Rear springs, why so fat ?

[fporro]

I'm just wondering if there's a particular reason for their shape?

[LJ851]

I'm just wondering if there's a particular reason for their shape?

It's a way of making a progressive spring in a small (vertically) amount of space.

[einstein57]

The longer the wire the better. The larger diameter means more leverage which requires a thicker wire. The thicker and longer the wire the smaller the standard deviation will be from any given point on the spring. That means a more consistent spring force. At least thats my understanding. I could be wrong.

[LJ851]

Spring rate is determined by several things. Wire diameter, coil diameter, distance coils are apart, and number of coils all play a part.

Linear rate springs have equal diameter, equally spaced coils over their whole length.

Progressive springs usually have closer coil spacing (softer) on one end or larger coil diameter in the middle (softer) and smaller coils on the end. This is what the E30 has front and rear.

The other option is to taper the wire diameter, but it is not common due to cost.