Swing axles etc

[C M Smith <cmsmith@xxxxxxxxxxxxxxx>]

Jacking of swing axles is a result of weight transfer forwards on lifting 
off, the rear suspension goes into droop, positive camber then the sideways 
momentum of the car tends to drive the wheel into the ground using the 
driveshaft as the lever arm. This is actually what was "wrong" with the 
Corvair that Nader found so inherently dangerous, as it can be if the 
driver knows nothing about how his car handles. If tire pressure were not 
kept to spec and a driver really overcooked it and then lost his nerve the 
result would be a rollover, or more accurately an engineered flip. Didn't 
the esteemed Porsche use swing axles in the 356? They had a similar 
reputation for biting back on lift off, a Porsche and VW tradition for 
years. The general rule is swing axles can be dangerous with too much power 
in unskilled hands.... but doesn't that apply to all cars?

Mercedes Benz reduced this vicious characteristic of the swing axle by 
cleverly lowering the roll center using  a transverse coil spring under the 
differential. If I recall correctly, only one universal joint was required 
as the other half of the axle was bolted up to the differential housing, 
the whole thing pivoting around one UJ.

Triumph improved the swing axle for the later Spitfire (Mark IV?)  also by 
lowering the roll center, possibly by mounting the transverse leaf spring 
on two mounts instead of one center pin, and dialing in a good deal of 
static negative camber at the rear. By starting negative the tendency of 
the rear to move into positive camber on forward weight transfer can be 
reduced, or even eliminated if stiff enough springs are used. Perhaps this 
improvement convinced Triumph to return to a swing axle for the GT6 in the 
last couple of model years. The Rotoflex (Giubo by Dunlop) and lower 
wishbone apparently didn't do much for the GT6 handling. Lots of negative 
camber would work great in the latest version, tire wear be damned. No 
Triumph ever had enough power to make the swing axle a serious issue unless 
the driver was totally ham fisted. VW Beetles also qualified as 
sufficiently underpowered to safely use swing axles. It is somewhat ironic 
that the superior power of the Corvair was what got ordinary folks into 
trouble.

Finally, another interesting characteristic of the transverse leaf spring 
is built in anti roll stiffness. The Corvette used a single leaf transverse 
spring in their irs, and I even think it was fiberglass. The Corvette, of 
course, has essentially the same rear suspension as Jaguars of the same 
era, except for the leaf instead of coilovers.

Cheers


Michael Smith
White 1991 164L
Original owner
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