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      08-23-2012, 09:31 PM   #1
Rodgers
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M3 Supplemental Manual -Tire Pressure

This is a long shot but would anyone know why BMW specifies tire pressure this high for speeds above 100 MPH? A BMW Chassis engineer would be preferred

M3 Supplemental Manual
35 / 36 sub 100 MPH and 44 / 46 greater than 100 mph

Let it be clear, this inquiry is just that, an inquiry into this one particular issue and not look-at-what-tire-pressure-you-are-running-cuz-its-awesome. I have read all of those threads and don't want one here.

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      08-23-2012, 09:34 PM   #2
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http://www.tirerack.com/tires/tirete....jsp?techid=72

"Because of the weight they bear, pneumatic tires' sidewalls bulge and their treads flatten as they roll into contact with the road. This results in dimensional difference between the tire's "unloaded" radius (i.e., between the center of the axle and the top of the tire) and its "loaded" radius (between the center of the axle and the road). The engineer's call the difference between the two radii "deflection." Increasing vehicle speed will cause the tires to deflect quicker and increasing vehicle load will cause the tires to deflect farther (if tire pressure isn't increased)."
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      08-23-2012, 10:26 PM   #3
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Thanks for the info. Very informative, this also explains the high load PS2's (dealing with deformation under various loads). I wonder if they were intended to be run at "nominal aka lower" pressure at these speeds, or if the chassis was designed for optimal performance at these pressures. I am basically looking for a good track pressure to run my tires at.

It would appear that holding the tires profile and shape under load is the goal here, and interestingly enough may not negatively affect the tread profile (surface contact patch at rest) as much I thought. Of course this pure speculation on my end.

Again, thank you.
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      08-23-2012, 11:56 PM   #4
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Quote:
Originally Posted by m3alabama View Post
Your tires will also wear much quicker if you have low pressures but drive 100 plus mph frequently. The heat that goes with the substantial increase in deformation really eats the tread away. Plus the deformation KILLS gas mileage.

I settle on 40psi on all tires, all the time. I drive mostly from city to about 80.
Correct, that constant deflection is not symmetrical in the sense of energy exchange. Hysteresis in the process means the tire heats with the working (deflection/distortion) beyond its ability to shed heat and the greater the amount and the faster the cycle (rotation) the more heat buildup the tire has to endure.
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      08-24-2012, 02:54 AM   #5
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Interestingly its not Michelin that sets the tyre pressures but BMW themselves. It certainly appears that they choose a slightly lowish pressure for everyday driving to aid comfort and ride quality - add five lbs to the std pressures to feel a distinct improvement in turn-in response during fast direction change.
High pressures are recommended for high speeds to reduce the tendency for the center part of the tyre tread to balloon out.
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      08-24-2012, 01:11 PM   #6
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The problem is that ideal tyre pressure for ride comfort, ultimate grip and turn in response are all different. Your tyre pressures for track usage will not necessarily be ideal for my road usage as I prefer a far tighter response to steering input over ultimate grip.
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      08-24-2012, 02:52 PM   #7
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Interesting thread.

Can't recall the door jamb pressures on my E92, but on my ZHP E46 I ran different front and rear (38/40 IIRC).

Are you guys saying that M3 needs a little increase up front to give a bit more grip?

I would guess this makes the car more neutral?

Bear in mind the only thing I've tracked is a play station. My car is my DD.
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      08-24-2012, 03:17 PM   #8
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I have kept myself busy with calculating tyre-pressure since 2008, with use of the formula that the tyre-makers also use, and got much to know about tyres .
You probably have low aspect ratio tires ( fi 235/40R18 the 40) and for those tires the maximum load that is given is to high or it needs a higher pressure to carry that load. This leads to more deflection of the tyre than it can stand at higher speed.
The tyre-makers know that too but would have to downgrade many tyres in their maximum load when they keep the reference-pressure the same.
Second is that you probably have reinforced/XL/Extraload tires wich referencepressure is 42 psi ( pressure needed for maximum load up to 100m/160km/h, for higher speed there is a system of highening up that ref.press.).
So for this higher speed the tires need that high pressure for savety of them.
Gripp and comfort are totaly gone then, but savety first.
The advice pressures are given for the Gross axle weight ratings , up to wich you wont load the car when only driver, so for that situation you can go lower.
Give me the maximum load and tyre type ( standard load or XL) and the GAWRs front and behind and I can give you a balanced advice.
Beter would be empty weight of car and motor in front or back, and how you load it in every day practice, so I can give you an advice for normal use.

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      08-24-2012, 05:28 PM   #9
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Quote:
Originally Posted by MMME30W View Post
Are you guys saying that M3 needs a little increase up front to give a bit more grip?
I would guess this makes the car more neutral?
A graph of psi against grip will be a kind of semi circle with peak grip at say x psi.
A graph of psi against turn in response would be a similar graph shape but with the peak at say X+5 psi.
So you have to choose what you want - for a DD you would probably feel the benefit of a sharper turn in more than you would miss a marginally lower level of ultimate tyre grip.
Best is to try adjusting the pressures and see what you think.
I run recommended plus about 5 psi front and rear and it makes the car feel more "immediate" especially when making fast steering inputs.
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      08-24-2012, 05:50 PM   #10
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Quote:
Originally Posted by SenorFunkyPants View Post
A graph of psi against grip will be a kind of semi circle with peak grip at say x psi.
A graph of psi against turn in response would be a similar graph shape but with the peak at say X+5 psi.
So you have to choose what you want - for a DD you would probably feel the benefit of a sharper turn in more than you would miss a marginally lower level of ultimate tyre grip.
Best is to try adjusting the pressures and see what you think.
I run recommended plus about 5 psi front and rear and it makes the car feel more "immediate" especially when making fast steering inputs.
Ok, thanks, will give that a shot.
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      08-24-2012, 07:12 PM   #11
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Seems like many people are running psi close to 40+. Is there any danger in this? Seems quite a bit higher than mfgr recommended pressure.
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      08-24-2012, 11:20 PM   #12
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Quote:
Originally Posted by SenorFunkyPants View Post
A graph of psi against grip will be a kind of semi circle with peak grip at say x psi.
A graph of psi against turn in response would be a similar graph shape but with the peak at say X+5 psi.
.
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      08-25-2012, 02:10 PM   #13
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Quote:
Originally Posted by gatorfast View Post
Seems like many people are running psi close to 40+. Is there any danger in this? Seems quite a bit higher than mfgr recommended pressure.
The recommended pressures are from the car manufacturer. The maximum safe pressure for the tire is determined by the tire maker.

The OEM PS2's have a max pressure of 51 PSI. The maximum safe pressure is stamped on the tire, and is determined by the tire manufacturers. This is NOT a recommendation, but it tells you the tire does not have any abnormal risk of failure up to that pressure.

From a practical standpoint, if you notice your tires wearing out prematurely in the center before they wear out on the edges, your pressure may be too high.
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      08-25-2012, 03:07 PM   #14
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Just looked, my 2009 E92 with optional 19" wheels states on the door jamb 33/35 psi, front/rear.
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      08-25-2012, 03:49 PM   #15
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Let me rephrase - I wasn't referring to danger in terms of the tire exploding but more in terms of lack of grip or increased/uneven tire wear.

Basically, for driving on the street why would one stray from the manufacturer recommended pressure?
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      08-26-2012, 06:42 AM   #16
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For better performance. The mfr recommendation is a based on a compromise of performance, wear and comfort for a mixture of highway and city use. A one-size fits all uses and all drivers recommendation.

On my car, I have run stock pressures for about 15k miles and notice the outside edges have more wear than the centers. Probably 5% more pressure would be good for my overall balance of driving.

Some may want to vary depending on the use, such as for track days or long highway trips or extended periods in town on rough roads.
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      08-26-2012, 02:36 PM   #17
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Quote:
Originally Posted by MMME30W View Post
Just looked, my 2009 E92 with optional 19" wheels states on the door jamb 33/35 psi, front/rear.
Just came back from setting car to 35.5 front / 37.5 rear. Will see how it feels.

Wow both front and rear were around 30 PSI, not good. Must get back to regular monthly check, been kinda busy and all moving this month.
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      10-14-2013, 02:58 PM   #18
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Sorry to resurrect an ancient thread, but I just became aware that BMW has revised their recommended pressures. The category of "Exceeding 100 MPH with 4 passengers" has been removed, so now it's just sub-100 with a full load (4 passengers and cargo, or 5 passengers and no cargo for the E90), and above 100 with a full load. That last category has significantly different recommended pressures, though there are other changes for the E92 and E93 as well.

These new values appear in an edition of the M3 Supplement that Candide13 posted (link to thread), which came with his M3 delivered in November 2011. My own Supplement from my car delivered in May 2011 has the older values. And of course, in some cases these numbers are different from what appears on actual doorjamb stickers; the sticker on my E92 shows the values BMW recommends for the E90.

Anyway, pic below with changes highlighted. Thought some of you might be interested.

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      10-15-2013, 08:26 PM   #19
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I'd have to check the manual, but if I am not mistaken mine says something like 36/39 and 41/44 ('12 E92 and came with Pirelli P-Zero). I am running 39/41 and rear tires show excessive wear at the center. With 41psi I don't think this should have happened. Replacing them this weekend by Michelin PS2. Car has now 8200 miles.
Front tires look better as far as thread, but are cupped and make a ton of noise.
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      10-16-2013, 07:07 AM   #20
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Quote:
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I'd have to check the manual, but if I am not mistaken mine says something like 36/39 and 41/44 ('12 E92 and came with Pirelli P-Zero). I am running 39/41 and rear tires show excessive wear at the center. With 41psi I don't think this should have happened. Replacing them this weekend by Michelin PS2. Car has now 8200 miles.
Front tires look better as far as thread, but are cupped and make a ton of noise.
You have excessive wear in the center because your tires are over inflated! Unless they changed the manual/doorjam in 2012, your pressure should be somewhere in the neighborhood of 35psi all around.
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      10-16-2013, 08:01 AM   #21
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I'm still trying to figure out what pressure to use for my aftermarket wheels. I know it's supposed to be a bit lower than mfg. For OEM wheels, I always ran +2 psi on top of what mfg recommends, and the car always felt great.
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      10-16-2013, 12:12 PM   #22
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Quote:
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I'm still trying to figure out what pressure to use for my aftermarket wheels. I know it's supposed to be a bit lower than mfg. For OEM wheels, I always ran +2 psi on top of what mfg recommends, and the car always felt great.

Do you really think BMW hasn't done a shitload of testing to find the perfect pressures? Run the factory spec. The butt dyno is not reliable.
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