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.
