Mmm, new rear springs would definitely help. The Rover in effect already has progressive rears thanks to the huge bump stop acting as a supplementary spring on full deflection.
Progressive rate springs are a difficult topic. Many modern designs use them, but to a purist designer they are a bit of a cop out. They work very well on dead smooth surfaces. Problems arise when the surface isn't smooth, particularely if there are big camber changes on a straight road. In these circumstances progressive rate springs can induce a certain amount of rear end steer.
The design availble for the P6 from Rover-Classics is well proven and could well suit you. I suspect only really enthusiastic drivers will notice any bump steer effect in the UK. I have chosen to go with stiffer single rate springs on my Thai car because I DO expect to drive it enthusiastically (!) AND the roads in Thailand do suffer from a lot of camber change irregularities.
There were heavy duty rear springs listed for the P6 for caravanning, but, as this was before variable rate springs had been thought of, they were stiffer single rate. It's easy enough to get a set made at, say, +25%, or you could adopt the method used contemporarily with the P6 of adding rubber blocks between the spring coils. Not sure where you'd find a set today though. With both routes you do risk changing the handling balance of the car, which is why people started going for the progressive ones. With progressives, handling in quiet driving is unaffected, but if you start pushing in corners you will find understeer at the limit is reduced or slight oversteer induced. With stiffer single rate springs then you will have reduced understeer throughout (I'd have said that was better - but it is very much to personal taste!).
Rear springs on the P6 are easy enough to change, so why not try a set and if you don't like them revert to single rate?
Sur D is looking splendid in your picture!
Chris
