MASCOT August 1984

THE ROADSTER FRONT - Rear Bodywork Cracks

Bill Haverly

Whether they are caused by accident or design, the cracks that occur in the rear bodywork of most working Roadsters are a common feature which has to be lived with. In fact the cause of the trouble is one of design, but it's a common simplification just to lay the blame on a weak rear body structure when the real problem lies further down.

Designing into a swept tail car a boot 3' 0" wide which recedes 2' 0" into the car, when the overall frame width is only 4' 0" was asking for trouble at the start. Clearly a big rectangular hole of this size adds nothing to the overall strength of the bodywork and the cracks which occur at the four stress points at the corners of the framing are the result of its movement. But these cracks are only the end results of a problem in which the main culprit is the chassis. All three versions of the Roadster chassis are strong and well up to the job, with a built in flexibility in their design. Their one weakness lies in the section to the rear of the rigid tubular cross member which sits two thirds of the way down the frame beyond which only a "U" sectioned pressed steel member crosses at the end. The distance between these two members is one third of the overall frame length and it's an area which has to support the rear axle and wheels, the petrol tank and most of the rear bodywork.

In use the chassis flexes probably more than the makers intended, and although this is a boon as far as roadholding is concerned, it shows up a weakness in the form of cracks which can occur just to the rear of the tubular cross member. Look out for these if you are in the habit of "hammering" your Roadster.

But as far as the bodywork is concerned, the chassis flexing transfers through the body framing to the stress points in the corners of the boot lid openings which in turn throws pressure on the metal skin in the area surrounding these joints. Multiply these slight movements a few hundred thousand times and something has to give - a crack appears and we have a case of what the professionals call metal fatigue.

Providing the ash frame is sound with no movement in the corner joints, welding is the only satisfactory way of repairing these cracks, but because only the symptoms of the problem are being dealt with, it's not likely to effect a permanent cure. How successful and how long the repair lasts also depends a lot on the skill of the welder, for old aluminium is not an easy metal to fuse. Well worth doing, all the same, and much better than riveting unsightly support plates onto the bodywork. Prevention is better than the cure though and one attempt in America at tackling the cause involved mounting two vertical supports between the rear cross member and the bottom corners of the boot lid frame, which for reasons already given didn't last long - they put direct pressure on parts they were intended to support! More ambitious schemes have included reinforcing the wood framing with metal brackets and strengthening the back end by replacing the inner wheel arch angle iron supports with square sectioned tubing.

My own 4AB has extra wedges of wood fitted in between the chassis main frame and the woodwork. These preventative mods usually work for a while, but in most cases the cracks reappeared after a few years of use. Working on the assumption that 'every little helps', though it pays to use the axle banjo as a base rather than either side of the main frame when jacking up the rear. As an experiment try jacking up from one of the rear chassis corner points until the wheel is three or four inches off the ground and you'll see just how much the frame flexes. This concentrated twisting movement puts considerable pressure on the boot frame corners and is worth avoiding unless absolutely necessary.

So overall there's not much that can be done to prevent these body cracks from occurring. In the end they have to be accepted and I'm reminded of some sound advice from NASCO President Corey Welty which just about sums it up. He said "There's only one surefire way to deal with the cracks at the bottom of your Singer trunk lid - ignore them!"

(The much maligned but irrepressible Rabbitt passed this way many years ago - in 1975 in fact. Ed. will investigate which Rabbitt did as reported at the time, but perhaps others have found permanent cures for this body cracking?)