Body mounting thoughts for the model A job

[idrivejunk]

Forgive me gents but I am going to cut and paste my words here, as written for another forum. Already got one idea I didn't have but the more the merrier and some of you guys certainly have the counsel I seek. Thanks...



A 1930 Model A coupe with reinforced, unitized body over tubular aftermarket chassis with near-stock modern 400 HP EFI / OD V8 automatic... is the subject here. Not mine, its a customer project at my job. Here is my idea / question:

Stock Model As have the body bolted directly to frame rails and a thin strip of padding is used between them. As for this custom built body and TCI frame...

Frame comes with rivnuts. Those are threaded inserts which are drawn tight with a threaded stud on a setting tool which is similar to a hand riveter. On top of the frame rails. They proved far too delicate to even make it past initial body fitting. If a bolt is bumped on one side, it tilts in the insert which will then spin in the hole on the tubular frame rails.

They can be replaced easy enough but I cannot imagine the body staying on the frame during a barrel roll. Extremely wide, soft, and low profile rear tires, tall vehicle profile, weight of added body structure, and occasional drag racing contribute to that concern. I know how easily that rollover scenario can occur, even just feathering throttle out of a burnout too abruptly. I can't see 8 or 10 3/8" rivnuts doing the job.

I can remove the nutserts (whatever ya wanna call those) and replace them. Option A.

Option B: Weld tabbed nuts to top of rails, the ones made with little wings to weld to.

Option C: Enlarge holes, sink a plain nut in the rail and weld it all around.

Option D: Plates with nuts welded fully to them, over the enlarged rivnut holes and secured with plug welds to top of rail.


None of those options offer sufficiently positive retention of the body for me, but this is my first rodeo constructing a prewar checkbook rod and I come from a collision repair background. Just a couple dozen weld dots or eight rivet thingys does not seem safe, all that can rip out or off in an impact due to inertia.

I want the body to sit about 5/16" above the frame and be attached with normal body mount cushions on stands welded to the sides of the rails. I think that satisfies my durability concern and as a bonus should improve ride and comfort from a noise, vibration, and harshness standpoint. Option E. Can sketch it if the idea is unclear.

Notes for guys familiar with Model As:

Aprons attach to floor rails now and do not extend over the frame rail like stock.

Front fender brackets would be attached normally but with a 5/16" thick bushing added on top.

I believe rad /grille will move up that far although 5/16" spacers rather than bushings would be appropriate.

In the current configuration, I figure if for example you tried to lift the car by the body, the inserts would rip right out of the rail tops. With normal plain big-thick-washered and rubber cushioned metal sleeved body mounts on stands welded to rail sides, it is my opinion that much more hairy wadded-up-car nightmares could be walked away from using Option E as opposed to taking flight in a jettisoned pod as is my fear with Options A through D.

Am I overkilling in thinking this? Which option would you prefer and do you have an Option F?

Thanks for the read, and in advance for the benefit of your experiences.

This is the chassis in question and I know you know I know you know what the body looks like.    

[enjenjo]

The Rivnuts you have there are the wrong type for that application.It is the type that shears in the middle when it is pulled up, like this one  https://www.mcmaster.com/standard-rivet-nuts  Look at the third row down, fourth one from the left, but they don't work well in thinner material because the outer piece does not create enough friction.

What you need is this type  https://www.mcmaster.com/standard-rivet-nuts  First row, top left  I have used these in the past in a race car, and they will rip a big holes in the frame before they will come out.

[Canuck]

Did mine with a version of your option "D", only instead of plug welding the plate to the top of the frame, I mounted it inside the frame, with the nut on the underside.  To pull it out you would have to pull the 3/16 thick x 1 1/2 x 3 plate thru the top of the frame.  Used the same idea for shock mounts on the front and fender , running board mounts etc.  I didn't want it to break.

Drilled holes in the frame top rail for the welds.  Pulled the plates into position with a threaded stud in the plate with wire attached, wire fed thru frame and out bolt hole in frame.  Grabbed stud and threaded a nut on to hold tight in position.


The rest of my mounting was similar to stock.  Frame top covered with fabric webbing and wood spacer blocks mounting into the sub-frame.  Webbing just keeps metal that sits on frame (Front fenders and splash aprons from squeeking.  Rubber bushings between frame and body sound like a good idea.

Bit of a pain but worked OK.  Sorry no pictures.

[idrivejunk]

The Rivnuts you have there are the wrong type for that application.It is the type that shears in the middle when it is pulled up, like this one  https://www.mcmaster.com/standard-rivet-nuts  Look at the third row down, fourth one from the left, but they don't work well in thinner material because the outer piece does not create enough friction.

What you need is this type  https://www.mcmaster.com/standard-rivet-nuts  First row, top left  I have used these in the past in a race car, and they will rip a big holes in the frame before they will come out.

They don't look to me like a split type but do look like the bottom ones, twist resistant steel. They are "resistant" like "moisture resistant" electronic devices or "theft resistant" vehicles. They were not made foolproof, I know that because I'm just the fool to prove it.

[idrivejunk]

Did mine with a version of your option "D", only instead of plug welding the plate to the top of the frame, I mounted it inside the frame, with the nut on the underside.  To pull it out you would have to pull the 3/16 thick x 1 1/2 x 3 plate thru the top of the frame.  Used the same idea for shock mounts on the front and fender , running board mounts etc.  I didn't want it to break.

Drilled holes in the frame top rail for the welds.  Pulled the plates into position with a threaded stud in the plate with wire attached, wire fed thru frame and out bolt hole in frame.  Grabbed stud and threaded a nut on to hold tight in position.


The rest of my mounting was similar to stock.  Frame top covered with fabric webbing and wood spacer blocks mounting into the sub-frame.  Webbing just keeps metal that sits on frame (Front fenders and splash aprons from squeeking.  Rubber bushings between frame and body sound like a good idea.

Bit of a pain but worked OK.  Sorry no pictures.

Fish wire! You said the magic words.  Thank you very much. Now I am thinking of tapping half inch thick plates to be held by a pair of plug welds next to the existing holes. But now that I think more, some 7/16" nuts on 16 ga plates would work and be much faster. That covers all the bases except the actual fishing. Rubber mounts would be nice but the idea is not catching on. Got idea in hand now though, much obliged.

[moose]

Matt what I have done is taken 3?4 round stock that was drilled and taped and weld them into the top of the frame rails. We then used rubber and wood shims to to square up door gaps

[idrivejunk]

Thanks, Moose. Thats another way I hadn't thought of. My latest revelation is that rivets would work instead of welds, to hold the plates inside the rail. That allows making the change without any welding on the frame.

[idrivejunk]

Keeping it simple, just welding in 7/16" nuts. There will be a strip of padding but all my rubbery mount ideas might detract from the overall rigidity of the car.