Fun with Tire Pressure

[gjhsu]

Hrmmm.... my tire pressure light has only ever gone on after a tire rotation, and a reset was all it took to clear. Have not noticed any actions in colder weather, but will report here if I see anything similar.

[schatzy62]

Originally Posted by gjhsu

Hrmmm.... my tire pressure light has only ever gone on after a tire rotation, and a reset was all it took to clear. Have not noticed any actions in colder weather, but will report here if I see anything similar.

Part of it depends on the outside temperature and the pressure you run your tires at.

The sensors on the Gen 2 cars will alarm at around at 27-28 PSI or about 18% lower than the recommended 33 PSI

and then the other factor is the outside temp. For every 10 degrees of temperature drop there will be an approximate 7-8 PSI drop in the tire.

So if you have them set at 33 PSI and have a 10 degree drop from the last time you checked you tire pressure then you MAY have an alarm.
If you have a 20 degree drop you should definitely see an alarm. I know this as i where i live we went from mid 40's over the weekend to the mid teen's yesterday and what did i see a TMPS warnign light come on. Adjusted the tires up to the recommended 33 PSI when cold and reset the sensors.

There is great rite up in the latest NAM Alliance Magazine about the TPMS systems in the Mini's

[Browser_23]

Originally Posted by schatzy62

For every 10 degrees of temperature drop there will be an approximate 7-8 PSI drop in the tire.

7-8 psi for every 10 degrees temp drop? I would think more like 1-2 psi. If it was as much as you say some areas of the country where there are 40 degrees swings in air temperature in a day would have a lot of flat tires every night. I have been as high as 90 degrees and as low as 50 since I got my car 5 weeks ago and never received a low pressure warning. if it was 8 psi then my 33 psi pressure would be 1 psi in the early morning or at 65 psi in the afternoon when it is 40 degrees warmer when the car has been sitting at work all day. Now I would believe 4-8 psi change over a 40 degree temp swing with cold tires.

[miniclubman]

Originally Posted by Browser_23

7-8 psi for every 10 degrees temp drop? I would think more like 1-2 psi. If it was as much as you say some areas of the country where there are 40 degrees swings in air temperature in a day would have a lot of flat tires every night. I have been as high as 90 degrees and as low as 50 since I got my car 5 weeks ago and never received a low pressure warning. if it was 8 psi then my 33 psi pressure would be 1 psi in the early morning or at 65 psi in the afternoon when it is 40 degrees warmer when the car has been sitting at work all day. Now I would believe 4-8 psi change over a 40 degree temp swing with cold tires.

Actually, a 10-degree change in the temperature would change the PSI by about 2%. For a tire with 35PSI, it would go to 34.3 PSI.
For any gas in a confined volume (like air in a tire), the temperature and pressure are directly proportional. A 2% change in temperature will result in a 2% change in pressure. Normal room temperature of about 70 degrees fahrenheit is 530 degrees above absolute zero. A 10 degree change would be 10/530, or a little less than 2%. Of course, if changing the temperature affected the tires ability to seal properly (which can happen, especially in cold weather), some air could leak out more quickly. But the change directly caused by a temperature drop would be very small. I think that the loss of tire pressure many see in cold weather relates more to the ability of the tire/rim seal to hold properly in cold weather, especially since rubber stiffens quite a bit in very cold weather.

[gjhsu]

Reminds me of The Simpsons....

'Lisa, in this house we follow the laws of THERMODYNAMICS!'

[redevil]

I have non-runflat winter tires size 205/50/16. What would be the recommended tire pressure for everyday driving?

Should I just go with the sticker on the door?

[corcoranwtnet]

Yes, but remember to check the tires. If you have 33 (cold) psi in the summer, it won't remain 33 (cold) psi in the winter. (I have a feeling the average daily temperature changes a lot more than 10 degrees between summer and winter where you live).

[blackjackmark]

If the TPM light comes on and then goes away, does that mean it's OK? It came on and stayed on once and I aired them up and reset it (first cold spell). Since then (probably 6 weeks ago) about 3 or 4 times I've been driving for a few minutes and it'll come on, then go off. I'm assuming that maybe as the tires warm up the pressure increases and then it negates the need for the warning?

[Arns]

Does lowering the itre pressure from 38 psi to 33-34 psi on Proxes 4's do any real harm to the tire wear? I shed the run flats becasue they were too harsh. The car sure rides a lot softer. I always wondered why tires are inflated according to the door frame scale versus the tire manufacturer recommnedation. Of course the tire store says inflate according to the door plate. That's when I hit my head.

[corcoranwtnet]

You inflate to the pressure on the car because the folks who design the car take into account the specifics of the car in spec'ing tire pressure. The tire manufacturer just give you maximum tire pressure so you won't blow it up...the optimum pressure for a given tire depends on the car on which it is installed.

[Little Bean]

So what is the proper tire pressure for an 09 clubman S with run flats?

[TheBigNewt]

Originally Posted by Little Bean

So what is the proper tire pressure for an 09 clubman S with run flats?

Look at the tire pressure plate on the driver's doorjamb. Mine says 38/38 with the original Dunlap RFs (although the manual says 36 as I recall, I never could figure that out). So I got new Bridgestone Potenza nonRFs and I still do 38/38. Same car, same weight. Same pressure. Also, I think tire pressure change with temperature is also affected by how much water vapor is in the tires. More water more change. That's supposedy why nitrogen results in more stable pressures (no water because it comes out of a tank). The nitrogen gas itself varies the same as room air with temperature.

[Little Bean]

Thanks. My door plate says the same but there is another door plate that shows lower numers like 32 front 34 rear or something like that. Boy were my tires low. Rides much faster now at 38 psi. But harder ride, I don't mind but the wife might. Too bad, it's my ride

[oldMGguy]

Evening, all. Just found this thread on run-flat tires. I've got a pair of "07 MCS, both with the factory run-flats. One flat so far, due to a very obvious roofing nail. I heard a funny sound, almost like a squeaky dry wheel bearing sound, for a few miles before the alarm triggered. Guessing that was the tire casing flexing on the rim. And yes, you really can't tell by looking at it that it's flat.

My understanding of the monitoring system is that the ABS computer is monitoring the comparative rotation speed of each tire. As the tire loses air, it loses a bit of diameter, and rotates faster than the other three. At some rotation rate differential, the alarm is triggered.

I've also discovered the run-flats (all 8) lose air much more frequently than traditional tires. I've had to top mine off monthly back up to the posted value.

[slinger688]

Originally Posted by oldMGguy

Evening, all. Just found this thread on run-flat tires. I've got a pair of "07 MCS, both with the factory run-flats. One flat so far, due to a very obvious roofing nail. I heard a funny sound, almost like a squeaky dry wheel bearing sound, for a few miles before the alarm triggered. Guessing that was the tire casing flexing on the rim. And yes, you really can't tell by looking at it that it's flat.

My understanding of the monitoring system is that the ABS computer is monitoring the comparative rotation speed of each tire. As the tire loses air, it loses a bit of diameter, and rotates faster than the other three. At some rotation rate differential, the alarm is triggered.

I've also discovered the run-flats (all 8) lose air much more frequently than traditional tires. I've had to top mine off monthly back up to the posted value.

When I had a front tire flat, I had the noise also coming from the front. I attributed it to the LSD.

07 MCS generally will use TMPS which is a monitoring of the actual pressure compared to the set point, not tire rotation. There was a cutover point where it was changed from tire rotation to TMPS but I am not sure of the exact point in time.

It does seem that the tire pressure loss is greater that normal tires. It could also be because of the TMPS in addition to the run flats.

[loud9266]

Originally Posted by DanF

When temperatures go down, so do tire pressures. You should get in the habit of checking your tires pressure at least every week.

Something I noticed when my car was new was the runflat tires did not hold pressure as well as a regular tire.

+1

[Chemdean]

I have run my tires at 33 psi and have not yet seen a warning indicator due to temp. fluctuations. When I rotated the tires, I did get a warning, but simply reset it and no problems.

[MinnieMama]

Originally Posted by oldMGguy

I've also discovered the run-flats (all 8) lose air much more frequently than traditional tires. I've had to top mine off monthly back up to the posted value.

I had a flat on the front drivers side the very first week I had Minnie. Obviously something very large punctured the center of the tire - the hole was huge. You could tell that it was much lower than the other tires looking at it (although it wasn't "flat") but driving it I hardly even noticed that anything had happened - I really like the run flats for that reason even if they are a little "hard".

Anyway, I've had the same experience with my run flats. I check them now once a month and almost always need to put in a little air - I run at 33psi and they are usually between 28-30 psi after one month.

[661CooperS]

We have run-flats and everytime we go to the beach our tire pressure light comes on. We have also had a problem with faulty valve stems.

We have replaced two tires in 30,000 miles due to the valve stems cracking at the base. Just wondering if anyone else has seen this.

[CarJon]

People tend to forget (or don't know) that tire pressure varies greatly with temperature. Especially for those of us up north the season has changed and the morning temps are significantly different from those of two months ago. If you don't regularly check and adjust pressure (add in cold, let out in summer) your tires won't be where they should be leading to poor handling, poor wear and other problems.
My light went red this am (practice what I preach and not what I do) - checked and they were a little low, added reset the TPMs and all is now set.

I don't know why MINI, etc. can't provide more info on tire pressures and which tire. My Chevy (ok it was a Corvette) gave me pressure by wheel and let me know specifically which one was low. I'm sure the info is in the computer somewhere they would just have to display it.

[BunsenH]

I only recently learned that "no news is not good news" regarding run flat tires. My light came on. I checked the tires and they were all at about 29 psi. After topping them off, I was surprised that the warning light was off without having to be reset.

Lesson learned: Check tires whenever gas is purchased.

[jbewley]

Bought my 09 MC in October last, went all winter and all summer with no low tire light. Last Saturady I get the light for the first time and there was no real temp swing that night. I check the pressure and three were exactly the same and on is "1" psi less. Added "1" psi, reset and all is well. I find that very odd.

While checking the manual I notice that they show an optional tool set with a compresser and a bottle of tire sealant. I don't remember seeing this when I built the car. Can this be purchased seperate or is it not available in the U.S.?

Jim

[flatlander_48]

Originally Posted by jbewley

Bought my 09 MC in October last, went all winter and all summer with no low tire light. Last Saturady I get the light for the first time and there was no real temp swing that night. I check the pressure and three were exactly the same and on is "1" psi less. Added "1" psi, reset and all is well. I find that very odd.

While checking the manual I notice that they show an optional tool set with a compresser and a bottle of tire sealant. I don't remember seeing this when I built the car. Can this be purchased seperate or is it not available in the U.S.?

Jim

It is called a Mobility Kit. I believe MINI has them as well as 3rd party folks on the web...

[miniclubman]

Originally Posted by CarJon

People tend to forget (or don't know) that tire pressure varies greatly with temperature.

Tire pressure varies directly with temperature. A 10-degree fahrenheit drop will cause only a small drop in pressure, as this 10-degree temperature drop is only about 2% of the total temperature above absolute zero.

[47leaf]

I have a Canadian spec 2007 Cooper and it has the same indirect TPMS system as the Gen1 cars. Direct monitoring is apparently not required in Canada at the moment although it is available on some US built cars.