This is a follow up to the Twin Shock Rotation/movement Around Mounting Bolts where the plot has thickened somewhat. I did some more digging as to why the distance between the top & bottom shock mounts are different on either side of the swingarm by around 2 mm. I first looked at the rear wheel and discovered it's not sitting vertical and has a slight rightwards leaning slant / - see pictures below.
Oh arse... so I then looked at the swingarm to see if that was out. And sure enough it isn't sitting exactly horizontal - especially around the point where it would be moving. Much lower down it's almost correct. There's no discernable movement in the swingarm pivot so what could cause this? My present thinking is either the swingarm bushes aren't parallel with the rear axle or is it possible for the eccentrics to be very slightly unaligned? I'm not going to think about the possibility of a twisted swingarm.... And it is level with a 2 mm washer under the rhs side i.e. the difference between the two shock mount distance.
Sorry . i get what you are trying to do but your method doesnt really show anything. If the frame was put in a jig no way would they be measuring from the datum points you are using like the top of a painted tube edge of a tire that might not even be round or true. You need to measure off the bores for the Axel and first of all make or sure the swing arm mounts are true IE the frame and so on. Are the shocks the same length, IE if you fit one does the other line up. If it is the swing arm its quite weak, its strength is in the wheel fitted and will flex quite easily but TBH you are playing with fire if you even try that.
It wouldn't surprise me if that's within(just) the acceptable manufacturing tolerance when built. As has been said you should really be measuring using the bore the rear axle passes through(possibly using the axle in place), and comparing it's orientation to another significant point on the chassis, preferably the headstock bore or possibly the engine mounts. But if you are convinced, then whipping the swingarm out and 'tweaking it' with some brute force will easily take the 2mm out. But I'd be making up a dummy for the rear wheel so you can have the rear axle/spindle properly nipped up in position whilst the tweaking is done.
Hey @Andy Bee In theory, you eccentrics cannot be misaligned, as the locating pins are bloqued in the groves machined in the rear wheel axle. Unless something has been brutally « forced » into place, all should be aligned. In addition to what @Paddy Barratt and @Nasher have already written, I can’t help but ask: is your garage floor perfectly flat? Cause if it’s not, your bubble measurements will be affected. Hence the need to measure everything for point to point on the frame.
Thanks for the advice & tips guys but first an answer to Paddy's question: The answer is no... I have two shock absorbers of the same length. If the rhs one is bolted to both it's top & bottom mounts and the lhs one bolted to it's bottom mount then it's top will be around 2 mm short of it's mounting. There is no way of fitting the bolt without resorting to some serious force being applied which I am reluctant to do. Perhaps I could fit the lhs one first (in the longer position) and then see if I could ratchet strap compress it a couple of mill to get the rhs one in. But again it's not something I fancy doing, or should really have to. It's also worth bearing in mind, as detailed in the linked thread, that when I removed the shocks the rhs top mount bolt was bent... I'm also thinking if I do crowbar them in to get them to fit would they then be running slight different preloads? (Btw I have also done a wheel alignment check and this shows the lhs gap at the front to be noticeably larger than the rhs gap.)
Did you fit the current shock absorbers, or were they on the bike when you acquired it? If you put them on did you have any problems fitting them suggesting the problem was there previously. Typically you would loose fit everything and then tighten so it may have been easier to loosely assemble things rather than fit one and tighten and then fit the other. Also, looks like a stainless bolt, so is the bolt strength the same as the original plated steel bolt, a bit of a concern that it bent like that! did you ever have any problems with the rear shocks bottoming out? Also, assuming the bolt bent 'upwards' due to a force being transmitted up through the shock absorber, does the bent bolt correlate to the side that the shock absorber is now 'too long'? Not sure if it's easy or not to remove the swing arm, but if easy may be worth removing it, fit the axles and eccentrics, sit the four axle ends on some equal sized blocks on a flat surface, and check the swing arm for any distortion. The bent bolt suggests it took a bit of a thump, possibly when you had your knee down....
I found out the swingarm is a bit of a pain in the butt to put back on these bikes, due to the « frame-excentric-shim-swingarm pivot-shim-excentric-frame » sequence… It requires time and patience to get to the point where everything lines up nicely allowing the pivot to slide in gently.
I've had the bike 40 years and before it was being laid up for about 25 of them (ho hum ) the bike had a complete chassis/engine refurb at Brancato engineering. When I got round to putting it back on the road, around 5 years ago, I stripped it down to the bare frame and yes it was showing the same discrepancy with the top lhs bolt showing witness marks on it's thread. I saw the same when refitting with new shocks with the top rhs bolt requiring a little more persuasion to fit but it wasn't anywhere near as bad as it is now. It's also worth noting during my refurb the (internal) swingarm bushes it's pin runs on were also replaced & reamed by a local engineering company. Correct it was stainless, as was the original fitted by Brancato that I subsequently removed when doing the refurb. I'm not sure and am a little disappointed with myself for not having the presence of mind to make a note when removing it. And correct again, the bolt was at the top of the rhs shock which is the 'longer' side. The swing arm has been removed and that's exactly what I'm looking to do. I'm thinking perhaps the reamed swingarm pin bushes might not be square to the axle & shock bottom mounts and all that spirit level shenanigans was just me having too strong a coffee this morning and barking up totally the wrong tree. There's deffo summat up though and after seeing the bent bolt I wouldn't be too happy riding it after forcing it all back together (with a new straight bolt) without some idea what's going on. There was real bad pot hole recently that I can remember, you know, one of those that you see too late, can't avoid and just have to brace, wince & oof... You may well joke, and although I don't quite get my knee down, I am partial to going round some of my favourite roundabouts a couple of times Which, of course, places stress on that 'longer' rhs shock.
So a bit of chassis measuring today. I installed the swingarm eccentrics & pivot shaft to allow me to measure it's position relative to the top shock mounts. One of the pivot pins was difficult to engage in the eccentric slot and was in fact very slightly bent - this wasn't the case when I initially installed it.... All a little rudimentary but it does show a similar discrepancy as seen between the (swingarm) lower & upper shock mounts. Bent eccentric pivot pin: If the ancient Egyptians can build the pyramids with a marked stick... the bottom is resting in the split in the eccentric & the top is marked at the centre of the bolt hole. Comparison of the two marks on the rhs bolt hole. I did swap the eccentrics & pivot from one side to the other & still got the same discrepancy i.e. the rhs measurement is smaller than the lhs.
Old bike frames are rarely totally true, I’ve seen it on quite a few classic jap bikes. It’s probably within tolerance of the day. edit: unless you’ve binned it of course!
I did drop the bike at slow speed many many moons ago on a diesel slicked island that popped a hole in the generator cover. It was this that prompted the original refurb at Brancatos. I'm beginning to suspect you are right - Verlicchi made the frames for Ducati (& Norton) and I've heard of similar tales.
Here's a picture of the bush wear marks on the pivot which are around 90° apart. You can just about make them out, top on the lhs & bottom on the rhs. I'm not entirely sure what that goes to show, some torque going on?, but it must show summat
@Andy Bee Everything you are showing us is pointing towards a not entirely true frame, isn’t it? Now on an older bike like this, that shouldn’t be a problem, should it ? What about your shocks? These seem to be new aftermarket ones. Can’t you adjust their length to compensate that 0.5cm difference, instead of having one pulling hard on the upper mounting bolt?
The shocks are Maxton and, unfortunately, are non length adjustable. And, yes, the out of true frame in itself is probably not an issue but I'm still unsure how it affects the working of the rear shocks.
It’s such a tiny amount that the shocks out i doubt it will make much difference and I’d be surprised if you noticed it when riding.
Could be wrong, but I'm struggling to accept there was a manufacturing tolerance of +/- 2mm for a dimension between the swing arm pivot centreline and top shock mounting bolts, even in the late 70's. If so, that must have been one shonky build jig! Now the swinging arm has been removed from the bike, and you have had the opportunity to check, would be interested in any measurements to determine the rear axle and swing arm pivot are parallel - or not.
You can adjust the length of your dampers by putting in a droop stop of the appropriate length. Typically it would be a nylon washer above the existing internal droop stop.
Aha! so I spent the morning measuring the swingarm. The dining room has recently had a laminate floor laid for which they used one of those pourable liquid resin floors so it is pretty much as flat as a flat thing, and it's the closet I've got to a engineering table. I used 4 sockets of equal height on which I rested the swingarm pivot and the axle. A check was made across all sockets to ensure they were level (with each other). Looking from above the swingarm pivot is parallel to both the axle and the rod I've inserted between the two shock mounts. However, the swingarm didn't rest on all four sockets at the same time and there was a noticeable gap between the rhs of the axle & it's supporting socket: The swingarm could be rocked back & forth across the top right & bottom left socket axis which shows one of the legs has bent slightly. It looks to be the right hand one, as that is the side that also had the bent shock bolt at the top. Sheesh it must have been some pothole.... How it was resting on it's own i.e. not level: And with me pressing down so the axle rested on the rhs socket:
As a preliminary investigation, I think what you have done is fine. However, there is no way that I would trust a resin laid floor to being flat to within any engineering norm. My thoughts are, you either need to take it to an engineering shop with a surface table large enough. (Even a milling machine bed would probably give you the accuracy you need) Alternatively a company or institution with a CMM machine could tell you precisely how much it was out. Straightening it, if the above confirms it is bent, will take some substantial jigs, presses etc if you want it done accurately. You would need accurate measuring equipment to determine it is then straight. In all, with the relatively small 2mm twist identified, it is something you are unlikely to be able to straighten accurately in your garage.
