[rothwem]

Does anyone know what the spring rates are for the E91s? Supposedly the coupe and sedan have a ~450lb spring in the rear, but obviously the back of the wagon is heavier than the sedan, so it would make sense that the rate is different.

I'm wanting to replace my stock rear springs with a 2.5" coilover style spring/height adjuster, but I want to maintain a similar spring rate to stock for now. I'm thinking that a 9-10" spring with a 500 lb/in rate is in the ballpark, but if anyone has measured it or has some sort of resource through BMW that would be awesome.

[relentlessone]

I can only speak from what I am running, but the BC Coilovers (I-17) on my 2007 RWD wagon use a 12k (672 lbs/in) spring in the back and I don't have any issues there. For BC, the wagons actually use the E93 convertible coilovers because I guess the weight of the folding roof is close to the weight of the wagon.

[Tambohamilton]

I'd like to know too... Maybe someone can even measure up a spring (free length, number of active coils, wire diameter) so we can take an educated guess at the rate?

[rothwem]

Originally Posted by relentlessone

I can only speak from what I am running, but the BC Coilovers (I-17) on my 2007 RWD wagon use a 12k (672 lbs/in) spring in the back and I don't have any issues there. For BC, the wagons actually use the E93 convertible coilovers because I guess the weight of the folding roof is close to the weight of the wagon.

Yeah, that makes sense that the E93 is similar to the E91, however, aftermarket coilovers rarely use stock rate springs, or even try to make the ride frequency ratio of front to rear resemble the stock values.

Originally Posted by Tambohamilton

I'd like to know too... Maybe someone can even measure up a spring (free length, number of active coils, wire diameter) so we can take an educated guess at the rate?

I wonder how much the powder coat on the springs (additional thickness) influences the measurements. I've also got access to a great measurement lab at work, but there's no way I'm bringing a spring in from home to measure the wire thickness. I'd get more than a few looks and I don't really want that. So its not going to be me.

[Tambohamilton]

All it would need is a set of vernier calipers and a ruler/tape measure. Can you borrow them?

I'd measure a spring if I had one off the car (sorry, not taking my car apart to find out...yet).

[Tambohamilton]

Just taken some measurements of my e91 330d SE springs, and used a formula to calculate the rates. My measurements are likely very approximate, so these figures are really just an indicator!

Front: 19984N/m ~= 114lb/in
Rear: 77104N/m ~= 439lb/in

[Tanuze]

Originally Posted by relentlessone

I can only speak from what I am running, but the BC Coilovers (I-17) on my 2007 RWD wagon use a 12k (672 lbs/in) spring in the back and I don't have any issues there. For BC, the wagons actually use the E93 convertible coilovers because I guess the weight of the folding roof is close to the weight of the wagon.

+1

[Thorin]

I'm curious about this. The Eibach springs are rated in the rear for 650 in/lbs. However Ohlins R&T MI00 are listed for e90/e91/e92/e93, but the rear is listed at 400 lbs/in. Can anyone give insight to the discrepancy?

[Tambohamilton]

What are the front spring rates? My guess is that ohlins are expecting more damping control, so don't need stiff springs to control the chassis

[Thorin]

Originally Posted by Tambohamilton

What are the front spring rates? My guess is that ohlins are expecting more damping control, so don't need stiff springs to control the chassis

Oh nice! Front spring rates are 343 lbs/in.

Front: 60 N/mm (343 lbs/in)
Rear: 70 N/mm (400 lbs/in)

[Tambohamilton]

Really that similar? OK, I have no idea. The wheel rates of those will be well out: front motion ratio is about 0.9, and the rear is more like 0.5... So rear wheel rate will be about 100lb/in, and the front more like 300...that seems pretty wild to me, so I'm not sure what their idea is.

[stevesingo]

E90 MR are 0.9 and 0.56 rear.