Coilover spring rates

[MarkT]

What spring rates are you running on your coilovers. Anyone help, or know of a guide?

Mark

[Brian28]

Depends on how you like the car to feel. 500 to 650 front, 300 to 400 rear. Softer rear if you have a rear anti roll bar. Everybody will prefer something different though, there isn't really a single "right" set up. Mess about with different rates until you find what works for you.

[MarkT]

I'm just after a starting point really mate. That sort of rate, I'm thinking 600 or so for the fronts and 350 rear. Just out of interest how long are the springs that you are running? I'm looking to convert my original struts to coilover units and run some gaz adjustable inserts.

[vladv]

I noticed the most coilovers are sold have softer front then rear. But for drift is prefered vice versa.
What advantages and disadvantages has harder front?

[GeoffBob]

Rear coil-overs aren't mounted in the same place as the normal springs, that's why they have markedly different spring rates! The suspension arms work like a lever and thus the location of the spring has a marked effect upon the "actual" rate measured at the wheel.

Front springs can generally be compared due to the fact that the location of the springs remains the same (namely around the strut), although vehicle weights may differ, and thus what's right for one car isn't necessarily right for another.

[djs325]

Run away from rear coilovers. Unless you massively reinforce the rear mounting points (i.e. tie them into a cage) and make up trailing arms with a double shear attachment for the dampers, you'll break something expensive.

I think you may have got your spring rates mixed up a bit. E30 fronts about 300-350lb/in, rears 400-550lb/in. The rear uses a higher rate because it is not working on a close to 1:1 ratio like the front springs. Because there is leverage involved with the trailing arms, you use a higher spring rate at the spring to achieve a similar wheel rate (the overall movement of the wheel). Spring rate will vary based on a number of conditions: vehicle weight and weight distribution, chassis stiffness, tyre construction and rate, desired droop, etc...

[MarkT]

I've heard this too. I.e softer front than rear.

With regards to mounting, the rear end is going to be turreted, welded into a custom cages cage and the rear arms are going to be reinforced with selective bracing

[UweM3]

Well, top of my head I know at least of ten E3o M3 with proper rear coilovers. NONE of them has had any turret failure, caged or not. NONE had anything snapped off at the trailing arm either. Grp A cars ran rear coilovers on the stock trailing arms.
I have a faint memory listening to a conversation of somebody really clued up about E30 suspension working out forces the the rear shock mounts has to take just with the shock attached and the main force was the rebound of the shock and not a possible spring force.
And the Z3 reinforcement plate is added to help exactly in this matter, stopping the shock to pull OUT of the tower and not the tower collapsing from compression forces.

So as long nobody can show pictures of a broken rear turret from a coil over shock I will say this is another internet myth.

[Brian28]

As I was saying Mark, everybody prefers something different . DJS I guess you're using the thicker front anti roll bar? 318is 600 front 275 rear works pretty well for me (coilover rear with standard rear arb). 320i had 650 front 500 rear (inboard) that was Ok too. 318i 550 front 400 rear (inboard) was a bit soft dry but really good in the wet. Some people use up to 800lb rear inboard, have a play and see what works for you, nobody can tell you what will feel best to you to suit your own driving style and how the rest of the car is set up.

[UweM3]

As I was saying Mark, everybody prefers something different . DJS I guess you're using the thicker front anti roll bar? 318is 600 front 275 rear works pretty well for me (coilover rear with standard rear arb). 320i had 650 front 500 rear (inboard) that was Ok too. 318i 550 front 400 rear (inboard) was a bit soft dry but really good in the wet. Some people use up to 800lb rear inboard, have a play and see what works for you, nobody can tell you what will feel best to you to suit your own driving style and how the rest of the car is set up.

cough cough I am running 1030lbs rear and was thinking to upgrade....

[handpaper]

Run away from rear coilovers. Unless you massively reinforce the rear mounting points (i.e. tie them into a cage) and make up trailing arms with a double shear attachment for the dampers, you'll break something expensive.

Not my experience. My rear top mounts have only a 1/4" steel plate to reinforce them; the trailing arm mounts are the standard 12.9 M12 bolts.*
8,000 road miles and 160 'Ring laps later, nothing is loose, bent or cracked.
If you're going to track your car, a decent cage is a good idea and should tie into the rear top mounts, but an E30 shouldn't fall apart without one.

I think you may have got your spring rates mixed up a bit. E30 fronts about 300-350lb/in, rears 400-550lb/in. The rear uses a higher rate because it is not working on a close to 1:1 ratio like the front springs. Because there is leverage involved with the trailing arms, you use a higher spring rate at the spring to achieve a similar wheel rate (the overall movement of the wheel). Spring rate will vary based on a number of conditions: vehicle weight and weight distribution, chassis stiffness, tyre construction and rate, desired droop, etc...

For track use I'd go for 550-700lb/in front, 300-400lb/in rear.
If you're using the OE spring location, multiply the rear rate by 2.8 to allow for the reduced leverage and movement, so about 800-1200lb/in.
An E30 is nose-heavy, a stripped E30 even more so. Spring rates should reflect likely static and dynamic loads.

If you're developing the car (and yourself), start at the soft end of these scales and stiffen if necessary - suspension should be as soft as possible. Bear in mind also that what suits others may not suit you.

* I've seen both 10.9 and 12.9 bolts in this location, was there an upgrade at some point?

[MarkT]

Thank you for an interesting factual post. I'm not using the OE spring location but am going to use the OE damper location.

That helps explain the confusion over the rear spring rates.

I realise that this area is purely subjective and down to application, but I was just after a little info on where to start. Looking at this thread it looks like I should be shooting for 600lb fronts and somewhere around 300lb rears. I guess it takes a few trackday's around the same track to hone the performance of the suspension and for that matter different tracks and surfaces will require different rates. Firmer for somewhere like Silverstone and fairly compliant for smooth 'B' roads and places like the ring.

Run away from rear coilovers. Unless you massively reinforce the rear mounting points (i.e. tie them into a cage) and make up trailing arms with a double shear attachment for the dampers, you'll break something expensive.

Not my experience. My rear top mounts have only a 1/4" steel plate to reinforce them; the trailing arm mounts are the standard 12.9 M12 bolts.*
8,000 road miles and 160 'Ring laps later, nothing is loose, bent or cracked.
If you're going to track your car, a decent cage is a good idea and should tie into the rear top mounts, but an E30 shouldn't fall apart without one.

I think you may have got your spring rates mixed up a bit. E30 fronts about 300-350lb/in, rears 400-550lb/in. The rear uses a higher rate because it is not working on a close to 1:1 ratio like the front springs. Because there is leverage involved with the trailing arms, you use a higher spring rate at the spring to achieve a similar wheel rate (the overall movement of the wheel). Spring rate will vary based on a number of conditions: vehicle weight and weight distribution, chassis stiffness, tyre construction and rate, desired droop, etc...

For track use I'd go for 550-700lb/in front, 300-400lb/in rear.
If you're using the OE spring location, multiply the rear rate by 2.8 to allow for the reduced leverage and movement, so about 800-1200lb/in.
An E30 is nose-heavy, a stripped E30 even more so. Spring rates should reflect likely static and dynamic loads.

If you're developing the car (and yourself), start at the soft end of these scales and stiffen if necessary - suspension should be as soft as possible. Bear in mind also that what suits others may not suit you.

* I've seen both 10.9 and 12.9 bolts in this location, was there an upgrade at some point?

[UweM3]

......matter different tracks and surfaces will require different rates. Firmer for somewhere like Silverstone and fairly compliant for smooth 'B' roads and places like the ring.

I disagree. I run the 1030lbs springs at the ring and find them too soft at some places.

These are the softest Grp N springs and are usually refered to as the rain setup (together with the softer of the two shock options)

[MarkT]

What location are you running your rear springs? Stock spring location or OE damper location?

As I said earlier and by others it's purely subjective.

[handpaper]

Uwe runs front coilovers and OE located rear springs on adjusable perches, here they are :



1030lb/in springs in this location are equivalent to 380lb/in on coilovers - still too soft for Uwe

[MarkT]

Ahh, that'll be it then!

[UweM3]

Uwe runs front coilovers and OE located rear springs on adjusable perches, here they are :



1030lb/in springs in this location are equivalent to 380lb/in on coilovers - still too soft for Uwe

handpaper, when I first installed this setup I was thinking OMG it will be rocksolid and bumpy but after getting used to it I am seriously thinking of going a tad harder. 1140lbs and the stiffer of the Grp N shocks. My back end is pumping at a few places.

[MarkT]

This because they've had a little time to bed in?

[ShakeyC]

Run away from rear coilovers. Unless you massively reinforce the rear mounting points (i.e. tie them into a cage) and make up trailing arms with a double shear attachment for the dampers, you'll break something expensive.

I think you may have got your spring rates mixed up a bit. E30 fronts about 300-350lb/in, rears 400-550lb/in. The rear uses a higher rate because it is not working on a close to 1:1 ratio like the front springs. Because there is leverage involved with the trailing arms, you use a higher spring rate at the spring to achieve a similar wheel rate (the overall movement of the wheel). Spring rate will vary based on a number of conditions: vehicle weight and weight distribution, chassis stiffness, tyre construction and rate, desired droop, etc...

Well said, with E30 rear top mounts these can fail and rear coilover will push through in worst case and with uprated rear top mounts i agree tie into a prop cage is required otherwise the coilover can punch through the turret. Hence the spring pads are the main rear load bearing parts the damper location just to control the bounce that is all on most BMW's.

[keri-WMS]

Springs and shocks exert different types of force on the chassis as the suspension act on them.

- Springs: they apply a "smooth" and steady long term (low frequency) force, only in one direction.

- Shocks: they apply a "hammering" and rapidly varying (high frequency) force which contains more damaging shock-loading energy, and in both directions.

Remember that the "bumps" encountered by a moving car are FAR higher forces than the "weight" of a car sitting on it's springs, and "bump" energy goes mainly through the shocks. The result of this is that the shock mounts have a hard life with higher forces in both directions which is an ideal environment for fatigue failures, whereas the spring mounts have a comparatively easy life with steady forces in one direction.

So fitting uprated SHOCKS not springs is the more potentially damaging change, and adding spring loadings to a rear shock mount isn't really an issue.

The real reason not to go coilover on the rear is the need to "slot" the holes to get the springs to clear - a stress raising way to weaken the shell.....

[ShakeyC]

Keri i think your bit confused here were talking about coilovers and top of the rear turrent is not a major load bearing part of the BMW car chassis. This is where ALL the rear forces go with coilovers, not spread between 2 points. In oe design the spring is doing most of the work and the dampers in oe case does most of its work in rebound not in compression. I have x-ray pictures in a book with several BMW chassis which clearly show this.

When going to rear coilovers your focusing everything at 1 point between the hub to top of the wheel arch/turret which was never designed to take such combined loads which is why rear top mounts are known to fail.

[keri-WMS]

Keri i think your bit confused here were talking about coilovers and top of the rear turrent is not a major load bearing part of the BMW car chassis. This is where ALL the rear forces go with coilovers, not spread between 2 points. In oe design the spring is doing most of the work and the dampers in oe case does most of its work in rebound not in compression. I have x-ray pictures in a book with several BMW chassis which clearly show this.

When going to rear coilovers your focusing everything at 1 point between the hub to top of the wheel arch/turret which was never designed to take such combined loads which is why rear top mounts are known to fail.

I'm not at all confused, I concede that the damper does less during the compression stroke than it does during the rebound, but that's still more high-frequency shock loading transmitted to the chassis under compression by the shock than the spring manages unless it gets coilbound - we need some numbers here and I'm afraid I don't have them (maybe Gareth or GeoffBob can help or have some reference material?). Also I'd be interested to see the pages of the book you mention to clarify this?

Also with a coilover a lot of the forces on rebound (that load the chassis between the separate shock and spring) in the OE setup are contained WITHIN the coilover shock and never even act on the shell!

Let me put it another way with a mental experiment! Imaging that you have an E30 spring on one kneecap and an E30 shock balanced on the other (both on a 2" dia steel disc so the knee area is the same). Imagine hitting the top of each one with a hammer at the same speed. The shock side knee will hurt like f--k as the deceleration of the hammer happens rapidly, ie it transmits a damaging shock to your kneecap. The spring will take longer to slow the hammer down, so there's less shock, and less damage (it won't hurt!). Swap your knees for an E30 chassis and you get the idea!

[handpaper]

handpaper, when I first installed this setup I was thinking OMG it will be rocksolid and bumpy but after getting used to it I am seriously thinking of going a tad harder. 1140lbs and the stiffer of the Grp N shocks. My back end is pumping at a few places.

Karrussel exit, Hocheichen dip and the compression after PF3? Mine, too.
Increasing the rear damping helped, but proper spring rates would have been better

[RPM]

A rear coilover will be fine on a rear turret, there are a large number of PBMW car with rear coilovers and no cage to the rear turrets, all heavily riding curbs

A standard rubber rear topmount will however fail. You really should look into an uprated rear topmount

Spring rates- what engine is in the car, will it run front and rear ARBs? if so what size?

[handpaper]

Just wondering, how many of the people posting doom'n'gloom about rear coilovers have actually fitted and used them?

Here are a couple of pics from my install; the first shows the spherical bearing top mount fitted below :



Note how wide, flat and thick the mount is - this distributes the suspension load widely and evenly.


Here's the turret from inside the boot :



The plate on top of the turret clamps it firmly to the mount, eliminating any potentially damaging movement in this area and reducing the stress concentration at the slotted holes.

[UweM3]

handpaper, when I first installed this setup I was thinking OMG it will be rocksolid and bumpy but after getting used to it I am seriously thinking of going a tad harder. 1140lbs and the stiffer of the Grp N shocks. My back end is pumping at a few places.

Karrussel exit, Hocheichen dip and the compression after PF3? Mine, too.
Increasing the rear damping helped, but proper spring rates would have been better

actually I am fine there. Breidscheid bridge comes to my mind.
I don't think my spring rates are off, it's more of a damping problem. This is a situation where an adjustable damper would be handy.

[UweM3]

So fitting uprated SHOCKS not springs is the more potentially damaging change, and adding spring loadings to a rear shock mount isn't really an issue.

That's what I said in my earlier post. I spoke to a guy I trust on this and he asked what shocks and worked a few numbers out.
And the mathematical result with the shocks I was using is putting far more stress into the mounts than added springs. And he was only working the rebound out, wasn't even interested in bump!

[caneswell]

Keri i think your bit confused here were talking about coilovers and top of the rear turrent is not a major load bearing part of the BMW car chassis. This is where ALL the rear forces go with coilovers, not spread between 2 points. In oe design the spring is doing most of the work and the dampers in oe case does most of its work in rebound not in compression. I have x-ray pictures in a book with several BMW chassis which clearly show this.

When going to rear coilovers your focusing everything at 1 point between the hub to top of the wheel arch/turret which was never designed to take such combined loads which is why rear top mounts are known to fail.

Known to fail by who? Have you ever seen one fail?

Everyone here that actually runs them seems to think they are fine.

Try another mental exercise. If you took the shocks off your car it would gently move up and down on its springs over undulations. However over sharp bumps the wheels would flail around all over the place, all the force that normally controls these high G accelerations of the wheel assembly go through the shock mounts. Force = mass x acceleration.

[GeoffBob]

One of the ways I remember the difference between springs and dampers (shocks) is that a spring resists displacement, while a damper resists velocity.

In other words the force exerted by a spring depends upon how far you displace it (the more you squash it the harder it pushes back) while the force exerted by a damper depends upon how fast you displace it (the faster you squash it the harder it pushes back)

This is inherently why the load path tends to be through the dampers over fast changing terrain (rapidly undulating lumps and bumps), while the load path is through the springs over large/high bumps which result in a large spring displacement.

Of course it is perfectly possible to encounter an obstacle which results in both a large and fast displacement, but we tend not to drive our cars down stairs

[handpaper]

Of course it is perfectly possible to encounter an obstacle which results in both a large and fast displacement, but we tend not to drive our cars down stairs

Some of us have been known to take the odd kerb here and there, though, Geoff

[ShakeyC]

I cant explain any more simply 2 seperate paths of load (normal e30 e36 e46 ect) vs 1 combined (coilover)

Even with uprated top mounts I have seen enough damage to put the geometry out and sometimes can be brought back with re-alignment and tracking. Worse cases without uprated top mounts to spread the loads punches through and deforms the turret. If you cant understand that i give up

[martinpallot]

I cant explain any more simply 2 seperate paths of load (normal e30 e36 e46 ect) vs 1 combined (coilover)

Even with uprated top mounts I have seen enough damage to put the geometry out and sometimes can be brought back with re-alignment and tracking. Worse cases without uprated top mounts to spread the loads punches through and deforms the turret. If you cant understand that i give up

Are you implying that you have seen someone upgrade to a coilover without upgrading the topmount? I also fail to see how a deformed rear turret could alter your geometry. It may alter the angle of the damper minutely, but not the geometry of the wheel itself. What I fail to understand is how someone can possibly say that there will be adverse effects with fitting coilovers when its how the works rally cars ran them. I'm sure that If failures were as common as everyone here claims then thay would have done it.

I,m sorry but until anyone shows me a picture of a failed or severely distorted rear turret as a result of fitting coilovers I just dont buy it.

[e21Jason]

E21's and e28's run coilover bodies as stock, and I ran uprated leda's in my e21 for 2 seasons of track days and did not have a turret failure and those turret are no stonger than an e30.

The only failures I have seen are due to corrosion.

Jason

[bullwinkle]

All very interesting. Before reading this I was about to order some KW coilovers but now I'm not so sure.
Does anyone know of any manufacturer offering decent, adjustable dampers (non coilover) for the rear? If not, I was thinking of looking at the BMW grp N but information is really hard to find and I'm not sure if the M3 set up will fit the rear of my car (318is).

Is there any reason that the rear turrets cant be plated to help take the stresses?

Will grp N front struts fit the 318is? Sorry, new to E30's

[Theo]

Group N front shocks and springs will fit your iS, but will be very expensive. This link may be of some interest: http://sandstromracing.se/motorsport/M3E30_GrN.pdf

For what it's worth the KW coilovers don't use a proper rear coilover unit, instead they use an adjustable spring perch located in the standard spring position.