280i Rear Springs

[MCWEDGE]

I tried searching, but could not get the details I'm looking for.

I just replaced the rear shocks on my '86 US 280i with the Gaz model with adjustable spring perches. I was going to replace the springs, but the ones that Gaz provided were significantly shorter than the originals. The originals, with the "red stripes", measure 13.75 inches, uncompressed. The ones from Gaz are 12.0 inches, and are labeled Tasmin 12 200. I assume the 200 is the spring rate, 200 lbs/inch. The wire diameter is vrtually the same between the two. The original springs were pre-loaded to a length of 12 inches.

Has anyone replaced the stock rear springs in a 280i with new ones, and if so, what length did you use? Also, do you think 12 inch springs could be fitted in place of 13.75" springs, and be properly adjusted for ride height and suspension travel.

thanks in advance,

Jim McPhee
Placitas, NM

[SEvans]

I can't really say about the spring length etc as I don't have Gaz shocks on mine. Have you spoken to the people who supplied them? It could be this is normal if the platform is higher. Cheers Steve

[adam quantrill]

If you are fitting like for like then it will sit lower. How much lower depends on the comression rate of the new springs. If you assume the new springs are 2x as hard it will sit almost an inch lower between the spring platforms and therefore slightly more at the wheels as they are a little outboard of this. If the speings are the same rate it will sit a LOT lower.

This might look a bit odd is the front is high (and it might rise up, too).

[MCWEDGE]

Thank you for your responses.

I believe the springs are very close to the same rate, based very subjectively on my ability to compress each of them a similar amount by leaning down on each of them, and because the wire diameter, and the number of coils/foot are virtually identical. Certainly if that is the case, and I install the Gaz springs and pre-compress them the same 2" that the originals were and are again, the car is going to ride almost 2" lower in the rear. That is not desirable or acceptable.

I am in communication with the vendor here in the states, and he is checking with Gaz to determine if they believe the 12" shocks they provided are the correct ones to replace the 13.75" originals. I was hoping someone on here had some specific replacement spring details.

[adam quantrill]

Yeah so the idea is that you set the platform up to set the ride height.

However if the spring rate is the same or similar this will mean you can carry less weight before the suspension crashes out over a bump. This is obviously not very desirable on an uneven road when you have a passenger and luggage.

You can of course set the dampers to a harder setting to compensate but then the car gets stiffer and less comfortable on long journeys, especially as they are the rear ones.

Or - use the new dampers with the old springs and get a refund on the short ones.

[MCWEDGE]

Adam,

Thanks for your input.

I wish someone who had fitted Gaz shocks and springs had responded. First-hand experience/knowledge is what I need most.

I heard back from the vendor here in the US, and he said that Gaz confirmed that the springs they provided are the ones they recommend for use on the rear of the 280i. I'm willing to install them, but I'd like to find out how much to pre-load the new springs to come close to maintaining the stock ride height. If I don't get that info, I think I'll start out with about 1.25", and increase it if necessary.

Jim McPhee
MCWEDGE

[Hayduke]

5 Factors affecting spring rate:

a) Material – metallurgy of the spring

b) Diameter of winding - bigger is softer

c) Thickness of the wire – thicker is stiffer

d) Length of spring – a short spring is stiffer

e) Number of turns per unit length – more turns per unit length is less stiff

I think you’ve accounted for C, D and E with C being equal, D being stiffer and E being softer. No reason to expect A is same, and B might be a hair off if the Gaz perch seat is different.

Your primary ride height adjustment is the perch position on the threaded damper body.

Then you need to account for the damping effect of the shock, it won’t affect ride height, but it’ll matter how the ride feels. Are yours adjustable?

I’d say that your subjective test result of equal spring rate isn’t definitive. There’s enough uncertainty in all this that I think you’ll save a lot of time with just trying them out and hope that Gaz wasn’t hopeless with their design for the 280i.

[BIG DUNC]

Remember, with that arrangement you can adjust them on the car. So fit them approximately correctly, then raise or lower them as required.

Edited by BIG DUNC on Friday 19th April 08:16

[adam quantrill]

I was thinking you could measure the spring rate quite simply with a bit of physics.

Get a broom and a bit of 2 x 4 a couple of feet long. Drill a hole in the middle of the 2x4 so it fits over the broom.

You'll need a friend with some calipers to measure the spring deflection.

Put the spring over the broomstick and the 2x4 on top, then put the broom vertical.

Measure the spring, then stand on the 2x4 and measure the spring again.

Get your weight from teh bathroom scales. The spring rate = the deflection divided by the weight.

Repeat with the other spring.

[MCWEDGE]

Thanks to everyone for your replies.

I took the stock rear springs off, and fitted the 12"/200 lb. Gaz springs. Since they are 1.75" shorter than the stock springs, I only pre-loaded .5". Car actually sits about .25" higher, measuring 10.25" from the ground up to the bottom edge of the rear valance. (1986 280i, Series 2.) The Gaz shocks I settled on three clicks up from full soft, front and rear. Rides great!

Edited by MCWEDGE on Monday 13th May 22:28