Poly Bushings - Good,Bad, Indifferent

Mike8675309

Registered User
It came up in another thread and thought if Duffy is up to it, let's discuss it here and maybe we can turn it into a FAQ.

I think Duffy was hinting at something in his previous thread post when he said that there are some rubber bushings in our cars that are best left rubber.

My thought is that part of this is to help support the suspension design that uses the flex of the rubber bushings to assist in keeping the tires planted properly on the ground. If you replace a compliant bushing with a hard bushing in a place where the suspension is designed to flex, you could run into issues with oversteer and odd tire wear amoung any number of things.

Certainly some of this flex causes issues for us with wheel hop in the rear suspension. But a good handling car is not necessarily one that can smoke it's tires at will.

So maybe the key is to figure out which bushings are valuable as Poly, and which ones would be best left alone as rubber?
 
I got the MN12performance front and diff poly bushings and solid rubber trans & engine mounts. I really like the diff polys but have not completed the install of all of the front ones but I've had the poly sway bars in for a while and like them to. In that kit, the strut to lower control arm bushings are Ford rubber w/metal inside anyway. I had to get those in the drivers side b/c I was getting a pop while braking and on hard left turns plus it was killing my tire on the inside. The originals had dryed & rusted so badly and I had to chisel & file them a lot to get the rust out. So far so good, I'll let you know if I ever get them all in. Good luck
 
Mike,

You caught my drift correctly. Every suspension has certain basic designed response to a given set of loads. Those loads include what happens in jounce and rebound as the wheels travel up and down as well as what it will do when loaded side to side (roll angle and such). On most cars the rear end is a solid axle and this designed response is much less complicated because essentially both wheels act a unit since they are directly tied together. On a IRS car the design is a complex affair and the suspension handling characteristics are very much affected by the bushings selected. The compliance in he bushings is used now to result in a known designed camber and toe curve as the suspension undergoes travel.

Suspension design has become an extremely complex business. Additionally large amounts of money is spent on both NVM development as well as handling and ride characteristics.

In the old days the bushings were very sloppily designed compared to the relative sophistication of today's version. The handling on many musclecar era vehicles was so poor that poly bushings actually helped. The vendors that sold them found they7 could make money so they pushed their product. The "old" guys that tried them on old cars reaped the benefits and helped "sell" the idea to the next generation. Nothing happens in a vacuum though. Designs evolve and improvements are made...even in the poly arena. (addition of graphite compunds to cut the binding problems and sweaks, greaseable bushings for the same reason) But the need for these poly bushings is extremely limited on a MN12.

You could tune the suspension a tad by using sway bar mounting bushings made from poly. That would increase the effective coupling of the bar to the chassis since the bushings are harder...same for the end links in the rear if you choose to go aftermarket there. I don't see a major issue with the poly diff bushings since they just cut down on the motion of the pinion end of the diff and do not directly affect handling but could cause some NVM. ( I have only these few bushings as poly in my 92).

Anybody else care to chime in?
 
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That pretty much covers the bushing evolution.

The reason this gets critical for handling is the structure of the vehicle. Back before the 60's, it was always body on frame which was solid and suspensions were pretty simple. With unibody, the whole vehicle flexes, changing the suspension geometry. As Duffy has pointed out, there's been a lot of R&D to deal with the flex.

That's also why there's after market suspension parts like the bushings, different shocks/struts and braces. To minimize flex, you'd have to install the biggest swaybars, braces, bushings and the biggest strenghthener of all, a rollcage.

Of course you'd also need a mouthpiece to keep your teeth from rattling out of your head. :D

Lee
 
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Before you go about stiffening the suspension in the MN12 at all, you must look at structural reinforcement. What will happen if you stiffen up the car is you will find that windows begin to leak, windshields crack, firewall cracks, rear floorpan cracks, etc. The unibody AND subframes are both not stiff enough to handle a non-compliant suspension. The above comments are based not on opinion or myth, but rather the experiences of individuals in this community.

Johnny Langton (see TCCOA) and Kenny Brown are examples of some of the people who make stiffeners for these cars. I would not bother with any "suspension" upgrades until you have first made these improvements. Your results will be muddied, and the car will be compromised.

Contrary to popular opinion sometimes, stiffening the body will benefit you in many ways. Less rattles and squeaks inside the car, a quieter and much SMOOTHER ride, and handling improvements that you wouldn't believe.


:)
 
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