Sunday, 22 August 2010

Having established that I'm perfectly capable of letting a week go by without managing anything on the Beemer, and hot on the heels of that establishing that I'm forgetful and easily distracted, then I suppose it behoves me to try and get my act a little more together.

One of the problems with Version 1 of the seat hump was that it didn't really lend itself to any immediately obvious rear light solution. Version 2 more was more or less crying out for a round rear light from the very outset. Despite my avowed intent to spend some money on this build, I still found that looking at motorcycle rear lights induced spasms in my wallet handling muscles when my eye happened upon the price of them. So I bought a Land Rover Series 2 one for an entirely amenable sum that was a little under £8.

Sadly, while the light unit would fit on the flattish part of the seat hump, once again, it didn't really look right. However, mounted on the curved bit of the seat hump, I looked a lot better, but stuck out too far. The obvious solution was to tunnel the rear light into the metal work of the hump, which meant holesawing it in the relevant place. When I turned the hump over, I remembered that I'd marked the centre line of the the centre section of the hump in felt pen, and further more I'd marked the point where it stopped tapering in towards the rear. I spotted all this after I made a wild stab at where the centre of the light should go with one finger, put the finger of the other hand inside the hum and opposite the first finger, and then turned the hump over, to find that the "blind" finger was on the point where the two lines I'd completely forgotten about crossed.

At this point, you might be thinking that was lucky, which in itself was true, but I have to point out that the more I practice, the luckier I get.

Once I'd made a dirty great hole in my new seat hump, I used the same 3 3/8" (I think) holesaw to cut out another hole from some of the 18 gauge steel I'd made the hump from, and kept the "washer" from the middle of the hole. With it firmly clamped down I lined up the pilot drilling and enlarged the hole in the middle with a 1 3/8" holesaw. Once I'd de-burred it, that gave me a base for the light to sit on. then I cut a strip of the 18 gauge that was about 5" wide, and something over a foot long.

Basically, I cut it so that it was wide enough to roll into a tube that disported horizontally would easily fill the hole in the seat hump, and I chose it because it was long enough to wrap round the piece of 3" diameter tube I found for a former. I used the 3" because steel tends to spring back from where it gets bent to, and I wanted a tube that my 3 3/8" washer would sit on the end of. This would form the basis for a plinth for the light, with the rolled tube having the same OD as the light unit's base.

At that point, since it was starting to look like it would all turn out happily in the end, I thought I'd best weld the three pieces of the hump together. I do possess a TIG, but I often can't be bothered to drag it all out of the corner, it costs a bloody fortune to run, and with my dodgy leg I have trouble keeping my body stable enough to do a tidy weld, so I MIG'd the seams by more or less joining up a series of tacks. Not ideal, and while I could have set things a little lower and used short runs of weld, that requires that you don't wobble about while you're welding.

The picture shows the raw, untouched welded seam on the left, and the right hand one has been dressed back flush with a sanding disc on the angle grinder.

Once the seat hump was fully welded and sanded, I took the light mounting I'd made and offered it up to the seat hump to get some idea of where I wanted it to sit. I decided on having the plinth protruding from the hump by what is probably a little less than 1/4". Having thought about how I was going to weld it all together, I cut the plinth off with a hacksaw so that there was about an inch of tube behind the face the light sat on. Then I took the off-cut and cut it down the seam so it would sleeve over the light mounting. I arranged this sleeve so that the open end of the light plinth was flush with the sleeve, leaving the face of the mount sat in the tunnel.

That whole assembly was then inserted into the hump and positioned so that it looked the way I wanted it too and I could mark the "tunnel" to the profile of the hump. Once I'd used the tin snips to trim the tunnel to that line, I tacked it in place and used the sanding disc on the grinder to finesses the edge before fully welding it in place.

By welding the open end of the tunnel and the plinth together, I didn't have a weld bead interfering with the fit where the light mounting face met the side of the tunnel. It might have been better to weld that up prior to welding the tunnel in, but welding it after, although it meant that I could get all the way round the join, meant I could still tweak the amount the the plinth protruded from the hump. With the light mounting dealt with, I folded the "spare" material I'd allowed at the bottom of the hump through 180 degrees and hammered it flat to give the open bottom edges a finished look. I do this on the edge of a flat surface with a pair of Vise Grips, working the lip a little at time and folding it through no more than 30 degrees at a time until it reaches 90 degrees, when it's time to start using the hammer on it. At the corners where I was trying to force a lot of metal into a smaller space and I had the weld to deal with, I folded it over to about 45 degrees, and carefully cut a "V" out of the lip with a hacksaw, taking care to leave enough of the lip to be able to make it fold the rest of the way.

That was, admittedly, a lot of work to mount a rear light, and, let's face it, the whole thing has been a bit of a rigmarole. And at that, it's still not over and done with as I've yet to make the base for the upholstery, make something to keep the worst of the water out of the hump, and work out how to attach all of this to the frame.

But, I think that in the end I managed to make something that looks like it belongs with the rest of the bike. I recently described the idea I have for the bike in an E-mail to someone as "looking like a BMW that's going racing, and not a race bike with a BMW engine". An essential part of that was that the seat unit looked like it belonged on the bike.

As I mentioned, there's a little more fiddling about to do, what with making a seat base, figuring out mountings, and so forth. When that's out of the way, I'll turn my attention to the frame and see if I can stiffen that up a little.

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