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petorius

Bultaco yokes

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I guess everyone has their own lockdown projects on the go at the moment. Mine is using up accumulated parts to put a Sherpa together using a late engine in a slimline (91B) frame. I am quite keen to get the bike relatively finished so that I can move onto other things, so far so good except I had some 35mm triple clamps that I thought would be suitable as they had quite a bit of offset but it would seem they have too much. As it stands with the rear shocks measured at 355mm (temporary fitment only to hold the bike together) the wheelbase measures 1335mm (52.5”). Standard is 1315mm (51.7”) on 340mm shocks I believe. Not sure I will be able to get this back by dropping the forks so wondered if anyone happens to have a spare set of aluminium yokes they would care to sell? I do have a set of Alpina yokes, these have no offset and not sure if they would work.

DSC_0020.JPG

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This is from memory.

355 vs 340mm shocks only have a very minor effect on fork angle and can be neglected.

My Alpina yokes are parallel and have approx 30mm offset and are more heavy then Sherpa yokes.

More offset means less trail

Sherpa yokes have a 1,5-2 degree rake reducing the trail. I have measured a number of them and 1,5-2 degrees is the best I can say.

When fork is springing down (Sherpa non parallel yokes) trail will increase making the bike more stable. So the bike will be little more stable when going straight downhill or applying front brake but turning might be more difficult

C-C distance is bigger on Alpina yokes. Alpina also has a different front wheel with 140mm brake.  Can be differences depending on model and I am not shure Bultaco were consistent all the time. You must check.

I can check tomorrow C-C Sherpa vs C-C for what I belive is Alpina

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I've always thought that the wheelbase given in the manual was a bit conservative as when I've measured my Bultacos in standard set up (a while ago) I'm sure they were over 52", even with the wheel forward which it rarely is due to chain length and tension. I'm working on an M151 and a 198 at the moment, both standard so I'll measure tomorrow out of interest. They don't feel that short to ride...

When you mention offset it sounds like you're actually referring to the angle of the forks to the steering stem ?  which isn't the offset as that's the distance of the forks from the steering stem. Your yokes look like Montesa ?  which probably have less offset than Sherpa yokes

As Carl said the later Alpina yokes have an offset but run parallel to the steering stem so would reduce the wheelbase slightly. I tried some once but because they were wider new spacers were needed and I couldn't be bothered and you'd have to stretch you mudguard brace to fit or fit it to the inside

I've ridden a Sherpa with the head angle tightened and which also had parallel billet yokes which I thought might affect the steering adversely but it steered fine and seemed stable enough

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The wheelbase changed over the years

M92 were just over 52

M124/125  150/151 were shortened to just under 52 with a slightly steeper head angle- apparently to bring them into line with the Miller frames

M159  just under 52  , from m183 onwards a longer swinging arm was fitted taking them back to just over the 52

 

I would personally try and get hold of some original ones just to eliminate that as a handling issue later down the track

Cheers Greg

 

 

 

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I measured 27.5 degrees steering angle on 199B and 25.5 on 199A. Perhaps precision in my setup but the sum of steering angle, yoke offset, stanchion rake and front wheel offset is little unusual. My 199A feels unstable when running straight. Bearings an everything is OK as far as I know.

C-C stanchions Sherpa is 165mm and C-C Alpina 170mm.

Wheelbase 199A (unloaded, rear wheel in front position) approx 1315mm.

Geometri.JPG

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While the frames are not all the same steering head angle, all the Sherpa T triple clamps have the same geometry however the early ones are made for fork tubes that have a tapered connection at the top. Your tubes being non-tapered at the top limit your choice of triple clamps to Sherpa Ts after about 1974.

If you are in a rush to get it finished, the Alpina triple clamps will work, but you end up with a lot more trail and less wheelbase and the front guard will be very close to the front downtube on full fork compression. Some people like the steering/handling with the Alpina triple clamps. It is very different to a normal Sherpa T.

Edited by feetupfun
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I run Alpina fork yokes in my 325 Sherpa, needs a bit of sorting to get everything to fit properly, the rear mudguard stay needs to have a different bend to miss clouting the frame down tube. I prefer the set up with the Alpina yokes, with the bar position right over the top of the steering stem. Feels more like a modern bike. Steering is an improvement for my liking. I used the Alpina front wheel with the larger brake drum, a pity it is 1kg heavier than the Sherpa wheel. Bye, Peter B.

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A friend of mine built a Sherpa using fork yokes from a Montesa Cota 247, these seem to have possibly a 1.5° offset or run out similar to Sherpa yokes but place the handlebar mountings over the steering stem instead of behind it which gives more room on the bike. This for me being a taller rider gives much better steering and control.

Steering bearing diameter is the same as the Sherpa, unsure if the lock stops or wheel spacers would need altering as the fork legs are slightly wider apart with the Montesa yokes.

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Just for interest, I ,measured the 151 and 198. The 151 came out as a touch over 52 with the wheel just forward of centre, the 198 was close to 53 with the wheel just behind centre 

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Thanks everyone for your comments, very helpful.

I had based wheelbase measurements on Sherpa sales brochures. According to the brochures all Sherpas have the same 1315mm (51.7”) wheelbase except for model 158, 1290mm (50.7”) and Model 80/91/92 at 1330mm (52.3”) not sure though if this refers to the early 91/92 which I would assume would be the same frame as model 80 or the late frame which I would have assumed was the same as the 124/125.

https://www.motoguapa.com/BULTACO/ArchivosBultaco4.html

My wheelbase measurement was taken from centre of rear wheel adjustment, with the wheel foremost on the adjusters it is 1330mm, so it would seem all is okay as it is and line with early Slimlines, perhaps with the Alpina clamps it would bring it more in line with later Sherpas. 

That is a good spot on the yokes being Montesa I did not know what they were from, I seem to recall they had an aluminium dome stem nut which I have managed to lose over time, this has caused me some grief as the stem has a very fine thread, 1mm pitch. I had found some listed on the well-known auction site but when it arrived it was not the size listed; I think I have found one now though.

 

Thanks again.

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If yokes are parallel, everything others the same, it would move the front wheel approx 20mm rearwards. Bike should be more stable when running straight.

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Talk to Ossy at Audit CNC, they make them for all of the classic lads around the country and may give you a better Geometry

Tel: +44 (0) 191 3718888

 

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I have a pair of yoke from a 1974 325 if that is any help. Also a complete engine and gearbox,pair of hubs and a tank / seat unit.

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3 hours ago, jon v8 said:

I have a pair of yoke from a 1974 325 if that is any help. Also a complete engine and gearbox,pair of hubs and a tank / seat unit.

Best answer so far

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