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I'm guessing that the TPMS measures in bar, not psi. Thus, when it converts from bar to psi within the number of decimal places of precision it has available, it never shows certain psi values. You can see the problem in the below conversion chart:
1.9 bar = 27.55722 psi
2.0 bar = 29.0076 psi
2.1 bar = 30.45798 psi
2.2 bar = 31.90836 psi
2.3 bar = 33.35874 psi
2.4 bar = 34.80912 psi
2.5 bar = 36.2595 psi
2.6 bar = 37.70988 psi
For when it measures 2.2 and 2.4 bar, respectively, it likely rounds up to 32 and 35 psi.
Astute... I can grok this. For me the TPMS is good enough... I just use a gauge to bleed pressure.
Well went to the dealer and left the car there on my trailer.
They will have to take the wheel off on the trailer itself since it is to sketchy to take the car off the trailer with no tire.
The only place they are able to get this rim is in Germany so it looks like the car might be there for a few days if the current wheel is bent in any way, not sure yet.
Overall they seemed pretty shocked by it, but funny enough the 2 front tires valve stems were also able to turn by hand (a bit tighter though) so not sure what to say since they did not change those, but I did just buy the car from them 2 month ago with only 200 miles so maybe this is something that should be checked as a CPO car, again not sure.
If there is one thing I would recommend at this point to anyone who tracks a GT3 or RS is to check the valve stems regularly to see if they are tight enough by trying to turn the nut by hand because it does not make sense that all 4 of my valve stems were loose unless there is some flaw that slowly loosens them after each track sessions because of the speed we are driving these cars.
You are very lucky to have kept the car relatively under control.
By mentioning the bent rims, I had previously gotten bent rims in various other cars while driving around my city- not just on the track. I'd have these annoying leaks. We see it in BMW, Audi, Porsche and McLarens...so basically all my dealerships have dealt with pothole contact creating oval rims. Occasionally a tire manufacturer will F up a mold and their tires are oval.
But the biggest thing with these tires (on a track) is that if there is anything causing the bead to unseat, you can rip the tire right off when you pull the car back onto the black stuff from the back side of a gator curb...there is a tiny chance you can catch the sidewall against the gator and pinch it just like getting something caught in a zipper.
I drove once with tire pressure too low. I could feel the rear floating a little, and i am sure i would have lost a tire if i pushed it. Sounds to me like a tire pressure issue, tbh.
Seriously... Will be part of my routine now when I check my tire pressures because this time I was lucky, if it happened at a higher speed turn it could of been really bad.
The TPMS is probably +/- 1 PSI compared to a gauge because I know that there are certain values the TPMS never seems to show on my car... I have never seen 31 or 34 psi on the TPMS... Only 30/32 and 33/35
the reasons for this is that the TPM measures in bar so they measure 2.0 2.1 2.2 etc
I think that internally they round to the nearest 1 decimal prior to doing the conversion to psi which prevents certain values from being shown
math sample:
1 14.5037738
1.7 25
1.8 26 (where did 27 go)
1.9 28
2.0 29
2.1 30 (where did 31 go)
2.2 32
2.3 33 (where did 34 go)
2.4 35
2.5 36 (where did 37 go)
2.6 38
Seriously... Will be part of my routine now when I check my tire pressures because this time I was lucky, if it happened at a higher speed turn it could of been really bad.
Just got a call from the dealer and they are going to repair the rim on their dime and asked if I pay for the tire which seems fine since the next big track will be Big Willow and will probably want new tires for it anyhow.
Here is the crazy part... The service manager decided to check all the valves on their GT cars they have in stock that they knew were tracked and found ALL of them to be lose and could be turned by hand so now moving forward they will be checking them on all there pre-owned GT cars.
The valve needs to be tighten down to 4NM so looks like I will be buying a new tool to check them on track days and I highly recommended anyone who tracks their Porsche even non GT cars should be checking them regularly as well.
Let me also clarify... It is the nut around the bottom of valve stem which holds the TPM in place on the inside of the wheel.
This nut does not actually have anything to do with the valve stem itself but when loose creates a leak around it.
I think it would be wise for all of us to check them and report back. Thanks for the update
Yes, would be curious to know as well and if this helps other people in saving there cars from a really bad situation, that would be a really good thing.
The valve needs to be tighten down to 4NM so looks like I will be buying a new tool to check them on track days and I highly recommended anyone who tracks their Porsche even non GT cars should be checking them regularly as well.
Let me also clarify... It is the nut around the bottom of valve stem which holds the TPM in place on the inside of the wheel.
This nut does not actually have anything to do with the valve stem itself but when loose creates a leak around it.
I own 7 Porsches, and I have discovered that at least couple of my Porsches over past 5 years had occasional air leaks, and the cause in most cases was a loose bottom nut and the strange part all of the looses ones were on the rear wheels. Perhaps too many launch control starts! So I decided to buy myself a piece of mind apply a small drop of LOCTITE 222MS (designed for Low Strength, Small Screws application) then retighten the nut back between (4 to 6 N.m or 3 to 5 ft.lb). Never had an issue of them coming loose again after I started doing so. It is just an insurance to keep the preload on that nut regardless of vibration, temperature changes, or extremely low tire pressures, etc.
so you are not alone or the only one experiencing this. But as others mentioned bent wheels, are probably the number one cause for tires deflating quickly on the track, but there is no harm from getting the extra insurance from air leaking past the valve stem. Good luck. Mark
My friends tire came off on a warm up lap at Thunderhill on the 2nd turn during the first session. Tires were newly mounted so we guessed it might be too much tire lube.
Also, these taller wheels don't have much side wall so it seems the wheel are more susceptible to bending.
I was following a GT3 during the recent Rennlist day at BRP and thru the esses his tire came off the rim. Here is a video of it.
05-22-2017, 06:17 PM
