CMP Solid Raising Subframe Bushings

[E46m3zcp]

I'm looking at putting reinforcement plates to address the RACP issue. I read CMP engineering's technical article about their solid "raising" bushings but I don't understand.

Here is what I do understand:
Plates will increase the distance between the CG and the subframe mount points. The height of the subframe w.r.t the ground will not change, so the height of the Center of Rotation (CR) will not be affected. Since the distance between the CG and CR increases, the torque exerted at the CR will increase, thus increasing body roll when cornering. Body roll will create positive camber on the inside during cornering, which is bad.

So far so good...

So how exactly are "raising" bushings fixing this problem?

What are they raising?
If they're raising the CR, for one wouldn't it be counterproductive - since it would increase the jacking effect during cornering? Also, how can they raise the CR without also raising the CG?

I don't get how these work. If someone does, I'd be grateful for the explanation


Another quick related question is: how much will plates affect the suspension geometry, really.... ? It's just raising by a few millimeters after all, isn't it? Does anyone have technical details about this?

[old///MFanatic]

When they say they are "raising", from what I gather is they're actually shortening the top of the new aluminum bushings. Because of the less bushing material at the top the chassis then sits lower on the axle carrier (subframe).
I have posted before the term "raising" isn't how I would describe it.

[STAATS]

I never thought about the weirdness in the terminology and it just kinda clicked when I saw them but they are essentially just shorter aluminium subframe bushings so they "raise" the subframe up closer to the frame... i believe they bring the subframe ~6mm closer to the frame. This just help correct the angle of the suspension arms on lowered cars... I wouldnt say its a project worth chasing but if you are doing the RACP job and/or replacing those bushings anyway then these are a no brainer if you are running ~6mm or greater lower (or planning to in future) than stock height. if you are stock height then normal aluminium bushes like from TMS etc are probably better as youre running more stock geometry

[AussieE46M3]

The term raising is referring to the subframe and all attached components being raised closer to the chassis.

For a car on flat ground, the subframe is raised further from the ground as the static spring length is the pivot point and what you would perceive as a constant as the weight of the car hasn't changed nor has the motion ratio.

The centre of rotation or roll centre is defined by the angle the rear arms are pointing in and thus, raising the inner end relative to the chassis has them pointing more upward raising the roll centre.

You are correct that the centre of gravity does also go up as the subframe and diff which are quite heavy are raised in the process however, when plotting roll centre it isn't a linear geometry but, parabolic so the rise in roll centre is disproportionate to COG and thus, they do end up closer together inducing a smaller roll moment (rotational force).

The more practical benefits of raising the subframe are found when doing so to lowered cars. You'll notice that these cars when stock sat incredibly high off the ground and many people run their cars quite a lot lower than that.

Once issue is track width. When you excessively lower a E36/E46 the track width increases so much that the cars have excessive toe in (physically cannot achieve ideal camber without aftermarket toe brackets) and also hyper extend the CV's in the axles and thus can blow the CV's easily. If you look up axle spacers it's a very common mod on drift/trackcars that are lowered.

The other is simply the angle of all the arms. As I mentioned above the roll centre as well as camber & toe geometry are not linear but parabolic (varying rate of change through linear displacement). What this means is that if your static position is further along the parabolic plot line the change in camber, toe & roll centre geometry you experience in bump is more dramatic for the same amount of vertical displacement than it would have been if the car was at a taller static ride height. The other issue is axle misalignment. The axles can also sit at quite an upward angle which can be excessive on the CV's in significant bump travel.

And thus, raising the subframe brings the suspension geometry (on a lowered car) back closer to it's designed range of motion and rate of change through bump and squat travel.

As I mentioned in another recent thread,

You'll see on the Team Shirmer cars and many other quality tracks cars that they employ eccentric spherical bearings in aluminium housing pressed into the upright and subframe on newer beemers to achieve a similar effects geometry wise. The benefit of raising the subframe is you get to keep stock rubber or sealed ball joints and correct axle misalignment while their approach uses exposed sphericals which offer a significant reduced service life requiring additional maintenance which is an acceptable compromise on a race car however, only corrects the geometry side not the driveline part. This is where they also modify the subframe and fit eccentric diff bushes to raise the diff relative to the subframe and thus body to straighten out the axles to the upright. This also flattens out the prop from the CSB to diff as well.

Fitting subframe raising bushes achieves all the changes the above mods achieve in a single part on a far smaller budget and with less complexity although, with the compromise that the subframe is higher up and some bolts are a little harder to get to.

As Staats said, the change is very small and thus it wouldn't be suggested that they replace another brand of solid subframe bush or would it be suggested that it be your first suspension mod or fitted to a car that will not be lowered at all compared to a factory ride height.

Another point, the subframe bushings are the same length as the factory as that cannot be changed due to the double shear bracket on the front two mounts however, where the bushings position the subframe along their length is up closer to the chassis.

[E46m3zcp]

Thank you for all the details. It all makes more sense. On your website, you say "To be clear, the need for adjustable camber arms when fitting these bushes only exists when the car is still at stock ride height." That part got me thinking that fitting these solid rising bushings would be a mod that could make sense at stock ride height. But what you seem to be saying here (and in other parts of your writeup on the website) is that it doesn't, really. It corrects the suspension geometry for lowered cars by positioning the subframe closer to the body.

I'm also assuming that the 2mm increase in the distance between the subframe and body introduced by RACP reinforcement plates has a negligible effect on suspension geometry. Do you think that's true?

In the case of an M3 used for street & track and maintained relatively stock (stock ride height at least), it seems that solid bushings (including rising ones) don't really make sense. The additional roll resistance doesn't seem to make up for the additional strain on the fragile subframe mounting points created by solid bushings (even if reinforced, it's still a design flaw). There are other ways to increase roll resistance that won't affect the strain at the subframe mounts (anti-sway bars or stiffer springs). But if we're talking about a race car, it's a different story.

[AussieE46M3]

Thank you for all the details. It all makes more sense. On your website, you say "To be clear, the need for adjustable camber arms when fitting these bushes only exists when the car is still at stock ride height." That part got me thinking that fitting these solid rising bushings would be a mod that could make sense at stock ride height. But what you seem to be saying here (and in other parts of your writeup on the website) is that it doesn't, really. It corrects the suspension geometry for lowered cars by positioning the subframe closer to the body.

I'm also assuming that the 2mm increase in the distance between the subframe and body introduced by RACP reinforcement plates has a negligible effect on suspension geometry. Do you think that's true?

In the case of an M3 used for street & track and maintained relatively stock (stock ride height at least), it seems that solid bushings (including rising ones) don't really make sense. The additional roll resistance doesn't seem to make up for the additional strain on the fragile subframe mounting points created by solid bushings (even if reinforced, it's still a design flaw). There are other ways to increase roll resistance that won't affect the strain at the subframe mounts (anti-sway bars or stiffer springs). But if we're talking about a race car, it's a different story.

Glad it helped.

The thing about camber arms and stock ride height is based on the camber curve. It's like raising the car slightly which does move the wheels into a more positive camber position. When lowered, this means closer back to stock. Stock ride height this means more positive that it should be. I think the max adjustment with these and stock springs & camber arms is -0.5' which isn't ideal for cornering.

I would say it is negligible depending on the application. For a stock car that just cruises around town, absolutely. If you're building something you want to really handle like a track car then it's worth considering when building your car. The stock geometry was designed for street use and as mentioned above things get a bit excessive when you lower it to a more performance oriented ride height so compensating for the plates and some is a good idea to reduce roll moment.

As I mentioned, this is very common practice on track cars such as drop knuckles which you can see from brands like wisefab or eccentric bushings if budget or rule limited.

It's an each to their own . If you're wanting to improve handling the subframe bushings are a very significant point of interest IMO and if one option has something to offer over another for the same money then why wouldn't you. You see lots of brands are selling more and more track focused cars for the street so things like this help to wind back the clock so to speak on how these cars handle to make them tighter and more nimble to compete with more modern cars.

Your last point I do have to disagree with. My belief is that solid subframe bushings are a must for any E46 (and just about any other BMW). The reason I consider them significant is that everything in the back end (including trailing arms on newer beemers) bolts to the subframe and thus replacing the rubber for a solid mount helps improve link rigidity and reduce wheel hop in the driveline. So when the body rolls and the springs & sway bars load up they jack the subframe causing more flex than just the body roll. This is something BMW figured out for themselves when they fitted aluminium subframe bushes to E92 M3 GTS's which was eventually replaced with a light weight tubular subframe with no bushings at all in the F8X and new G8X generation of M cars.

Yes, E36 and E46 has the chassis issues however, they can be addressed and on most cars this issue doesn't exist at all. If anything, the subframe bushings are so much more effective on the E36 & E46 because of how narrow the subframe is. The bushings are so much more prone to flex as they have so little leverage to resist driveline forces.

The bushings also massively improve the feedback to you in the driver seat which IMO also helps to improve the pleasure of driving and the engagement and as the diff still has it's own bushings there's no (noticeable) compromise in NVH.

I think it's a go try it for yourself and see thing. I put Power Flex poly in my M3 the first time I had it apart and it felt no better than stock while the solids really transformed the car for me. I think once you drive a car with them, you'll notice the difference and realise why even BMW went that way.

[E46m3zcp]

Great info here, thanks for the write-up By what amount is your bushing raising the SF?

I guess one last thing I have in mind is: okay, suppose I lower the car and use the rising bushings to fix the rear suspension geometry. What about the front? I'm by no means an expert but I think the MacPherson strut assembly is particularly sensitive to ride height variations. Is there a similar way to correct the geometry in the front you'd recommend?

[AussieE46M3]

Great info here, thanks for the write-up By what amount is your bushing raising the SF?

I guess one last thing I have in mind is: okay, suppose I lower the car and use the rising bushings to fix the rear suspension geometry. What about the front? I'm by no means an expert but I think the MacPherson strut assembly is particularly sensitive to ride height variations. Is there a similar way to correct the geometry in the front you'd recommend?

About 8mm compared to stock rubber. 6mm with plates fitted. This is essentially the limit as the subframe cannot get any closer to the chassis.

Front is a great point, as the roll geometry on a McPherson strut is even generally even less ideal than the rear.

Unfortunately due to the front subframe not being on bushings and hard mounted straight to the chassis there isn't any way to raise the inbound end up like we do in the rear so the alternative is outside down.

To do that you've got 3 options.

1, Is custom front control arms and outer tie rod ends that use either a block of aluminium (like on many drift angle kits) bolted to the underside of the upright or large misalignment spacers to lower the pivot point relative to the wheel centre. Downside is non-standard arms and exposed sphericals. Examples of this are the SLR kits or Wisefab.

2, An angle adaptor kit which is similar to the block they use in angle kits in that the pivots points are lower however, this is done by pushing the upright out so the pivot points are staggered increasing scrub radius, camber and track width. An example of this is the relatively cheap angle adaptors offered by brands such as PMC. Whether changing wheel offset and camber position at the strut tops can help compensate for the negatives I do not know.

3, A drop knuckle. Which is a custom knuckle/upright with the pickups for the lower control arm and tie rod lowered in a custom designed upright. I don't know of anyone doing this for E46 but I do know of the Strom Motorsport piece for E92's which has caught my eye however, is pretty expensive at $3695 USD while the rear bushings are roughly $225 USD

https://www.strommotorsports.com/uprights

I was contemplating making something like #2 for the non-M e46 drifters and can experiment on how it lasts to see if the same can be done without the steering angle and Ackerman being changed for M3's.

[Tbonem3]

How about, don't lower your car beyond 13.5"/13". Bonus points for reducing shock mount thickness for reclaimed shock travel.

The raising bushings make sense to me when plates are added.

[r4dr]

How about, don't lower your car beyond 13.5"/13". Bonus points for reducing shock mount thickness for reclaimed shock travel.

The raising bushings make sense to me when plates are added.

The 6mm with plates is about a quarter inch. Isn't 13" rear ride height more than a quarter inch lower than stock?

[Tbonem3]

Not sure what you're getting at. I was just saying rather than spend a bunch of money and brain power on "fixing" the car from being too low, just don't go too low.

[r4dr]

Not sure what you're getting at. I was just saying rather than spend a bunch of money and brain power on "fixing" the car from being too low, just don't go too low.

My point is, your metric of "not too low" of 13" rear ride height is more than 6mm lower than stock. So wouldn't these bushings help correct the suspension geometry closer to stock geometry even at that ride height?

[Thoglan]

They seem like a no brainer if you're lowered to the extent that most people are when running these cars.

Next time I'm dropping the rear subframe I'll grab them. CMP seems to have quality stuff too from the things I've bought from them

[Tbonem3]

So what