By the time you read this, the beauty pictured above will be at Salon Prive; but how was the frame made? Allen tells all...
This month Allen gets frame building!
Ihad recently finished making the six-cylinder engine and wheels for my RC374 and I was really looking forward to getting on with the frame. Before I could start making it, I needed to finish modifying the forks, machine up a headstock and make a swingarm. The lower fork modifications were completed when I trial fitted the hub which involved welding on new brake anchor points, removing the lower mudguard mounting lugs and reducing the length of the fork seal holders, so I just needed to shorten the fork tubes to make them the right length. To do this I cut 40mm off the top of each fork leg, but in doing so, also removed the internal threaded portion that is used to secure the forks to the top yoke. I was able to re-use the two bits of fork tube with the internal thread by machining down the outer diameter until they were a push fit back into the top of the fork tubes then I welded them in place. The bottom yoke was cleaned up to remove unwanted casting ridges, lugs and the steering lock. I then cut off the raised handlebar mounting lugs from the aluminium top yoke and welded up the two holes that were left behind. Finally I dressed the surface with my Dremel and rotary burrs to produce an ‘as-cast’ looking finish. The next job was to make the headstock for the frame so I ordered a piece of thick wall steel tube and machined it on my lathe to Honda CB77 internal dimensions and overall length but to look similar to the RC174 on the outside. Once the headstock machining was complete the CB77 bearings were pressed
into the headstock and then assembled in the yokes. The forks were then placed into the yokes followed by the front wheel, to complete the front-end sub-assembly. The next job was to make a swingarm. My initial thoughts were to make one from scratch, but looking at the CB77 swingarm I could see that is was similar in many ways and could easily be modified. The main difference was that the pivot points were designed to fit on the outside of the CB77 spine frame so I cut the swingarm in half, reshaped the halves, then cut a portion out of the cross brace to reduce the width at the pivot end. The two halves were welded back together with a new one-piece bearing pivot tube and the plates were then welded on the top and bottom for added strength. The shock mounting points will need to be modified as well but I can do that later while making the frame so that I can position the shocks at the correct angle. I trial fitted the rear wheel in the swingarm, reduced the length of the wheel’s spindle and machined new spacers to centralise the wheel, I then positioned the brake plate, attached the torque arm and welded a new anchor point onto the underside of the swinging arm. With this complete I now had the front and rear sub-assemblies ready to make the main frame. The frame for my RC374 would be an open type frame with the engine suspended underneath as a stressed member, but due to the design of the FZR engine it was not possible to stress the cylinder head as part of the frame like on the original RC174. I would have to
extend the frame discreetly down the front of the engine to meet the crankcases where there were two lugs strong enough to take the load. With this in mind I decided to make the frame from T45 chrome manganese steel because of its high tensile strength. This material was used to construct the airframe for Spitfires due to its excellent strength to weight ratio, and importantly for me it can be TIG welded without any need for further heat treatment. I worked out how much tube I would need and placed an order with Tube Bender Ltd in Rugby. While I was waiting for the tube to arrive I started to link up my sub-assemblies ready to make the frame. I used a spare set of standard FZR250 crankcases as a jig: I set about making the lower frame assembly that would join the rear of the engine to the swingarm. I worked out the dimension from the gearbox sprocket centre line to the swingarm pivot using the side view photo in my reference book, then cut out cardboard templates that would link them together maintaining this dimension. These templates were then trimmed and shaped to look as close to original Honda as possible while still mounting to my
“To make the frame, I made a jig to hold the sub-assemblies in alignment, perfectly fitting the 55.5in wheelbase.”
six-cylinder engine. When I was happy with the shape I cut out four identical pieces from 2mm thick steel sheet using my angle grinder fitted with 1mm cut off discs. After deburring with a file I set up the four pieces on my milling machine to drill three holes for the cross tubes and mounting bolt internal support tubes. I then machined internal support tubes and cross tubes on my lathe, making them stepped at each end to ensure correct linear alignment, and also a tight fit in the side plate holes. The parts were then pressed together and bolted to the engine and swinging arm, checking that the centre line of the engine matched the centre line of the swingarm while I tack welded all the joints. The lower frame assembly was then removed to finish the welds. I fitted a spare output shaft and front sprocket into the crankcases so that I could trial fit the chain later on, then I bolted the lower frame assembly to the rear of the crankcases followed by the swingarm and rear wheel. The alignment was checked with a straight edge and spirit level to ensure that the wheel was in the middle of the engine and vertical in relation to the base of the crankcases. I was also pleased to see the wheel span freely and the front and rear sprockets aligned perfectly. I put on the chain and set the tension, then checked to see that the chain cleared the lower frame cross tubes at the upper and lower limit of the rear suspension travel. The next job to do was to make a frame jig to locate and hold the front and rear sub-assemblies in alignment while I made the main frame. I bought some box-section steel long enough for both wheels to sit on and be held at a 55.5in wheelbase, and the forks at the required angle. Several locating points were then welded onto the main spar of the jig to locate the crankcases bolting points and headstock. The front and rear sub-assemblies were then bolted on to the frame jig and final alignment was checked using a piece of string and a spirit level to ensure both wheels were aligned and vertical.
Swingarm with new torque arm anchor welded in place.
Cardboard template and pressed-up side-plates.
Cutting down the fork tubes.
Cutting/reshaping a CB77 swingarm.
Front and rear sub-assemblies mounted on frame jig.
Lower frame assembly.