By the time you read this, the beauty pic­tured above will be at Sa­lon Prive; but how was the frame made? Allen tells all...

Classic Motorcycle Mechanics - - CONTENTS -

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.

Swingarm parts.

Cut­ting down the fork tubes.

Cut­ting/re­shap­ing a CB77 swingarm.

Front and rear sub-as­sem­blies mounted on frame jig.

Rear sub-assem­bly.

Lower frame assem­bly.

Frame jig.

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