[philcrombie]

Has anyone ever cut springs to make the car sit lower? I have heard of people doing this before and looking at the front springs design, the end of the spring appears to not be 'finished' in any particular way - so you could potentially cut it.
My aero has a definite 'nose up' look to it (I think the rears have sagged a little) - I'm happy with the height at the back but want to drop the front about 20mm to match.
The only other issue is that it you don't really know how much you need to remove to drop the car by x amount, so chances of it going pear shaped are high...
Any thoughts welcome

PS - Abbott springs have dropped in price by 60 notes just to add to my dilema(s)

 

[sj]

Yes, I did it on a 1990 9000 turbo, quite sucessfully. I bought some '-30mm' springs which turned out to be -55mm on fitting and were no good on real roads due to the frequent bumpstop contact. I calculated the spring rates and found that I could lower the front by about 20mm and the rear by about 25mm while keeping the spring rates within 10-15% of the originals. (using standard dampers)
I thought of doing this again on my 1995 9000, after grief with supply of springs from TSL, but on checking the size and calculating rate of the standards, I found that the later model springs were much stiffer than the earlier ones.
This meant that I could either maintain a reasonable rate and have negligible lowering, or have useful lowering and a rock-hard rate. I chose to not bother and persevered with TSL.

 

[whitesaab]

Car springs generally are designed with closer would coils at the top to gradually increase the spring rating as the suspension compresses.

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

 

[sj]

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.

 

[philcrombie]

Hi - thanks for the replies. I'm just trying to get my head around the fact that reducing the spring height by say 20mm will increase the spring rate (read firmness?) I would have thought the firmness would be the same???
Also, even if it did change, it should only be very slight/hardly noticeable? (as much as 15% suggested - sounds high)

On the point of actually cutting them, I would be pretty sure that holding the spring in a vice and using a fresh grinder would cause no issues (using the vice as a heat sink to prevent any changes there)

The question is really that it will probably take me a coupe of attempts to get the springs correct and its a lot of work each time...for potentially not much in return. I also agree with your aftermarket spring comments - I doubt they are really tested and there have been quite a few unhappy spring buyers on this site to date. I'll either go Abbott or not at all. (todays conclusion!)

 

[carrera]

Shortening the spring will increase the spring rate. here's an explanation http://www.allpar.com/eek/coils.html

When you look at the springs you will see that the coils at the top and bottom of the spring are different to those in the middle. This helps seat the spring and retain in in position.

I have used http://www.grahamgoode.com/ to order custom made springs for my race car in the past.... but you need to specify the spring length, internal diameter and rate. They are not as expensive as you would think, but you do need to know the answers to those three questions. 1 & 2 are easy, number 3 is the difficult one.

In other words, I would forget about getting the grinder out.