Saturday, January 24, 2009
I was deep into an offramp during the commute home earlier this week and I felt the rear suddenly come loose. Judicious steering and brake inputs settled the car in short order but at the time it felt very strange...not anything like a typical oversteer condition I've experienced on the road or the track. The rear felt particularly unsettled throughout the recovery.
I initially suspected I hit an icy patch but the roads were dry and clear, if not a bit salty. On the next straight patch of road I performed a couple of quick lane-change maneuvers to see if I could duplicate the conditions but was unsuccessful. I drove the remainder of the way home thinking perhaps one of the (original) trailing arm bushings decided to give out at the ripe old age of 172K miles and that caused the arm to flop around a bit under load, affecting the rear alignment in the process. When I got home I walked around the car a bit but found nothing out of the ordinary aside from what I perceived to be a slightly greater bulge in the sidewall of the left rear tire. Given the effects of cold temperatures on tire pressures I didn't think much of it at the time.
This morning I decided to brave the 37 degree air and wash the car to rid it of the 60 pounds of salt it had collected over the past week. As I got around to cleaning the left rear rim I noticed that the bulge in the tire sidewall had grown. I knew full well what this meant and it didn't take me long to find the nail, which had been squarely driven into the tread of the tire about 2" from the sidewall. While some might interpret this as bad news I was actually relieved that the damage occurred where it did because it meant the tire could be salvaged with a plug patch and remain in service until the end of the season -- at which point I had plans to decommission the entire set anyway.
Of course, having discovered this on a Saturday at around Noon meant scrambling to find an open tire shop. To make a long story short, the third shop I tried agreed to patch the tire just before they closed for the day so I quickly got the car into the garage, put the rear up on jack stands, pulled the tire, and brought it over to the tire shop. Since I brought the tire to them "loose" and they didn't have to jack up the car or put it up on their lift they only charged me $15 for the fix. About an hour later I had managed to get the tire back on the car, all the tires brought up to my winter tire pressure of 33 pounds, and a short test drive completed successfully.
So what did I learn today?
Track junkies know full well that one way to increase an otherwise neutral car's tendency to oversteer is to decrease the tire pressure in the rear relative to the front, by a few PSI. My episode on the off ramp proves that this works. Incidentally, the effect of greater pressure in the rear is predictable -- less oversteer (or more understeer) -- and this is one of the main reasons why BMW now insists (to a fault, in my opinion) that tire pressure be as much as six PSI higher in the rear.
Mileage: 171600, Labor $15