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MG MGB GT V8 Factory Originals Technical - Aluminum floor

Have started to build a track/road car based on a 1976 roadster and am keen to make it as light as possible. Its pretty rotten so im having to replace a lot of panels. Just fitted a complete sill to one side
Has anyone replaced the floor with ally?
Thought I could stiffen things up by welding small box sections running between sill and trans tunnel then fit an ally floor with rivets.
With all the rust im chopping out its pretty light at the moment.
Planning on fitting sebring arches and when the bodywork is done swapping the running gear over from my 4.6 GT.

Mark
Mark

Mark:
I don't think that you would save any weight after you added box sections to strengthen the floor and body. I believe that riveting aluminum to the body would add any strength to the chassis unless you used aircraft type rivets and a rivet gun and bucking hammer.
I would be concered about the possibility - no, the certainy! - of galvanic corrosion. When two disimilar metals are in contact in the presence of an electrolyte (such as water), corrosion will increase on the least noble material(the anode)and decrease on the noblest(the cathode. This increase in corrosion is called galvanic corrosion. Since aluminium is less noble than steel, this can be a serious problem. Add salt to the equation and it becomes even worse.
Phil
Phil O

Mark, aluminum is great... but this isn't a great application for it. If you're really determined, there are two-part adhesives for bonding carefully cleaned bare steel to carefully cleaned bare aluminum, and you should investigate them. They aren't cheap, but they can be very strong if applied correctly. Normally they're used WITH rivets, but the rivets are mostly for alignment and clamping while the adhesive cures.

Carroll Smith's book "Prepare to Win" is a wonderfully informative book you should read before cutting any more metal... It includes a lot about how rivets are used in real race cars.

Pay special attention to seat mountings.

The embossments in the original steel floor panels added more rigidity than you'd probably expect. They had other benefits too, like reducing noise propagation. The characteristics if aluminum, including cost, vary quite a bit with alloy and heat treatment. Some alloys have more corrosion and fatigue issues. But the bottom line is, it'll be very difficult to approach the performance of steel with aluminum without significantly increasing material thickness and subsequently compromising your intended weight savings.

A rusty roadster with an especially big v8, and you want to occasionally put it on track? It sounds to me like you should consider a full cage and/or really substantial body reinforcements (like maybe welding the doors shut). A torsionally stiff chassis will help with both suspension tuning/handling and getting power to the ground.
Curtis

Thanks for the replys.
Curtis
I fully intend to strengthen the chassis and thought adding a steel ladder underneath would do this.
I was intending to use say 35x20mm square tube going right across at the back to pick up seat mountings then a 35x35mm square tube to replace the original crossmember with more tube further forward. If necessary I could then fit triangulated braces.
Surely this would be a lot stiffer than standard without a floor fitted.
The ally could be quite thin as the ladder is doing all the work.
Phil
Hadnt thought about Galvanic corrosion. If the ally was painted with etching primer and a form of underseal beforehand and fitted on some sort of sealer this would prevent metel to metal contact, would that solve the problem?

The car will be used mainly on track only needing to drive on the road to get to the circuits.

Mark
Mark

Mark,
Any form of physical barrier will stop the galvanic corrosion Phil mentioned.

The Carroll Smith book is well worht a read as are any by that now deceased author. Very practical and useful with enough theory to get other things sorted.

Cheers , Pete.
Peter Thomas


Mark,

I know from experience that welding box tubing to an MG's existing thin, fatigued, rusty, steel is tricky. (My GT's seats sit on 1" by 1" by 0.063" box tubing that's welded-in and reinforced, and I also have a full cage fitted.) You should certainly plan on fabricating gussets and brackets to reinforce joints and spread loads.

Triangulation is a good strategy, but I really think you'd be making a big mistake if you don't also think of the floor as a "stressed skin".

To work as a stressed skin, the floor needs to be securely attached to the rails and rungs of your "ladder", and hardware-store pop-rivets aren't adequate for that by themselves. The Carroll Smith book explains which pop rivets are much, much better than others... and it also talks about structural adhesives. Adhesive could serve as a barrier for galvanic corrosion, but you'll want to look for rivets that won't conduct or corrode, and that'll seal out water. Aluminum has its uses, but you might actually find steel is better suited for this job. You might even consider pre-galvanized sheet. (DON'T weld it though. The fumes are quite toxic.)
Curtis

Thanks Curtis
Ill get a copy of that book.

Mark
Mark

Extract of magazine Historic Race & Rally december 1992 relating to the MGC GTS torsional rigidity problems. For further weight saving, some body panels were pressed in aluminium sheet.
« I think RMO’s stifer torsion bars were provoking an inherent deficiency of these lightweight aluminium bodies they lacked the torsional rigidity of the standard steel shells. Basically, their engine bays needed stiffening to stop them twisting”.
“RMO has a unique front-hinged bonnet. We had problem with the bonnets coming open and flapping on their safety straps because of movement of the aluminium bodies”
“A report from pre-Marathon testing at Thruxton shows another drawback of the lightweight body : “Fit safety catches on doors as body distorts”. Simple, sliding cupboard catches remain on both cars.”
“ A weighbridge ticket from July 1968 shows that MBL turned the scales at exactly 22cwt when weighed in Marathon trim, ready to roll, compared with a dry weight of 23.3cwt for the standard MGC GT.”
Regards
michel

Michel, Thanks for that interesting note.
I am surprised the weight difference is so little ( about 5% by my extimation)especially since the aluminium body was always touted as being "ultra light weight" by many.

Cheers , Pete.
Peter Thomas

Two problems with comparing this to the MGB are that one, the MGC has a different front suspension and crossmember which arrangement was recognized as less stiff than the MGB, and second that the fenders being hinged at the front also removed rigidity. For an interesting weight comparison look at the weight difference between a Range Rover and a comparible steel bodied SUV. The difference is likely to be much less than you think. It's much different than comparing a steel MGB hood to an aluminum one for weight. Note the aluminum hood is very noticeably more flimsy, though lighter. Where aluminum really comes into it's own is in structures built from the ground up to take advantage of it's properties such as aircraft where thin sections and tempered alloys are stressed along their strongest axis, usually in tension. Few other applications will show much of a weight advantage unless they are overbuilt to begin with, and car body components usually must handle stresses applied simultaneously along multiple axis' which can usually not be done without adding weight.

But if you really want to reduce the weight, use alloy panels and strengthen the body perhaps the best approach is to construct a highly triangulated tubular frame from round high strength thinwall tubing in either a steel or aluminum alloy and then hang aluminum panels on it. But of course now we are talking about a completely custom hand built car which is an expensive and lengthy undertaking. The MGB unibody is heavy, that is true. It was overbuilt, and that is the thing that makes it such a good choice for a V8 transplant. There are only a couple of weak areas and they are easily reinforced, making it capable of handling engines of surprisingly high horsepower. My suggestion to you is to repair the rust with original panels, brace the forward leaf spring mounts or at least the right hand one, make sure there are no cracks around the front crossmember mounting bolts, and build in sufficient horsepower to take the place of any weight advantage you think you might have had by doing differently. It's hard to go wrong with that approach.

Jim
Jim Blackwood

This thread was discussed between 29/05/2005 and 02/06/2005

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