Pressure (cold tire)

No, it was your wife's fault

Originally Posted by Snipe656

No, it was your wife's fault

Your cold snipe.

The important point of this is that she "noticed" the warning light and "attempted" to look at the tire. In the world of women drivers that is freaking AWESOME.

So basically you are bragging about how well you trained her?

Originally Posted by Snipe656

So basically you are bragging about how well you trained her?

I'd pretty much say so. Much better than the time she drove my Yukon around with the emergency brake set. Had no idea what that little red light meant!

I've had the D for two years now and I think she has driven it all of one time so far.

Classic call:



my old girlfriend would never want to drive my D, for fear of doing something she would regret.....

A couple things to keep in mind for decreasing pressure. Like several mentioned, the gas molecules diffuse through the rubber. Also, pressure decreases with decreasing temp. There is another wierd behaivor to consider . The tire EXPANDS as it cools off and this further decreases the pressure. You have the same amount of molecules occupying a larger volume after rubber expansion. Ask me how i know this? Space shuttle tires decreased 0.8 psi for every degree of temp decrease. We watched the aluminum rim temps during cold flights. Coolest flight got down to -36F

The youtube audio is great. Sad to know it is actually real.

BB_cuda,
The company I work make the main engines for the shuttle!

Would the tire pressure decrease have more to due with the alumimun rims cooling off than the tire actually shrinking?

Regardless, we should always be checking tire pressure every month...

Originally Posted by BB_cuda

A couple things to keep in mind for decreasing pressure. Like several mentioned, the gas molecules diffuse through the rubber. Also, pressure decreases with decreasing temp. There is another wierd behaivor to consider . The tire EXPANDS as it cools off and this further decreases the pressure. You have the same amount of molecules occupying a larger volume after rubber expansion. Ask me how i know this? Space shuttle tires decreased 0.8 psi for every degree of temp decrease. We watched the aluminum rim temps during cold flights. Coolest flight got down to -36F

BB_cuda, you guys are running around 420 PSI nominal?

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


Cheers
D.

Originally Posted by johntube

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......

There are two methods of determining a low pressure situation and I think BMW has used both in the 3 series over time. One uses the abs sensors to detect a long term variation in rotational speed (ie longer than the difference when taking a corner) detecting a low pressure tire by it's increased RPM. This system would not detect low pressure in all the tires, since there would be no rotational difference large enough to trigger a warning.

So far as I know, all our d's are equipped with sensors in the wheels that communicate via wireless to a controller. This system actually reports pressure and temperature but BMW does not choose to let us see the temperature or individual pressures, only throwing a warning if a tire is more than 20% low. Bear in mind that it is important to reset or "init" the system after inflating all the tires correctly, because if is was reset before with other values, it will retain them.

Yes, A8540TDI, the Canadian D's have the ABS-based FTM (flat tire monitor) rather than TPMS. The system works well, though. I've had it report two problems accurately and timely. The first was a slow leak from a crack in my winter alloy wheel due to a large pot hole and the second was a fast leak due to picking up a box-cutter blade fragment in a summer tire. The slow leak triggered the warning at about 28-30 psi (I was running the high-speed 41/49 pressures, so about 10-12 below the set pressure). My fast leak was a bit more pressure lost, by the time I pulled over about a minute after the warning caption, I was down to about 15 psi.

As a matter of procedure, I check my tire pressure about bi-weekly, and reset my FTM system about once every month to keep it calibrated and responsive.

Regards
D.

With the available technology I fail to see why BMW would not choose to provide at least the individual tire pressure. In my car without i-drive, all I get is a summary alarm that any one of the tires may be low.

I once got another alarm, not sure what it was but I thought for sure it was low tire pressure, and I spent quite a few minutes walking around my car kicking my tires trying to figure out which one it was. Turned out it was some other alarm, I think my blinker light.

If I had the actual tire pressure readout (like I have in my Tahoe), I would have quickly deduced it wasn't a tire at all.

Even with the i-drive all you can tell is which of the 4 tires it may be.

Even our Honda minivan provided the actual tire pressures for each tire.

Considering BMW has it's roots as a brand catering to the enthusiast, I'm surprised at their reluctance to provide much useful information to the driver. All of the following would be easy to display on the iDrive screen:

Individual tire pressure and temperature
Oil pressure
Oil temperature
Water temperature
Boost pressure
Fuel consumption in gallons/hour

I would like to have that info to hand and wonder if there is an after market plugin to the OBD II port that provides. At our meet up, Flyingman mentioned a Garmin device that via Bluetooth, transmits some data to a navigation unit. I will check this out!

TDI,

The Garmon ECOROUTE, about $100 after buying the garmin GPS for $250-$400, gives you a nice gauge display where the user can select what they want displayed and where.

Some photos to consider: I get Cooling Water Temp, Boost Pressure, Charge Air Temp (after Turbo and intercooler I assume), air flow (in oz./sec), battery voltage, MPH and RPM, plus a few more like throttle position (I'm still not sure what that means). This info has helped me understand a bit more about our engines, for example sitting at indle in D vs in N, big difference in fuel consumption, same for with A/C on or off.

No Lube Oil Temp or Tranny Temp (which my Tahoe also has, hint,hint,hint)

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

Cheers
D.

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.

Originally Posted by DnA Diesel

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.

Originally Posted by Flyingman

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.

Originally Posted by Pasa-d

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.

<<<<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

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.

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).

Originally Posted by BB_cuda

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.

Originally Posted by DnA Diesel

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.

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

Cheers
D.

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!

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