jc2

Sam, how did you clean out the mid-section?

I've done a bit of research on this. As I suspected, it seems its all to do with chain-pull torque/effect under power, which extends the rear suspension & stiffens it up, making it less supple over undulations potentially causing loss of grip. A longer swingarm reduces that effect so potentially offers more grip - most noticeable under acceleration/full power on rough ground. It also means less tendency to wheelstand, since it reduces the effect of raising centre of gravity from the extended suspension. And it means more power is available for drive since less is lost extending the swingarm & raising the C of G. This of course is what Vesty was after when he modified his Bul in the early 80's. I hasten to add that a longer swingarm is not the only thing that affects chain-pull effect on grip & Vesty's bike used several mods to achieve his aim.

I recently scored a 75 247 Cota. Any tips for set-up & improvements?

Interesting comments but too many are off the track. Perhaps I didn't state clearly in the original post what I meant. I'm not referring to extending a swingarm on an existing design. That obviously affects wheelbase, weight distribution & sometimes suspension travel all of which are going to affect grip. I mean, all other things being equal - weight distribution, wheelbase, power characteristics, wheel travel, damping & spring rates - why is a longer swingarm design said to give more grip? I've heard/read it repeatedly applied to trials bikes, MX bikes & motoGP bikes, but perhaps no more so than regarding trials bikes which are my main interest now. Can anyone shed light on it?

Can anyone tell me the theory/reasons why longer swingarms are said to give more grip?

I think you nailed it MRB. On a little more research it seems the change in pivot point actually happened from the M80 to M91 models - it just didn't make the brochures till the M124 models. (The same happened w the pursang brochures when they shortened the wheelbase on the M103 Mk6 but the brochures didn't change till the M135 Mk8) The spanish on the SherpaT (M124/5) brochure I believe can be translated: "thanks to a small advance in the fork" (meaning rear fork, ie swingarm)

Thanks for your responses guys. If anybody's got any more input, I'm still listening

Gents, I've noticed on the bultaco brochures (on Todotrial website) that the model series 124 & 150 have 1315mm wheelbase listed as opposed to 1330mm for earlier Sherpas. Can anybody tell me: 1) is that true - ie did they really have shorter wheelbase? 2) if so what was changed from previous models (M91, M80 etc)? Was it shorter swingarm, steeper rake, less offset on the triple-clamps or what? There is a bit of an explanation on todotrial, but its in spanish & I can't read spanish & it doesn't make any sense attempting to translate with my spanish dictionary or with google translation. Can anyone translate: "gracias a un menor avance de horquilla" Any help greatly appreciated

Hi Gents, New chum here. More or less new to trials. Was into MX back in the day, then VMX. Needing something a little more sedate lately but no less interesting. Have background in spanish bikes, but Feetupfun recently gave me a taste of a range of twinshock trials bikes incl TYs & KT. Loved every minute of it. Thanks Dave, yr a great ambassador for the sport!