Classic Cob From the shed
Many a discussion and articles have been written since Phil Irving advocated that parallel twins should have crankpins set at 76 degrees as opposed to the usual 360 degree crankshafts. Somehow, decision makers at the head of the British Motorcycle manufacturing establishments were reluctant to make changes until it was all too late. They failed to address one of the parallel twin’s worst features; that the 360 degree crankshaft, with pistons rising and falling together, is badly imbalanced. The worst enemies of these crankshafts is primary forces, created by the inertia of both pistons as they are forced to slow, change direction and accelerate at the top and bottom of their strokes. Undesirable secondary forces also arise, some acting in an upward direction at the end of each stroke, and therefore being strongest as both pistons hit Top Dead Centre together, and others acting downwards in mid-stroke. Irving arrived at the 76 degree because in a typical twin, where the connecting rod is twice the length of the stroke, that configuration results in one piston being at TDC when the other is moving at its maximum velocity. Each time one piston is experiencing TDC inertia, its companion is compensating by moving at its fastest. And it is working at pushing the crankshaft around. Irving also noted that this set up would require a much lighter flywheel. Few have actually undertaken the task, although it is not all that difficult for those who possess engineering skills. It is believed that the first in Australia was undertaken by Lee Kernick who used a Norton crankshaft which was cut and machined to form 76 degrees. Another is Bill Wolvey who I was able to meet up with and discuss one of his machines which he has modified. Although Bill falls into the self-taught category he is not short of practical engineering skills and ability. Bill tells me that he was influenced to undertake the modification after a discussion with Phil Tandy. Bill’s machine is a Unit Triumph engine in a locally made flat tracker frame of which he did make a change to the steering geometry. It is fitted with a 5 speed cluster, T140 barrels and connecting rods and the ten stud head is fed with a pair of Amal Concentrics. A SRM belt drive and clutch pressure plate are used. Bill made his own moulds for the fibreglass fuel tank, seat, guards and side panels. He has fitted an oil filter which actually filters prior to the oil entering the engine. Bill uses a Tri-Spark ignirtion made in Adelaide by Steve Kelly. The crankshaft and cams are in EN26. He made his own flywheel and used a balance factor of 48 percent. This is consistent with what Irving stated and is the same as that used in a 500 unit Triumph modified in the UK. He has ridden this machine to Tasmania and back without any form of trouble. This engine has now been together for some 15 years with one rebuild to the top end. Bill started this machine for me and a quick dash across the paddock showed me that it was a get-upand-go machine, with not the usual Triumph throaty exhaust note but more like a Ducati. Whilst the engine was merely idling over I thought it was just at the usual 800 to 1,000 rpm mark but the rev counter was indicating just on 1,500. It certainly did not sound that it was running that high, but Bill advised that it was a correct reading and it is just that the engine likes to rev higher than normal. The 500 unit model modified in the UK had to have a higher sprocket fitted because of the tendency to rev harder. This resulted in the engine running at 4,430 rpm against the previous 5,343 rpm in top gear at 70 mph. This machine has also proved its worth where Bill used it in sand track racing at Broadford, Goulburn and Ponde. No doubt there are probably other worthy ideas in Bill’s head but his saw mill operation is very time demanding for the maintenance required to keep it operational. If only the heads at Triumph, BSA, Norton and others had possessed the vision and desire to make sound judgements and take on board beneficial technological changes, things may have turned out differently. See you next issue, Pete
Bill Wolvey and his ‘76º’ Triumph. You can get in touch with Pete at...
The 76 degree crank.