poly diff bushings: alternative to super stiff AKG 75D?

[nando]

talked to a few other people and they all say the same thing - this makes the rear diff really, really loud. If it were more of a toy car than it is already I'd just leave it alone (I love the solid feel of the rear end), but I can't stand driving on the freeway with this thing, especially since the peak resonance frequency is at 70mph which also happens to be the local speed limit.

So what else is there that I can slide right in? I know, stock, and it's near the top of the list but the nice thing about poly is I can slide it in there by hand and be done with it.

Since I'm dropping the subframe (again! fuck!) I'm also going to lower it a little bit (raised it up last time) to gain 1/2 degrees of negative camber putting me at -2.5. Anyone know how much I have to move the subframe to gain 1/2 a degree? I'm thinking about 1/2"?

[whiltebeitel]

What about a little sound deadener? Would that help?

[Aptyp]

try BMPD/Korman ones. Perfect compound for application, and price isn't crazy... Read urethane ones...

I ran IE race and only time it started feeling unsafe was around 125mph, but that was more due to other stuff, other than bushings.

[nando]

What about a little sound deadener? Would that help?

No, it's not just the sound, it makes the whole car vibrate at the resonance frequency of the diff.

I'll look into the korman bushings, thanks.

[Aptyp]

Buy them from BMP, it's exact same bushing, but Korman marks it up by $15+

[gearheadE30]

I have the AKG in my car, and it doesn't seem to be that loud... Seems like the other e30s I've been in have been a similar volume. I dunno. Otherwise, BMP is the way to go, if it is actually softer.

[Brew]

Have you run the car on the lift to see if there is maybe a bad bearing in the diff or a noisy wheel bearing? Rear wheel bearings can be tricky because they gradually get louder and if you drive the car often you may not notice until it gets obnoxious.

[Aptyp]

^so true, one of my friends didn't notice until it was too late and bearing outer race overheated from friction and cracked trailing arm. but that was on e36, but bearing is the same.

75D are very hard, but the noise you usually hear is through subframe mounts, and diff bushing.

[nando]

Have you run the car on the lift to see if there is maybe a bad bearing in the diff or a noisy wheel bearing? Rear wheel bearings can be tricky because they gradually get louder and if you drive the car often you may not notice until it gets obnoxious.

Wheel bearings are new.. the diff is old but it was silent before the solid diff bushing. that was the only part that I added that was stiffer than before, the subframe bushings are actually a little softer.

[Dominic49]

i run a solid alum bushing and have hardly any diff noise

[xLibelle]

oh wow... it just hit me that the driveline noise i have could be somehow related to this.

the diff is held in at three points, to the body. The two subframe bushings and the diff cover. if these damper points are worn to the extent of easy deflection, everytime torque is applied, the subframe twist ever so slightly and dislocate a true (straight) driveline and the universal joint obtain excessive wear.

I have replaced the diff to find the same noise. Each of the bushings look a little worn. So if i solidified the three subframe mount points, that would keep the noise down a little... if there were a problem and you replaced just the diff cover bushing, the noise could be amplified due to continued subframe deflection.

Dominic for example doesnt have the problem maybe because he doesnt have other issues with the driveline to be concered over.

these are just some suggestions that came to me as i've been trying to tackle this niose problem.

[xLibelle]

do we have a thread going about the different poly stiffness values and what kind of values are good for what?

[dirty30]

My delrin is not very noisy at all. Yes diff whine and road noise has increased but even with the carpet out (used to be a daily with full interior) it is still not that noisy. Put a stock e30 M3 diff mount in a be done with it. Upgrading to race parts without compromising a little (alot is a better word) is impossible. You can't strap into a pair of skiis in Hugo Boss dress shoes.

[nando]

I am wondering if maybe my driveshaft is out of balance or something.. everything is pretty new though (within 10-30k), CV joints are still tight, etc. Was thinking of taking the driveshaft out, drive to the local shop in FWD mode and have it rebalanced.

[nando]

My delrin is not very noisy at all. Yes diff whine and road noise has increased but even with the carpet out (used to be a daily with full interior) it is still not that noisy. Put a stock e30 M3 diff mount in a be done with it. Upgrading to race parts without compromising a little (alot is a better word) is impossible. You can't strap into a pair of skiis in Hugo Boss dress shoes.

It's not that I don't expect some increase in NVH. I've been driving with all poly bushings for years.. it's just that this is a little too much.