Spring Length

[steverrquattro]

I am currently making some DIY coilovers. I have opted for the Bilstein dampers and approximately 300lb springs rates for both front and rear.

The question that I'm trying to work out is the spring lengths required. The original spring lengths are 12.5" front and 11.5" rear and I want the car to be lowered.

One other factor is that there is around 80mm adjustment on the front and approximately 110mm on the rear.

I have spoken to Demon Tweeks and they say to go for a one length spring rather than have helpers. Some of the feedback I've had from peoplle who have these SWB URs are running between 7" and 8" with helpers.

Having no previous experience of this, I'm just looking for someone to point me in the right direction i.e.

1. A one length spring, but shorter than the original and move the collar up so that when the car is off the ground the spring will not dislocate. Having said that I'm sure this will effect the damper performance as it will not allow it to fully compress due to the spring.

2. A shorter spring and helpers to make sure that when the car is off the ground the spring will not dislocate. Obviously this will compress under normal load and therefore make the car lower......in which case I could have stayed with the original set-up?

Help and advice please.

[Rusty]

You can't really run a single length 300lb spring on stock length dampers with the car lowered and it still in tension when the car is in the air. There is a school of thought that suggests it's unlikely that the spring will come unseated during driving conditions. Up to you really. I have a spreadsheet somewhere to calculate spring lengths, helpers etc. I'll drag it out later on.

You could get the dampers internally spaced really; that'll solve the issue.

[Rusty]

As promised, here's a version of my Excel spreadsheet, which you can use to calculate a coilover setup. You'll need to measure the various things and guess the weight as best you can for a quattro rather than an S2.

It's a bit of an odd calculation, as the spring seat adjustment is critical to the calculation but the ideal value can't be calculated until the end; hence the button at the bottom. Check the spring length is OK once you've done that... To be in tension, the uncompressed length of the spring stack needs to be greater than the uncompressed strut length. If your version of Excel supports it, there are some conditional formatting rules which *should* help you out. The spring seat adjustment can also be used to manually alter the damper uncompressed length, as if you were internally spacing the damper.

You'll see from the attached file that without helper springs, your spring stack is short by a bit. Only an extreme take-off would see the suspension uncompress to that degree and on 300lb springs, the sag is almost there but on say 500lb springs, you'll have a much larger shortfall.

[re2500]

just for ref my KW v2 use 170mm length springs 60N/mm all round with helper springs.

[Kroegarn]

Another reference: gotten hold of a used FK Automotive kit with 170 mm springs + helper spring, spring rate 80N/mm.

Since it is a avant the rear is built with a car specific spring. Progressive so I´m not sure if the calculations are correct but I get between 50-70N/mm (mostly depending on how many not actively used "coil laps"(?)). Will be used with Koni dampers, which hopefully will fit.

I´m leaving the conversion to lbs and " for you to do

[Error404]

I'm looking at this as well, cheers for the spreadsheet Andy, was looking for that the other day!
Useful info there on what KW use and their set-up is pretty tried and tested from a spring length / helper perspective.

I need to get some springs and then I'm going to send my Konis off the get them dyno matched before I build them up.