Thanks to all who answered my first post. Surprisingly, it would appear that the US market XR4Ti was not so much more softly sprung than the eu models, at least not the 3 door Cosworth Sierra. That is, until you take into account that the Merkur was considerably heavier at over 2900 lbs. curb weight.

I was able to get some spring rate numbers from a local rally suspension guru. He spec'd the Escort Cosworth at 353.4, the 4dr Sapphire Cossies at 290.7 and the Sierra Cosworth at only 268 (all in lb/in). The XR4Ti is harder to rate as it was a progressive spring specified at 257/503 lb/in much of which was taken up at the softer end just by lowering the car off the jacks. As the STA design greatly affects the numbers at the wheel, the wheel sees only 42% of the spring rate. The Merkur XR4Ti basically only "saw" around 105lb/in at the wheel, and the 3dr cosworth would only give me 268 x 42% = 112.6 lb/in. I'll pass on these.

It is interesting that the stock rear Merkur springs are very close to front springs on earlier model Camaros. These progressive springs became so popular for circle track racers, that they are currently produced by a company called Hypercoil (and probably others) in various heavier rates and marketed by numerous specialty retailers. This includes at least one specialized to Merkurs in the US.