Technical Model A Float-a-Motor

So I've been reading up on Float-A-Motor rear engine mounts ['30 Tudor Banger] and it appears there are considerable pros and cons.
One thing that seems to come up is the FAM contributes to frame sagging.
I can see how the original mounts can act as more of a "crossmember" [which is a good thing] as there is less room for "play" and the FAM would allow more movement in that regard but I'm not seeing how it would contribute to frame sagging. I mean it's the same amount of weight in the same place.
What am I missing?
Also, is the rear transmission mount that comes with the kit necessary? Is that needed as the FAM allows more up and down movement?
Any comments on the pros and cons of FAM are welcome.
Robj

 

I've had to straighten two frames from original cars, it's common since the weight of the transmission is hanging behind the rear motor mounts and the torque tube transfers movement to the back of the transmission. There have been several articles over the years in the Restorer magazine on straightening sagging frames, I've had a Float-A-Motor on my stock 29 coupe for well over 20 years with no problems. I don't think the FAM causes any more problems than the stock rear motor mounts.

 

If you want a deal on a FAM, I have an unused one on the shelf. Killer deal.

That said, I've had no less than 3 dozen frames come in and out of my possession over the years. I've seen sagging/bent originals and a few had evidence of rough history to match.

What the FAM won't compensate for is an already bent or abused frame although you can get one mounted in a frame easier than the original hard mounts (those just won't go back in on a bent/warped frame without getting the frame corrected) they'll wear out (as designed) when conditions force them too.

 

My frame had a crack right to the rear of the rear motor mount. It was "repaired" by welding up the crack on the outside and welding a 5/16 plate to the bottom of the frame. The resulting sag was never repaired. I cut the weld, V-ed it, raised the frame in that area [to remove the sag] and welded a new bottom frame piece in about 10 " long. [I'm working on a bare frame]
I also made 2 plates, 15" long to fit inside the frame where the rear engine mount is located. With this the stock engine mounts will no longer fit. So I got the FAM thinking I would weld up the bolt hole [for the rubber donuts] on the frame portion of the FAM and drill a new hole slightly outboard of the old hole.
I put a cove in the end as I had read that a "hard end" , or a partial boxing of the frame, could generate cracking. Plan is to weld these in with 3/4" of weld, skip 3/4, etc.. The smaller holes are to plug weld to the frame.
Seemed like a great plan until I stumbled on the perceived cons of the FAM.

How about the FAM mount that goes on the rear of the transmission that came with the kit. Is it necessary to use that if using the rear FAM engine mount? What does that do?
robj

 

If you're referring to the mount that goes on the U-joint housing I didn't use it on either of the FAM kits I installed, I guess it was to replace the U-joint housing tang on the early U-joint housing. The 29 Roadster Pickup I'm working on now is a pile of miss matched parts, when I took it apart I noticed the U-joint housing was held on with 1/4" bolts, the transmission was an early 28-29 with a later U-joint housing which has a different bolt spacing. I did have an early housing in the spare parts and used it, here are both styles of housings. As you can see the tang on the rear housing serves no purpose and was eliminated in 1929.

 

I've had FAM on all my A powered vehicles plus the '32 style front engine mount. Over the years, the FAM rubber will get harder than a hockey puck and needs to be replaced. Bert's Model A sell the replacement rubber cushions.

 

A lot of old wives' tales get repeated, and one which won't die is that the frames of pre-WWII cars were designed to flex. The truth is subtler: those frames were designed not to break when they inevitably did flex, despite everything the designers did to prevent them from flexing — because within the parameters of the format there was only so much they could do.

It's been around for a while. I recently encountered it in a British book on the subject dating from 1957, along with its frequent corollary that the springs on such frames ought to be as stiff as possible. If that were true, why do the springs comprise stacks of flat leaves and the frame rails veritable beam sections, rather than the other way round? Who would design a frame to flex and then go to all the trouble to equip it with springs designed not to flex? — and go about it in such an inefficient way?

No, early frame designers didn't lightly pass up any opportunity to get a frame to flex less, and one of the things they did was to make major mechanical components like engine blocks and gearbox cases effectively part of the structure, by designing those components with wide-spaced rigid connections to the frame. Take a look at "divorced" gearboxes, whose cases were often integral with two successive frame crossmembers, the idea being that moving the gearbox rearward subjects only the part of the frame aft of the back of the gearbox to multiplied engine torque. A shorter portion of frame is torsionally stiffer. And the bit of frame between the engine and the gearbox sees only straight engine torque.

So, the rigid engine mounts on an A are actually doing a job: they are reducing the effective length of the frame rails for the purposes of structure. Making the rear mounts soft makes the entire frame floppier.

Not that a stock A's frame is anything but floppy, so it's probably six of one and a half-dozen of the other.

 

I guess the obvious question is why not make the frames from a slightly thicker metal. Obviously one answer is cost but the second could be the inability to stamp thicker metals. The other may be the designers never expected them to be around this long and what was done was "good enough".

As far as the frame sagging with the FAM, I'm not sure I buy that as a cause unless, the FAM allows the frame rails to twist while the more solid connection resists this movement.

I can see where the design of the stock mount does create more of a "crossmember" than the FAM.
That being said, I think I going to go ahead with my 3/16" reinforcements on the inside of the frame, but I'm not going to use the FAM. I'm going to cut the original motor mounts and reduce their length by the 3/16's so they'll fit.
Thanks to all for their input. It definitely helped me think this through.

robj

 

I have restored several Model A's and every frame was tweaked in some way. The weak point is just in front of the center crossmember, as much of the stress is caused on the drivers side by the location of the battery, weight of the transmission and motor and the position of the driver and 1930's poor roads. The frame for the 1929 that I recently restored was sagging 3/8" in that location. We set the frame up on "I" beams and chained it down and used hydraulic jack's to get it straight.