Body Mounts...Rubber or Poly?

[79_GBody]

I've got the body off the frame on my g-body while I box the frame and I'm going to replace the body mounts. The car is going to be a driver with some minor budget suspension and brake upgrades. I was wondering from others with poly body mounts what type of ride it is...alot more vibrations in the car etc.? I've done searches and some say the the ride is fine some say they switched back to rubber because the ride was too rough. The rubber mounts are about $230 unless someone else knows where to get them cheaper for a g-body and the polys are about $130 maybe less depending on where you buy them. Help me out please :hmm:

[83hurstguy]

poly.

[MarkM66]

Poly.

New cars have no frame connected to a body via bushings, and they ride just fine.

[silver69camaro]

I'd do solid. Get some 2" 6061 bar stock and go at it.

[jerome]

with the amount of flexing that I'm seeing with the S10 (similar frame) I'm afraid to put in solid bushings, I think either the mounts on the body would bend or the glass or cab welds would crack

Try putting one jackstand on each side in the rear and then a couple of blocks with a 2x4 longitudinally under the k-member, and then rock the frame side to side, you'll see what I'm talking about.

[silver69camaro]

with the amount of flexing that I'm seeing with the S10 (similar frame) I'm afraid to put in solid bushings, I think either the mounts on the body would bend or the glass or cab welds would crack


That's normal, all pickups should flex like that. The bed needs to move (somewhat) independently from the cab especially when it's loaded, or big problems can occur. Only a couple pickups in the last 50 years where a unitized cab/bed assembly, and that method didn't last long.

A full frame sedan/coupe can have big benefits from solid bushings. Similar to a unibody with solid mounts and subframe connectors.

[Bobby Schulz]

Poly or solid!:attn:

[racereno]

I'm at the same place with my G-body Cutlass...I have the poly mounts on hand but I keep wondering. I have'nt seen a real world comparo on this yet...just heresay. New cars are much more complex in their flex engineering from the factory so it's hard to compare.

[79_GBody]

Almost all of the newer cars are unibody so they wouldn't have the same affect as using a solid mount on a full frame car would it. Solid mounts on a full frame street car seem like overkill to me...

[MrQuick]

Almost all of the newer cars are unibody so they wouldn't have the same affect as using a solid mount on a full frame car would it. Solid mounts on a full frame street car seem like overkill to me... Plus the suspensions are tuned for it.

Poly's do transmit more NVH but you have to make the sacrifice somewhere. Solids...I don't see the need. Minimal gain at best.

[DAK]

I installed solids in my g-body 2 years ago. I had to change to softer shocks right after. I tried poly suspen bushings in the rear but there was way too much road noise in the back so I changed just the back 4 bushings to new Moog rubber. I think the solids did a great job of firming up my car.

[Norm Peterson]

Eventually I ended up with 88A poly bushings in most of the places on my '79 Malibu. A little more NVH sneaks through, but not a huge amount more (the difference is about like running 6 - 8 extra psi in the tires IIRC). That's the way the car was autocrossed, and there were no issues with either metal or glass cracking.


Norm

[MarkM66]

Almost all of the newer cars are unibody so they wouldn't have the same affect as using a solid mount on a full frame car would it. Solid mounts on a full frame street car seem like overkill to me...

Unibody as in front and rear frame sections tied directly to the body?

[silver69camaro]

Solid mounts on a full frame street car seem like overkill to me...

IMO a chassis can't be too stiff, street car or not. Make the suspension do the work.

[Norm Peterson]

At some point, I'd start shifting my attention from overall chassis stiffness to places where abrupt changes in stiffness exist. For a full-frame car, one such area would be the frame rails at the body/frame attachments closest to the firewall. You go from [combined frame + body] to just [frame] over only half the length of the body mount.


Norm

[jaybee]

Norm, what are your thoughts on that? Perhaps diagonal braces from the firewall down to the frame rails near the suspension pickup points?

[6'9"Witha69]

Only a couple pickups in the last 50 years where a unitized cab/bed assembly, and that method didn't last long.You mean like 61-63 Ford F100s?

Poly or solid. Only ways to go.

[83hurstguy]

At some point, I'd start shifting my attention from overall chassis stiffness to places where abrupt changes in stiffness exist. For a full-frame car, one such area would be the frame rails at the body/frame attachments closest to the firewall. You go from [combined frame + body] to just [frame] over only half the length of the body mount.


Norm

Definitely. You can really maximize torsional stiffness gains using this method. When we were going FSAE chassis work back in my school days, we had the chassis in Ansys and would try to look at data of deflection vs. longitudinal position to find where the most deflection was coming from (of course, on these cars it can be fairly intuitive). General rule of thumb we used in optimization was to have the chassis an order of magnitude stiffer than the difference in the front and rear roll rates. This would allow the chassis to be tuneable with the suspension.

Anyways... overkill for this thread, lol...

I put poly's in my T-top H/O and love the way the car rides. The rod ends in the rear UCA's put a little more rear end noise into the car, but it's not too bad. They were very cheap too.

[silver69camaro]

You mean like 61-63 Ford F100s?


Yeah, those were cool lookin'.

[Norm Peterson]

Norm, what are your thoughts on that? Perhaps diagonal braces from the firewall down to the frame rails near the suspension pickup points?Torsion is sort of a 3-D problem, so you need some sort of bracing in more than just one plane. Bracing back to only sheet firewall metal won't help much, as the sheetmetal isn't rigid enough to develop the resistance that attracts loads into a tube, so you'd at least need some sort of lateral tube, and probably tie through that into maybe the tranny tunnel or A-pillar. I think you also need to do something about the pair of 90° "wiggles" that the frame rails make as they transition from being more or less under the sills to clearing the wheel wells to the inboard. It may be strong enough as is, but if I (at ~180-ish lbs) can pry the chassis frame down by enough to replace the rubber body bushing with poly using only one hand on the pry bar while I swap the bushing material with the other, it's not what I'd call rigid.

83hurstguy - there must be something semi-universal about "10% effects and smaller being negligible for first cut analysis". That's about what I've settled on, just playing around with structural analysis in general. To go much stiffer probably puts you into the realm of diminishing returns, stiffness vs added weight anyway.


Norm

[Blown353]

The poly mounts I used on my car were extremely high durometer... it barely took any rotation of the bolt after contact with the body to achieve spec torque on the body bolts (35 ft/lbs.) With a very objective (cough, cough) educated fingernail hardness test they were almost as stout as Delrin.

So yes, run either poly, plastic (delrin, etc), or solid of some kind. Going from fairly fresh rubber body bushings to poly bushings on my car made a very noticeable improvement in the "solidity" of the car. A bit of increased NVH but the stiffening of the overall frame/body combo was worth it.

One thing about the poly mounts though is they don't include the metal sleeves & washers, you have to reuse the old steel bits from the factory rubber mounts. If they're in bad shape then you'll have to find/make/buy new hardware.

[83hurstguy]

83hurstguy - there must be something semi-universal about "10% effects and smaller being negligible for first cut analysis". That's about what I've settled on, just playing around with structural analysis in general. To go much stiffer probably puts you into the realm of diminishing returns, stiffness vs added weight anyway.


Norm

It has to do with a "springs in series" equation from what little I remember (where the overall rate is summed by 1/kt = 1/k1 + 1/k2, etc...) All our chassis judges seemed satsified with that number, and we never had a problem with the car being unresponsive to bar changes. I'm not sure if Smith mentioned it in one of his books or not. Definitely one of those "semi-universal" things though...

[slimneverdies]

So 79 g-body!! What did you decide to go with poly or rubber? Im at the same point in my build.

[jackfrost]

So 79 g-body!! What did you decide to go with poly or rubber? Im at the same point in my build.

i didn't think there was still a question.

go poly.

[GBodyGMachine]

Poly

[Peti Boi]

go with SOLID!!!!!

[slimneverdies]

The poly is actually cheaper. I was still asking cause on another site a few guys with el camino's went with some poly and some solid. I have this car completely apart and dont want to take it apart again because of binding issues or some annoying sqeaking noises. Im spending alot of money on this car and would hate to have road noise because of bushings.

[Peti Boi]

mine doesn't squeak and the body feels real stable

[79_GBody]

So 79 g-body!! What did you decide to go with poly or rubber? Im at the same point in my build.

Yeah I ended up going with the poly...I haven't driven the car yet, but the body is back down on the frame. Now here comes the fun part of running the new brake and fuel line that ran on the inside of the rail. Thinking about just taking it somewhere and having it done.