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Thursday, August 21, 2014

Doug's Domain

Doug Vetter, ATP/CFI

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Saturday, October 11, 2008

Worn Rear Tires + Wet, Off-Camber Corner = Uncommanded Oversteer

If you've read my Long Term Review of the E36, you may recall the following quote:

"...it's just cheaper and easier to put four new tires on the car when the rears are spent -- even if the fronts appear to have some life left in them. This costs money, yes, but your greatest enemy in any modern rear-wheel-drive sports car is worn tires. Don't scrimp on tires, or you'll wind up in the weeds."

(Image: Depiction of loss of control in turn)Little did I know that I'd have the opportunity to prove that theory correct recently. A few weeks ago I was on the way to my brother's house, following the same route I'd used for years. It had rained earlier that morning but the roads were mostly dry at this point. I was certainly in no particular rush to get to my destination and I wasn't hot dogging it as I had my mind on other things. However, as I managed to navigate one of the last curves on the route all hell broke loose.

Just as I passed the apex of the left turn and started to take out the steering input to straighten the car I felt the rear start to rotate out to the right as the tires lost traction. My instincts and training on the skidpad at the BMW Performance Center kicked in instantly and once I realized a small amount of counter-steering wouldn't bring the rear back in line I added more. And more. And the rear kept rotating. Everything felt like it was happening in slow motion, but it didn't take long before I was looking out the passenger window at the direction the car was traveling down the center of the road. The steering was at full opposite lock as the car crossed the double yellow line going sideways and continued to rotate past 90 degrees. At this point I knew I couldn't save the turn so I used the only tool I had left at my disposal -- I hit the brakes. Normally this wouldn't be a particularly smart move but shortly thereafter I felt the rear shudder and rotation start to slow down as the car approached the 180 degree point. Even as the rotation slowed the car's momentum was still carrying it sideways a bit so I came to rest with the car parallel to the road, in the opposite lane about two feet off the road into some grass and mud.

I took a few seconds to let out a few choice words before I carefully applied a little throttle to make sure I wouldn't dig myself a hole in the mud and quickly made the k-turn necessary to get back on my way. And that's when it dawned on me -- as I saw some traffic approaching I realized that I had just dodged a bullet. Not only did I manage to avoid hitting a nearby telephone pole and a residential mailbox in the vicinity but I was extremely fortunate that there was no traffic in the opposite lane approaching the turn. If that were the case I would have most certainly hit another car, lost my baby, and likely been charged with careless driving. My insurance would have gone through the roof and I would have received nothing for the car...because I had finally taken collision coverage off the car less than a month earlier because (get this) the economics no longer made sense. Talk about a Marty McFly moment.

When I got to my brother's house I took a close look at the passenger's side of the car and found only a bit of mud on the right rear wheel which I promptly washed off.

So here are the lessons learned:

  1. This is clearly the result of worn rear tires, a slightly wet road, and an off-camber corner. Re-read the quote above. There's nothing wrong with trying to stretch a set of tires but it's important to be aware of the effects worn tires have on the dynamics of the car. BMWs are famous for their 50/50 weight distribution. This is normally a perk as the car remains balanced through turns, favoring neither understeer nor oversteer (particularly with a "square" wheel/tire configuration like I choose to run) but when the car is near the limit of traction in wet conditions worn rear tires can shift the tendency toward oversteer...which is exactly what happened in this case.
  2. This serves as a practical example of the difference between the early ASC-based traction control on the E36 and the DSC traction control with its integral yaw-rate sensor installed on the E46 and later model BMWs. Had I been driving the E46 under the same conditions it's quite likely that DSC would have stopped the rotation before it developed by independently controlling the rear brakes. All the E36's traction control does is reduce throttle if it senses a 2 MPH or greater differential in the RPM of the front and rear wheels, but as it's theoretically possible to have the front and rear wheels traveling at more or less the same speed while traction is lost, ASC won't prevent or arrest the yaw if it develops. I should point out that ASC is present on E46 and later vehicles, but it's only one component of the traction control system known as DSC.
  3. Road designers in this country need to buy a clue from Germany, who actually know how to drain roads properly without using 40 degrees of crown. Got me?
  4. Off-camber corners suck.

I had plans to swap these tires with the snow tires on December 1st as I did last year but as a result of this incident I'm now planning to swap on November 1st. Even if we don't expect to see any snow until December or January, average temperatures should be well within the optimal operating range for the winter tires.

Microfilter

I had to take the E46 out on an errand and happened to pass by my dealer so I picked up a Microfilter for the E36 and and later installed it. The only interesting thing to report about this is that I can now do it in about 5 minutes. The dealers charge 0.6 for this job because the official instructions from BMW advocate removal of the glove box, but even if none of the techs do it that way they'll still charge you book labor. 0.6 * $110 + tax is $71 saved by doing it myself. I also saved 10% on the part but that was largely offset by the 7% SST so I wound up paying $38 for it.

Maintenance Schedule Sanity Check

Mileage: 166983, Parts: $38, Labor Saved: $71