E36 dedicated Track car - Rear Subframe Bushings - AKG Aluminum or 75D Poly?

[timsen]

Looking for recommendation for rear subframe bushings for a track-only E36.
I'm fix on getting AKG bushings but not sure if I should go with the Aluminum ones (#SF36L) or Poly 75D (#SF36D).

I know the aluminum ones are harder to install, are they still worth it? Or are the poly bushings that hard that there isn't any difference in handling?

Any feedback appreciated!

Thanks,
Tim

[testify]

I just got a set of 75D AKG subframe and diff bushings in the mail, and was actually a bit surprised how hard the 75D poly actually is. Definitely the stiffest poly Ive seen. Feels every bit as hard as a piece of aluminum in your hand. Quality seems spot on too. Good looking, solid product with correct tolerances.

[JPJ]

To put 75D in perspective, from "bowlingball.com":

Durometer - A device used to measure the hardness of a bowling ball. The USBC limits the hardness to be no less than 72D.

[NeilM]

I wouldn't use either of those options without first strengthening the chassis area where the subframe mounts. This is a known failure point for the non-M E36 chassis, which lacks the reinforcements fitted to the M3.

See http://www.turnermotorsport.com/p-39...ement-kit.aspx

Neil

[timsen]

Chassis reinforcements will be installed at the same time.
Any other thoughts on poly vs. Aluminum?
It sounds like people go mostly with the poly?

[Inflame]

I went poly 75D everywhere except rtab which I chose 95A, all from RevShift.com

[loftygoals]

First, let me say that the reinforcements are definitely a must.

As for bushing material, my thoughts, what I use, and what I recommend to my customers for the subframe is aluminum. So my reasoning for this is simple: almost every rear subframe mount point failure I've seen at the shop was on a car with tired factory bushings. My theory has been that soft bushings allow the subframe to deflect and unevenly distribute the load on the mounts. Thus, making the mounts solid actually helps distribute the load. Although 75D and Delrin are seem hard as a rock, they both will still deform given enough time--aluminum, not so much.

My general rule is that if it is a mount or doesn't move, make it aluminum. If it moves use a spherical bearing. If it moves and sphericals aren't available, consider 75D. My one exception to this rule motor mounts. After running solid aluminum and every poly hardness under the sun, I've come to the conclusion that some give in motor mounts helps keep the motor from vibrating itself to pieces. For a motor you want to eliminate as much movement as possible but also retain some dampening. With my current personal race car build I'm going to be trying Vibra-Technics race mounts.

-bj

[Frank@AchillesMotorsports]

We keep the chassis reinforcements in stock as well as other E36 & E46 reinforcement kits!
E36
http://www.achillesmotorsports.com/R...25649-5678.htm

Others
http://www.achillesmotorsports.com/B...ent-s/1874.htm


Also have E36 & E46 Aluminum Mounts in stock.
http://www.achillesmotorsports.com/A...-arsfb-e36.htm

[apexin2]

First, let me say that the reinforcements are definitely a must.

As for bushing material, my thoughts, what I use, and what I recommend to my customers for the subframe is aluminum. So my reasoning for this is simple: almost every rear subframe mount point failure I've seen at the shop was on a car with tired factory bushings. My theory has been that soft bushings allow the subframe to deflect and unevenly distribute the load on the mounts. Thus, making the mounts solid actually helps distribute the load. Although 75D and Delrin are seem hard as a rock, they both will still deform given enough time--aluminum, not so much.

My general rule is that if it is a mount or doesn't move, make it aluminum. If it moves use a spherical bearing. If it moves and sphericals aren't available, consider 75D. My one exception to this rule motor mounts. After running solid aluminum and every poly hardness under the sun, I've come to the conclusion that some give in motor mounts helps keep the motor from vibrating itself to pieces. For a motor you want to eliminate as much movement as possible but also retain some dampening. With my current personal race car build I'm going to be trying Vibra-Technics race mounts.

-bj

Not to take this thread to a different direction, but this leads me to a question regarding the correct choice for bushings. I am planning on replacing the rear sub-frame mounts of my 95 m3 and I am headed to the direction of power flex bushings (purple - 80A). Based on the previous post, would this be a solid investment? The car sees auto-x frequently and some track use, but not daily driven. I would still like to maintain some streetability. Any input - good or bad would be appreciated.

Thanks,
Sean

[loftygoals]

would this be a solid investment?

Pun intended?

Rubber bushings are there for NVH issues--they have no performance benefit. Just think about it, why would you want the item that holds all your suspension to move around? We align autocross and track cars to 1/16" precision or higher, but subframe deflection from soft bushings could be more than that.

If a car is still street driven, decide how much NVH you can live with and go from there. Just be realistic with yourself--subframe bushings isn't a job you want to do twice. I've always been fine with solid suspension mounts on a car that I don't daily drive and just use occasionally on the street. As I've gotten older, I've grown to appreciate softer bushings on my daily driver, though.

-bj

[NeilM]

Not to take this thread to a different direction, but this leads me to a question regarding the correct choice for bushings. I am planning on replacing the rear sub-frame mounts of my 95 m3 and I am headed to the direction of power flex bushings (purple - 80A). Based on the previous post, would this be a solid investment? The car sees auto-x frequently and some track use, but not daily driven. I would still like to maintain some streetability. Any input - good or bad would be appreciated.

Thanks,
Sean

The PFX 80A/purple subframe bushings, although stiffer than stock, are entirely streetable. From an NVH standpoint you'll notice little difference.

Neil

[apexin2]

No pun intended Thanks for the input...Leaning towards solid now...and after reading the labor procedure for these, I don't want to have to do them twice. Thanks again for the input.

[timsen]

I decided to go with solid mounts too, AKG

[pbonsalb]

The newest M cars have solid mounted rear subframes. I run the PF Purple rears, but were I to do it again, I would go with PF Black or AKG 75D. What I would not do is solid mount both the subframe and diff on a street car. One or the other should have some isolation. Or both a little.

[NeilM]

The newest M cars have solid mounted rear subframes.

I saw that too. Of course the rest of the rear suspension support system is likely configured to suit; I'd bet the diff is well isolated, for instance. Still, it's interesting after all these years of isolated BMW rear subframes.

I run the PF Purple rears, but were I to do it again, I would go with PF Black or AKG 75D.

Me too, in fact I'm actually planning to switch them out for PFX blacks while doing some other rear end work.

What I would not do is solid mount both the subframe and diff on a street car. One or the other should have some isolation. Or both a little.

Yeah: I'm keeping a normally isolated diff. I don't see that having the normal small amount of diff wind-up affects anything else much, whereas subframe motion affects handling directly.

Neil

[apexin2]

The newest M cars have solid mounted rear subframes. I run the PF Purple rears, but were I to do it again, I would go with PF Black or AKG 75D. What I would not do is solid mount both the subframe and diff on a street car. One or the other should have some isolation. Or both a little.

Still undecided, but after reading more and more about subframe mounts, PF purples (at least for me and my intentions) look to be the best fit. NVH is a consideration as I still enjoy the Sunday afternoon cruise without having to take 2 advil the next morning for my back. Not looking for an all out track car quite yet.

[Crustashio]

Has anyone run 75D subframe bushings with stock bushings? Does the diff stay quiet on the highway? I keep waffling between poly diff bushings with OEM subframe ones, or OEM diff bushings with poly (75D) subframe ones. E36 M3, mainly an autoX/track car but is driven 2 hours to the track and at least 1 hour to most autoX events. I've heard lots of complaints about poly diff bushings WITH poly subframe bushings.

[LivesNearCostco]

I'm having same debate but my E46 gets lots of street time. Going to go with poly subframe mounts either way, but debating amongst AKG 75D, PowerFlex 95A (black), or AKG 90A. THere doesn't seem to be a PowerFlex purple option for the E46 non-M, only yellow (80A) or black (95A). My diff mounts are Meyele HD rubber... not sure if will keep them that way or change them to poly. No plans to go solid anywhere, though 75D sounds like it's very stiff.

[Inflame]

75D is like hard hat material, very stiff but my car is strictly a race car and will barely see any street time. The 95A has small amount of give to it, comparing the 2 side-by-side if my car was a street car, I'd go 95A.

[NeilM]

I would not consider harder diff bushings for anything other than a trailered track car. NVH downside is maximal, and benefit is minimal.

Dissenting arguments welcome, but I'd want to know what benefit you've achieved.

Neil

[n3985]

I would not consider harder diff bushings for anything other than a trailered track car. NVH downside is maximal, and benefit is minimal.

Dissenting arguments welcome, but I'd want to know what benefit you've achieved.

Neil

Strange when I called AKG they actually said for a streetable track car to do 95A for the subframe and 75D for diff to prevent it from moving.

[berny2435]

Strange when I called AKG they actually said for a streetable track car to do 95A for the subframe and 75D for diff to prevent it from moving.

that's intersting.
Engergy will be dissipated to the weakest link typically. no deflection at the diff will put that tornsional strain into the clutch, driveshaft, quibo and subframe. 75D will cause the subframe to get more of a work out. I decided against aftermarket stuff back there b/c plastic deformation of the subframe bushings just sounds like a nightmare and therefore the only options in my opinion are solid aluminum and OE rubber for subframe. DD/AutoX/HPDE car so I just ordered OEM. May not be ideal in the long run for time trial but I'll worry about that when I actually become competitive.. .

[Fair]

This is a tricky question... if it is a pure race car then you should think about aluminum, but definitely reinforce the chassis if it isn't an '96-99 M3 to begin with.

And when it comes to rear chassis failures I agree 100% with ClownShoe... it always traces back to sloppy OEM bushings. If it is soft enough to move it will fatigue the metal given enough load and time.



The real question is.... are you REALLY building a 100% race car? Most people I ask say they are until I dig further. Well, I'm going to drive it to and from the track, but that's it! Well, I do some fun weekend driving, but that's it! Well, its still registered and insured, but its really a race car.


If we are ever upgrading subframe bushings to a BMW it is usually only because we're doing rear subframe reinforcement repairs

Why does this matter? Because the added NVH from solid subframe mounts is considerable. And for street use solid aluminum mounts will bang and clank and drive you nuts, just like spherical LCA bushings, spherical RTABs, etc. Those really only belong on pure race cars, and the maintenance interval is much shorter. As long as you know that going in.


Here's a "dedicated track car" that is very fast, but still sees some street use - so we went with the firmest PF bushings

90% of the time when I'm told about a "dedicated track car" the thing still sees at least light street use, and in those cases I feel they will benefit the most from the "Race" stiffness poly mounts from places like Powerflex (we sell them, as do many other BF vendors). For an E36 need part numbers PFR5-3606B & PFR5-3607B.



We install a lot of these PF bushings in E36 and E46 chassis rear subframes and haven't had anyone complain about the noise and haven't seen long term failures, either. They are, in my mind, the best of both worlds. These will be MUCH stiffer than the floppy sloppy OEM rubber bushings but not quite as rock solid as the aluminum bushings. Even for pure street cars I will always try to talk the customer into the "street" poly PF bushings, just to keep the movement under control to prevent (another) series of rear chassis failure cracks.

Just my 2 cents.

[apexin2]

How about running AKG 95A/PF - purples (80A) with solid diff mounts? Too much one way or another (NVH and stress on the rest of the drive train)? Has anyone tried this set-up or something comparable? At the end of the day, I am not looking to ever build my car into an all-out track/race/auto-x machine.

More information from anyone who has run a set-up comparable would be greatly appreciated. I figure the more I ask, the more information I can base a good decision on.

Thanks in advance.

[papiichulo]

I have akg 75d subframe bushings, stock rtabs with limiters and stock diff mounts. No increase in Nvh. Car was actually smoother.