Rubber vs. Polyurethane, What Gives?

December 13th, 2006 by William

As is often the case, some routine maintenance on the Jeep has quickly become a philosophical debate on several 4×4 forums that I post on. My Jeep has developed a pretty severe drive line vibration which I believe to bad transmission mount combine with old motor mounts allowing everything to shift around a lot under load. The quick and easy solution is to replace all the existing rubber mounts with new rubber mounts, but then I found some vendors that make polyurethane mounts for my CJ7.

So what exactly is polyurethane? Prothane, a company that deals in polyurethane auto parts, gave this brief definition: “Polyurethane is a term used to describe a wide ranging family of elastomers (any compound exhibiting the characteristics of natural rubber; stretchy and elastic.). Poly meaning “many” and “urethane” the classification of the chemical structure. Polyurethane or urethane for short, is used as a solid cast material (bushings). Polyurethane can be as soft as a rubber band or as hard as plastic.”

Polyurethane transmission mounts.

Sounds wonderful to me. The polyurethane seems to offer the benefits of rubber–elasticity and give–but also hold up better to deterioration and provide an extra degree of stiffness that rubber lacks. In fact as soon as I found out about the polyurethane mounts I was basically sold. Then I found out that the cost about three times more than the standard rubber mounts.
On further exploration of the subject I began to question the ability of the urethane mounts to be both more rigid than rubber and as flexible as rubber at the same time; impossible.

After posting to a couple of 4×4 bulletin boards (CJOffroad and Pirate4×4)this is what I learned:
1. Polyurethane mounts are much stiffer than rubber and therefore hold the drive train in place much better. But this also means that a lot more noise and vibration is transmitted to the driver and vehicle from the engine and transmission.

2. Rubber mounts are softer and absorb vibration better, but they deteriorate rather quickly, especially when engines leak oil on them, compared to polyurethane mounts.

3. Most Jeep guys don’t seem to care about added noise and vibration in their trucks.

A rubber transmission mount will turn your Jeep into a Cadillac

To address my third finding first, I don’t generally care about a little noise or vibration in my Jeep, but a lot of vibration is pretty annoying, especially since my CJ sees a lot of daily driver miles. Many people responded to my worries of the added vibration of polyurethane mounts by saying “it’s a Jeep not a Cadillac”. This statement is true to a degree but I find you can quickly start using that as an excuse to drive a poorly put together vehicle. I know because I used to say that myself when my engine was spurting oil and my drive shaft was about to fall off. It is much nicer to drive a vehicle that has minimal squeaks, rattles and vibrations, and it is possible to make a Jeep pretty quiet. So while the urethane may hold things in place better I like the give and absorption of the rubber mounts better. Score 1 point for rubber.

The rubber mounts will deteriorate faster however, even without oil pouring on them, but the question is how fast? I’ve never changed the transmission mount in twelve years of owning the Jeep and have only done one motor mount (young and stupid or I would have done both) about nine years ago. Now that I have an engine that doesn’t leak oil that lifespan may increase. For me that interval is perfectly acceptable. No points awarded either way.

Now cost is always a concern for me. As an automotive writer and mechanic I’m not exactly making the big bucks, so trying to justify paying $90 for a set of polyurethane mounts instead of $30 for rubber mounts is pretty tough. I mean I could buy a lot of Top Ramen noodles with $60 in pocket. Score 1 for rubber.

With some Ramen in my stomach I will really appreciate my Jeep’s Cadillac ride.
The rubber mounts have claimed a two-nil victory over polyurethane for my particular application. I’m a happy to say I will be driving a quieter Jeep for another ten years with a tummy full of cheap noodles, and by then who knows what cool products will be on the market to do battle with rubber.

-Bill Mertz

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One Response to “Rubber vs. Polyurethane, What Gives?”

  1. Jon Says:

    Great article. It seems that among most auto enthusiast crowds, poly mounts are always just touted as “better.” Nice to see some consideration of the drawbacks. I don’t think I’ve ever used them myself for the same reasons you stated. I got an incomplete set of poly spring bushings with the other spare parts that came with my Jeep, which gave me an opportunity to feel the material. Certainly didn’t seem flexible at all, and I could only imagine what the jarring effects would be.

    “Polyurethane can be as soft as a rubber band or as hard as plastic.”

    This statement leads one to believe they could make a poly with the flexibility more like that of rubber. I wonder if this is true or if it’s simply always that hard?

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