Until recently that I took on the interest in making the Mini more fun, I only have cursory notions of how car suspension works. When I decided on the set of Swift Sport Springs I was puzzled by the rear spring being stronger than the front.
The Swift Sport Springs are 5.0 kg/mm 280 lb/in for the front, and 5.5 kg/mm 308 lb/in or the rear. It all make sense once I recognize the spring rates themselves tells you nothing until you take into account the suspension geometries. Each car is different and front is different from the back. There are two main factors that must be taken into account. One is the lever arm effect, and the other the incline angle of the spring. In reality a precise calculation for a range of suspension travel is more complicated due to the geometry of the suspension design.
Here is a suspension worksheet from Koni.
On some cars you can find the motion ratios on the web. I try to find them for the R53 Mini and what I found is quite useless and questionable. Most people cites 0.9 to 1. Just from reviewing the photos of the front and rear suspensions these numbers look grossly too high.
the front strut type suspension
the rear trailing arm type suspension
Based on very very rough estimate from these two photos the front is closer to 0.7 and the rear 0.55. In any event, the point of this post is the spring rates provide very little information unless one also understand the specific suspension design of the car. For the Mini, a lot more weight is allocated to the front, but yet the rear springs have higher spring rate. When properly calculated, the wheel rate at the rear has to be lower than the front by a considerable amount.
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