Benz-O-Rama

The SL felt pretty sloppy the other day, so when I got home I checked all the tire's pressure. I normally check once a month or so, so you can imagine how shocked I was when I found this:

RF - 12 lbs
LF - 23 lbs
RR - 24lbs
LR - 18 lbs


Now, I normally run 38lbs up front and 40lbs in the rear on these super low profile 20" tires.

For the life of me, I have no idea why the pressures were all over the map, and I'm certain I'm not leaking anywhere.

Anyway, I corrected the pressure and left her in the garage. I don't drive the car that often, but on Friday when I took her out, I felt a bad vibration up front. CRAP! Here we go, another bent rim, most likely caused by the super low tire pressure, right? Wrong.

I took all the wheels off today, full prepared to see bends. But what I found was that the RF inner sidewall had failed in two places. I'm guessing its from driving the car with super low air pressure. With these low profile tires, sometimes it's hard to recognize low pressure. The sidewall is so damn short anyway, plus it's very sturdy, so there's not much sagging.

Anyway, with the temps rapidly dropping, so will your air pressure if you park outside or your garage is not climate controlled. This is a reminder to all of you, to keep an eye on your pressure. Here's what happened to mine:



















The car looks so sad now, rolling on stock wheels. But oh man, how smooth the ride is...

V12Godspeed

Ouch..thats got to hurt man...Sorry to see that.

tgoss

Glad to see nothing negative other than those bubbles came out of that man! Stockers sure aren't as blingy but certainly don't look bad

*Edit: Just checked mine 28 on both fronts 30 on both rears.... looks like I'm a little low, thanks for the reminder Clay!

Yacht Master

TPMS, not working?

Benz-O-Rama

Yeah, the good news is, I'm in no hurry to replace it. It's only a couple hundred bucks, but since these wheels won't go back on the car until spring, I have time to dig for change in the couch.

This is when I wished I had a set of SL65 wheels laying around. These wheels and tires barely fill the fenders wells. Oh well, rolling on 18's with all-season tires makes it feel like a lexus, compared to the 20's.

Not equipped.

moosejaw

i had the complete inner sidewalls on both tires collapse on me while the car was in the driveway. The tires just deflated when it got cold. New tires come next week

Das Geld 2

Bull****, That's foul play.

No way your pressure would drop so sharp and unevenly all around.

RF - 12 lbs
LF - 23 lbs
RR - 24lbs
LR - 18 lbs

AMGPilot

Tires do not deflate "When it get's cold"...

they deflate due to leaks in the wheels, valves or small punctures in the tire

220S

Yeah, this puzzles me too. I've stored my P-cars for over a year in the past and never had pressure fluctuations, both cold and hot weather. I pump them up to 50 psi just to avoid flat spotting, but they always remain at 50 psi after sitting a long time (which is the whole point of pumping them up to 50 psi in the first place.)

The only time I ever had low pressure on any tire was due to one tire with a tiny puncture issue (had a very slow leak that I couldn't detect until 5 months)

MRAMG1

Then I guess you failed physics my friend.

My favorite equation for cars is

PV=NRT

So when T temperature drops, P, pressure MOST also drop as the V, Volume of the tire stays constant.

Most of the time this occurs from people NOT checking their tires frequently, read this as EVER. So back in March when they went for their first joy ride they get air in the tires. 8 months later they are low

Okay sorry guys, its the engineer me.

Back to business.

PS: NEVER trust your eyeballs for pressure guys, espically with LOW profile tires. Most people can NOT tell 20 psi from 40.

AMGPilot

While yes you can use that equation. it still doesn't support the claim of deflation, the tire will still have the same VOLUME of air. (in our case aprox 7.5 cubic feet @32psi and that will weigh 32 grams.

A 68F temp change will change the pressure only 22% not the 69% observed by the OP. 22% of 38 is 8.36psi and 22% of 40psi is 8.8psi

you would need ~ 212F change in temp to affect the pressures observed


Nice try mr. engineer

Yuille36

Thanks for the heads up, due to temperatures falling I need to recheck my air pressures in both cars.

MRAMG1

I never quoted his loss

I simply stated that your statement was incorrect, which it is.

Don't get your undies in a bunch over it my friend.

As far as his loss, it would probably be from lack of checking, as I stated above, valve stem leakage as they always do leak a Little, and of course the plain leakage through the rubber itself. AND YES the temps helped it to become lower to, as you pointed out above with your math.

turbodude

A good rule of thumb is every 10 degrees =1psi. so if you fill in 90 degree weather and the temp this morning is 40 then you lost 5 pounds. Unless you use nitrogen.

MRAMG1

Sorry my friend, but Nitrogen does not matter either for PV=NRT

No matter what gas you would use, helium, nitrogen, acetylene, air, CO2, argon, etc, etc. The pressure drop would be the same for all of them

Talking ONLY about temperature changes.

Different gases DO have different sizes, and as such would leak out at different rates over time.

tbone

My car didn't come with TPMS either. Like Clayton, I drove on low pressures, ruining my tire. I've since installed this Hella TPMS. It's great. It gives real-time individual tire pressures and individual tire temps. I have the gauge installed in the ashtray compartment.

http://www.tirerack.com/accessories/detail.jsp?ID=51

NCE500

My experience has shown that tires tend to lose about 1 pound of inflation for every 10 degree Fahrenheit drop in ambient temp.

I'm picky about tire pressure, had to top everything twice this week as temps dropped outside.

Clayton, for what it's worth I have found 28 front/29 rear cold to optimal on my SL with the stock size OEM rubber.

Benz-O-Rama

my gas flap says 26/F and 28/R for the OEM tires. I think that's a bit low for the speeds this car is capable of, even at stock HP. I run my 18s at 29/F and 32/R for a bit of peace of mind. I've been known to go at least 1 or 2 mph above the posted limit.

The car feels sooooooooo much smoother on the stock wheels. 20's look great, but they introduce so much road feedback into the cockpit.

06E55

Had this happen to me on the way to work a few weeks ago. Found out that the wheel did not have the TPM sensor installed!!!

220S

Like everybody says the rule of thumb is around 1 lb per 10 degree drop. And don't forget that proper tire pressure is based on the ambient temperature at any given time (which is what mfgs mean by "cold temp") So just measure the pressure before heating up tires on the road. Pretty obvious.

But these huge differences in each tire don't have anything to do with temp changes. Something else is going on. Like I said I've stored cars (with low profile tires) over a long time and tire pressure always remains even.

KLR CLS

Great reminder. I had the same thing happen to me earlier in the summer. Good thing falkens are cheap.

Benz-O-Rama

Agreed.

I'm pretty good about checking pressure, and I'm confident that there's no leaks in any of the tires. I think I checked the week of last the MIR rental. I wonder if some teenagers were having a bit of fun when the car was parked somewhere.

Roupin

Just got my TPMS warning up today too after a quick drive. I naturally run low psi in the rears but mine were down to 21 when the temps dropped today.

Your 211 is brown nosing your RR.

MRAMG1

Just a little reminder to everyone.

My S which sits in a heated garage at 68F, and ALWAYS has it tires at 35F and 30R, happen to spend the night outside.

Well, at 28F here in Pittsburgh, my TPMS went off like the fourth of July in the morning. The rears dipped down to 24PSI.

Guess what guys, that is a delta of ONLY 40F and it caused a 6PSI drop or a 20% change.

AS I said before, CHECK your tires during the cold months.

PS: The above posted had his math wrong, aka you have to use ALL of the correct inputs, aka proper measurements/values

Yuille36

I checked the air pressures in both ym E63 and C63 and they were both low the C63 TPMS were going off, but my E63 doesn't have them, and my rears were at 28 PSI. So this was a good thread to remind me to check them.