Body Mounts

[Guest 69GS]

I have a 69 buick GS 350 2 door coupe currently in the middle of a frame off resto, i am ready to put the body back on the frame and i can't find any info online or in my chassis service manual about the 4 none bolted mounts (mounts #4 and #5) these mounts are referred to as "steady rests" in the original 1969 chassis manual and they give torque specs for the 10 bolted mounts. My question is can you put bolted mounts in these locations or will it cause the body to crack or affect handling? Mounts #4 have caged nuts in the body already but were never used so i ordered the extra mounts and hardware from energy suspension to bolt these two spots (most forums say they upgrade to bolted mounts at these two spots) but the #5 mounts (top of the main rear cross member that the upper control arms bolt to for the rear axle) do not have the caged nuts in the body but there are holes in the body brace and also access holes to put the nuts through. I am already going to add two bolted mounts at #4 going from a total of 10 bolts to 12 bolts(he he just like my rear end) but i wonder if going with all 14 would be detrimental. I want this car to be as rigid as possible but i don't want to crack the body or possibly the glass. Can anyone tell me why these 4 mounts were only "steady rests" and do they need to stay that way?

Edited August 15, 2011 by 69GS (see edit history)

[NTX5467]

This is the first time I'd heard of this situation, but it's also the first time I'd considered A-body body mounts, too.

The closest thing I could find was a '72 Pontiac service manual, which also covers the LeMans and Grand Prix body styles (A-body and G-body, respectively . . . both of which are "intermediate-chassis" platforms). On page 2-5 of the "Frame and Body Mountings" section, it has a comprehensive list of the body mounts and their (at that time) GM part numbers. In position #4, for convertible and station wagon, there is a full pair of mounts listed, but for #5, just the upper mount is listed. Other body styles have a #4 mount that is just the upper mount. The cross section picture of the mounts, single upper cushion and the full upper/lower cushion with bolt/washer is also there, too, which indicates that recesses in the frame will locate the cushion between the body and the frame.

As I mentioned, this is the first time I've delved into the A-body world of frame-to-body mounts for intermediate-chassis GM cars. There can be several reasons for using only the upper mounts in these locations. One could be assembly costs, but I really suspect there are some good reasons related to the engineering of the vehicles.

Frames and bodies are not nearly as inflexible as you might suspect, especially the ones which used stamped steel frame sections (unlike the much more recent "hydro-formed" frame sections of light duty trucks). As the Pontiac manual states, both the frame and body combine to form a rigid structure, with the rubber isolation between the frame and body (in "body on frame" construction) resulting in metalllic transmission paths of road noise and other chassis operational sounds being highly diminished . . . i.e., a quieter ride. Two somewhat flexible units combine to make one stronger unit.

In any flexible unit, there will be harmonics which can develop in certain operational modes. PLUS, the amount of flex is typically designed-in as it must be controlled for the longer term durability of the combined unit. Conventional wisdom might indicate that everything needs to be nailed down solid, but in so doing that would move any flex to a location where it's not meant to be (in the vehicle's structural design).

The #4 mount is a full mount on the convertible and station wagon chassis for a reason . . . plus, the manual notes that a convertible frame is a "boxed" frame rather than the normal "C-section" frame, which can help compensate for the strength lost when the steel top is removed from the body structure. Therefore, the convertible body has its own set of unique harmonics and "bending" from the closed body types.

With regards to handling, those rearward mounts would, I suspect, not have any real impact on the vehicle's handling as that would be more determined by what happens at the front end of the frame (which is also not nearly as strong as you might suspect, as the crossmembers on similar Chevrolets have been known for their "sag" with time and age, which then affects front end camber alignment issues, especially on the big-block cars, as I understand, AND why aftermarket upper front control arm shaft kits have compensation for such designed into them). "Slop" in the rear suspension is controlled by bushings which locate the various pivot points of that suspension system, not specifically by the frame itself.

In one respect, if you chopped the frame into two sections, leaving the #4 and #5 mount areas out of the mix, you'd have what a uni-body Firebird/Camaro has for frame attachment with two subframes. Therefore, if you think of the addition of the #4 and #5 body mounts as cushions for the "subframe connectors" (which the F-body cars probably needed from the factory), then having just the upper cushions in these "mount" locations can make some sense. Additional body supports and "flex absorption areas" to both support the body and also decrease any harmonics which might be magnified by this area of the body's (a "closed" body in your case) floor pan.

There might be some specialized applicatioins where having solid body mounts or using every possible body mount location might be advantageous (as in an all-out, single purpose racing vehicle, which will usually have a full roll cage in it anyway), but for an everyday street-driving vehicle, I'd say that the factory guys knew what they were doing much more than we might give them credit for in designing a vehicle chassis/body combination that would take a beating on uneven road surfaces and still be quiet on smooth road surfaces.

Therefore, I'd be of the orientation that you should duplicate the factory production body mount/cushion set-up your vehicle was produced with. Including using factory-style rubber mounts rather than polyurethane mounts, too.

One change I might recommend is to use Grade 8 bolts for each of the bolted mount locations. One reason is that they'll be more resistant to long-term corrosive elements than the softer Grade 5 bolts might be. Observe all factory recommended torque specs in this situation, too. Only downside is that that'll make the nuts (suspected to be Grade 5) more prone to stripping the threads with the harder bolts, which is why the factory torque specs being adhered to is highly important.

So, my recommendation would be to put it back like you found it.

Regards,

NTX5467

[Guest 69GS]

Thank you NTX5467,

The info you provided was very insightful and will help me make my decision.

On the topic of urethane vs rubber, i have researched this topic for a long time just like every other facet of this project and there are mixed opinions on the subject and also depending on the intended purpose of the vehicle. The majority votes for urethane, for most applications(overall platform stiffness is increased which equates to better handling and road noise in some cases is reduced or eliminated) and usually only siting rubber as prefered if looking for a concours restoration and like you said, alot of the engineering was also heavily based on cost reduction to increase total profit. I have been a red seal Mechanic for 16 years and i know better than most that the engineers, especially now design components of all vehicles to meet a minimum standard unless related to safety or emissions which 90% of the budget is focused on, but if you take a ring gear for example in the 50's and 60's these were overbuilt the metalurgy was far stronger than todays now with all the testing they can do they adjust the process to meet a very minimum standard (ie make it just past warranty with normal service)to keep cost down and to gain profit on repairs or new vehicle sales.

It is a fact that the stiffer the chassis (modern unibody cars are incredibly stiff) is the better your suspension can work ie modern corvettes and S.L.A front suspension is still the prefered technology in performance applications not macpherson struts (these were designed to lower production cost and to combine the system to one replaceable unit with a high parts cost to the consumer but a quick change design to keep labor cost at a minium if failure happend within the warranty period.

My chassis service manual also has detailed pictures of the mounts cross sectional view and there respective locations and torque spec ranges 30-45 lbs ft and also a description of there intended purpose but there is no litrature on the purpose of the four steady rests, i know the convertibles used the #4 mounts and also the wagons and in fact my hard top has the grade 5 caged nuts already in the body at these locations but not at location #5 and most builders add the two bolted mounts at these two spots (#4) but no one has said anything about location #5. From what i gather most of the chevrolet A bodys didn't have bolts at location #1 just a steady rest but the outer hole #2 had a bolted mount unless it was a convertible or the higher end monte carlo's but all the buicks i have come accross have bolts at #1 and #2 so i think it depends partly on level of production ie buick being higher up than a chev or pontiac, partly on road noise isolation and somewhat on cost ie urethane is more expensive than rubber but it is also used heavily in todays modern cars and trucks because of its longevity which seems like a contridiction but they are expensive to replace and also there superior properties in controlling harmonics.

I do agree that these A body frames are very flexible with the body off and the body, contrary to popular belief is incredibly stiff and strong if you have a hard top, convertibles were notoriously flexy and thats why they got the extra mounts and boxed in side rails these usually had the cradle sag partly because they all had 400 cid or larger engines (buick GS did anyway) but my datum plane measurements on my frame are dead on with the secs in the chassis bible, but on further consideration, befor i add these mounts at locations #5 on the frame i think i will box in the rails first to limit the frame flex on its own so as to not add stress to the body structure once bolted at all 14 locations (originally 10 bolts and 4 steady rests)like i said most builders go with 12 bolts and 2 steady rests because there seems to be no liturature or someone who has tried it but i have read some reviews on solid aluminum mounts which have been sited as incredible for all out performance applications and are not as harsh as one would believe when it comes down to noise and vibrations.

Please consider also that in the west we have a heritage of soft plush highway yaghts that generaly give little or no feedback to the driver whereas in europe there cars are very small tight and ridged with incredible feel and feedback which gives the driver complete confindence over vehicle controll and they handle alot better then there western counterparts and thats all due to our different lifestyles and our sprawling landscapes and long not so winding roads but if you want to make an all out performer you must stiffen the platform increase front caster keep camber level with slight gain on cornering (ie caster) and diving (ie S.L.A front suspension) and preferable independent rear suspension with also the S.L.A setup (not macpherson struts) but this dosen't stop NASCAR which took the A body design chassis and improved it by making it stiffer ie complete upper rollcage and increased the length of the lower rear controll arms considerably and of course they use all tubular suspension arms and delalum bushings (no give or flex) and heim joints to prevent binding in the rear suspension when the axle has to articulate. but don't get me wrong they are based on the A body design only every NASCAR chassis is custom fabed frame and rollcage is one unit.

Thank you again for your insight into the enginering and i will definately consider your recomendations.

Edited August 16, 2011 by 69GS (see edit history)

[NTX5467]

Thanks for your additional comments and information.

One of the things I've considerd over the years was something of a hybrid system of body and suspension pivot point mounts. A mixture of urethane and rubber, depending upon the location.

For example, I'd keep the rubber bushings in the lower control arms for isolation, but would consider urethane for the upper control arm pivots. Similarly, use rubber for the upper body mounts and "steady rests", but urethane for the lower mounts. Keeping some "flex", but in a limited manner.

As I later discovered, cradle sag even happened on 1955 Chevys. If you look at the aftermarket upper control arm shaft kits and it mentions "improved adjustability", you can bet it's got some allowances built into it for cradle sag.

I concur with the max-caster suspension alignments. This puts the outside wheel "braced" against the outside turn due to the increased negative camber at that outside wheel and increased positive camber on the inside wheel. It seems that almost all modern front suspensions have this in them, now, but it used to be just Chryslers that were that way (with normal caster settings). As the car leans, this means the outer wheel is more perpendicular to the road surface, for better cornering capabilities. For the super-wide tires of modern times, this might be a little too much such that only the outer sections of the tire are in contact with the road surface . . . which the ultra-short sidewalls can't compensate for in their flex range.

I always thought the transverse composite leaf spring on some of the middle-80s Corvettes was a very neat way to do things and still have control arms. There used to be an adapter kit to convert the 2nd gen Camaros/Firebirds to that set-up, plus composite rear leaf springs (ala the 1980s Chevy Astro Van) for the rear springs on these same cars.

First we had the SLA front suspension with short (normal) spindle height. Then we got it with TALL spindles, with "coil over" spring/shocks. Never did understand the rationale behind that, but I've noticed that many newer cars have that arrangement.

I've determined that the factory will run things right up to the jagged edge of reliability in how they do things. Sometimes, it makes perfect sense to do things that way, other times, it can cause an unhappy customer. Computer modelling/testing has allowed many castings to be lighter, strong, and durable. But if we'd tried that with a die grinder, we might have ended up with something for the scrap iron heap. Be that as it may . . .

When the earllier polyurethane suspension bushings came out, I asked my wheel alighment associate and his reply was "They squeak", and laughed. A year later, the graphite-impregnated bushings were introduced, to allegedly combat that situation. Later still, the greaseable polyurethan bushings were introduced, along with the Del-A-Lum variations.

In the mean time, I opted to stay with rubber everywhere. I'm not road racing and can get enough steering response to suit me with increased tire pressures and better tire choices, even some front/rear sway bar packages, too. My orientation is that if polyurethane fails, it cracks . . . if rubber fails, it just sags. If I want stiffer rubber bushings, I can use a trick I found in an old car magazine . . . insert some smaller finishing nails into the rubber to stiffen it up. That was from the later 1960s.

One place I have no problem with substituting polyurethane for rubber is on sway bar link bolts or control arm attachments. BUT I've also had some failures with them, as did a friend that used them on everything he had, when the urethane shatters before or after the bolt kit deposits itself on the road way. Never had that happen with pure rubber for those link bolt grommets.

With a little modification, these same polyurethane sway bar link bolt grommets can also be used as upper shock stud mount grommets. Obviously, it would put the shocks in their "high speed' damping mode a little quicker than with the rubber grommets.

If you might desire to hide your urethane bushings, you might spray a coat of "FlexSeal" on them to give them that rubber look.

Your car. Your money. Your dreams. Pursue as you might desire.

Regards,

NTX5467

[Guest 69GS]

I definately agree with you on the failure of urethane and the fact that they squeek thats why for any torse elastic bushing ie upper and lower controll arms in the front suspension and rear controll arms or leaf spring eyelets i went with rubber, im only using urethane on the body mounts which only compress (no twisting going on)

The main reason urethane squeeks is that in a controll arm setup the bushing is rubbing on the metal when the arm moves up and down unlike rubber which is fused to the metal collars thus creating what is called a torse elastic bushing the rubber twists within the two steel collars instead of sliding against the steel and that is why all suspension bushings must be torqued at ride height only, if you tighten them with the suspension hanging when you lower the car back down they can tear or at the very least shorten there lives and will also cause the vehicle to site a bit to high.

thanks again

I will post my reviews after i do some testing of my own(maybe im the first guy to do this )so i hope anything i come up with will help other guys in there decision to improve on the original design of there car.

[NTX5467]

The issue of doing the final torqueing of ALL suspension bushings at ride height is something that many might have missed along the line. Too easy to nail things down with the car supported on stands or a lift. This results in all of the flex being all one direction, rather than centered in the range of motion of the suspension part . . . plus the possibility of decreased service life. Having the bushing's flex centered in the range of motion lets things act as they need too.

Keep us posted, please.

Enjoy!

NTX5467