Clas­sic Cob From the shed

Old Bike Australasia - - CONTENTS -

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.

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