[turugara]

I've seen a true rear coilover setup for the E9x, mainly on track/race cars.

What's the benefit? Why don't we all have that type of setup?

Example:

[cvc 22349a]

That set up might tear the subframe away from the chassis and would require extensive reinforcement of the shock towers and solid subframe mounts.
Those spherical joints don't look so hot btw.

[CJ421]

Actually I think the coilover rear would be less stress on the subframe.

The inboard spring setup puts a substantial amount of stress on the subframe. The spring sits in the camber link (which is connected to the subframe) and is connected at the top to the body; so every time the spring compresses, it's acting on the subframe (literally pushing it away from the body).

The spring has to be quite stiff to begin with because the spring's inboard location means the wheel rate is 1/3 of the spring rate (there are exact numbers on this forum somewhere). In the front the wheel rate is 1/1.
That's a problem because it greatly limits how much lb/in we can use in the rear. The maximum rear rate is about 800 lb/in; evidence for this is that none of the coilover kits available for our cars or the M3 go above that in the rear. We can increase the front spring rate FAR more than the rear, but to maintain a front-rear balance we can't go nearly as high in the rear and that limits the front.

So if you switch to a coilover type rear, your wheel rate is much closer to one (about 4/5ths I imagine) and that means you can increase the spring rates back there a lot more (and therefore the fronts too). The spring will now put much more force on the body of the car (no longer on the subframe) so I think cvc is right that some reinforcement will be needed in that area.

Edit: I don't know what is involved in the rear coilover conversion. VAC Motorsports runs it on their E92 M3 w/ Nitron NTR R3s. And I know there's other benefits from what I've researched so far. I stopped looking into it after I realized I was not going to go with a really high spring rate.

[dvpouldar26]

The reasoning behind a rear coilover is because of the motion ratio of the rear suspension. In current form, the motion ratio is about 50%, meaning that with an 800lb spring in the rear, there is an effective rate of 400lb at the wheel. With this current geometry, an extremely high spring rate will need to me run in the rear, however, this causes a lot of stress on the rear subframe. The subframe would need to be reinforced. By going to a rear coilover, the spring is moved out closer to the wheel, increasing the motion ratio. In doing this, the spring rates that can be used are not limited by how much force the subframe can handle. This is the same reason why so many subframe problems were seen in the e46 m3's. Hope this helps.

[turugara]

Originally Posted by CJ421

I stopped looking into it after I realized I was not going to go with a really high spring rate.

This is only partially true. Edited for truth.


I shouldn't have stopped looking into it because eventually I know I going to go with a really high spring rate because racecar.

[turugara]

Originally Posted by dvpouldar26

The reasoning behind a rear coilover is because of the motion ratio of the rear suspension. In current form, the motion ratio is about 50%, meaning that with an 800lb spring in the rear, there is an effective rate of 400lb at the wheel. With this current geometry, an extremely high spring rate will need to me run in the rear, however, this causes a lot of stress on the rear subframe. The subframe would need to be reinforced. By going to a rear coilover, the spring is moved out closer to the wheel, increasing the motion ratio. In doing this, the spring rates that can be used are not limited by how much force the subframe can handle. This is the same reason why so many subframe problems were seen in the e46 m3's. Hope this helps.

So it's better for the subframe?

[CJ421]

Great post dvpouldar26.

Originally Posted by turugara

So it's better for the subframe?

Unquestionably. I added some more info to my post above.

Subframe problems:
http://www.m3post.com/forums/showthread.php?t=659693
Page #2 is where there is good info.

[turugara]

Very cool. I'll inquire as to how much work it'll take to get this done. I don't imagine much given all that we've done already CJ.

[dvpouldar26]

Originally Posted by CJ421

Great post dvpouldar26.

Unquestionably. I added some more info to my post above.

Subframe problems:
http://www.m3post.com/forums/showthread.php?t=659693
Page #2 is where there is good info.

+1. I can't tell you how many cracked and damaged subframes i've seen on e46 m3's because of this reason. A friend of mine is running an 1100lb rear spring with motons on his e92 m3. I think this is the highest spring rate I have seen run on the rear of these cars. KW CS's come with 800lb rears out of the box.

[turugara]

Talked to VAC. You need extensive shock tower reinforcement and not a mod recommended on a street car. Otherwise it is very likely that you may damage the chassis.

[CJ421]

Originally Posted by turugara

Talked to VAC. You need extensive shock tower reinforcement and not a mod recommended on a street car. Otherwise it is very likely that you may damage the chassis.

Thanks for the report.

Going off that train of thought - given reinforcement is required, it makes me reconsider doing solid subframe bushings. I really do think there needs to be some flex where the subframe connects, there otherwise there'd be too much stress on where the shock connects to the body.

[turugara]

All I know is that at my current setup level, Im limited by tires and personal skills. Perhaps when because racecar with full aero in a year or so, I'll revisit this.

Originally Posted by CJ421

Thanks for the report.

Going off that train of thought - given reinforcement is required, it makes me reconsider doing solid subframe bushings. I really do think there needs to be some flex where the subframe connects, there otherwise there'd be too much stress on where the shock connects to the body.

[CALWATERBOY]

Originally Posted by turugara

Who's selling this setup?

Or is it home-grown?

[turugara]

I know a lot of high end brands are like moton/AST, JRZ's, or nitron are offering it.

[CALWATERBOY]

Originally Posted by turugara

I know a lot of high end brands are like moton/AST, JRZ's, or nitron are offering it.

Looks like Ground Control

[DaveOH32]

Originally Posted by CALWATERBOY

Looks like Ground Control

How so?

The shock is an inverted Moton.

[Doyle]

I would avoid full true coilover until you run a full cage. The added rigidity will allows the higher spring rate to function better. Think of the body as a third spring.

[mike-y]

Originally Posted by CALWATERBOY

Looks like Ground Control

That setup is using an Eibach Race Spring (ERS line), which just happen to be used/sold by Ground Control - but anyone can buy them for custom setups, similar to what HPA does with the swift line.

[CALWATERBOY]

Originally Posted by DaveOH32

How so?

The shock is an inverted Moton.

Spring color & hat.

But like Mike-y says, OEM'd by Ground Control; in this thread wasn't a Ground Control setup.

[HP Autosport]

Less stress on the subframe bushings and the sheetmetal around the subframe mounts, but more on the shock towers. You will likely need to reinforce the towers or run a cage.

More consistant spring rates(no spikes) since you will no longer have the slight bending of the springs due to their current location between the lower camber link and the frame of the vehicle and the arc the lower camber link must travel through.

You can also swap this coilover setup(spring and shock) in minutes.

The list goes on and on...

[ajm8127]

Looks good, Andrew. Why no tubes to the structural support seen at the top of the first picture?

[ajm8127]

Originally Posted by AndrewLang

Do you mean the rear parcel shelf? I didn't see anything up there of significant structural integrity worth attaching to, although this is just my opinion. The other issue was cost and weight. The owner probably would have prefered I use less tubing to save weight but this is what I felt was needed at a bare minimum considering the lack of the cage. The way his build is going I expect him to come back for a half or full cage in the near future so I wanted to design the rear bracing structure to work well with a cage.

There is a piece of sheet metal formed into a 'U' channel shape that runs along the top of the inside of the trunk from one side to the other. I remember thinking it looked structural after looking inside my trunk. I would think what you have is fine though, I was just curious.