Tech Talk

The lighter the bike, the eas­ier it is to ride – the CDB pre- 60 Mxer sheds more pounds.

Classic Dirtbike - - Contents - Words and pics: Tim Brit­ton

If only I had an al­loy tank I’d be faster… bet­ter… cooler. Rarely mis­taken for the King of Cool the edi­tor does have a Tri­umph with an al­loy tank now.

There are a num­ber of ways to re­duce the weight of a typ­i­cal Bri­tish road bike if it is go­ing to be used in off-road sport. The eas­i­est way is to pull the en­gine and gear­box out and slot them into a bet­ter frame, but not ev­ery­one wants to do that. Ob­vi­ously, al­loy rims, smaller hubs, al­loy mud­guards and re­mov­ing the lights all con­trib­ute to shav­ing off the pounds. Do­ing this got the CDB 650 Tri­umph down to 345lb, but there was still a way to go and much more po­ten­tial to lose more weight. The tanks – oil and petrol – were still steel orig­i­nals and as such quite heavy, so the an­swer was to have al­loy ones in their place. Prob­lem be­ing all the ones I saw were not quite right for a pre-unit Du­plex Tri­umph frame. Ac­tu­ally, no one re­ally ex­pects to use such a frame in com­pe­ti­tion any more but our ar­chive has a nice photo of John Giles air­borne on just such a bike. Close in­spec­tion shows the tank is much smaller than the one on our Tri­umph. A word with John brought the in­for­ma­tion it was a US com­pe­ti­tion tank – an op­tion from Tri­umph in the early Six­ties and rare then… 50 years on the sit­u­a­tion hasn’t im­proved.

In the dim and dis­tant past I served a join­ery ap­pren­tice­ship so work­ing with wood isn’t a prob­lem, and as the ap­pren­tice­ship was served in a steel mak­ing area it was as­sumed at some time I would have to make pat­terns for cast­ing, so mak­ing a wooden tank for­mer was an easy op­tion. This isn’t a dif­fi­cult process, time con­sum­ing by hand, but not dif­fi­cult. Essen­tially it is screw­ing a load of bits of wood to­gether and carv­ing them to shape with a few ba­sic hand tools such as a sur­form, block plane and a san­der. I guess these days rapid pro­fil­ing and that bril­liant new thing – 3D print­ing – could make a for­mer too. The idea is a panel beater has some­thing to work to.

The tank buck was of­fered up to the frame reg­u­larly, un­til the shape was to ev­ery­one’s lik­ing; it is amaz­ing how even a few mil­lime­tres off here and there can al­ter a shape dras­ti­cally. Once it was right we nipped it off to a lad who knows a thing or two about mak­ing things in alu­minium and let him loose. The re­sult is very nice and now there is more steer­ing lock too.

From a few scraps of ply­wood…

…to a thing of ex­quis­ite beauty.

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