E30 M3 spring rate question
Discussion
I've been running KW competition coil overs on my E30 for a while and whilst they're pretty much spot on for track, when it comes to fast road use they're a bit too stiff. Actually it mainly the rears that are too stiff and it's just a little too skittish for my liking. What I'd like, is some rear end squat and better rear end traction so I'm thinking a softer spring is the way to go?
KW V2's typically come with 390lb front and 440lb rear, while the competition spec (hence true coil over rear) are 1000lb front and 1120lb rear. I did try and find any markings on the current springs but had no luck.
I did see the ex Prodrive car that was for sale recently, where they quoted spring rates from their road/rally set up but KW said they were far too low to be correct. I have asked a few other sources but they seem to just run with whatever spring rates were supplied from the manufacturer.
So, what spring rates are people running?
What free length spring is ideal and what difference does it make?
I also wondered what shorter drop links people use when needed?
I appreciate that whatever rates I go with, they're going to be a compromise between road and track unless I want to start swapping out springs for specific events, but I figured it would be good to just get some experience and opinion from other owners. Mind you, there aren't that many people who actually track their E30 M3's anymore.
My KW's as they were when I first bought them.
'true coil over' rears.
KW V2's typically come with 390lb front and 440lb rear, while the competition spec (hence true coil over rear) are 1000lb front and 1120lb rear. I did try and find any markings on the current springs but had no luck.
I did see the ex Prodrive car that was for sale recently, where they quoted spring rates from their road/rally set up but KW said they were far too low to be correct. I have asked a few other sources but they seem to just run with whatever spring rates were supplied from the manufacturer.
So, what spring rates are people running?
What free length spring is ideal and what difference does it make?
I also wondered what shorter drop links people use when needed?
I appreciate that whatever rates I go with, they're going to be a compromise between road and track unless I want to start swapping out springs for specific events, but I figured it would be good to just get some experience and opinion from other owners. Mind you, there aren't that many people who actually track their E30 M3's anymore.
My KW's as they were when I first bought them.
'true coil over' rears.
Edited by e30m3Mark on Friday 30th March 10:46
Are your rears true coil over, as shown in the pic?
What is important is the wheel rate, not the spring rate. Due to the geometry the springs don’t act directly on the centre of the tyre contact patch. This is referred to the motion ratio (MR). That is wheel travel : spring compression.
The E30 M3 Motion ratios are;
Front 0.9326:1
Rear 0.64:1 (Stock spring location)
Rear 1.04:1 (True coil over)
Converting spring rate to wheel rate is MR*MR*Spring rate.
The motion ratio changes the wheel rate proportionately, for example a stock front spring is 100 lb/in and this gives a wheel rate of 87lb/in while at the rear, the stock spring is 300lb/in which is 123lb/in at the wheel.
What you can see is that as stock, the rear is stiffer than the front at a ratio of 0.71:1.
I we take your 390 lb/in fronts they have a wheel rate of 339.2lb/in. To achieve the same ratio front to rear you need a wheel rate of 477.7lb/in (339/0.71), spring rate of in the stock location 746lb/in and 460lb/in with a coil over spring.
So at 440lb/in at the rear you are slightly softer than a stock front/rear balance.
For reference the GpN springs give wheel rates of 591 front and 444 at the rear. Which is a ration of 1.31:1 as opposed to 0.71:1 of stock.
I don’t know why BMW went so soft at the rear, I can only guess that in also lowering the car, the effect on roll centres meant that the front needed stiffening to retain the overall front/rear roll stiffness balance.
Perhaps there is scope to soften the rear down to equal front rear wheel rates, so for a coil over spring, that would mean 315lb/in.
Don’t discount damper values and bumpstops either. A lot of folks running lower than stock with Bilstiens have found their bumpstops have been supporting the weight of the car, not the springs.
I am of the opinion that you shouldn’t need super stiff suspension on a road car that is occasionally used of track. The handling characteristics of the E30M3 are partly a result of the geometry changes under pitch and roll. Eradicate pitch and roll and you lose some of the adjustability.
What is important is the wheel rate, not the spring rate. Due to the geometry the springs don’t act directly on the centre of the tyre contact patch. This is referred to the motion ratio (MR). That is wheel travel : spring compression.
The E30 M3 Motion ratios are;
Front 0.9326:1
Rear 0.64:1 (Stock spring location)
Rear 1.04:1 (True coil over)
Converting spring rate to wheel rate is MR*MR*Spring rate.
The motion ratio changes the wheel rate proportionately, for example a stock front spring is 100 lb/in and this gives a wheel rate of 87lb/in while at the rear, the stock spring is 300lb/in which is 123lb/in at the wheel.
What you can see is that as stock, the rear is stiffer than the front at a ratio of 0.71:1.
I we take your 390 lb/in fronts they have a wheel rate of 339.2lb/in. To achieve the same ratio front to rear you need a wheel rate of 477.7lb/in (339/0.71), spring rate of in the stock location 746lb/in and 460lb/in with a coil over spring.
So at 440lb/in at the rear you are slightly softer than a stock front/rear balance.
For reference the GpN springs give wheel rates of 591 front and 444 at the rear. Which is a ration of 1.31:1 as opposed to 0.71:1 of stock.
I don’t know why BMW went so soft at the rear, I can only guess that in also lowering the car, the effect on roll centres meant that the front needed stiffening to retain the overall front/rear roll stiffness balance.
Perhaps there is scope to soften the rear down to equal front rear wheel rates, so for a coil over spring, that would mean 315lb/in.
Don’t discount damper values and bumpstops either. A lot of folks running lower than stock with Bilstiens have found their bumpstops have been supporting the weight of the car, not the springs.
I am of the opinion that you shouldn’t need super stiff suspension on a road car that is occasionally used of track. The handling characteristics of the E30M3 are partly a result of the geometry changes under pitch and roll. Eradicate pitch and roll and you lose some of the adjustability.
Edited by stevesingo on Friday 30th March 16:51
Steve Rance said:
Do you not have any adjustment on the shock unit?
I do and although we softened them off the rear just doesn't have as much compliance as I would like and is just too skittish on anything but smooth tarmac.(incidentally, yes Steve mine is the true coil over rear pic above)
I run Leda coilovers on my M3 and from memory run much softer springs at the rear than the front. I will need to check, but I am fairly sure my rear springs are around half the poundage of the fronts. The shocks also have adjustable damping so I wind these back for road use and up for track use. I can pretty much set the car up to be neutral, by adjusting the shocks. I seem to remember that a few of the guys I used to track with went to a setup like you have and also found their cars a bit snappy at the limit.
stevesingo said:
Are your rears true coil over, as shown in the pic?
What is important is the wheel rate, not the spring rate. Due to the geometry the springs don’t act directly on the centre of the tyre contact patch. This is referred to the motion ratio (MR). That is wheel travel : spring compression.
The E30 M3 Motion ratios are;
Front 0.9326:1
Rear 0.64:1 (Stock spring location)
Rear 1.04:1 (True coil over)
Converting spring rate to wheel rate is MR*MR*Spring rate.
The motion ratio changes the wheel rate proportionately, for example a stock front spring is 100 lb/in and this gives a wheel rate of 87lb/in while at the rear, the stock spring is 300lb/in which is 123lb/in at the wheel.
What you can see is that as stock, the rear is stiffer than the front at a ratio of 0.71:1.
I we take your 390 lb/in fronts they have a wheel rate of 339.2lb/in. To achieve the same ratio front to rear you need a wheel rate of 477.7lb/in (339/0.71), spring rate of in the stock location 746lb/in and 460lb/in with a coil over spring.
So at 440lb/in at the rear you are slightly softer than a stock front/rear balance.
For reference the GpN springs give wheel rates of 591 front and 444 at the rear. Which is a ration of 1.31:1 as opposed to 0.71:1 of stock.
I don’t know why BMW went so soft at the rear, I can only guess that in also lowering the car, the effect on roll centres meant that the front needed stiffening to retain the overall front/rear roll stiffness balance.
Perhaps there is scope to soften the rear down to equal front rear wheel rates, so for a coil over spring, that would mean 315lb/in.
Don’t discount damper values and bumpstops either. A lot of folks running lower than stock with Bilstiens have found their bumpstops have been supporting the weight of the car, not the springs.
I am of the opinion that you shouldn’t need super stiff suspension on a road car that is occasionally used of track. The handling characteristics of the E30M3 are partly a result of the geometry changes under pitch and roll. Eradicate pitch and roll and you lose some of the adjustability.
Hi Steve, you seem to be next level in your knowledge of calculating the correct numbers for the true balance of the car. I would be thrilled to get my E30 M3 handling as well as it can be. I am considering a set of Fortune Auto coilovers as they have been pretty big in time attack nowadays. I was curious about your opinion on them or if you would recommend other coilovers. I'm also curious if the numbers for the coilovers that they recommended to me for street and occasonal track use below seem correct in your opion. Your feedback is appreciated. They replied recommending the following:What is important is the wheel rate, not the spring rate. Due to the geometry the springs don’t act directly on the centre of the tyre contact patch. This is referred to the motion ratio (MR). That is wheel travel : spring compression.
The E30 M3 Motion ratios are;
Front 0.9326:1
Rear 0.64:1 (Stock spring location)
Rear 1.04:1 (True coil over)
Converting spring rate to wheel rate is MR*MR*Spring rate.
The motion ratio changes the wheel rate proportionately, for example a stock front spring is 100 lb/in and this gives a wheel rate of 87lb/in while at the rear, the stock spring is 300lb/in which is 123lb/in at the wheel.
What you can see is that as stock, the rear is stiffer than the front at a ratio of 0.71:1.
I we take your 390 lb/in fronts they have a wheel rate of 339.2lb/in. To achieve the same ratio front to rear you need a wheel rate of 477.7lb/in (339/0.71), spring rate of in the stock location 746lb/in and 460lb/in with a coil over spring.
So at 440lb/in at the rear you are slightly softer than a stock front/rear balance.
For reference the GpN springs give wheel rates of 591 front and 444 at the rear. Which is a ration of 1.31:1 as opposed to 0.71:1 of stock.
I don’t know why BMW went so soft at the rear, I can only guess that in also lowering the car, the effect on roll centres meant that the front needed stiffening to retain the overall front/rear roll stiffness balance.
Perhaps there is scope to soften the rear down to equal front rear wheel rates, so for a coil over spring, that would mean 315lb/in.
Don’t discount damper values and bumpstops either. A lot of folks running lower than stock with Bilstiens have found their bumpstops have been supporting the weight of the car, not the springs.
I am of the opinion that you shouldn’t need super stiff suspension on a road car that is occasionally used of track. The handling characteristics of the E30M3 are partly a result of the geometry changes under pitch and roll. Eradicate pitch and roll and you lose some of the adjustability.
Edited by stevesingo on Friday 30th March 16:51
Front Suspension: 137MM shock stroke, 270MM shock body – *7kg front spring rate (Swift) if you go with Hyperco it would be 400lb spring or 7.14kg, I would recommend adding helpers/assist springs to this, I would recommend Swift / Hyperco upgrade* - 6-inch main spring with Swift helper or Hyperco Tender spring.
Rear Suspension: Standard shock specs – 11kg rear spring rate (Swift) or Hyperco spring rate would be 10.7kg or 600lb springs *Swift / Hyperco upgrade 5 or 4-inch spring*
The mother of invention is necessity. That is the only reason I posted your quote, I wanted to understand it.
I have no experience of Fortune Auto, nor do I know anyone who has.
Presumably, after looking at their website, it seems they list coil overs for the E30, but not E30 M3 - which has a different steering knuckle. SO, it might be worth checking if they do indeed make a coil over for the E30 M3.
I'm not sure I understand their units of spring rate.
What I would say ir, 400lb/in front and 600lb/in rear (in the stock location-not true rear coilover) would be very stiff. 4x stiffer on the front and 2x stiffer on the rear.
What is the use case proportion of road vs track vs competition?
I have no experience of Fortune Auto, nor do I know anyone who has.
Presumably, after looking at their website, it seems they list coil overs for the E30, but not E30 M3 - which has a different steering knuckle. SO, it might be worth checking if they do indeed make a coil over for the E30 M3.
I'm not sure I understand their units of spring rate.
What I would say ir, 400lb/in front and 600lb/in rear (in the stock location-not true rear coilover) would be very stiff. 4x stiffer on the front and 2x stiffer on the rear.
What is the use case proportion of road vs track vs competition?
Edited by stevesingo on Friday 16th February 19:46
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