Sherpa T chain offset

[michaelmoore]

I've got wheel alignment problems with my Model 159, even after straightening the bent swing arm and triple clamp. I've measured and drawn up the swing arm and cast aluminum rear engine mount and it sure looks like the engine case center is supposed to be on the centerline of the bike with the swing arm centered on it too. Yet with good sprocket alignment (fresh sprockets too) to get the rear wheel aligned on the front wheel the rim has to be pulled over towards the sprocket to the point where I've got serious concerns about the amount of thread engagement between the spokes and nipples on the right side of the wheel.

Perhaps the new Buchanan spoke set is off, with the long spokes on the right not having been made long enough. They seemed pretty similar to the spokes I took out of the wheel.

Several people have told me "it is just a trials bike, don't worry about wheel alignment" but while I could do that if it is only a mm or two, 10-15mm is getting a bit out of hand. Other people have mentioned that they suspect Bultaco may not have been too concerned about wheel alignment when they made the bikes, though I'd like to think they'd show a reasonable level of concern. Since the swing arm/pivot/damper mounts seem to be pretty symmetric around the engine case split I'm going to presume they meant to hold the rim on the centerline too.

Since the engine is out I figured I'd verify the sprocket offset from the case split.

First I measured the (mildly worn) motor mount widths on the engine:

front 1.230-1.235"

upper rear: 1.230-1.235"

lower rear: 1.220"

31mm = 1.2204", 32mm = 1.25984 (1.260)"

So I'll use a nominal width of 31-31.25ish mm

I set the engine on 2-4-6 blocks (hardened and ground steel blocks 2" x 4" x 6" sold as a matched set) at front and upper rear motormounts with the sprocket side of the engine upwards. I then used a screw jack to bring the bottom motor mount upper face to the same (basic) level as the other two mounts, measuring all this with a vernier height gauge.

The 520 sprocket has roughly a 1mm shoulder on each side (which would make it a 525 sprocket at 8mm (5/16") full thickness when adding 1mm to each side of a 6mm sprocket). I wasn't going to disassemble the left cover to measure it exactly and I'm working off the outer face of the sprocket anyway, since that needs to be in plane with the outer face of the 6mm wide rear sprocket.

The outside of the 6mm thick sprocket to case centerline is 73.5mm as the best approximation measuring from the outer face of the upper rear motor mount. That would make the sprocket centerline 70.5mm. Would they have set an even number for the centerline (like 70mm)? 70-70.5mm is a .020" difference which is pretty negligable for chain alignment. Since the measurements were coming out closer to 70.5mm than to 70mm I'm willing to accept the former as the dimension for the sprocket centerline that the factory was intending to use.

So if you are building a rear wheel and you want to presume that the engine is on the bike's centerline you can subtract 1/2 of the rim's measured width from 73.5mm and have the offset from the outer face of a 520 sprocket. Or subtract it from 67.5mm if you are measuring from the flange on the wheel that mates to the inner face of a 6mm thick sprocket.

If someone has a Bultaco service bulletin or similar "official" document that calls out a sprocket offset dimension I'd be interested to hear what they said it should be. I don't know what kind of manufacturing tolerances they held on machined parts like the engine motor mounts vs welded/built up parts like the rear rim or swingarm pivot or rear axle plates.

FWIW I've got 190mm for the nominal width at the rear axle for the backing plate/hub/spacer (or speedo drive) assembly. So you should be able to take an assembled wheel (at least for a 159, I don't know if Bultaco changed things on earlier or later models), subtract the rim width from 190mm, divide the result by two and then have the rim be that number inwards from the outside face of the backing plate or the outside face of the spacer on the right side of the wheel if that is easier for you to measure.

Presuming of course that the rim is really supposed to be centered.

ETA: I forgot to mention that the 159 sprocket is counterbored to slip over the flange on the hub, so depending on your bike's arrangement (counterbored or not) you may need to adjust to get a good offset dimension for the flange on the hub if building a wheel without a sprocket on it.

cheers,

Michael

Edited December 21, 2010 by MichaelMoore

[motovita]

I think the most important thing to worry about is clearance between the tire and the exhaust on the right and the chain tensioner and guard on the left.

I agree with your friends who've suggested that you may be overthinking things a bit. I almosted passed beer through my nose when I read the part of your post about looking for a Bultaco service bulletin.

Hope to see you in the sections, I'll be on the blue 199.

[02-apr]

Leaving aside any production quality problems you refer to, the Sherpa was designed by Sammy Miller who expressed the opinion at the time that a trials bike was easier to balance if the wheels were slightly offet. That being the case it was designed such that the offset needed to accommodate the chainline was not seen as a problem and the wheels do not share a common centreline.

[michaelmoore]

Did you ever see any clarification from SM on why he thought that? I can't think of any reason why having the wheels out of line could be of a benefit, unless maybe you were doing flattrack/speedway and wanted to bias things in the only direction the bike ever turned.

cheers,

Michael

[swooshdave]

Did you ever see any clarification from SM on why he thought that? I can't think of any reason why having the wheels out of line could be of a benefit, unless maybe you were doing flattrack/speedway and wanted to bias things in the only direction the bike ever turned.

cheers,

Michael

Sure, makes the bike less likely to tip over at the super slow speeds of trials. Just as you can keep the bike upright by slightly turning the front wheel, having the wheels offset add to this when you are going straight.

In this exagerated example of the contact patches you can see how the front wheel will reduce the tendency for it to fall to the left and the rear wheel will reduce the tendency for it to fall to the right.

Bikes that don't have the wheels perfectly in line is not uncommon. For example the Norton Commando also was like this and it was known for pretty decent handling.

[kramit]

Sure, makes the bike less likely to tip over at the super slow speeds of trials. Just as you can keep the bike upright by slightly turning the front wheel, having the wheels offset add to this when you are going straight.

In this exagerated example of the contact patches you can see how the front wheel will reduce the tendency for it to fall to the left and the rear wheel will reduce the tendency for it to fall to the right.

Bikes that don't have the wheels perfectly in line is not uncommon. For example the Norton Commando also was like this and it was known for pretty decent handling.

Every time I follow a Ford Truck, I notice the rear wheels don't track the same as the front.....must make them easier to Balance?

[brt650]

Hello Michael,

Just measured my 159.I relaced my wheels with stainless spoke kits from Bultaco UK last year.

From the very edge of the rim to the swing arm both sides is 37mm. The right rear spacer is 30 mm long.

A centre line through the engine case's lines up with the centre of the tyre.

By the way. Harley davidson motorcycles can have a offset of between 8 to 16mm from the centre line of the frame to the centre line of the wheels. I set my Harley at zero off set. I don't notice anything wrong with the way it goes around corners.

Have a merry christmas everyone.

Cheers

Brian

P.S. Hey Sherpa 325 any chance of you completing an event on your own twinshock bike?

[swooshdave]

Every time I follow a Ford Truck, I notice the rear wheels don't track the same as the front.....must make them easier to Balance?

Get them up on two wheels and you'll know.

[feetupfun]

Get them up on two wheels and you'll know.

I do like the way you think Dave

David Lahey

[02-apr]

"Did you ever see any clarification from SM on why he thought that? I can't think of any reason why having the wheels out of line could be of a benefit, unless maybe you were doing flattrack/speedway and wanted to bias things in the only direction the bike ever turned."

Michael, out of line was not mentioned. Out of line means pointing in different directions whereas offset means running parallel but on different centre lines. The article (and I'm not going to go through the pile of period publications in the attic to check) said, if I remember (and it has been the topic of conversation over the years so hasn't faded too far from memory), essentially that having the wheels offset gave a wider base on which to balance. I'm not saying I necessarily agree with this or that the small measurements involved in this context would make an appreciable difference but SM was probably better placed to appreciate the subleties.

He must have had something right in his ideas as I recall him saying in the last few years that the wheelbase and steering head angle of a modern Gas Gas were the same as the original SHS on which he rode his first SSDT. Only forty years or so of development to go full circle.

Edited December 24, 2010 by 2/4

[michaelmoore]

The right rear spacer is 30 mm long.

The length of the spacer I made to replace thr speedo drive that was on my bike is 33mm. I wonder if the speedo drive was not correct, or they normally used a 30mm spacer and just jacked things apart in the unlikely event someone wanted to run a speedo.

One person measured their 250 rear wheel and came up with the same offset I found on the countershaft sprocket, so with that and the various reports of "wheel/engine/swing arm are centered" I'm going to presume that the engine is intended to be centered in the frame and the wheel aligned to the centerline.

Turning the bike into a two-track vehicle instead of a single track vehicle sounds questionable to me, even if SM did advocate it. Two parallel tracks doesn't sound any better than the wheels never being in line (and if the rear wheel is cocked to the side at some point the front wheel will be turned off center just enough to make it parallel to the rear wheel so it should act just the same as having the entire rear wheel spaced over).

I think the next step is probably seeing if I can get the wheel offset without worrying at all about spoke thread engagement. Then I'll mark the spokes and pull a few and see how much longer spokes I need on the right side to get enough thread engagement. Buying half of a spoke set might do the trick.

I was looking at the front wheel and noticed a LOT of thread is visible above the nipples on most all of the spokes. I guess I should check those for spoke engagement too.

The bike has been through a lot of hands before it got to me so it is difficult at times to tell what is factory specification and what is Dreaded Prior Owner work.

cheers,

Michael

[swooshdave]

Turning the bike into a two-track vehicle instead of a single track vehicle sounds questionable to me, even if SM did advocate it.

cheers,

Michael

Just ask yourself one thing, are you a better trials rider than SM?

I know what my answer is if someone asked me that.

[michaelmoore]

Ace's can ride on stuff that mere mortals will struggle on. Generally, something responsive enough to be on the edge of immediate disaster for anyone else is just barely responsive enough for someone like Rossi, Bou, etc.

Motorcycles are intended to be single track vehicles. I don't want to have to remember different modes of turning response that vary with the direction of the turn.

A gyroscope would be best to help my bikes balance. Or maybe a better rider.

cheers,

Michael

[02-apr]

Michael you sound as if you are as fussy as I am about these things but I long ago gave up worrying about this with trials bikes because I have never found one where everything lined up properly and the other forces (rocks, camber etc) trying to put the bike off line are, at trials speeds, greater than a wheel being slightly misaligned. Tyre wear isn't really a factor at these speeds. A twisted swing arm was very common in the old days and most folk didn't feel the effect of the off-vertical wheel.

I usually just try and get the chainline right by sighting along the chain - nobody actually believes the swingarm or snail cam marks do they? - and live with the resulting wheel alignment (unless it is really bad when I might go half and half) as the wear on the chain is probably the biggest effect of misalignment.

Even Japanese production tolerances aren't that great. I wasn't happy with the tyre wear on my then new SV 650 and knew that the chain line was not perfect. I never had satisfactory results using string but when I finally found a good reliable straight edge (fluorescent light tubes, advice courtesy of Kevin Cameron who will not be unknown to you) I discovered that with the wheels correctly aligned (and sharing a common centreline) the sprockets were parallel but offset by 8mm. Measuring the shoulder on the outside of the rear sprocket showed it to be 4mm so reversing the sprocket such that it was on "the wrong way round" cured the chainline problem.

Given that the motor really had to be in correct relation to the frame and the chance of the rear hub being "too narrow" was nil then the real cause was a mystery. Interestingly later batches of the SV had no shoulder on the rear sprocket - so presumably only run 4mm offset with no means of correction (I haven't had the chance to measure one to see).

Edited December 25, 2010 by 2/4