| Re: 1985 C4 - almost frame off |
Subject: Re: 1985 C4 - almost frame off by Vette91 on 2014/4/21 11:14:20 We got a '91 spring but did not consider that the late springs are 1/2" wider. So we need the retainers made of aluminium, not of steel. Will take a few days to get these and as we are on vacation some time around Easter, I have some time to share some thoughts why you definitely need a late spring if you are converting the early C4 to later model wider suspension and should not shorten the early spring ... Some facts: - the front spring rate of '84-87 L98 C4 is 54 to 65 N/mm (except Z51) - the front spring rate of '88-91 L98 C4 is 93 N/mm (except Z51) - '88-91 spring is shorter - '88-91 suspension is wider - according to Hooke's law: F = -k * x (or F = k * |x|) where F [N] is the resulting force vector k [N/mm] is the spring rate, a constant depending on material and construction. Negative because the force the spring exerts is in the opposite direction from its displacement. x [mm] is the displacement vector. So what happens if you shorten the spring? It will get stiffer because of the lever rule. In principle, that is what we need and why the '88-91 spring is manufactured shorter. But cutting 1/2" would not be enough. That is why the geometry of the '88-91 spring is different - it is a little bit wider and formed in a slightly different curve. According to the the lever rule, another thing does happen: Due to the wider suspension (more precisely the position of the hub of the wheel relating to the retainer of the spring), the lever acting on the spring does extend. That is the important stuff! If you keep a spring with a rate of 54 N/mm instead of using a 93 N/mm, it will compress way more than it should. Shorten the 54 N/mm spring can not cover that! So the car would get lowered by approx. 40% without a lowering kit. In addition due to the soft characteristic of the spring the overall handling of the car would fall off in quality. Beside a poor handling, due to the increased dynamic compression all the time the wear of the tires could increase. So getting the proper spring can avoid a lot of problems ... |
