Originally posted by black&whitesaab:
[qb]I'm sure that someone out there has done it but as a engineer I would have quite a few reservations :
I wouldn't want to mess around with changing the springs rating without knowing the consequences. If you shorten a normally rated spring you will increase the likelihood of the car hitting it's bump stops - newly manufactured lowering springs are produced with higher spring rating than normal springs to compensate for their shorter length
The machining would have to be accurate and, given the characteristics of the material, the only suitable technique would be grinding. To allow safe grinding operations to be carried out you would need to ensure that the spring was retained in such a way as to prevent movement. This would likely involve the need to make some sort of jig.
Given that you would have to grind them to length you would then have to have them heat treated to re-establish the correct temper as required for spring steel to operate as per it's original specifications.
The bottom line is - just buy them [/qb][/b]
As another engineer,
your point on increasing rate to compensate for free length reduction would be logical, but I have purchased two sets of springs for Saabs (9000 and 9-5) where the new rear springs were actually of LOWER rate than the originals. Of course, in cutting down the spring you do increase the rate by reducing the number of active coils.
I agree grinding is the only practical method of cutting down, but using a thin slitting disc, the heat affected area is very localised and certainly very well within the unstressed area of the spring where it is resting on the spring seat. Accuracy of cut is not that critical, as the manufactured springs frequntly have very roughly cropped ends and are not identical to within a millimetre or two - easily achievable in a vice with a hand-held grinder.
The big issue is whether you can achieve what you want by cutting out of the standard spring.
Aftermarket springs always alter the rate in one way or another, but whether they are made of the same quality of steel is debatable. I have a set of Vogtland springs lying in my shed, where the rate is much higher and the spring stresses are hugely higher than standard due to the drastically reduced size of the spring (length, coil diameter AND wire diameter are all reduced)
I strongly believe that there are many spring manufacturers out there who may well know how to design and make springs, but who pay little attention to their application on any particular car.
By all means buy them, but don't presume that just because the manufacturer or distributor says so, that they are ideal for your car.
Springs are easily measurable and the rate calculable, so you can at least compare new with old, unlike dampers, where you just get a sealed unit which you can only really assess by installed feel.