bultacosteve Posted January 19, 2009 Report Share Posted January 19, 2009 This is my first post on this forum but I have been viewing the topics on the board for about a year. I own several Bultaco's and I am very knowledgeable about the brand and many of the models they produced. I have been riding an M159 Sherpa for the past 5 years in vintage events. This past year I have been using it in the vintage class but we ride junior type sections. Myself and the other vintage riders find the junior sections more than manageable on the older bikes. Last year I got an M199B sherpa t and I am just now finishing off the restoration. I use that term rather loosely as all I have done is blast and paint the frame, new tires, new cables, brakes, and all engine internals have been replaced. I have fitted a Mikuni flatslide and have it dialed in very nicely In any case one of the problems I have with the Sherpa models is how long the wheelbase is. I have read several articles on swingarm placement and I am aware of how critical it is but nobody has ever really commented on the tiller like effect that the front end of these bikes have. Basically from the M158 series right to the end of production the bikes were designed at a time when long rocky climbs were the norm. Thus making a long stable wheelbase very desirable. And here in western Canada we do have lots of rocky sections where the sherpa will really show its true colors and perform the way it was intended. However riding the bike in the modern junior class I find that it doesn't have the turning ability that I would like. I am thinking that I would like to take the front end off of a M116 Alpina. This would effectively reduce the wheel base by about 25-30mm. Has anybody out there tried doing this?. Also has anyone tried to actually change the steering geometry on their Sherpa by either making their own frame or removing the steering stem on the frame and welding it a degree or two steeper. I wouldn't say that the Sherpa isn't a quick turner ..... it is, its just that I would like something a with a little bit better turning manners. Any thoughts from some of you knowledgeable folks would be appreciated Steve Quote Link to comment Share on other sites More sharing options...
swooshdave Posted January 19, 2009 Report Share Posted January 19, 2009 I assume the M116 Alpina is the one with the Pursang forks? I have heard that putting those forks on a Sherpa is unadvised as that it becomes highly unstable at speed. There were some recent pictures of a Bultaco which had the swinarm pivot moved forward. That and a shorter swingarm would do the trick. Quote Link to comment Share on other sites More sharing options...
woody Posted January 19, 2009 Report Share Posted January 19, 2009 I'd try a simple change before you go altering the chassis and that is to fit longer rear shocks, or extending your own, which will also have the affect of pulling the front end in a little. Due to the design/layout of the Sherpa frame there is little room for pulling in the Sherpa forks on the head angle or by reducing the offset with different yokes as the front mudguard will hit the frame downtube if it is moved rearwards even slightly. I've never really understood the 'stability' theory of the Sherpa. I find they can get off line the same as any bike up rocky streams/gullies but because they have a lot of lead on the fork angle you need a lot of room to get back on line, KT kwak is the same. I have a 199b and apart from the 10mm longer shocks it is standard. Steering in tight stuff is fine, although it has a lot of lead on the forks, the turning circle is tight. I'm more concerned about improving the crap front forks than steering. The bikes that had the swingarm pivot moved forwards by Commerfords here in the UK were done to improve rear suspension travel and grip, not turning. They had longer swingarms to compensate for the pivot moving forward. You really don't want anything less than 51" wheelbase otherwise your never going to keep the front down. You can get that wheelbase by running the rear wheel as far forward as possible. Quote Link to comment Share on other sites More sharing options...
bultacosteve Posted January 19, 2009 Author Report Share Posted January 19, 2009 (edited) Yes the 116 alpina has the pursang type triple tree. Stability could be an issue with the wheelbase pulled in but how much so is anyone's guess until I've tried doing it. With the wheelbase pulled in the fender could interfere with the frame down tube and obviously could be quite dangerous. I find in rocky creek beds that the M159 is very solid, but the comment about the bikes being knocked off line easily is well founded. I assume my M199B will be the same because the wheelbase is identical. I find that if I have approached the section right and have the right line that the bike behaves superbly but if off line you're in for a heap of trouble because trying to right the bike only ends up in over correcting which puts you off line even more. Also the 326 engine has so much snort that I find at times it can put me off line as well. I am going to try the swingarm mod later this year. I notice that the frame has to be altered to incorporate the pivot being moved forward. As well the rear engine mount would have to be reconfigured. In the latest issue of classic dirt bike there is an article on a gentleman from the uk that did a superb build on a 198A bike that incorporates the swingarm mod. How far up does the engine have to be moved if at all? does anyone know?. I'm going to get a season of riding the M199b before I try anything too radical. Longer shocks seems the sensible way to go. Getting the front forks to behave the way I want is at the top of my list. I might try some racetec emulators for the front forks. These are basically a damper that goes underneath the fork spring and can be tuned to adjust rebound damping. Stock fork springs are a weak point on the Bultacos as well. Has anyone machined weight off their primary flywheel and if so how much?. And afterwards did it need to balanced?. Also I am using new betors on the rear of the bike. Any suggestions on a better shock as I'm quite sure they betors are the same quality they were 30 years ago. Steve Edited January 19, 2009 by BultacoSteve Quote Link to comment Share on other sites More sharing options...
woody Posted January 19, 2009 Report Share Posted January 19, 2009 It's not the wheelbase with the Bultaco that causes a problem, its wheelbase is about the same as any other bike, most are between 51.5" and 52.5". It's the steering geometry that makes it a bit lazy when trying to get it back on line after you've lost it. A bike with quicker steering can be brought back on line quicker. Ossa MK2 has the same wheelbase as the Bulto as near as dammit and it is really agile up rocky streams as it has quick(ish) steering compared to most other bikes of the time. The MK1 Ossa was shorter at 51" but for the MK2 they added about 1.5" to the swingarm to help keep the front down on climbs and aid grip. It steers exactly the same as the MK1 as the geometry is the same but the MK1 will turn a tighter circle because of the shorter wheelbase. Either is a very agile bike and can be ridden really slowly up rock streams/gullies and basically placed wherever you want it to go. So the wheelbase isn't the problem. With the Bulto they have to be ridden quicker and more in a straight line up that type of section due to the lazier steering. Can't say either one is any more stable though. To be accurate, no bike is going to be stable if the rider can't hang on to it and hold the line, they'll all fly all over the place. Best thing to concentrate on is getting the suspension working as best you can front and back as that will give the best benefit up rock sections. The damping in Bulto forks is too soft and it's hard to stop them topping out on rebound. I've even had them at the point where I've too much oil in (and this is 30 weight) so that they hydraulic on compression but still top out on rebound.... Not much you can do with that. I cured it by fitting some Ossa MAR damper rods as the Ossa forks work better. The rear suspension mod is done by tipping the engine upwards very slightly due to the shape of the rear of the engine, in order to allow the swingarm pivot to be moved as close to the engine as possible. Don't know the exact measurement by how much they pivot it upwards but it's not much. They didn't use the standard swingarm on this mod though, a longer one was used, otherwise the wheelbase would be down to under 51" and that is without moving the steering back. As regards the flywheel weights, why do you want to lighten them. Do you really want a 325/3400 Bult to pick up any quicker? The trade off is that you will lose the additional flywheel inertia that gives the engine so much plonk at low speed. You'll end up with an engine that just wants to stall at slow speed and which will require constant use of the clutch to prevent that. I'd leave it alone to be honest. Or if you must experiment, try the flywheel and crank weight from a 250 both of which are lighter as at least that is a bolt on/off solution. For rear shocks I'd use Falcons and ask for an additional top mount which is 10mm or so longer so that you can experiment with the difference longer rear shocks make to the steering. Easier than cutting frames. Quote Link to comment Share on other sites More sharing options...
swooshdave Posted January 20, 2009 Report Share Posted January 20, 2009 Yes the 116 alpina has the pursang type triple tree. Stability could be an issue with the wheelbase pulled in but how much so is anyone's guess until I've tried doing it. With the wheelbase pulled in the fender could interfere with the frame down tube and obviously could be quite dangerous. It's not a guess as to what happens as it's been tried before. Unless you only believe in what you try yourself. Quote Link to comment Share on other sites More sharing options...
bultacosteve Posted February 2, 2009 Author Report Share Posted February 2, 2009 Ok since the Alpina triple clamps are a no go how about trying a set of pursang non leading axle forks. This would leave the steering stock but bring the front wheel back slightly. Has anyone tried doing that on their Sherpa's Steve Quote Link to comment Share on other sites More sharing options...
big john Posted February 2, 2009 Report Share Posted February 2, 2009 Well I'm not as fit as I used to be, but... ...try hanging more off the back end of a Bultaco rear end steering if you like, that helps overcome the front end, it worked sometimes for me but all of the time for a certain H.M. Lampkin! I do have a model 199B with a revised swing-arm pivot set up (right Mr. dabster?) Big John Quote Link to comment Share on other sites More sharing options...
swooshdave Posted February 2, 2009 Report Share Posted February 2, 2009 I wouldn't go messing with the forks when slightly longer shocks might do the trick. Quote Link to comment Share on other sites More sharing options...
bultacosteve Posted February 3, 2009 Author Report Share Posted February 3, 2009 My bike is set up stock right now. AndI'm not going to hack it apart or change anything drastic until I have a season of competition on it. And I will try the longer shocks first. In our club there is a fellow that has a modified M99 frame. He has removed the lower frame rails and steepened up the steering geometry. As well he uses a pursang front end with non leading axle front forks. His is a very well sorted out Sherpa and he rides intermediate type sections with good success. Having said all this I was just looking for any info that you guys could help me with. At this point with how scarce 340 parts are; changing anything drastically from stock probably isn't such a good idea. Also I am on the 4th over piston with this bike. Are 340 cylinders available in the UK?. Because there sure isn't any here in north america. Steve Quote Link to comment Share on other sites More sharing options...
swooshdave Posted February 3, 2009 Report Share Posted February 3, 2009 Also I am on the 4th over piston with this bike. Are 340 cylinders available in the UK?. Because there sure isn't any here in north america. Steve Hugh's or Mobley didn't have any? Quote Link to comment Share on other sites More sharing options...
nigel dabster Posted February 3, 2009 Report Share Posted February 3, 2009 new liner? Quote Link to comment Share on other sites More sharing options...
woody Posted February 3, 2009 Report Share Posted February 3, 2009 Yes, new liner or find a later 325 cylinder and bore it out - you won't notice any difference (with early 325 cylinders the liner is too thin to take the bigger piston) As regards handling it's personal preference really, what suits one person won't suit another so it's difficult for other riders to make suggestions. Everything has been mentioned in previous posts so it's up to you which way to go. You may find that longer shocks is all you need. If the bike with Pursang front end suits you then go for that mod, at least it's bolt on/bolt off with no hacking. The Pursang forks must have been modified for damping though as they're not suitable for trials as they are. Quote Link to comment Share on other sites More sharing options...
feetupfun Posted February 3, 2009 Report Share Posted February 3, 2009 If you use Pursang in-line forks in Sherpa T clamps, in a standard Sherpa T frame, set to provide the same height for the front end, you will probably have interference issues with the front wheel and the front downtube when the forks are compressed. If you use long fork tubes to provide the required clearance for the wheel, you will make the front higher and the steering even slower than it is already. You may be on the right track with the geometry change though. A couple of us experimented with our KT250s last year and one experiment was very similar to what you are suggesting. We fitted TY250 (in-line) forks and wheel into the KT250 clamps. KT forks have a similar axle offset to Sherpa T forks. The front end ended up being very close to being the same height as standard. The effect was that the KT then steered like a TY250 (steering became quicker). That experiment worked OK though because there is more clearance for the front wheel on a KT than on a Sherpa T. I like to have bikes set up to feel correct for the period though so it is back to being a normal KT again now. There really is nothing "wrong" with a Sherpa T, they just require a different riding style and it is hard initially to swap between something like a TY250 or an OSSA and a Sherpa T, but if you spend time on the Bully and ride in a way that suit the bike, they do feel great to ride. I totally agree with Woody and whoever else said it on the path you should take - longer shockies or shockie extensions will make the world of difference to the steering and it is so easy to do! A bloke over here made up neat-looking flat steel brackets for the bottom shockie mounts of his M198 that fit over the bottom shock mounts and bolt to the holes just ahead of the shock mounts - no welding required and no damage to the frame. Instant quicker steering! I rode it like that and the bike that the steering reminded me most of was my 250 Godden Majesty (and they steer pretty well) Quote Link to comment Share on other sites More sharing options...
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