With our BMW 318is project now done—and a stint at VIRginia International Raceway in our bag—it was time to start having fun driving the car. Our first real shake-down trip came when, after displaying the car in our booth at the Carlisle Import show, we drove it through the mountains of…
Have we mentioned we really, really like this car?
You have a couple of times! But that is ok. Us mere spectators like it also!
It seems counter-intuitive that stiffening the rear sway bar would take the twitchiness out of the rear end. Is this a BMW thing? On most cars, stiffening the rear end would cause more oversteer (or less understeer), right?
losendos said:It seems counter-intuitive that stiffening the rear sway bar would take the twitchiness out of the rear end. Is this a BMW thing? On most cars, stiffening the rear end would cause more oversteer (or less understeer), right?
Right; increasing roll stiffness increases load on already loaded tire, which coefficient of friction has typically already peaked. However, BMW trailing arm geometry was never ideal, and 318is is not real heavy, so reducing suspension excursion for improved geometry may have helped more than increasing tire load hurt (with better camber).
Exactly - Carroll Smith and Fred Puhn's books are great if the car has a well-designed suspension in the first place. But as I learned when autocrossing an MGB some 20+ years ago, adding a stiffer front anti-roll bar REALLY reduced understeer. Why? The B's front end has a pretty hefty positive camber gain under roll, for gobs of safe, confidence-inspiring understeer. Reducing body roll reduced camber gain, which in this case was a good thing, as it meant the tire rolled onto its sidewall less than with the stock bar.
The "hot trick" back then -- legal in E Production road racing, at least -- was to move the inboard suspension mounting point UP and OUT, tilting the lower control arm so that body roll gave more NEGATIVE camber. The Butch Gilbert chassis I put a "good school and regional motor" in back in the day stuck very well in the front, compared to my old autocross MGB.
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