# Pressure (cold tire)



## johntube (Sep 28, 2010)

Haven't checked the air pressure on my D in some time, so tonight, went into my unheated garage (38 F) with my compressor and air gauge in-hand to determine if she needed air.

To my surprise, the fronts had 32 psi and the rears had 34 psi......

* Lesson-learned:* Check pressure on cold tires once a month and fill to 38 psi front and 41 psi rear


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## Snipe656 (Oct 22, 2009)

Strange that your car did not prompt you. Mine seems to do it for the slightest drop in pressure and it is actually rather annoying.


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## Flyingman (Sep 13, 2009)

Mine never indicates low pressure, I do keep it rather high, 41-42psi, but only check it perhaps once every two months or so. We don't have any extreme temp changes.

I much rather have the actual tire pressure reading than just a summary alarm though, like my Chevy Tahoe.


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## johntube (Sep 28, 2010)

Car never prompted me, but the tire pressure was "equal" but low in all the tires. Doesn't the BMW system warn you based on the rotation of differences of the tires, versus the pressure in individual tires? I'll have to take a look at the manual today......:dunno:


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## Snipe656 (Oct 22, 2009)

johntube said:


> Car never prompted me, but the tire pressure was "equal" but low in all the tires. Doesn't the BMW system warn you based on the rotation of differences of the tires, versus the pressure in individual tires? I'll have to take a look at the manual today......:dunno:


I have had mine warn me for all tires before. Whether they were equally low or not is debatable.

I too wish the system alerted the actual or estimated pressures. I honestly ignore the system after our first winter with massive temp changes that kept setting the warning off. I am glad that after the software update I got that the system now at least tells me which tires are low. Before it just told me that a tire was low and was up to me to figure out which one or even if it was multiple ones.


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## bimmerdiesel (Jul 9, 2010)

johntube said:


> * Lesson-learned:* Check pressure on cold tires once a month and fill to 38 psi front and 41 psi rear


now reset TPMS. Alarm depends on what it has in its memory. Last time when you did a reset if all tires had low pressure then it will consider it as safe. Also I think you see warning when pressure is 15-20% lower of reset pressure.


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## Alpine300ZHP (Jan 31, 2007)

Flyingman said:


> Mine never indicates low pressure, I do keep it rather high, 41-42psi, but only check it perhaps once every two months or so. We don't have any extreme temp changes.
> 
> I much rather have the actual tire pressure reading than just a summary alarm though, like my Chevy Tahoe.


That 42-42 psi is high. I run 36-38 psi front and rear. That pressure got me almost 40k miles out of the tires on my 08 335i.


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## KeithS (Dec 30, 2001)

I just investigated this. As per goverment regulations TPMS alerts when you lose more than 25% of the pressure it was last reset at. Given that it transmits to the car pressure information in .1 PSI increments as well as temperature, it would be nice if the actual pressure was displayed (could be done with a simple no cost software change).

Of course for those of us with real winters, pressure will go down with decreasing temperatures. I have non sport and am currently running 34 fronts 38 rears. Overall I like the non sports ride (and smaller - less prone to damage wheels), but would have liked stiffer shocks. That little extra bounce, especially from the rear over bumps is a bit annoying. 

A bit early to tell for sure but looks like the rear tires are wearing 2X faster than the fronts. But with the torque of the d, not surprising.


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## Snipe656 (Oct 22, 2009)

My father n law runs nitrogen in his tires to try and avoid these pressure losses with outside temp changes. He claims it really works, I still am hesitant in his findings. I'd happily switch my BMW to nitrogen though if it in fact really does work just to avoid the TPMS alerts.


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## swood02 (Nov 30, 2010)

Inside door jam on my 2011 335d M-sport says 35 lb. front, 42 lb. rear.


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## Scooner8 (Mar 20, 2011)

2011 335d with sport pkg: Inside my door jam say: 42 lb. front & 51 lb rear if going over 100 mph. I usually keep it 10% off the "max". 51lb is ridiculously high but I'm assuming it's because of our massive torque on that rear tire.


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## KeithS (Dec 30, 2001)

Scooner8 said:


> 2011 335d with sport pkg: Inside my door jam say: 42 lb. front & 51 lb rear if going over 100 mph. I usually keep it 10% off the "max". 51lb is ridiculously high but I'm assuming it's because of our massive torque on that rear tire.


Think the over 100 MPH tire pressures is determined by lawyers. They want to make sure we do not have a tire failure at high speed. Once your rolling at 100 MPH+ torque really has nothing to do with it.


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## Flyingman (Sep 13, 2009)

Snipe656 said:


> My father n law runs nitrogen in his tires to try and avoid these pressure losses with outside temp changes. He claims it really works, I still am hesitant in his findings. I'd happily switch my BMW to nitrogen though if it in fact really does work just to avoid the TPMS alerts.


Nitrogen as per Boyles Law!:rofl:

I'm not going to pay for something that is essentially free. Isn't air already like 78% Nitrogen?:dunno:

It has larger molecules so won't leak, doesn't expand/contract as much with temperature variations. Won't "oxidze" the inside of your tires.

Give me a break!:jack:


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## Snipe656 (Oct 22, 2009)

I am right there with you but he swore up and down he has had zero tire pressure alerts since doing it two years ago. Maybe his sensors are broken


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## johntube (Sep 28, 2010)

It would seem that checking your tires more often makes more sense than putting Nitrogen in them. For the $$, doesn't make any sense, to me at least.

For tire pressure, I would aim for higher pressures in the winter to avoid rim damage.


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## Snipe656 (Oct 22, 2009)

johntube said:


> It would seem that checking your tires more often makes more sense than putting Nitrogen in them. For the $$, doesn't make any sense, to me at least.
> 
> For tire pressure, I would aim for higher pressures in the winter to avoid rim damage.


If the nitrogen truly does not bleed down, which I honestly still am having a hard time believing. Then it boils down to how much your time is worth to you. I'd personally pay the $50 or so it costs at the local tire place if it meant avoiding dealing with topping off tires with every major temperature change. It also means I'd probably trust the tire sensor alarm more because quite frankly as much as it has gone off in the past it is not something I get worried about now days when it goes off.


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## slickdevil08 (Nov 4, 2011)

Just put air back in it. I had.nitro. In my first car and was always having to mess around with the.presures because.it takes longer to warm up

Sent from my SPH-D700 using Bimmer


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## KeithS (Dec 30, 2001)

With temperature changes and regular "air" your should not be dropping 25% in pressure that would trip the TPMS. Even with nitrogen you should be manually checking your tire pressures. The one advantage I can see with nitrogen is protecting the TPMS sensors themselves. As it should have 0 moisture in it, would prevent any corrosion that may happen to the TPMS electronics.


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## Flyingman (Sep 13, 2009)

Read this. Be sure and read the comments below as well.

http://www.popularmechanics.com/cars/how-to/repair-questions/4302788

And this:

http://news.consumerreports.org/cars/2007/10/tires-nitrogen-.html


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## Snipe656 (Oct 22, 2009)

KeithS said:


> With temperature changes and regular "air" your should not be dropping 25% in pressure that would trip the TPMS. Even with nitrogen you should be manually checking your tire pressures. The one advantage I can see with nitrogen is protecting the TPMS sensors themselves. As it should have 0 moisture in it, would prevent any corrosion that may happen to the TPMS electronics.


It happened the past two winters in my car. By temp changes I mean fluctuations. Like one week in say the 70s then next freezing then next in the 70s and so on. The first winter I had this car the weather fluctuated really bad. The second winter not so much and this one is looking to be even more mild.

But again I do not believe nitrogen works past this.


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## DnA Diesel (Jul 31, 2010)

Flyingman, does the manual describe what the "Engine Load" actually is? (% max torque?)

Cheers
D.


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## Pasa-d (May 7, 2011)

I use an aftermarket Smartire system in my truck and have had it for several years. It was previously installed in the car the BMW replaced.

Not only does it give you individual pressure readings, it also measures temperature and corrects the pressure readings accordingly. From my experience with many types and brands of tires, a high quality tire in good condition will lose only about 1 psi per month. When I saw pressure going down more than that rate in a tire I'd take a close look at it and generally find a thin nail or staple deeply embedded in the tread somewhere, often barely visible.

One of the cool things to do with it was to watch tire temperature and pressure increase through the course of track session.


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## Flyingman (Sep 13, 2009)

DnA Diesel said:


> Flyingman, does the manual describe what the "Engine Load" actually is? (% max torque?)
> 
> Cheers
> D.


I don't think the manual describes this output, but I understand it is the turbo boost. I don't see how they would know "torque" unless it is interporlated from other data.

It measures air flow which is a function of the turbo, which in turn can be interpolated as engine load, and thus torque.

But again I am reaching on my understanding.


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## Pasa-d (May 7, 2011)

Flyingman said:


> I don't see how they would know "torque" unless it is interporlated from other data.


They absolutely know torque, at least what the typical engine does without modifications. They develop the torque curve of the engine on a dyno under a range of all parameters and store this in the ECU. When you push on the gas pedal you are actually giving a "torque request" which the ECU then tries to fulfill given all the parameters it can adjust. This is important because the engine and transmission talk to each other and the transmission requests torque modifications, usually a torque decrease, when it shifts so as to make the shifts smoother and easier on the parts. It is also part of the emissions, ABS, cruise control and other functions.


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## Flyingman (Sep 13, 2009)

Pasa-d said:


> They absolutely know torque, at least what the typical engine does without modifications. They develop the torque curve of the engine on a dyno under a range of all parameters and store this in the ECU. When you push on the gas pedal you are actually giving a "torque request" which the ECU then tries to fulfill given all the parameters it can adjust. This is important because the engine and transmission talk to each other and the transmission requests torque modifications, usually a torque decrease, when it shifts so as to make the shifts smoother and easier on the parts. It is also part of the emissions, ABS, cruise control and other functions.


Correct, but "Torque" is not actually being measured, it is a calculation based on other inputs. That is what I was trying to say.

Now I'm wondering if the engine load is not the same as torque, as it is quite high when you are sitting at idel in drive and then when you accelerate it quickly goes to the top of the scale.


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## BB_cuda (Nov 8, 2011)

<<<<BB_cuda, you guys are running around 420 PSI nominal?

-36ºF doesn't seem that cool...high tropospheric flight is around -70ºF.>>>>

We built thermal models of the whole orbiter so the tires and rims were driven by the local radiant temp within the MLG compartments. Nothing to do with the troposphere or stratosphere temp. This is vacuum of space at 213 nautical miles above the Earth. The rubber responds a little faster to cooling profile than the rim as its thermal mass is lower.

Concerning pressure. The tires would be loaded at 430 psia (at room temp=75F) about 60 days before flight. The tires would loose about .1 psi per day waiting to launch. Typically, about 6 to 9 psi would be lost by the time launch happens. Then the tire pressure vs temperature is somewhat set. Typical eqn was P= 0.8*T + 358. The last term would change depending on how long before the launch. It is called the y-intercept for those of you who remember algebra.

The -70F business pertains to when you are still flying in the atmosphere at the aircraft/spacecraft is being convectively cooled. When on-orbit, there isn't external convection, only radiant and conductive heat losses. Sorry, heat transfer is my game


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## BB_cuda (Nov 8, 2011)

Concerning the leak rate, it is specific to this giant tire on the shuttle. Its rate is probably high compared to a normal passenger car tire as we are talking about 425-430 psi here. The pressure difference between inside to outside will drive the rate of gas diffusion through the rubber. 40 -50 psi (car tire) is 1/8th the other so the rate is slower. I have observed 1 -2 psi per month on my truck tires. Haven't had the 335D but 3 months with fall weather so no steady temp to make conclusion from.


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## Flyingman (Sep 13, 2009)

I would say all my tires lose about 1-2psi a month. Sometimes one may lose a bit more.

I try to check tire pressure about once every two months or so.

With the Taho I can read it every time I drive it. Very convenient, (hint,hint).:thumbup:


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## DnA Diesel (Jul 31, 2010)

BB_cuda said:


> Concerning the leak rate, it is specific to this giant tire on the shuttle. Its rate is probably high compared to a normal passenger car tire as we are talking about 425-430 psi here. The pressure difference between inside to outside will drive the rate of gas diffusion through the rubber. 40 -50 psi (car tire) is 1/8th the other so the rate is slower. I have observed 1 -2 psi per month on my truck tires. Haven't had the 335D but 3 months with fall weather so no steady temp to make conclusion from.


Ah, not a bad guess then. I just ran your pressure loss rate against an assumed 65ºF baseline, dropped it to Rankine to go absolute, then divided by 0.8psi/ºF/R to backwards calculate the ~420-ish PSI baseline pressure.

Yeah, I wasn't quite sure where your statement about -36ºF was relative to, between convective effects during tropo/strato/mesospheric transition, or exospheric ops where its going to stabilize with conductive/airframe and (negative) radiation. I was using -70ºF as a tropopause avg temp, although the STS went through that region pretty quickly. Had a friend who did interesting air-breathing ops and his rig had some pretty cool tires running some fairly high pressures and interesting alloyed compounds to keep temperatures as well as nitrogen diffusion low.

Good stuff...well less recalling the emotional scars from repeated solution of dU = TdS - pdV during Thermo classes... 

Cheers
D.


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## Flyingman (Sep 13, 2009)

DnA Diesel said:


> Ah, not a bad guess then. I just ran your pressure loss rate against an assumed 65ºF baseline, dropped it to Rankine to go absolute, then divided by 0.8psi/ºF/R to backwards calculate the ~420-ish PSI baseline pressure.
> 
> Yeah, I wasn't quite sure where your statement about -36ºF was relative to, between convective effects during tropo/strato/mesospheric transition, or tropospheric ops where its going to stabilize with conductive/airframe and (negative) radiation. I was using -70ºF as a tropopause avg temp, although the STS went through that region pretty quickly. Had a friend who did interesting air-breathing ops and his rig had some pretty cool tires running some fairly high pressures and interesting alloyed compounds to keep temperatures as well as nitrogen diffusion low.
> 
> ...


You'd think that for the price of the shuttle they would have auto air refill system! Trucks and busses in South America have that!:eeps:

Heck, for what it costs they should have RFTs all around!:loco:


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## A8540TDI (Jan 2, 2011)

Well, if they put RFT's on the shuttle they may need an additional million pounds of thrust to get into orbit with all the extra weight!!!


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## johntube (Sep 28, 2010)

Too bad there are no more shuttle launches, my company would be more than happy to provide new rocket engines to push "run-flats" into orbit.....


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