At 02:56 AM 6/8/2007 +0000, Eric Peterson wrote:
>"I'm considering putting a roll bar in my MGA. Moss doesn't seem to list one. If there's information here on your site, that would help too. I just can't find it."

Well now there is.

Roll bar is a very tough issue in the MGA, as it generally requires cutting holes in the steel body tonneau behind the seats. A competition roll bar with angle brace may also render the rag top useless in standard form. Some years ago I was considering installing a roll bar, for a few rather odd reasons related to autocrossing with SCCA (Solo-II). One reason was, I was considering installing some form of shoulder harness setup, but this is not allowed in an open car unless a roll bar is installed first. Another reason was, the higher speed Solo-I on an open race track requires a roll bar in an open car. In the end I decided to stay in Stock class (or Street Prepared) for Solo-II, and settle for using a common lap belt, and never running Solo-I. Just as well, because a couple of years later the race tires I was using went out of production, and I was effectively legislated out of the SCCA race business (long story).

But meanwhile I had designed a roll bar for the MGA to meet SCCA minimum standards for Stock and Street Prepared classes in Solo-II and in Solo-I. This started with a single hoop just aft of the seats, required to be within 6-inches aft of the helmet of the largest driver (but of course you don't want to hit your head on it, so figure 3 to 6 inches behind your head. The reason for the roll bar being behind your head is because there are special exceptions for a convertible that may not require the roll bar to be much higher than your helmet. These rules were changing every year or two for a while, but once you get a roll bar installed and approved it will generally be "grand fathered", meaning it will be legal for all eternity thereafter regardless of how the rules may change. This is to avoid forcing people to modify the cars every year in a big way to meet new changing rule requirements.

One of the items that changed periodically was the height requirement for the roll bar. First it said "minimum of 3-inches above the helmet of the tallest driver, and not more than 6-inches aft of the helmet of the tallest driver". This is the most common spec, and it usually doesn't change much for many years, but there are exceptions for convertibles.

For "open cars" (convertibles), the rule varied from time to time ranging from 2-inches above the helmet", to "minimally at least as high as the helmet", to "3-inches above the helmet or as high as possible within the confines of the convertible top but not lower than the top of the helmet". All this was intended to give convertible owners a chance to use a roll bar without having to modify the convertible top, and still being able to use the convertible top with the roll bar in place. As fate would have it, the first time someone gets hurt in a rollover accident the rules are subject to review and change. Eventually they may have decided that the minimum height of a roll bar was more important than allowing use of the rag top without modification. Modern requirement may now say "3-inches above the helmet", and you might have to modify your convertible top to fit over this roll bar if you want to use the rag top with the roll bar in place.

Bottom line is, read the rule book first for whatever race organization you may want to run with.

If you just want a roll bar for street use, and it doesn't have to conform to any particular race group rules, then you can install whatever you like. This leads to things like the Mazda Miata or BMW Z3 double hoop roll bar that looks like a letter "B" lying on its back, with the hoops barely as high as the passengers' heads, and no particular strength requirements. They are cute, but no telling what happens if the car ever actually rolls over.

For myself, and for a street version, I might hedge a bit on the height, but I certainly want it to be strong enough to bear the load if it ever encounters a roll over. As such, I would start with the single hoop with legs spaced as wide as possible and the top of the hoop positioned as high as possible within the confines of the standard convertible top. The result of this was that the vertical legs pass through the MGA steel body tonneau at the far wide extreme of the interior space just aft of the cockpit trim rail corner pieces behind the doors. The bottom of the hoop legs then sit on top of the frame ahead of the rear inner fenders near the wide corners of the battery cover (possibly just in front of the rear plywood bulkhead).

Additionally the top curve of the roll bar hoop would be formed exactly in the shape of the rear bow of the convertible top frame, such that the roll bar actually supports the top fabric in place of the rear bow. The rear bow can be articulated forward to lay against the next bow forward (so you don't have to remove the rear bow).

Then you need two fore/aft angle tube braces that should be positioned as wide as possible about at the top end of the side curves of the main hoop (just where it transitions to near level on top). This works out to be very close to the division fabric between the windows in the three window rag top, which is a very nice place to put the rear braces, so as not to obscure you vision in the rear view mirror. Also as a matter of convenience, position the rear angle brace tubes to follow the inside of the rag top closely down and back to pass through more holes in the body tonneau just ahead of the transverse steel anchor strap in the back edge of the rag top.

This coincidentally runs the angle tubes down into the boot aft of the rear bulkhead panel, where the bottom ends of these tubes terminate directly on top of the main chassis fore/aft frame tubes (above the leaf spring area), just inboard of the inner fenders. Ah-ha, so far so good, four legs and four holes in the body tonneau. These holes will end up being generally elliptical, and you might trim them with plastic panel edging strip.

As a point of rule for SCCA, for Prepared class and Modified class the roll bar can be welded in place, or may be bolted in place. The difference is that a welded in roll bar is considered to be a chassis stiffener, and this is not intended to be allowed in Stock or Street Prepared classes. So for Stock or Street Prepared class the roll bar has to be bolted in, not welded directly to the chassis. To do this you need a pair of flat plates at the bottom end of each tube, one welded to the tube and to be bolted to the other plate. The bottom plate can be welded to the frame or may be bolted to the frame, given adequate structural support.

In the case of a unibody car, the plates would need to sandwich the body sheet metal between the two plates, and the plates would have to be large enough to spread the expected load over a large enough area of the body to take the expected load without tearing the body apart in event of impact.

If this isn't contorted enough already, you quickly realize that once the angle tubes pass through the body tonneau and are welded to the main hoop, the roll bar assembly can never be removed without cutting it to pieces. To allow for removal of the roll bar, the rules commonly allow for pinned or bolted slip joints. A slip joint would involve a closely fitting tube one size smaller to slide inside of the main tube with minimum 6-inch overlap, a minimum of one cross bolt to hold it together (on each side of the slip joint). Also the ends of the main tube must abut against each other and against the bottom mounting plate.

You can also build it with a slip joint just below the point where the angle brace meets the main hoop near the top. In theory then you might be able to un-bolt the slip joints at top and bottom ends of the angle braces, also unbolt all of the bottom mounting plates, and possibly be able to remove the bulk of the roll bar assembly leaving only some traces of the base plates or some holes in the body and/or frame.

However, in actual practice with the MGA, when the tubes pass through the body tonneau at odd angles, you might not be able to disassemble it even with a multitude of slip joints. As such, you might figure that after you install the roll bar it would be a permanent installation, never to be removed. Then you can forget about all the slip joints and get back to simple basics with a main hoop and two rear angle brace tubes with flat plates at the bottom ends. If you arrange to bolt it to the chassis you might later be able to unbolt it and remove the body from the frame with the roll bar still stuck in the body. Otherwise, if you ever want to remove the roll bar you use a Sawzall or a cutting torch.

There's more. This design with one main hoop and a pair of fore/aft angle braces is known as a "street roll bar". This may allow full and regular use of the MGA convertible top. Surprise! Yes, it can work. But the street roll bar will generally not conform to competition requirements, as it might be crushed and deformed from one side. For street use you might decide this is better than nothing (or at least it makes you feel better when driving).

For a competition roll bar it is generally required to have an additional transverse angle brace tube running from very near the base of the main hoop on the passenger side to very near the point of attachment of the driver side fore/aft angle brace to the main hoop (near the driver's head). Installation of this transverse angle tube under the main hoop will require a fifth hole in the body tonneau panel. This tube will also pass across in front of the space where the convertible top is normally stowed, making it impossible to raise or lower the top in the normal manner. Bummer.

Tentative solution to that problem is to install stake pockets on the inside of the body where the top frame normally attaches. Then when you want to lower the top, you detach it all around, fold it all together up high, then lift the entire assembly out of the stake pockets, fold up the side legs, and insert the folded assembly past the roll bar transverse angle tube into the normal stowage space aft of the seats. Oh, what a tangled web we weave. Another alternative may be to adapt the "erectable" top from the MGB, where you detach the fabric cover, then lift the top frame out of the stake pockets, pull apart slip joints in the cross bows, and toss all the top frame parts into the boot.

For a racing style roll bar there are some alternatives to cutting so many holes in the body tonneau. Rather than installing the fore/after angle braces rearward into the boot, you can install on each side a long angle brace that runs from near the top of the vertical leg of the main hoop (before it starts to curve inward) running down and forward to run just under the end of the dashboard, terminating on the upward angle part of the side frame in the foot well. These side mounted angle braces will intrude rather dramatically into the passenger space, generally voiding normal use of the doors, and therefore generally inhibiting any use of the rag top. Aside from being tough for ingress and egress, when seated in the car you would basically have one shoulder tucked under the side angle brace tube, and it would be tough to reach that arm forward to hold the steering wheel (without getting nasty bruises on your arm from the side brace tube). This would almost certainly require installation of a very small steering wheel, perhaps 12-inch diameter or smaller.

By now you may see why I gave up on the idea of a roll bar in the MGA for street use. It would be a royal PITA having to pass the tubes through the steel body tonneau aft of the seats.

You can review pictures of various MGA built for racing to see what real roll bars look like. Nearly all of them will involve holes in the body tonneau, and will preclude the use of the convertible top. Then again, there are people who install a cut down Brooklands windscreen and will never use a convertible top anyway. So maybe it comes down to how you intend to use the car.