Quote:
Originally Posted by 86_IT
Looking at rear LCAs like the SPL units where there are 3 shock mounting holes am I correct in stating that mounting the shocks in the outermost hole (closest to the hub) will effectively increase the rear spring rate/wheel rate?
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Moving the shock mount towards the hub would increase the motion ratio which would put more spring force (weight) on the spring, but the spring rate (on a linear rated spring) would remain constant throughout. It would only compress the spring further than it would if it were mounted on a shock mount more towards the control arm pivot point.
For my car I've only spec'd out the rear for coil-overs, but with the factory control arms the motion ratio is .76. The way I measured this was:
On the rear of my (2013) FRS D1 is 13.50" and D2 is 17.75". Dividing 13.50" by 17.75= .76
I haven't weighed my car on scales, but all this means is that (say for example) each rear corner weighed 630 lbs. on scales. You would subtract the unsprung weight of wheel/tire/brakes/half of control arm, let's call it 100 lbs., giving you 530 lbs. on that corner. With the motion ratio that 530 lbs. divided by .76 would give you the actual force the spring would see of= 697 lbs.
Typically on a street car you'll want to see 25%-30% of the springs free length compressed by the weight of the car, so changing the mount position inboard or outward on a control arm will effect the compressed spring length. Ultimately, it's the dampening of the shock that controls the motion/ride quality of the car, though.