MGB Chassis Reinforcement

MGB Chassis Reinforcement: "How to Give your MGB 6-pack Abs"

This article appeared in The British V8 Newsletter - Volume XII, Issue 3 - September 2004

by: James Jewell and Andrian Akhurst

Ok, so your MGB doesn't really have abdominal muscles, but this article will show you how to beef up your roadster's mid-section, which many have argued is a weak point when converting to a high-torque engine.

The MGB is one of the earliest production unibody, or monocoque automobiles in the world. Legend holds that Abingdon went to great lengths to over-design the car to ensure its strength and rigidity in the face of insufficient unibody design data. Keep in mind that the computer was not available as a design tool to auto houses in 1960, and that there wasn't a large body of data available on this new technique (outside of the aerospace industry). So the resulting design was quite stiff, even by today's standards.

Anyone who has ever driven a post-1979 Ford Mustang will have to agree, and I'm not even talking about the convertible! To this day, the only definitive design flaw to have shown up in the 'B (not including the 400lb, 90Hp 4-cylinder) is the 'Crack of Doom' on the door skin of 90+ percent of roadsters. An informal poll I took of MGB drivers who had been in accidents shows that the little 'B' protects the driver quite well in all but the most serious accidents. Then again, by definition, I couldn't interview people who had been killed in an MGB, so my data may be lopsided. Still, of all the people I spoke with, only one spoke of a known fatality in a 'B'. If any of you faithful readers has a good or bad MGB accident story, or know of any tragic accidents where you believe the driver would have been safer in another car than the MGB, please e-mail or write me. I may do a future article on that topic, as well as ways of strengthening the 'B'.

While the B's structural design appears to have rust as its only mortal enemy in cars with moderate output, there is a potential Achilles Heel for those who push torque to the maximum. It is the front leaf-spring perch that transfers all the torque at the rear wheels into the forward force acting on the car body. All of that force is, in effect, pushing on the car at the flimsy sheet metal intersection of the floor and the rear bulkhead. My gut tells me the forces in play are nowhere near the yield strength of mild steel, but the flimsy nature of the nearly-flat sheet metal in that area of the car will flex, and flexing ultimately leads to fatigue and cracks, not to mention alignment problems. If you've ever stood on the bare floor-pan of your MGB during a restoration, you'll have no doubt noticed the flexing (unless you are standing right over the central cross-member (jacking point). While I have never personally heard of any measurable deformations or structural failures in B's with engine conversions, my inner engineer tells me that routing all acceleration forces through a single layer of sheet metal and some spot welds is marginal at best, and it appears that the engineers who spawned the RV8 came to the same conclusion, as we will see momentarily. The way to avoid this potential problem is to alter the load path so that acceleration forces act through more rigid portions of the car that will better distribute those forces. In addition to the stock leaf-spring suspension, this approach is also applicable to any 3 and 4-link rear suspension upgrades that use that front leaf-spring perch as a mounting point.

MGB Body Stengthening

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Before we try to solve the problem, let us consider the design of the 'B' and how forces travel throughout its body. Even though the MGB is of unitized construction, there are portions of the design that resemble traditional frame rails. Starting at the nose of the car, there is the front sub-frame which handles loads from the front bumper to the middle of the cockpit, including all front suspension loads. At the middle of the car, the sub-frame ties into the central cross-member which runs from side to side connecting both the left and the right "frame rails" and the all-important rocker panels. As just about every non-California MGB has had rusted-out rocker panels at some point in its life, we're all familiar with the role they play in keeping the body from bending in the middle (one could argue that the transmission tunnel aids in that role). There is also a rear sub-frame that ties into the aft end of the rocker panels. But here the problems begin. The forward leaf-spring perches don't tie directly into the previously mentioned rigid members. There exists a flat, unobstructed, open stretch of sheet metal (floor panel) between the spring perch and the central cross-member. The path of least resistance would be to run some kind of rigid member straight from the perch to the cross-member. As luck would have it, this method works well, and the DIY community can either buy parts in the aftermarket, or fabricate something themselves. The load-path now starts at the spring-perch, runs along the new member (distributing some stress to the floor-pan at every spot-weld along its length) and ties into the central cross-member where the remaining load is distributed to the previously mentioned structural pieces.

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As I mentioned earlier, the RV8 engineers also felt that this was a potential weak-spot, and so they designed a load-bearing member that runs the same span. It also allows the use of anti-tramp bars and handles the extra twisting load they introduce. If you are tied into a good RV8 or British Motor Heritage supplier, you can buy the RV8 factory part. If you are handy with a sheet-metal brake, you can make yourself a new structural piece. If you are like the rest of us, you can send off some money to Adrian Akhurst in Australia, and he will make you a set. I chose the last method, and the rest of the article will cover that installation, though I suspect that the other two methods will follow a similar enough process.

As each MGB is a little different than the next, I will intentionally omit dimensions. For this project, they are easy enough to measure on your own vehicle.

When I first opened my package from Australia, I was a bit surprised to only find two simple channel pieces. I guess I had expected something more complex, like the RV8 factory piece, but considering how inexpensive the purchase was (about $25.00) I had little to complain about. What makes them well worth their price is that they are bent to the correct dimensions and that they are made from galvanized steel. Keep in mind that the zinc fumes produced from welding galvanized metal can make you very, very sick and the appropriate organic respirator or preferably a forced-air respirator should be worn.

Adrian Akhurst body reinforcement

When I held the parts up to the car, they seemed far longer than necessary, and it wasn't obvious to me how I would attach them at their ends to the spring perch and cross-member. A quick e-mail to Adrian resulted in a series of pictures that explained what I needed to do, far better than words alone could have done.
    Adrian Akhurst top hat

To get there, a bit of "tin-smithing" is required. The first step is to cut one end of the channel along each fold and bend back each of the resulting three tabs at a 90 degree angle. How far back you cut is up to you, but I wanted the center tab to be long enough to wrap over the central cross-member. After the length of channel is welded into place, the top tab can be hammered down over the top of the cross-member and welded in place. I use the generic term "weld", but its up to you to decide if you wish to spot, plug or stitch weld the parts in place. Because I have a MIG welder, I'll be spot-welding these in place, with the addition of structural epoxy between welds to keep water intrusion to a minimum. The next step is to measure between the cross-member and the base of the spring perch. Mark a line on the channel at this distance from the point where the three tabs start. The material from this line to the untouched end of the channel will have to be cut and shaped to "box-in" and wrap around the spring perch. There are a number of different ways to do this, and my way may not be the best way.

unibody reinforcement

frame stengthening

As with any other bodywork, you'll have to remove some undercoating before you begin, as well as the seats and carpeting, if you wish to avoid MGB Flambé.

You should also take the appropriate measures to rustproof such as using weld-through primer on the interior surfaces, as well as one of the better rust-preventative top-coats, away from the welds. You can apply Waxoyl or similar gunk to the inside through a drill-hole after welding. On the outside, treat it like any other part that will be exposed to water and salt. And remember what I said about welding through zinc! As you can infer from the pictures, you have the option of adding anti-tramp bars to the concoction. Page 56 and 57 of Roger Williams' "How to Improve MGB, MGC & MGB V8" give detailed plans for making a set of your own reinforcements along with many pictures of the one on the RV8. His plans also account for anti-tramp bars.

You can also visit "The Autoist", Tony Barnhill at his web site and see his take on these parts, which he calls "top hats." His web site is:

Adrian Akhurst makes a lot of other goodies, including a Panhard rod and remote oil-filter bracket that I'll review in a future issue. He can be contacted at:   Shipping costs from Oz are a bit pricey, so consider a bulk-buy for your local club to distribute the pain. Now get out there and drive!

Disclaimer: This page was researched and written by James Jewell. Views expressed are those of the author, and are provided without warrantee or guarantee. Apply at your own risk.

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