ninjabones

I'm hoping somone here can explain a very strange issue I discoved with one of my wheels last week.

Mounted new tires (kumho ecsta MXs) about a month ago. Everything was fine over probably three hundred miles of nice country cruises and short errand runs. However, last weekend, I found an open parking lot and tossed the car around for 30 minutes or so. When I got home, I found that one of the tires was nearly flat.

I pulled the wheel (three piece BBS), inspected the tires (no nails, glass, etc), looked very carefully for any cracks in the wheel (found nothing), remounted it, and overinflated to about 45 lbs. After a week in the garage, no sign of a leak.

I then took the car out for a few cautious trips and no issues whatsoever. However, I took the car back to the empty parking lot again... tossed her around for about 5 minutes, lost 2 lbs in the same wheel, another five minutes, another 2 lbs, etc.

Looks like the wheel/tire is losing pressure only when pushed aggressively. My thoughts are one of the following is happening:

1. Tire did not seal correctly
2. Valve stem seal is bad
3. Seals on the three-piece wheels are bad
4. Crack in the wheel that opens only when under extreme stress

No way for me to really test these theories (e.g., soapy water along the seals, valve stem, etc), as there is no pressure loss under normal situations.

Let me know if any of you have any bright ideas on how to troubleshoot this (hopefully before VIR next month).

cabrio993

I vote for #3.

1,2 and 4 would make the tire loose air regardless if is rolling or stationary.

You can try inflating the tire to 50psi or close to its max inflation and see if you can force the leak even when stationary with some soapy water.

chris walrod

I had a similar situation -- ended up being the valve stem on a set of track wheels/tires. Wiggle the valve stem and it would leak a little -- wiggle it again to reseat it, the leak would stop.

CornerCarver

+1

Paul E. Dodd

I had the same thing under autocross conditions once, turned out to be the valve stem core. $5 for a valve stem kit at the local auto parts store and it was fixed.

Paul

ninjabones

I bet it's the valve stem.... The other three wheels have the same type valve stem. This wheel has a much shorter and different style valve stem.

How do you replace a valve stem? I'm assuming the tires have to be removed, in which case I'll just let my shop deal with it. Anyway, definitely worth trying before sending the wheels back to BBS for rebuild.

AOW162435

Well, only one in this case, but yes.

Andreas

avidtest

This issue happened to me more than once w/ different sets of Kumho V710's on different wheels (once on a brand spanking new set of wheels), but only in AX or track conditions - most recently at Buttonwillow. The last time it happened at BW, it turned out to be a fault with the tire (or from when it was mounted). There was a very small nick at the bead of the tire. It could only be seen when the tire was removed. It held pressure fine in most circumstances....until the tire was subjected to high lateral loads, diminishing pressure at each successive corner at increasing rates until finally, the remaining pressure would vanish suddenly and alarmingly.

I never found out the cause of the problem on the previous sets of v710's/wheels, but suspect it was the same issue.

ninjabones

This actually makes the most sense to me.... I can't figure out how a valve stem issue would only appear at high lateral loads. I asked Bill Dougherty on the phone and his first question was what type of tires was I running. He said that he'd seen this on occasion with certain Hoosier tires. Anyway, I made a mistake and had my local porsche dealer mount and balance the tires for me. I thought that this did not technically break my vow about never letting a porsche dealer touch my car (as I just dropped the wheels off for them to work on).

Marlon

Alternatively - take the whee/tire to you favorite tire shop.