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Wednesday, October 01, 2014

Doug's Domain

Doug Vetter, ATP/CFI

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Wednesday, December 20, 2006

CSL Replica Valve Stem Issue Followup

I brought the wheels to my technician yesterday and picked them up this morning. The good news is that my tech was able to tighten the stems by simply deflating the tire and breaking the outside bead only, and doing so was easy enough because the mounting lubricant hadn't completely bonded the tire to the wheel. Since the tires weren't shifted on the wheel and we were able to use the original stems he didn't have to rebalance the tires.

Upon closer inspection, the stems still move around a bit, but they don't leak. This proves my assertion that there is a "sweet spot" of sorts, achieved at a specific torque, that is tight enough to prevent leaks, but loose enough not to split the o-ring. I still do not know what the "official" torque is, and I don't think I ever will know, apparently because the manufacturer assumes this is too trivial an issue to warrant any concern.

The assumption is that installers will just "know" how to install them properly, and if not, it's the installer's fault. That attitude is strangely reminescent of programmers I've worked with that write bad code, don't document anything, and then just expect the customer to "deal with it" in the field. I just hope for the manufacturer's sake that they start providing the necessary information to new customers or they may find themselves the target of a lawsuit when someone loses their car (or worse) over this. Think it can't happen? Think Firestone and think again.

All I can do at this point is communicate what I've learned so this doesn't bite anyone else:

  1. If your car is not equipped with a tire pressure monitoring system (TPMS), check tire pressures daily for a few days following the installation of new wheels, tires, or valve stems and then once weekly thereafter as part of normal maintenance.
  2. If you have metal valve stems, try to move them around a bit. If you hear ANY air leaking, get them to a tire shop and have them tightened. Centrifugal force -- and in particular, wheel hop due to rapid acceleration or bumps in the road -- can push the valve stems out and cause a leak. The leak may be only a momentary thing, but the effects are obviously cummulative and can have disasterous consequences on a high performance car if left unchecked. A BMW ain't no minivan.
  3. There's nothing wrong with rubber stems. In fact, I prefer them because they naturally seal against the inside of the wheel (they're a press fit), while metal stems rely on proper torque and one or more o-rings to maintain the seal. The KISS principle applies here as well as anywhere.

And speaking of tire pressure, it's not entirely clear what pressure I should be running in these tires. The E36 came with 15, 16, and 17 inch wheels over its lifetime, and the inflation data for those sizes is available on the door frame. The only data I have for 18" wheels comes from my E46, which specifies 32 lbs front / 38 lbs rear, but that is hardly applicable to the E36 due to differences in vehicle weight and wheel / tire sizes. I'm considering 34/36 lbs to start, and plan to closely watch tire wear, particularly on the rear. I used to run the 16" tires at 32/34 and have noticed the Pilot Sport A/S has worn a lot more in the center of the rear tires. This could be due to the softer compound used in the center of the tire, overinflation, or perhaps a little of both.

Mileage: 137180