technique can get you in the ballpark.

Below is a synopsis of my tire pressure notes from last Sunday.

Autocross Tuning Notes
April 26, 2009
Dunlop Direzza Z1
93 degrees, sunny
Early morning tire pressures were: Front: 36psi Rear 35 psi.
Afternoon (at event) pressures had risen to: Front 42psi Rear 41 psi.
--Adjusted pressures to Front 36psi Rear 35psi (took out a lot of air)—
At 36 Front/35 rear, overall grip was great. Car was slightly understeering.
After run #2 at these settings, adjusted pressures to 36 front 34 rear (-1psi rear) and car tended to oversteer more. Still mostly neutral, but inclined to oversteer more easily and felt less planted (rear). Rear grip less at 34psi than 35psi.

For the final run, adjusted pressures to 37Front / 36 Rear and car felt good.
All 4 run times were within 0.3 seconds.

woodym3
04-29-2009, 06:49 PM
Noel offers a lot of good information here. He did not mention what affect the sun shining on one side of the car might have.

Not necessarily recommended is my method: Set the pressures where you think you want them in late March. . . .that's it.

noelleslie
04-29-2009, 08:41 PM
Not necessarily recommended is my method: Set the pressures where you think you want them in late March. . . .that's it.

Yeah... once you reach GRAND PUBA level, then you don't adjust your tires anymore... you feel the pressure and the course adjusts to you... but that takes a lifetime..

Like some 10th degree black belt sheeat.

The rest of us just fiddle with pressures while cursing under our breaths that Woody is 1.2 seconds faster :)

Ethan Blank
04-30-2009, 02:13 PM
On a somewhat related topic, what gauge do people use/recommend?

As between the accutire gauge and the Psiclops, any thoughts?

Accutire: http://www.amazon.com/gp/product/B000RN4GT6 (http://www.amazon.com/gp/product/B000RN4GT6)
Psiclops: http://www.tirerack.com/accessories/detail.jsp?ID=56&cat=Air+%26+Tread+Depth+Gauges (http://www.tirerack.com/accessories/detail.jsp?ID=56&cat=Air+%26+Tread+Depth+Gauges).

mf44
04-30-2009, 02:50 PM
On a somewhat related topic, what gauge do people use/recommend?

As between the accutire gauge and the Psiclops, any thoughts?

Accutire: http://www.amazon.com/gp/product/B000RN4GT6 (http://www.amazon.com/gp/product/B000RN4GT6)
Psiclops: http://www.tirerack.com/accessories/detail.jsp?ID=56&cat=Air+%26+Tread+Depth+Gauges (http://www.tirerack.com/accessories/detail.jsp?ID=56&cat=Air+%26+Tread+Depth+Gauges).


I'm no expert, but from just looking at those websites I can say:

the first one looks to be a pain because it doesn't have a hose, so you need to be bent down to the tire valve.

and the second one seems way complex. Maybe it's a good idea, but just seems like more stuff to break to me.

Edit: not to mention that first one doesn't even have a bleed valve. No good there.

Pete
05-01-2009, 09:04 AM
I bought a $14 dollar model with a hose and bleeder valve. Nothing special but it works well.

http://shop.advanceautoparts.com/webapp/wcs/stores/servlet/product_Tire-Pressure-Gauge-G-H--Meiser_1920154-P_N3192_A|GRP2068____

1996 328ti
05-01-2009, 12:02 PM
Just don't drop any of them.
Mine has a rubber covering around around it.

Zcruizer
05-01-2009, 08:48 PM
Pete, I have that very model. Even red to match my car. These are fine gauges, but 1996 328ti is right, even though mine has a rubber covering, I make sure it doesn't get beat around. I had a digital model but that one had a delayed reading. The mechanical ones are instant, and the bleeder function is great too.

Car54
05-02-2009, 08:33 AM
Autocross Tuning Notes
April 26, 2009
Dunlop Direzza Z1
93 degrees, sunny
Early morning tire pressures were: Front: 36psi Rear 35 psi.
Afternoon (at event) pressures had risen to: Front 42psi Rear 41 psi.
--Adjusted pressures to Front 36psi Rear 35psi (took out a lot of air)—
At 36 Front/35 rear, overall grip was great. Car was slightly understeering.
After run #2 at these settings, adjusted pressures to 36 front 34 rear (-1psi rear) and car tended to oversteer more. Still mostly neutral, but inclined to oversteer more easily and felt less planted (rear). Rear grip less at 34psi than 35psi.

For the final run, adjusted pressures to 37Front / 36 Rear and car felt good.
All 4 run times were within 0.3 seconds.

But you'll need to somehow turn this all into cold tire pressures so that you can get setup for your first run. You may find that your cold tire pressures are not evenly less than your hot pressures and that can be course dependent.

I like Woody's approach!

Nick325xiT 5spd
05-03-2009, 08:49 AM
Longacre gauges are excellent. I also get accurate results from the cheap pencils. For some reason, the moderately prices in between gauges have never worked out well for me.

forrealb50
05-03-2009, 06:25 PM
After all this I ended up running late and had no time to add any air. The car still ran great and didn't notice any issues with the OEM pressure recommendations.

What helps a lot is having the DTC on this time around.