The previous page described the process for remaking the side and rear sections of the tub. Here I describe the corner sections and the shaping of compound curves as the section needs to be curved in a vertical direction (to match the curve in the back panel) and in the horizontal direction to join the sides to the rear. Very careful measurement and marking out on the original panel was needed to develop the pattern of the required shape to be cut out from the flat sheet - but this required further work as the first piece took shape and the metal started to move around in unexpected ways.
To develop the pattern I marked out the original rear weld seam at 25mm intervals and drew a line at right angles from it towards the side panel. Each of these lines was measured and plotted onto the flat sheet giving the final shape to be cut. I added 16mm to the areas where it met the mudguards as this needed to be bent over and shaped, plus I added a generous 100+mm at the top of the panel “just for the hell of it”. As it turned out, I needed to raise the short side by 60mm to correct the mating along the mudguard and I needed to add 12mm to the width on the long side because as the panel takes shape, this edge goes from being straight to pinching is at roughly the mid point and requires trimming to bring it back to aligning perfectly with the back panel for welding.
Part 2 on making a replacement tourer rear section
Pattern layout marks. How the pattern looks.
The other essential for making the corners is the construction of a “buck” to check that the correct shape is being formed. I made mine from 4 pieces of MDF cut to fit the inside profile of the corner section and slotted together making the whole thing reversible for the other side.
Buck fitted to old tub Completed buck on bench.
Shaping is started by bending over the flaps along the mudguard edge and shrinking them in which results in most of the curve needed in the horizontal plane forming the corner joining the side to the back. I use a hand powered shrinker which is a set of serrated jaws operated by a lever which squeezes the metal together. The alternative is to make the curve by hand but this crinkles up the bent over flaps which need a lot of hammer work to iron out and still maintain the curve.
Close up of the shrinker pulling in the metal and the shape after shrinking
Once this stage is done the metal needs to be stretched out in the centre of the panel to create the compound vertical curve. To achieve this I had a leather bag containing 30 kilo of lead shotgun pellets made up which is used under the sheet to absorb the mallet blows whilst having enough give to allow the metal to stretch. I probably could have used a sand bag but I wanted the shot bag for future projects. The Mallet I used was a plastic “Bossing” mallet (pear shaped) as this stretches the metal evenly regardless of the direction of the blow. Bossing mallets are hard to find and the only supplier I found that had them was a company called RS Components – I suspect Blackwood’s would also carry them.
The end result of the mallet attack on the panel looks like a bag of walnuts and this is rectified by passing it through the English wheel. This process of stretching and wheeling is done gradually over several stages, checking the results against the buck to ensure the shaping is not being over zealous.
Shot bag and bossing mallet The bag of walnuts
After wheeling the lumps have gone !
The remaining process on the corner panel is the shaping of the curve at the top where it is fixed to the timber work. My first attempt destroyed a lot of the previous work as everything is working against the curves already in the metal and it forces it to flatten out again. It is not an impossible task but since the metal is required to shrink a great amount I had to resort to heating with the oxy torch in the end and was still not happy with the result. My hammers and other tools coupled with my lack of skill just didn’t seem to want to smooth out the lumps and dings. I decided in the end to cut the top curve along the centre and fabricate the rest of it as a separate section and weld it in. The original panel was done without the need to do this but there may have been I suspect some specialist tooling involved.
The top distortion Rectified top tacked in
Finally the whole thing is clamped into place on the car and the seams tacked together ready for final welding and metal finishing. All in all this was a very satisfying project to carry out as apart from the top of the curves being done as a separate piece I have an exact replica of an original tub – with a lot less work than would have been required had I pursued the repair of my original.
Everything tacked together and ready for final welding