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MG MGA - Polyurethane versus rubber

I have never used polyurethane bushes in a front suspension. I have always used the rubber type. With the claims for the longer life of polyurethane you might wonder why.
I am concerned with the different way in which they operate. The rubber bushes must be fitted with the suspension loaded to the normal weight. This is because neither the inner nor the outer suface can move relative to the metal after the locating nuts are tightened. During operation the rubber distorts but does not turn.

Harder polyurethane would be expected to turn against the metal surfaces during operation. Yes? No?

The rubber bushes act as a type of spring oscillation damper, additional to the hydraulic dampers. They also give a less harsh ride than the harder polyurethane.
The polyurethane are longer lasting and oil resistant, and may give a more precise suspension alignment, but I am concerned that the polyurethane movement may also wear the metal parts, in the same way that synthetic oil seals can wear crankshafts, axles etc.

Do members agree/disagree?

Mick
Mick Anderson

A quality poly bushing will come with a stainless steel inner sleeve. It is this sleeve which the bushings rotates upon, not the outer metal part of the suspension component (A-arm, leaf spring, shackle, etc). A poly bushing without an inner stainless sleeve is worthless in my opinion. Not only will it never operate as smoothly as one with a sleeve (or as smooth as a rubber one for that matter) but it will wear far more quickly.

Theoretically I believe you can tighten a poly bushings while in the air, but I still see recommendations in manufacturer's installation guides to do it under load. That's the way I do it, just to be extra safe. These things aren't cheap!

For the record, I use Superflex bushings whenever possible although I'm about to install a set of Superpro which are reported to be the same.
Steve Simmons

I was really happy with the handling and ride when I replaced the rubber with poly. Hard to put into words for me, but it was much more solid, sure and, well, 'modern'. Now, four years later I am starting to notice a familiar occasiaonl clunk going over bumps, got to look at that.
Steve
Steve Meline

I received my front suspension rebuild kit with poly bushings yesterday (Moss), and it did not include any sleeves for the bushings. But, I already have a set of the V8 bushings that I am planning to use, and only ordered the kit with poly bushings because it was cheaper than the kits with rubber bushings, or the V8 bushings that I already have.
Del Rawlins

Del,

You say that the poly bushes from Moss do not have steel sleeves. Can you check if the inner diameter of the poly bush needs to have a steel sleeve fitted to make it the correct diameter, or is the bush the exact size to fit directly on the suspension shaft without a sleeve?

Are the V8 bushings poly or rubber?
If poly, do they have steel sleeves?
If they have steel sleeves, can they turn separately in the bush?

Mick

Mick Anderson

Mick be wary of some Aussie poly bushes not fitted with steel sleeves! My first set did not use these and yes they are nowhere as good as the sleeved versions. Some Aussie MG dealers don't like poly and only sell the non-sleeved versions. David at Abingdon Motors in Brisbane can supply sleeved ones at a good price, he can be contacted on 07 38442881
regards
mark
Mark Mathiesen

Mick, I will measure the poly bushes later when I have time, but when I looked at them yesterday it looked like they are sized for use directly on the shaft. I don't, however, have a shaft that is conveniently located for measurement. My car's suspension is still assembled, and the parts from my parts car are buried in boxes.

The V-8 bushes are one piece units made from rubber, with steel outer and inner sleeves molded in. The outer sleeve is a press fit in the lower control arms. I'm not sure if the suspension motion is supposed to be pure flexing of the bushes, like with the standard bushes, or if the inner sleeve is supposed to move on the shaft. I suspect that it is supposed to be the former, but often ends up being the latter in practice, based on my observation that the V-8 bushings are extremely stiff, without a whole lot of rubber to flex.

I don't see any reason why a guy couldn't make his own steel sleeves for the poly bushes, and bore them out slightly. A lathe would be ideal for this work, but a good hand with a drill press and careful setup could accomplish the same thing. Some guys I went to school with restored a Jeep back before the poly bushes became ubiquitous, and they made their own suspension bushes on a lathe from round poly stock.
Del Rawlins

Mick

Check out the SuperFlex bushings. They offer a variety of hardness ratings, with one within 10% of the stock rubber bushings (if I remeber correctly).

FWIW

Larry
Larry Hallanger

Interesting about the sleeve vs. non-sleeved. Most that we purchase are unsleeved.

Keep in mind that an unsleeved may over many-many years cause wear that would not otherwise happen, but they will be far easier to remove. MGBs are not terrible to remove, but usually requires cutting with interesting methods. Try removing these on a Spridget. You basically have to mess up the bodywork or A frames to remove them. Frankly, for a spridget, the stock units with sleeves are a horrible little job to remove after going through road salt zones such as colder climate-salt on road Northern areas.

Ride of "Prothane" brand: Better.

Feel of the system: better.

Amount of feel annoying? I have had no complaints through our shop and have only heard in passing about people who don't like them- most of them heard from a friend of a friend of a friend about how bad they were from what I have gathered, but I am VERY willing to listen to anyone who has anything negative to tell me.

Replacement rubber bushings from what I see, are not as good as the OEM units when new. Generally speaking, the rubber is not as hard and does not seem to be as good, but then this is more speculation since I can only see NOS units that went unused. I was in kindergarten when the last MGB came into this Country!

When possible, we install the uprated bushings. Prothane is the units we generally install.

The SOLID plastic bushings are usually something we do NOT use except for something that is a non-road use car. Those could potentially give some wear, crack and break or just transfer too much road feel... I have only used them once or twice and just didn't care for the installation or end use.

Hope this helps,

-BMC.
BMC Brian McCullough

This thread was discussed between 23/12/2007 and 27/12/2007

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