New rear bumper

Time to get the car back on the ramps! Having replaced the US front bumper with my own slimmed down version, it was time to replace the rear with its enormous over-riders (and substantial weight).

I ordered a GRP bumper from Smith & Deakin. This came in three parts (like the original UK version, I think). Since I didn’t have a UK frame to fit the cover on, I had to make one myself - reasonably strong but lighter than the original. This ought to be easier than the front bumper as the rear is pretty much straight (apart from the end returns).

I bought a piece of rectangular section aluminium tube, 50 x 100 with a wall thickness of 3.2mm. Having temporarily fixed the end sections on with two pairs of M3 nuts and bolts, I judged that the tube needed to be 1450 mm long which, fortunately, turned out to be just right!

I clamped the tube in place (see pic left), tried out the bumper over the tube and played around with the height until it seemed that the bumper would be in the right position once it was fixed to the tube. I then marked out the position of the six fixing holes. The holes in the brackets are elongated in the up/down sense so there is some built in adjustment available.

I centre-punched the positions and drilled 6 mm holes through both sides of the tube. I then used a hole saw on the outer face to provide access to the fixing bolts on the inner face. Then I enlarged the inner holes to 7 mm and tapped them to M8 thread with a tap held in the drill chuck to ensure the thread was straight. The idea here was to be able to secure the bolts in the tube so that the GRP cover could be fitted to the tube and then the whole assembly offered up to the car without needing spanner access to the bolt heads. I should have realised this would not work as it is not possible to tighten the bolts into the 3.2 mm tube thickness sufficiently to resist the twisting of the bolt, as the nut securing the bumper to the bracket on the car, is tightened. The bolt just unscrews slightly and then loosens which is no good. I altered this later to (hopefully) solve this problem.  

Left: turning the chuck by hand to thread the fixing holes as described above.

Left: the tube temporarily bolted to the car.

Next, I needed to fix the GRP to the tube. I decided to cut some 6 mm ply wood to fit as snugly as possible in the bumper. Then, I drilled more holes in the tube and screwed the ply to the tube, at this stage, with four wood screws.

Then I screwed the bumper temporarily to the ply (through holes in the GRP which would later be filled). I needed some temporary packing pieces to hold the bumper in the right position while I screwed it in place.

Left: you can see the five screws used to temporarily hold the bumper to the strip of ply.

Left: I have unbolted the bumper assembly from the car ready for the next phase which involves providing proper fixings for securing the bumper to the tube and fibre-glassing the plywood in place (which is next).