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missfire · 02-01-17, 03:27 AM

Can your mechanic not work it out for you?

If nobody has stock rates you need to know the sprung weight at each corner ideally (i.e the weight of the shell, engine etc. everythng above the spring, not axles, wheels etc. and how that weight and forces are multiplied up by the geometry). Then work out how much extension there should be in the suspension to get the spring height and decide how much sag you want, to get your ride height. Then you divide the sprung weight at that corner by the number of inches of sag you want, to get the spring rate. (So a 500lb spring will compress one inch for every 500lb applied to it, if it's a linear rate spring, which it will be on a coilover). The irony being that the best way to work out the sprung weight is to borrow a set of known rate springs and measure the sag. And equally, an inch of travel at the spring will translate to more than that at the wheel because it's further out from the pivot point etc. so using borrowed known rate springs is by far the easiest way to get it all to work out unless you're a maths, engineering and physics guru!

So with a weight of 1000lb per front wheel, plus leverage, an educated guess or experience from the s2 forum may not be far off? Just depends how much sag you want. I'd be searching german ebay for a full kit of some kind, or maybe even better, a set of replacement stock struts?

That sounds utterly confusing reading it back but if you had a noddy car setup vertical load on the spring straight into the ground with no wishbones or arms, 1000lb per wheel and you wanted four inches of sag, you'd need 250lb springs, but it's not that simple, hence easier to work it out using someone else's springs or someone with a good returns policy, lol.

02-01-17, 03:27 AM	#2
missfire making wooden things Classic Audi Club Member Join Date: Dec 2009 Location: cow land Posts: 8,154	Can your mechanic not work it out for you? If nobody has stock rates you need to know the sprung weight at each corner ideally (i.e the weight of the shell, engine etc. everythng above the spring, not axles, wheels etc. and how that weight and forces are multiplied up by the geometry). Then work out how much extension there should be in the suspension to get the spring height and decide how much sag you want, to get your ride height. Then you divide the sprung weight at that corner by the number of inches of sag you want, to get the spring rate. (So a 500lb spring will compress one inch for every 500lb applied to it, if it's a linear rate spring, which it will be on a coilover). The irony being that the best way to work out the sprung weight is to borrow a set of known rate springs and measure the sag. And equally, an inch of travel at the spring will translate to more than that at the wheel because it's further out from the pivot point etc. so using borrowed known rate springs is by far the easiest way to get it all to work out unless you're a maths, engineering and physics guru! So with a weight of 1000lb per front wheel, plus leverage, an educated guess or experience from the s2 forum may not be far off? Just depends how much sag you want. I'd be searching german ebay for a full kit of some kind, or maybe even better, a set of replacement stock struts? That sounds utterly confusing reading it back but if you had a noddy car setup vertical load on the spring straight into the ground with no wishbones or arms, 1000lb per wheel and you wanted four inches of sag, you'd need 250lb springs, but it's not that simple, hence easier to work it out using someone else's springs or someone with a good returns policy, lol. Last edited by missfire; 02-01-17 at 03:59 AM.