ALFAGTV6.COM

[Mats]

You made a Center Bearing isolator out of solid poly??
Why would you do that? Hardly a performance option and if you have even a teeny bit of unbalance it will go straight into the body. Massage GTV6?

[MD]

Mats

I think you may have a misread there. I think the issue is a substitute material for a rear engine mount and not a centre bearing support for the drive train, then again maybe I have misread it...

BTW If it is about the rear engine mount, agree with poly for a racer but for a road car it may cause too much noiseand vibration.

[Mats]

Firmed up the rear engine mount and center driveshaft bearing housing with some black polyurethane.

Pretty clear to me.

I don't think PU has even remotely the same durability as the rubber mounts either.

[x-rad]

Poly very durable(it is outdoor sealing material and remains flexible + tough through wide range temps) and still flexible and tough enough to take up vibration. It is under compression load which it is good at.

Main thing is original mounts were sagging (so bad the driveshaft was tapping on something up front)...so I removed them...centered the inner pieces(Used a stick of appropriate thickness to hold metal center in proper place, which was removed and the gap filled)

...and then filled in the big gaps with poly(took a few days and several layers)....works great...cost $7 and has lasted two years no problems

The original rubber is still in the mounts...things are just centered now

The poly is more of an assistant to the mount (a big help)


AND REMEMBER...cost $7

[Mats]

The difference between durable as in "house durable" vs. "car durable" is that in the case of the car the PU will se temps up to about 130 degrees centigrade and vibrations and such, on a house the worst thing is some temp change, very stationary stuff.

PU bushings are not mounted on production vehicles for a reason, their durability sucks.
Also, by filling that bearing support with PU my guess is that you went way over the spec for the support stiffness and you might be leading driveshaft vibrations into the car if you are unlucky.

[zambon]

It might lead to vibrations if the driveshaft isnt smooth to start with. That said, I think that having the engine and transaxle floating all over the place in bubble gum rubber can cause premature vibrations too because it causes shaft alignment problems.
In any case, durability shouldnt be an issue here unless the poly has a chemical reaction with the stock rubber in the mount.

Then again, I once cross bolted my front trans mounts in an attempt at improving alignment and I got some nasty vibrations as my reward, so I know vibrations can be born of such experimentation.

PU bushings are not mounted on production vehicles for a reason, their durability sucks.

Mats, you seem to hate poly bushings. I have seen you post about them before. Have you ever actually had problems with poly bushings failing prematurely? I always thought that manufacturers didnt use poly because it is a few pennies more expensive or because most car buyers like the insulated feeling.

[Mats]

Not personally, I haven't put PU bushings in difficult places though. Rotational under load is the worst and I know of people having thrashed a set of PU bushings in less then 100Km (at the track, but hey, the stock bushings can take it, for years and years).
Come to think of it my de-Dion front bushing was starting to make some noise just before I sold the car, not many years since I installed it and few miles.

I have a set of inner bushings for the upper control arm but I didn't think they could take the punishment so I never mounted them to the car.
Anyone wanna buy?

[zambon]

I dont see the need for poly bushings in situations where the stock bushings work well. There are, however, a few stock bushings and mounts on our cars which are seriously bad even out of the box. The castor arm ones are the most obvious example of this. A dense rubber bushing would work much better than the perforated bubble gum but that isnt on the market. A polyurathane one is available, however... I know that you prefer the ball joint for that one, but that is a modification.

The point is that I feel that some of the original rubber bits on our cars are not dense enough rubber. For this reason I am interested in experimenting with the modification of stock bushings to increase stiffness. I have had my eye on the rear engine mount and rear transmission mounts as good candidates.

X-rad- Please report on how the filled rear engine mount works out. I amvery interested.

[x-rad]

rear engine mount: Works great, engine much better located and stays in place pretty well when you try to push or pull it (unlike before)... I do have a minor shaft vibration at 3500 rpm

polyurethane:
depends on the type but the PL stuff is pretty good to 200F(what I used wasn't 3M but PL)....3M also makes 'military' grade poly

3M poly sealant(a little better than PL): Test Conditions Test Method Properties
Viscosity Mixture 68°F (20°C) Brookfield 100,000 cps
HBDVI* CP
Work Life 10 g, 1/6" thick @ 77°F (25°C) PEC 60 seconds
Tack-Free Time 10 g, 1/6" thick @ 77°F (25°C) PEC 15 minutes
Full Cure Time 10 g, 1/6" thick @ 77°F (25°C) PEC 12 hours
Hardness 24 hours @ 77°F (25°C) ASTM D2240-91 72 Shore A
Lapshear 24 hours @ 77°F (25°C) ASTM D1002-72 600 psi
Temperature Range Continuous PEC -60°F (-51°C) to
250°F (121°C)

[brookstroutr]

I just read this post about the combo mounts. About 1990 out of desperation, myself and some fellow BMW guys who tracked are cars alot were trying to find something for the front and rear control bushings. Poly for the cars were available at the time and the were the same as the rear motor mount that being non-solid rubber and they were notorious for short life span. We took and filled the voids as x-rad mentioned.

I gotta tell ya Mats, they lasted for about 12 years with a loooot of track time until I sold that one. The ride wasn't bad or harsh which I agree about putting them only in strategic locations. I didn't put the on top of my dampers in the GTV6 nor the upper a-arms. The last poly part I put on the M635CSi were a set of front upper strut mounts for camber for the track. Great for the track, but it a little too far for my street liking.

[brookstroutr]

OOPS! Just saw a typo in my last comment. Correction - Poly bushings were NOT available for our cars at the time...