Cars I crewed for -- and won with
Hi Nic,
My experience with engineers and cost cutting tells me someone at VW said "We can save two spring plates per car if we go back to singles!"
My knowledge of suspension (all from Porsche-BMW-Mercedes) tells me the dual plates are much stiffer and better at locating the rear diagonal arm.
Porsche 911's use of a single plate tells me they wanted the springiness of the plate in flex, to allow the toe-in a semi-trailing arm creates as it moves. Rear toe-in creates understeer. This is a good thing in a VW or Porsche for most drivers.
However, a double spring plate is rigid, and would have to create much less toe, possibly even zero toe, as it moves. I like that, as it is consistent with the racing suspensions we had on our 930's.
With a set of good hard rear torsion bar and diagonal arm pivot mount bushings and dual spring plates, you have a very predictable rear end. Add to that the lower rear roll center of the IRS, and you have a first class setup.
It makes the single spring plates seem like a "learning suspension" before you get into a car with a pro arrangement.
FJC
Point taken, FJC.