yeswife

Hi all,
so everytime select the Tyre pressure option on the on-board computer, it dosent display anything. It just asks me if I want to "Run Flat Indicator active" Restart with OK"

After I restart it and check it again later after driving, it displays the same thing.. it wont display the all 4 wheel pressures or anything..

am i missing something here ?? in the instruction manual, it saids it should show the pressure on the multifunction display after a few minutes of driving.

help please ?

jcfay

that is totally odd. no run flat tires here in the US, and if you don't have run flat tires than you certainly shouldn't be seeing that menu item.

MJ50

maybe u have passive TPMS...

Mr Jones

Yeswife, we seem to have a different system here in Australia. Correct me if I'm wrong. Run flat in this case doesn't mean run flat tyres. I think it refers to if your tyre is going flat or losing air. I recently had some tyres fitted at a place MB recommended and the guy said the system measures wheel revolutions vs distance travelled. If your tyre pressure is down slightly the wheel will do slightly more revolutions over time, enough for the system to detect a change. It then prompts you to check the pressure. So it doesn't actually know what the pressure is, just that it is less than when you reset the Run Flat indicator. I had a slow leak in my front tyre and it prompted me to check.

Mr Red

Yep, I can confirm this. I had a screw in one of mine & it prompted me to check pressures.
I also talked to MB about tpms for a second set of wheels & they told me the system just measures revolutions not pressure monitoring.

Mr Jones

Okay, so is it only that way in Australia? Anybody?

Wobble64

used to be the same in Germany several years ago. Not sure whether they switched to in-wheel mounted sensors by now.
Nothing wrong with the tire diameter approach in my book. Cannot run out of battery, as the in-wheel sensors do (they transmit the reading wirelessly to a receiver in the car and hence need power from a battery)

jcfay

The "wheel diameter" hypothesis doesn't make sense to me. Even with a slightly deflated tire the diameter and distance traveled by the wheel doesn't change (the tire would have to slip on the wheel for that to happen) - right? Also, tire wear drops diameter and that's normal and that would also trip the sensor. I may be missing something here. Not that it really matters.

I am a fan of the TPMS system, however. It's nice to be able to keep my tires right where I want them since I've got low profile tires and not stock wheels.

Wobble64

jcfay,

it is not a hypothesis, but a proven method of determining tire pressure indirectly. Whenever a tire loses pressure, the tire will become flatter, i.e. the thickness of the rubber under the rim decreases. That in turn reduces the effective wheel radius, so the wheel needs to rotate faster in order to cover the same distance (you are right, the tires travel all the same distance on a straight line).

Quantitatively:
1. a standard C63 rear wheel has a diameter of 635.7mm (255/35/18, i.e. 18x25.4 + (255x0.35x2) = 635.7 mm). So it's rolling radius is 317.9 mm
2. When the rubber between rim and road reduces by 1/4, the new effective rolling radius is 295.5 mm (the rubber is now 66.9 i.o 89.3 mm)
3. So the effective rolling radius is 1-(295.5/317.9)=7% smaller and hence the wheel rotates 7% faster at the same speed than the other tires.

It works. However is not very quantitatively accurate, especially with very low profile tires.

jcfay

This makes no sense to me. The diameter of the tire hasn't changed. Again, not that it matters that I just might not be able to comprehend a proven principle. I think about it like the same patch of tire has to contact the road in either (full inflation or low inflation) case. But I'm probably missing something. I get how a smaller diameter/radius wheel/tire combo figures in, but here we're not talking about a change in those parameters.

Wobble64

Maybe this helps: when a tire is completely flat, the car rolls essentially on its rim. The effective wheel diameter approaches the diameter of the rim, which is smaller. So it needs to rotate more to cover the same distance.

Does that start to make sense?

jcfay

Thanks. I guess (can you tell I'm a scientist?). I think then you get significant tire deformation as well, but that obviously occurs to a lesser degree with underinflated tires as well, so perhaps that's the key. I'll take your word for it

Wobble64

Exactly. The lower the pressure the more the tire deforms/flattens. That is the principle used when there are no TPMS a sensors in the wheels. Since nowadays almost every car has ABS they all have very accurate rotation speed sensors, so they can just use that available signal to determine a tire leak indirectly.