ALLEN MILLYARD
Gets the frame sorted on.
Irarely do any drawings or sketches when I make my bikes because I like to create parts from an image in my mind, but there are occasions when a drawing is unavoidable. I had recently ordered a length of 28mm T45 chrome manganese steel tube to see if it was possible to bend it at home with my old pipe bender. There is a difference between pipe and tube – pipe is measured by its internal diameter and tube is measured by its outside diameter, so the dies on my pipe bender didn’t match exactly with the diameter of the tube. I remembered an old trick where you pack the tube with sand and weld discs onto each end to help support the tube wall while it’s being bent. I was quite surprised how hard the T45 tube was to bend and even with the sand packed inside it would bend a bit and then start to ripple on the inner curve. “I’ve met my match,”i thought to myself, the main frame rails would need to be bent by a specialist firm. I contacted Tom at Tube-bender Ltd (07583 682814), who said that would be no problem, I was to send him a drawing and they would create a CNC program to bend the two frame rails on their CNC tube bender. Making a drawing was going to be easier said than done. I had a photo in my reference book and a vision of what I wanted to make, but now I had to draw it out on paper with all the critical dimensions for someone else to bend the frame rails 100 miles away. I decided the easiest way was to make a dummy frame rail out of 28mm mild steel exhaust tube so I had something to look at to finalise the shape and make sure it lined up on my jig. I bent the tube on my pipe bender and wasn’t worried about ripples or an exact radius, as long as the
straight portions were correct because I could determine the required centre line radius (CLR) using cardboard discs that I could overlay the tube and measure once they’d been trimmed to match the CLR I was aiming for. When I was happy with the fit I then made a full-size cardboard template that replicated the frame rail contours, and from that I made a full-size drawing that I could photograph and email to Tube-bender Ltd. A few days later I received a CAD file containing images and critical dimensions of my frame rail for me to authorise before the frame tubes were bent. All seemed okay, so I endorsed the files for bending to commence. While I was waiting for the frame tubes to arrive I set about making a pair of clip-on handlebars. To do this I cut two lengths of straight tube from some old handlebars and machined them to length on my lathe. The clamp tube was machined from sections of old Kawasaki H2 fork tubes. I like to use material I have laying around if possible. The outside diameter was just right but I needed to bore out the centre to slide over the CB77 fork legs. I then filed a cut out in the end of length of handlebar to match the clamp tube outer diameter and then cut the clamp tube with a hacksaw to allow it to expand and slide over the CB77 fork leg. The last job was to machine eight lugs, four were drilled and tapped M6 and four were drilled M6 clearance, these lugs were then welded in pairs onto the clamp tubes. The length of handlebar tube was then welded to the clamp tube at the desired angle. This was then repeated on the other handlebar at a mirrored angle. After a bit of filing and cleaning up with emery cloth the clip on handlebars looked great, and were trial fitted on to the forks. The main frame rails had now arrived
and I eagerly unpacked them, they looked really nice with smooth bends and a perfect mirror image of each other, so I took them out into the garage to try them on the jig. After a bit of trimming to fit the headstock and the lower frame sub assembly they fitted perfect and were clamped in place using several G-clamps ready for final alignment checks and welding. I remembered my friend Dave Lenton had mentioned a while back that he had a hand-operated mandrel bender but he only had dies for 25mm diameter tube. Dave had imported it from America to build a TZ350 frame a few years ago. After a bit of experimentation with some scrap tube I found it worked effortlessly to bend 25mm diameter T45 tube, so I could use it to make the rear frame rails. Once both rear frame rails were bent to shape they were mitred to fit the main frame tubes, clamped in place and welded. I then cut the central top tube, several cross tubes and the pair of tubes that run from the top shock mount to the swingarm pivot and one piece was bent into a hoop to weld across the rear of the frame. Each piece was mitred or shaped to fit, then welded in place checking alignment and position at each stage. The next job was to make two upper shock mounts and several webs to strengthen areas where tubes meet each other, replicating shapes and positions as detailed in my reference book. I first cut out card templates and used them to mark out 2mm-thick steel sheet. Once cut out and trimmed to size they were welded directly on to the frame. The top shock mounts were made from three pieces of 2mm thick steel welded up into small sub-assemblies and then drilled to allow 12mm threaded shafts to be pressed in and welded for the shock absorber to locate on. I then clamped them in the correct position and welded them in place. The shocks were then trial fitted and positioned while the lower shock mounts were relocated on the swingarm ensuring the shocks were at the correct angle as depicted in the photos in my reference book. The upper engine support tubes were then made and welded in place. The frame was now virtually complete and could be removed from the frame jig and the moment had come to see if my six-cylinder engine would fit. The design of the RC racing frame is excellent for fitting the engine, I simply had to position the engine under the frame, lean it forwards, align and slide in the lower engine mounting bolt then pivot the engine up resting it on a block of wood while sliding in the upper rear mounting bolt. The engine fitted perfectly so I tightened the two bolts and removed the block of wood. I still had several jobs to do on the frame but I will cover those next time. I pushed the bike outside for the first time rolling on its own wheels and lent it against the fence for a photo and to stand back and admire my evolving RC374 in the light of day.