Plywood chassis bonded to 'glass body (think "Marcos GT1800") project

[bartbrn]

I'm working on a project that should make LBC purists and hot-rodders
alike gape, gasp, and guffaw: I have one of Bill Bonadio's "Dio"
splashes of the 1959-60 Bill AMes/Dewey BROhaugh AMBRO fiberglass
bodies, originally intended for a Triumph TR-2/3 chassis or whatever
it would fit on (the body will accomodate wheelbases from 88" to 101",
thanks to the trim-able center body pieces), to race in SCCA
"Modified" classes. (If you've never seen one, it is dimensionally and
sculpturally about as close as a couple midwestern racers could get to
a '57 "Knobbly" Lister without generating lawsuits from Brian Lister.

I'm sending this message to three different user groups:
rec.autos.antique (because of the expertise here on the '64 MGB
running gear I'll be using, and because of the obvious connection to
the wonderful plywood-chassied Marcos cars), rec.aviation.homebuilt,
because I can't think of another group that is likely to have as much
knowledge of stressed-plywood and fiberglass construction, and to
rec.boats.building, because of the wealth of experience there with
plywood and fiberglass craft that don't leak or fall apart in the
middle of the pond.

I've been trying to get started on this project since I got my
"Dio" (AmBro splash) fiberglass body in 1997 (aahhh, why hurry?), but
miserable interruptions and daunting pre-tasks like actual money-
generating work and my Wreck of the Hesperus garage have kept me in
the "design phase" for 14 years -- during which time some of you may
have even seen a post or two of mine.

Well, I'm 63, retired, have my garage half-cleaned (my wife's half,
unfortunately...), I've discarded entirely the idea of building up a
steel-tube chassis to drop a small-block into it, and after much
research, doodling, modeling sections, I have come to the conclusion
that the plywood semi-monocoque and 1800cc MGB engine and drive train
are the way to go -- the Marcos GT1800 used (mirabile dictu!) an
1800cc Volvo engine, the same one as in the late, lamented Volvo P1800
(aside from the PV544, the best-looking car they ever made. But I
digress...)

I have long had an abiding admiration for Jem Marsh and Frank Costin's
Marcos semi-monocoque plywood-chassis/bodies, especially the GT-1800,
and I found, in Herb Adams' excellent book on chassis design, a
stressed skin "2 box plus integrated monocoque side sills and
bulkheads" design very much like that of the Marcos GT1800. Adams'
design is in steel, and was used for a high-end Cobra replica that
wasn't any more exciting than any other Cobra replica: except that its
stressed-skin-and-bulkhead chassis had HUGE torsional rigidity (as did
the original Marcos and Marcos GT)

My running gear donor is a '64 MGB-GT that was rotted to within an
inch of its life (so don't fret, "rebuild it from Heritage panels even
if all you have is part of the floor" enthusiasts -- it didn't have
ANY part of the floor (did have some nice sellables, like the aluminum
bonnet, working Smiths heater, several extra 60-spoke wire wheels,
etc.), but for MY purposes, and at $250 for the whole MGB-GT w/
complete drive train, INCLUDING the 50-mile tow to my garage, it
seemed like the perfect basis, especially as the entire front
suspension and steering rack are mounted to a bolt-on crossmember
(which itself is mounted to the MG's unit body). By using the front
crossmember as a whole unit, all that's necessary is to make sure the
crossmember is mounted to maintain the correct kingpin inclination, as
is perfectly illustrated in the Bentley manual.

Anyway, that's my double-bastardized sweaty-fingered project of the
decade. If anyone happens to know of a good website that might be able
to help me with the dreary details of this project, please let me know
(I'm thinking plywood boat and experimental aircraft builder's
forums), I'd be most grateful for any links. BTW, the origination of
the Marcos plywood monocoque had a LOT to do with Frank Costin's
previous engineering work at DeHavilland, makers of the plywood
monocoque DH.98 Mosquito "Wooden Wonder" -- the near-"stealth"
invisibility of the plywood Mosquito to "Freya" and various FuG German
radars made it an excellent Pathfinder and Night Fighter. Wonder if it
works against police radar?

I'm not building this to fool anyone, or to represent it as an actual
original AMBRO (which has been tried, by the most unlikely person one
could think of), it's just going to be a driver. My plan to get it
street licensed by the anal-retentively-regulatory CT DMV is to use
the matching chassis and engine S/N plates, and call it an "MG
Special," like the Lester-MG. Will it work? At 63, will I live long
enough to complete it? Who knows, but as my distant -- VERY distant --
relative Robert Browning said: "Ah, but a man's reach should exceed
his grasp, or what's a heaven for?”

Sorry for the long post. If anyone's interested, I'll be putting up
pix of the project elements, design, and build on Photobucket or
FlickR.

Any suggestions -- other than "get thee to a loony bin" or "go soak
your head" gratefully accepted.

Peace

Bart Brown

[myfo...@yahoo.com]

There shouldn't be anything too difficult or dangerous about what
you're describing. Pick your plywood carefully oak might be one
choice, unless you run across something else, and select the
fasteners/fastening system carefully. You'll want some "give" to the
system, so it doesn't self-destruct over a few potholes.

[bartbrn]

Thanks for the very welcome advice -- I'm going to do it pretty much
just like Marcos did it, although I'm not sure if I want to go to the
expense of marine plywood as they did: they used marine ply primarily
because the glue between laminations is not supposed to have ANY gaps
(I don't know how you'd tell, non-destructively, except by X-Ray).
Anyway, to make up for the rigidity lost by having an open cockpit,
Marcos used a triangular boxed sill of ply, glued at every joint with
1" x 1" spruce corner bracing, and epoxy-glassed to the fiberglass
body's undersill. I'm doing the same, except my sills are much deeper
(the Marcos' were about 5"-6") separate fiberglass panels with a
regular and continuous 10" radius under-curve. The main top fiberglass
body pieces continue this 10" curve radius, so, without offsetting the
flooring up or down, the body is 20" deep at the cowl. The rear body
panel is about 4" higher, but still connects as a smaller bulkhead to
the undersill).

The other main strength of the Marcos cockpit was a very deep central
drive tunnel that connected front and rear bulkheads of spruce-
reinforced ply. Mine will be similar, with a central drive tunnel of
the same 10" height as the side sills, and tied into epoxy glass and
ply -- not just ply -- cowl bulkheads immediately fore-and aft of the
cockpit opening. There will be latitudinal enforcements, epoxy-glassed
inside each side sill. Three of the enforcements will be continuous
from sill to sill across the cockpit: one at the rear bulkhead, the
next under the driver's knee position (like in a Porsche 550 Spyder),
and finally the front cowl bulkhead -- these epoxy-glassed ply
enforcements will tie the left and right sills, central tunnel, and
fore and aft cockpit bulkheads together, more than making up for any
loss of torsional rigidity from the "big hole in the middle" cockpit.

The two sides of the drive train tunnel angle open as the tunnel goes
forward through the front bulkhead, to accomodate the engine and
transmission, and, with the footwells, the outer sides of which are an
un-jointed continuation of the inboard cockpit interior sides of the
boxed-in sills, will also be boxed and angled to provide steering
clearance for the front wheels, and provide a multi-laminate fore- and
aft-braced connecting point for the front MG cross member. In front of
the MG cross member, there will be another glass and ply bulkhead that
will provide additional stiffness for the engine bay, and separate the
engine entirely from the radiator, which will be angled and mounted
forward, along with the 13-row Serck oil cooler, in a closed extractor
tunnel that will take advantage of the low-pressure area produced by
letting a radiator air exit into the "knobbly-style" center hump of
the bonnet, the low pressure area helping to pull air through the oil
and water heat exchangers. For low speeds and idle, a thermo-coupled
12" electric "sucker" fan will be mounted just behind the radiator.

From the rear cockpit bulkhead back there will be "stressed skin" ply,
spruce reinforcement, and epoxy glass torsion boxes. Immediately
behind the cockpit will be the fuel tank, and for the rest, I'm
keeping the leaf springs and Armstrong lever shocks, the attachment
points of which to the ply torsion boxes will be reinforced on both
sides by 1/8" steel load-spreading pads. The only change to the rear
suspension will be a polyurethane-bushed straight pivot (i.e., not a
"rose" or "Heim" joint) welded steel "A" arm, each "leg" of which will
mount to the torsion box in vertical and horizontal alignment with the
left and right front leaf spring-eye mounts. The apex of the
fabricated "A" will converge on another urethane-bushed straight
pivot, confining rotation to 90° to the car's centerline, which will
attach to another weldment tied into the top of the differential. This
fabricated "A," thus mounted, will control lateral axle movement
better than the leaf springs alone, and will also prevent axle "wind-
up", both in acceleration (which will hardly be neck-snapping with a
pretty bog-stock MG 1800) and braking, which will have to be carefully
set-up, taking into account the considerable weight reduction, and the
single-puck stock Girling front brakes.

All exposed ply surfaces -- like the woven-glass-and-epoxy-sealed 3/4"
plywood floor (also tied into all torsion boxes and sills with which
it comes in contact) -- will be sealed and painted with Valspar.

Sorry for the long explanation, but I was writing this out also as a
plan to which *I* can refer when I get lost!

Thanks for your interest!

Bart

[Mike]

On 10/19/2011 12:50 AM, bartbrn wrote:
> Thanks for the very welcome advice -- I'm going to do it pretty much
> just like Marcos did it, although I'm not sure if I want to go to the
> expense of marine plywood as they did: they used marine ply primarily
> because the glue between laminations is not supposed to have ANY gaps
> (I don't know how you'd tell, non-destructively, except by X-Ray).

I used some marine plywood for a project years ago because I was told it
has no voids (IE. empty knotholes in the plys). I did not encounter any
in the couple of pieces I used.

Furniture grade plywood, on the other hand, has a number of interior
voids. I've used lots of sheets and they have all been that way. That
is not necessarily a strength issue in furniture, but you do have to be
careful with screw placement. Construction grade plywood also has
plenty of voids.

Good luck with your project.

Mike