Updated: Nov 18, 2021
When you first start your journey into the custom motorcycle world , no part is more fun and makes you feel like you are actually a custom bike builder more than taking the angle grinder to a frame and having at it. However, as I have said before, you can't just go hacking at the bike. This is especially true if you are planning to significantly alter the subframe. This bike, like my others is going to be a single seater. So, it's necessary to cut the pillion section from the frame. In this case I am planning to keep the shock mounts where they are. I want to cut off the stock, pressed steel, section of the frame just behind them and close the rear tubes with a seat hoop. Cutting the frame here should open up the original seat tubes that are forward of the shock mounts to allow me to do just that.
There is also a structural, box gusset in front of the shock mounts that I wanted to remove too. So anyway, after a careful long stare at it all, I reached for the angle grinder. I had considered the seat size and length I required, the alignment to the tank and how the seat cowl was going to sit in proportion to them both. You see, this is the point that you have to have worked out your lines and the fit of your major components. You don't want big gaps between your tank and seat or have them at odd angles. You also don't want to end up with a seat that's too short or a cowl that's way too long. it all starts with this cut. No pressure then...
After I had cut out the box gusset and the rear section, for peace of mind, I tacked in a temporary cross section across the back. As I still had a little repair work to do on the underside of the frame, I didn't want the heat from the welder distorting the frame because I had removed some of the key structural parts.
So, out came the MIG welder for the first time in this build. A few years ago I invested in a decent MIG. It was the first, serious piece of kit that I added to my workshop. I certainly could not be doing anything like the work I am now without it. It's a Lincoln Bester 190c. They come in around £500 but by the time you have factored in Gas, regulator, wire and the necessary protective wear, you are probably looking at around £750 to get properly set up. It has been worth every penny though. It was a steep learning curve and takes time and practice and I still consider myself to be a terrible welder. However, I am now at the point where I can be confident in the integrity of my welds while at the same time make them look pretty when I need too. I have a lot to thank Ricardo Churchill for here. He is one of the best fabricators I've seen, (the man built his own canal boat for heaven's sake), and he has been very generous with his time advising me on voltages, gas flow and wire speeds for the different jobs I was attempting. Other than that though, I guess I'm self taught. Learning by doing and making all of the mistakes along the way.
Now, where was I? Ahh, the seat hoop! There are two ways I could have gone here. Classic Honda CB's are a popular choice for custom bike builders so there are a number of aftermarket, pre-fabricated seat hoops on the market. I even managed to pick one up from that popular Brazilian rain forest on same day delivery?!
The majority of them though, like this one, involve you making a diameter reduction. 25mm to 20mm to be exact. 5mm doesn't seem that bad on the face of it but it's very noncable. The idea is that the new hoop fits inside stock tube so it can be easily and securely welded in place. However, I didn't want a visible reduction in tube size. The other problem is that they are made with a standard, 180° semi-circular bend. I wanted to do something a little different for this build. So, an aftermarket hoop was almost certainly out of the question. I started thinking about making my own and toyed with the idea of using a pipe bender to create one. I grabbed a piece of 25mm steel tubing and headed down to see my friend Bob Moore at UPB. I could write an entire book about Bob. He's an awesome bloke with so much knowledge. I, quite literally, learn something new every time I speak to him. Amongst many other things, (including e-bikes and a successful parts business), Bob builds top end Trial bikes that use pre 65 Triumph Cub engines. What is really great (for me, probably not Bob) is that his unit is only 500 yards from my front door! So, we both talked through the pro's and cons of using the pipe bender and whether or not we could achieve the tight radiuses I wanted over such a short distance. You see, I wanted the hoop to turn 180° using four, 45° angles rather than one continual radius. However, I wanted these angles to be quite soft. We actually decided that the tube bender probably wouldn't work but that we might give it a go anyway. So, we agreed to meet the following day to see what we could do. Not convinced however, I was still chewing it over that evening and it suddenly occurred to me that I could just cut and weld it. I went straight out to the the workshop, got out some card and my technical drawing kit and started doing the maths.
Before long I had it drawn out 1:1 scale on a piece of clean white card. I have this cool little Evolution Power Tools - Mitre Saw with a mild steel cutting blade. It's a great bit of kit and cuts really accurately. I used it to carefully cut the tube lengths and angles I needed. The tricky part here was getting the sections with 22.5° mitres on both ends exactly lined up. If they were out even slightly, the hoop would end up wonky and unusable. The child's plastic set square came in really handy at this point as my carpenters one was too big for the job. It helped me get the pieces true in the saw clamp before each cut. Once the pieces were cut, it was simply a case of laying them flat on my welding table, holding them in position with magnetic holders while I tacked them together with the MIG. It was working but looked a little too sharp at this point. I knew that the welder would nock off the sharp corners though so, I kept at it.
After a while I had what was starting to look like the hoop I had in mind. it still needed refining and polishing up but it was beginning to look and feel like one piece of steel tube rather than the five sections I had cut out earlier. I took a couple of photographs and sent them to Bob with the line; Cut & Weld. He replied almost immediately saying he had the exact same thought about an hour before and was about to call. He used to build his exhausts the same way. Sectional! I'm not sure why neither of us got there sooner but we may not have at all, if I hadn't gone to talk the problem through with him in the first place. I would have almost certainly waisted hours and money, with a pipe bender, failing to get the required result.
So, a good evenings work all round. I now had a custom seat hoop that had the right aesthetic and was made out of the same, 25mm tube as the rest of the frame. Of course, happily, this also meant I could move straight on to the next job... Fitting it to the bike.
A couple of days after posting this, the MIG welder I so proudly spoke of went bang. I mean it really went bang. Three times! Luckily it has a two year warranty and the guys at United Welding in Peterborough were excellent about it. They swapped it out for a new one with another two years warranty on it. Couldn’t ask for more really but I thought I should mention it.
I’m pretty sure it was metal dust from grinding that killed it. I would often neglect to turn it off when I reached for the angle grinder. The cooling fans were then just sucking up metal powder and in hind sight, that is clearly not good. So, more than likely my own fault but a lesson well learned.