Got it, thanks Jonb94118 & flink. Reviewing AKG web site catalog, I see the E46 subframe bushings (75D and 90A) have aluminum center sleeves but the E36 bushings (75D and 95A) do not. Hence my confusion--I've been looking only at their E46 catalog.
E46 (including M3) 75D subframe bushings:
SF46Dlg.jpg
E36 75D subframe bushings (all E36 except 318ti)
SF36Dlg.jpg
I just installed the 95a, the front bushings seemed to take a lot of turns before hitting 40ft-lb. The rears not so much. Not sure why that was.
I plan to check them frequently...if they come loose I'll just go back to stock. Not really interested in putting aluminum bushings in a car that gets street driven a decent amount.
Local to me, RevShift, used my rear end to make their E36 Subframe bushings... Cut lips, trimmed for the welds and aluminum sleeves.
This has been a helpful thread. My subframe was recently found to be cracked and so a replacement is being installed with new bushings for the subframe and diff. The previous owner installed solid aluminum diff bushings with OEM subframe bushings. This may be the cause of my subframe failure as all the torsional drivetrain load was being transferred directly to the subframe bushings/mounts.
I like the theory of going with stiff/hard subframe bushings in order to create a solid foundation for the rear suspension. I would couple these with a softer poly diff bushing set, maybe 95A. I want some drivetrain windup/compliance to help keeps things smooth and tame NVH.
Does this sound logical? Any additional input/advice? Any long term durability risks?
This is for a street legal track car that is only driven to/from events. I'd rank my priorities as #1 durability, #2 performance, #3 comfort
Thanks
That's exactly what i said on post #68, but i haven't heard from anyone. I'm buying all bushings end of this month or beginning of next. so we shall see how it does.
The 95As are a good choice.
I think your conclusion is 100% correct.
First let me address NVH. This is extremely subjective. I apparently tolerate a lot of NVH. Maybe it is all the laps I've done in pure race cars, but NVH means has to be pretty bad before I notice it. I've had customers that can't stand their cars after switching to poly subframe bushings from the softest end of the scale. I personally don't notice them. Most street NVH is the kind of stuff I'm programmed to ignore at the track as "normal". So I can't comment on how much NVH will be present with compound ##X, but inherently the stiffer the compound the more NVH. That's just basic acoustics.
Looking at it from a mechanical perspective, anything that attaches suspension should be as solid as the driver can tolerate. Solid aluminium bushings in a subframe turns a part that induces torsional stress on the chassis into a brace for the chassis. On a track focus car, I personally make everything that doesn't move solid aluminum and everything that does a bearing. Why? Because I want the suspension to move in consistent predictable ways. Now on some areas of the chassis, like front strut mounts, this can increase the momentary forces. Thus hitting a curb with a OEM strut mount will transfer less energy to the strut towers than if you have race oriented camber plates with spherical bearings. On a the rear subframe this can be true too, because the rubber isn't there to absorb the energy (aka convert it to heat). The difference is that the solid bushings in the subframe distribute the load over multiple points, unlike the previous strut tower example.
As for the drivetrain, I think movement is more tolerable. The stiffer the mounts are the less energy is wasted and makes it to the wheels. Thus solid mounts are good for extracting that last fraction of a hp. I think it comes at a cost to the mechanical components, though. I typically do not like running aluminium or 75D mounts for the drive train. One thing I do recommend is matching compounds. If you main one part of the drive train really stiff, some energy from drivetrain shock will be transferred to other softer areas of the drivetrain. Thus I like engine, tranny, and diff mounts to have similar properties. They don't have to be identical, but in the ballpark.
Now, will qualify my statements as theories. I haven't done extensive research on this. This is "barstool engineering" that I've utilized with my cars and customer cars with good results. YMMV.
So in short, I would think no major issues would arise from doing solid subframe mounts and softer differential mounts (I wouldn't mix aluminum subframe with OEM diff mounts, though). The softer differential mounts will help with drivetrain NVH, but won't reduce road and suspension NVH. Excluding NVH, I think you have a good plan.
-bj
2002 M Coupe | 2000 M Roadster w/ LS | 1998 328i w/ S54 | 1987 325is w/ M52 | 1985 735i Turbo
Thanks for the thorough response. I still can't make up my mind on if solid mounts are better or worse for the integrity of the unibody/mounting points.
On one hand with solid mounts, the energy from a bump in the road that makes it to the subframe will be transferred directly to the unibody. However, all the mounting points are rigidly fixed, so is the energy dispersed or shared over the mounting points? Does having them all fixed prevent local flexing/fatigue at a single point?
On the other hand, soft mounts will allow the subframe to rock relative to the unibody. Could this result in one mounting point taking more of the energy? Could the rocking effect cause a push/pull fatiguing effect on the mounting points?
Alas, I'm an electrical engineer...
It seems that most subframe mounting point failures are seen in cars with worn out stock bushings. However, most cars have worn out stock bushings... Anyone see a car with solid mounts have unibody/subframe damage from stress/fatigue?
Honestly, things like bumps will mainly be absorbed by the device that is designed to do just that: the damper. There can be and is a torsional load placed on the wheel that gets transferred to the subframe as well, but as you assumed this is dispersed across the subframe and unibody with solid mounts minimizing the flex and impact on any single mounting point.
Your last question is spot on. As torsional forces are applied, they are typically transferred at the rear of the subframe where the rear control arms attach. It is not surprising that when we see cars with unibody damage, the rear mounting points are always the worst. I've seen many cars with rear unibody tears. I've seen some with both rears and one front. I've seen some with all 4 corners cracked or torn. What I have never seen is only the fronts damaged. To me this indicates that the rear takes more force, and it is only after the rears fail or the bushing is extremely fatigued that the fronts fail.
We've done a lot of repairs and inspections. I've never seen a car with non-OEM mounts with this failure. Although we have done quite a few of these, I don't know that my sample size is large enough to draw definite conclusions, though.
-bj
Last edited by loftygoals; 12-21-2014 at 01:07 AM.
2002 M Coupe | 2000 M Roadster w/ LS | 1998 328i w/ S54 | 1987 325is w/ M52 | 1985 735i Turbo
Well this definitely reinforces my thoughts behind the selection of aluminum subframe bushings and 95a differential bushings. I could post my thoughts about them once they are installed, but i will be installing more items along with these so i don't know if i will be able to accurately say what they did.
My e46-M see’s between 15-20 track days a year, maybe some TT time too. Currently the car has most of the interior all bushings are OEM with TCK DA’s and limiters on the RTAB’s. Other than the suspension and breaks, the car is all OEM.
I would like to upgrade the car for “track use only” other than being driven to and from the track.
Prior to reading this thread I was thinking of installing AKG 75D mounts for motor, trans, and diff with Turner aluminum sub-frame bushings. However after reading this thread I’m thinking it might be better to go with solid aluminum everywhere as over time this will likely become a caged track car.
Interested to read others thoughts?
Last edited by suss1173; 01-11-2015 at 12:15 AM.
Solid mounting is crazy. Tremendous increase in peak stress in return for which you get nothing. Something has to flex - would you prefer that it be the bushings, or the sheet metal?
AKG's 75D e46 subframe bushings have the center ferrules so they should be OK. I can't imagine why AKG omitted them from the e36 bushings - their e36 318ti/Z3 bushings have the ferrules.
Personal feeling:
Agree with your choice of Al subframe mounts.
Disagree, however, with solid mounting the drivetrain (this includes delrin/75d/nylon). You dont want the vibration or noise or mental anguish of wondering why your diff is so effing loud all of a sudden, associated with solid stuff. Especially when you used the words "over time" when describing the car's path to racecardom.
There is no detriment to the drivetrain moving around to absorb vibration (within reason). It is designed to. That's why there are also rubber isolated center bearings, rubber flex disks, dual mass flywheels, etc... all trying to reduce NVH for the passenger and reduce shock loads for the system itself. It's not like some compliance in the mounts is adversely affecting suspension geometry like flexy subframe mounts are.
When it comes to the drivetrain, let the big dog eat! I say stick to mid high Shore A durometer poly (AKG or whomever your favorite is) or HD rubber (BW Gr N replica engine mounts for instance).
[2004 330xi/6] Orient/Natural :: 117-142k :: ZSP :: ZPP :: ZCW
[1998 M3/4/5] Cosmos/Black :: 113-125k :: TCKline D/A (500/600) :: GC Plates :: RD Sways :: GC Rear Arms :: ZHP Rack :: 3.64 Diffsonline :: PF FCAB :: BW RTAB :: AKG Subframe :: TMS Pulleys :: AA & Borla :: XBrace :: TRM C2s :: BW lines :: DTC60 :: Safety Equipment
Past:
[1995 M3] Avus/Dove :: 141-242k :: S52 OBDI :: M50 manifold :: 3.5 HFM :: Turner Chip :: XBrace :: SS Lines :: Turner RTABS :: Vogtland CS :: Bilstein Sport :: Z3 rack
[1999 M3] Cosmos/Black :: 65-87k :: TCKline S/A (400/500) :: Turner plates :: UUC Front Sway :: PF RTABs :: AA Intake :: Borla :: XBrace :: TRM C2s :: CL RC6E :: Safety Equipment
[1993 325is] Brilliant/Black :: 135-139k :: Bilstein sports :: Eibach sways and springs :: Dinan camber plates and chip :: Borla :: LSD
[1983 320i] Safari/Brownish :: 219-242k :: homebrew CAI :: some rust :: multicolor body panels
I'm in the early stages of turning my car into a streetable track/auto-X car. In preparation for doing a rear subframe reinforcement and in typical "while you're in there" thinking I've been reading through dozens of threads about rear subframe bushings across multiple forums and I'm a little frustrated by the wide variety of opinions on types of bushings to use (yay internet ). Since this is the most recent thread on the subject, I'd like to post my thoughts and keep the discussion going.
Majority opinion seems to be go hard as possible on subframe and keep diff softish. How hard or soft is bit less clear. And of course there's the few persistent voices claiming that any poly in subframe is a bad idea as well as a few people who have diff and subframe hardness reversed. The really odd thing about that last one is one of the big voices on solid diff bushing with OE rubber in subframe is Turner Motorsport, who I've presumed up to this point to know what they're talking about.
One of the other interesting things I came across was someone who posted elastic modulus values for all relevant materials. I looked up and ran some of the numbers myself and they more or less matched. Rubber varies in stiffness, so it's hard to say for certain, but 95A is about as strong as the stronger rubbers. 75D's modulus is a few times higher, really tough polymers like delrin are a couple orders of magnitude higher, and aluminum is a couple more orders of magnitude on top of that. I found this pretty interesting as it shows just how soft the softer poly compounds are, but it also indicates that 75D is not nearly as hard as a lot of people seem to think, but it explains why most people didn't have much NVH problems with it compared to aluminum.
For my purposes, my autocross class rules prohibit replacing anything with solid metal that wasn't solid metal before. That eliminates metal bushings, but for a streetable car that's probably ok anyways. The revelation that 95A is roughly equivalent to the rubber it's replacing eliminates the softer polyurethane compounds. A fair amount of anecdotal evidence sprinkled throughout the forums indicates that quite a few people who put the Powerflex purples (80A) were later disappointed in some way with them and makes sense based on the technical information I've managed to digest.
For subframe mounts, it seems my only option is poly (75D or 95A) or stock rubber. The vast majority seem to have beneficial performance and and wear from poly of some kind, though a few had problems with deformation. Since the stock rubber is known to be problematic over the long term, I'm inclined to go with AKG 75D and hope for the best, a few bad experiences with deformation not withstanding.
For differential, I'm inclined to leave stock for now. I can't do solid even if I wanted to due to class rules and 75D seems to be stiff enough to cause a little diff whine. I'll check the current bushings closely for wear and if replacement is required, it seems sticking to rubber or 95A is best.
Rubbers span a range as does poly. You said it yourself. Short of measuring, you can't know how stiff the rubber is in that specific application. Throwing 75D or whatever at all of the mounts and all the bushings as some think is prudent may leave you with a larger before/after stiffness delta on some mounts than others.
Buy a 50 dollar Shore A tester on amazon and do some poking (prolly a cheaper one but I didn't look too hard)
[2004 330xi/6] Orient/Natural :: 117-142k :: ZSP :: ZPP :: ZCW
[1998 M3/4/5] Cosmos/Black :: 113-125k :: TCKline D/A (500/600) :: GC Plates :: RD Sways :: GC Rear Arms :: ZHP Rack :: 3.64 Diffsonline :: PF FCAB :: BW RTAB :: AKG Subframe :: TMS Pulleys :: AA & Borla :: XBrace :: TRM C2s :: BW lines :: DTC60 :: Safety Equipment
Past:
[1995 M3] Avus/Dove :: 141-242k :: S52 OBDI :: M50 manifold :: 3.5 HFM :: Turner Chip :: XBrace :: SS Lines :: Turner RTABS :: Vogtland CS :: Bilstein Sport :: Z3 rack
[1999 M3] Cosmos/Black :: 65-87k :: TCKline S/A (400/500) :: Turner plates :: UUC Front Sway :: PF RTABs :: AA Intake :: Borla :: XBrace :: TRM C2s :: CL RC6E :: Safety Equipment
[1993 325is] Brilliant/Black :: 135-139k :: Bilstein sports :: Eibach sways and springs :: Dinan camber plates and chip :: Borla :: LSD
[1983 320i] Safari/Brownish :: 219-242k :: homebrew CAI :: some rust :: multicolor body panels
95A basically feels like OEM to me. I have 95a subframe with stock diff bushings and it feels like any other BMW with fresh bushings. If I did it again I would do 75D subframe, but I have pretty much abandoned streetability manners in this car.
On my dual-duty E46 car I put in AKG 95A diff bushings and 90A FCABs. Bought AKG 90A subframe bushings but haven't installed them yet. RTABs are stock with Vorshlag nylon (or Delrin) enforcers.
For the E36 I put in Powerflex black RTABs and plan to do 75D subframe bushings with 95A diff bushings, but haven't bought them yet. Ordered UUC poly FCABs but they're back-ordered. The AKG E36 subframe bushings don't have the cylindrical metal cores and I'm wondering if that's okay or if it will lead to unusual compression or wear on the poly bushings (the AKG E46 subframe bushings and the PowerFlex E36 subframe bushings do have metal cores).
Last edited by LivesNearCostco; 01-13-2015 at 04:12 PM.
[2004 330xi/6] Orient/Natural :: 117-142k :: ZSP :: ZPP :: ZCW
[1998 M3/4/5] Cosmos/Black :: 113-125k :: TCKline D/A (500/600) :: GC Plates :: RD Sways :: GC Rear Arms :: ZHP Rack :: 3.64 Diffsonline :: PF FCAB :: BW RTAB :: AKG Subframe :: TMS Pulleys :: AA & Borla :: XBrace :: TRM C2s :: BW lines :: DTC60 :: Safety Equipment
Past:
[1995 M3] Avus/Dove :: 141-242k :: S52 OBDI :: M50 manifold :: 3.5 HFM :: Turner Chip :: XBrace :: SS Lines :: Turner RTABS :: Vogtland CS :: Bilstein Sport :: Z3 rack
[1999 M3] Cosmos/Black :: 65-87k :: TCKline S/A (400/500) :: Turner plates :: UUC Front Sway :: PF RTABs :: AA Intake :: Borla :: XBrace :: TRM C2s :: CL RC6E :: Safety Equipment
[1993 325is] Brilliant/Black :: 135-139k :: Bilstein sports :: Eibach sways and springs :: Dinan camber plates and chip :: Borla :: LSD
[1983 320i] Safari/Brownish :: 219-242k :: homebrew CAI :: some rust :: multicolor body panels
It has been mentioned elsewhate. Not sure why, but thats how AKG makes them. Personaly, I would want the metal center.
[2004 330xi/6] Orient/Natural :: 117-142k :: ZSP :: ZPP :: ZCW
[1998 M3/4/5] Cosmos/Black :: 113-125k :: TCKline D/A (500/600) :: GC Plates :: RD Sways :: GC Rear Arms :: ZHP Rack :: 3.64 Diffsonline :: PF FCAB :: BW RTAB :: AKG Subframe :: TMS Pulleys :: AA & Borla :: XBrace :: TRM C2s :: BW lines :: DTC60 :: Safety Equipment
Past:
[1995 M3] Avus/Dove :: 141-242k :: S52 OBDI :: M50 manifold :: 3.5 HFM :: Turner Chip :: XBrace :: SS Lines :: Turner RTABS :: Vogtland CS :: Bilstein Sport :: Z3 rack
[1999 M3] Cosmos/Black :: 65-87k :: TCKline S/A (400/500) :: Turner plates :: UUC Front Sway :: PF RTABs :: AA Intake :: Borla :: XBrace :: TRM C2s :: CL RC6E :: Safety Equipment
[1993 325is] Brilliant/Black :: 135-139k :: Bilstein sports :: Eibach sways and springs :: Dinan camber plates and chip :: Borla :: LSD
[1983 320i] Safari/Brownish :: 219-242k :: homebrew CAI :: some rust :: multicolor body panels
Precisely. I retorqued mine probably 5-6 times. It started out at maybe 3/4 of a turn after one session, ended up at 1/16th of a turn after a weekend. But something was still moving. It's quite disturbing to have that happening so I had Tony@TCD toss them and install powerflex bushings.
I went with AKG 75D subframe bushings and AKG 95A diff bushings. I haven't driven the car yet, but I'll report my experience.
Not sure what to do about the lack of center sleeve in the subframe bushings, I'll play it by ear i guess. The shop who installed them said that's their standard for race cars and they don't have any issues with them.
I emailed AKG a few months ago and they said it they were designed that way on purpose. Maybe it's to let the OEM washers squish the bushings against the body bit? That wouldn't worry me so much with 75D bushings because they are pretty stiff. Just seemed weird because their E46 poly subframe bushings do have the metal sleeves for both 95A and 75D.
That said I haven't heard or read of any problems with the AKG E36 poly subframe bushings.
Last edited by LivesNearCostco; 01-13-2015 at 10:19 PM.
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