[TR] What's the general life expectancy for rubber bushes for LBCs?

[Michael Porter]

On 4/6/2015 5:27 PM, Sujit Roy wrote:
> I changed them 14 years bac on my Stag This were the days before I 
> heard about polyurethane.
>
> Are polyurethane bushes really fit and forget?
>
>

This is almost one of those questions that is never going to get a 
satisfactory answer.  We start with the presumption that the cars spoken 
of are anywhere from 35 to 65 years old, and while the suspensions are 
similar, they all behave differently from model to model, because of 
engineering changes over time and differences in weight distribution, 
roll rates, state of repair, general care, etc., and with a few 
exceptions, the cars were bought used over a considerable range of time, 
with no certainty of what had been replaced by previous owner(s) or at 
what intervals.

All polymers--including rubber--have different rates of resistance to 
deformation, typically a figure on the Shore durometer scale. The higher 
the Shore number, the stiffer the elastomer.  And, the higher the Shore 
number, the rougher the ride, because the stiffer the elastomer, the 
more road shock it transmits to the body and the controls.  Typical 
replacement parts were analogs of factory items, but that wasn't always 
true, because of cost/supply issues, temporary shortages of oil or 
natural rubber inputs, etc., depending upon when the replacement part 
was produced.  Compounding the problem is that factory and replacement 
parts are subject to varying usages and mileage.  A part that may have 
worked wonderfully for twenty years on the car driven occasionally in 
the summers by an enthusiast on well-kept streets will have failed in a 
few years on a daily driver on rough roads.

So, choosing a replacement part today is pretty much dependent upon what 
you expect to do with the car.  Gentle driving, shows, etc., a standard 
rubber replacement will do fine.  If you are more demanding, some grade 
of polyurethane may be more suitable. Even then, you should think about 
the stiffness you really need.  Too stiff and you'll find the car 
unpleasant to drive. Too soft, and there's more deflection in the 
suspension than you'll think wise. The advantage of polyurethanes today 
is that they are a little less likely to degrade if soaked in oil, 
compared to most mixed synthetic/natural rubbers.

Very generally, if you want to get out and drive the car with a little 
vigor, I'd guess that anything of about 55-65 Shore is okay, whether 
rubber or urethane.  If you're going to baby the car, well, almost any 
rubber replacement is probably okay, and will last a fair time as long 
it doesn't become oil-soaked.

I put polyurethane bushings all around in my GT6 ten years ago, probably 
of 75-80 durometer, and while they haven't degraded at all, from the 
very start, the car felt like I was driving on cobblestones everywhere. 
By contrast, I put new control arms (with rubber bushings) and struts in 
the fourteen-year-old BMW last fall and the car is firm and relatively 
silent going over bumps, and the braking shimmy is gone with new 
bushings (definitely a problem with E39s), and the parts I replaced were 
clearly originals, and had only started to present problems at the 
12-1/2 year mark.

Everything wears. I doubt you're going to find a part on the car that 
you can replace which will then last forever.  Most grades of rubber are 
probably better for comfort, and most grades of urethane/polyurethane 
are better for stiffness, and will be subject to a little less 
degradation from environmental attack.  All that said, you should pick 
the item that suits your driving habits and your intentions for the 
car's use.


Cheers.




-- 


Michael Porter
Roswell, NM


Never let anyone drive you crazy when you know it's within walking distance....