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Old 06-06-2017, 08:23 AM   #83 (permalink)
Bicycle Bob
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Appliance White - '93 Geo Metro 4-Dr. Auto
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Swing axles and rear steering.

Originally Posted by sidecar View Post
among the influencing factors was the swing axle of up to late 70's designs. This meant that the car tended to push on the now near sideways rear wheel, and by the attribute of the swing axle folding it under the car, reducing tyre contact patch and raising the rear of the car and therefore CG even higher, further degrading stability.

Should anything contact the most forward rear tyre at this instant causes a snap roll, even if the driver induces corrective steering it will matter not, the car is bluntly 'out of control'.

If one is lucky, the car will fully invert to traveling backwards, its most stable condition.

because I dont think there is any hope that the center of pressure or center of resistance is going to be anywhere near aft of the the CG, where it needs to be for stability.

I think the outrageous safety record of this car speaks for itself. I know these cars are still popular to this day, and I have owned a few myself. Im quite used to the arguments that somehow the data is wrong and these are cheap to run well 'engineered' cars. But a car that will kill you has no saving graces, its just lethal and unnecessary.
Another great sin of the swing axle is the considerable track change and consequent scrub as it goes over a bump. This can start a slide as the effective traction rises and falls. Mercedes persisted for a while with the low-pivot swing axle, using a common hinge point below the differential, but that complicated the half-shafts as much as other systems, and only reduced the problems.

The basic handling of a tail-heavy car can still be understeer, with chassis tuning. The most basic modification is to add a strong anti-roll bar at the front. BTW, doing that would move the jacking point forward, without affecting the cg. On the VW, the driver moves the cg forward when under way. My point was that anything light and fast should be stable even without input from the ground. Sometimes it drops away too fast, and sometimes it is covered in wet ice. The Tatra gained a tailfin to help, but not enough.

To re-design a car to normally run in reverse with rear steering, you don't change the geometry. If you reverse the caster to keep the wheels trailing, once you do get into a steady - state turn, it wants to increase, not straighten out. The only change needed is a steering damper, to control the short-term wandering.
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