Ever Wondered Just How Necessary PSM Might Be?
This is a great video, and it showcases just how quickly things happen in a racing Porsche… including bad things.
While making my completely ignominious Canadian Touring Car Challenge at Mont-Tremblant a month ago (more on that later) I noticed this gorgeous 993 GT3 Cup running in the Porsche-only IMSA racing series with which we were sharing the track. Quite the looker and although I would conservatively estimate that 20% of all club-racing Porsches use the Gulf color scheme, all the way down to 924s, this one looked really sharp.
This morning, I saw that Jeff Lacina of TrackGuys had posted this video and I immediately recognized the driver: Dr. Bob Seitz. Dr. Bob has won plenty of races and he’s no rookie; still, it’s instructive to see how quickly a rear-engined Porsche can turn around on you.
It’s also important to note that rear-engined cars without swing axles understeer by default. Early Volkswagens and Porsches earned a reputation for “snap oversteer”. This happened because as the car leaned over in the turn, the suspension would suddenly change the angle of the tires on the road, drastically reducing the grip. Since a car leans over on its suspension at a rate determined by spring rate, it was therefore possible for a driver to enter a corner at a set speed and then experience sudden oversteer as the car settled onto the springs and pulled the wheel out from under him. That’s genuine snap oversteer, as opposed to the “snap oversteer” you hear about nowadays, which equates to “I managed to be a bigger idiot than my idiot-proofed car could predict.”
Modern Porsches don’t have drastic camber change in corners. The front end is lightly loaded and as a result steering input at the front end tends to be followed by the rear end after a slight but discernible pause. At the true, genuine cornering limit of the car, that time lag can cause problems with correction. I think that’s what happened here, although only Dr. Bob knows for sure.
PSM and the other Porsche stability aids are designed to address this behavior, which is why it’s not a good idea to turn them off on-track just to be cool. Learn the car using the blinking light as a guide to problematic inputs before you throw caution to the wind. Of course, for older cars like the one shown below, there’s no PSM, so as I found out this past weekend, it’s useful to have a coach with you on-track, even if he sits in the back seat.
More by Jack Baruth
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That 993 is a 996 ;-} Although I doubt it has PSM. PSM would not have gotten the job done in any case as it would undoubtedly be turned off on race day (funny how race car drivers don't like stuff that hits the brakes for them.) A combination of PTV and PDCC would have probably prevented the need for that harsh steering input to begin with however. I love videos like this, I find it fascinating to try to sus out what initiated the "snap" (prior to the steering input obviously.)
My vote is oil but I have never driven a rear engine car at that speed. The engine sounded fabulous. Watching him suffer I was thinking, "Please don't self extricate - please don't self extricate." I've seen bad things happen when disoriented and injured people stagger around after a collision; even in the weeds. I hope he's ok. Good presence of mind to shut off the engine.