length of SB swinging arm bushes

[Guy Wilson]

I recently, wisely or unwisely bought a 1960 A10 Gold Flash in in largish chunks in number of boxes…
The swinging arm has been removed and the silent block bushes removed.
I’ve read as much as I can on the forum and I know there is lot of debate about these things and how they work and what actually moves etc.
My understanding again from what I’ve read on the forum, is the inner part of the bush is held against the frame and the outer bush is a tight friction fit against the swinging arm tube. The movement comes from the give in the rubber in-between.. (I think Richard L disagrees with this understanding)
I have a replacement pair of bushes and before I pressed into the swinging arm, I thought I’d check them against the new spindle in the frame. They don’t appear to fit and are too long by roughly 4 or 5 mm as you may be bale to see in the photographs attached..
I’m pretty sure the frame is true, so I’m assuming the bushes are too long and I’ve read the the quality of the replacement bushes does vary..
The swinging arm is at the engineers so I can’t check the width of the swinging arm tube. I’ll maybe be able to do this by Tuesday.
Does anyone know the exact length the SB bushes are supposed to be? Mine appear to be 121mm on the inner bush tube..
Many thanks
Guy

[RichardL]

Guy,

Thanks for the mention. As I recall, when the discussion on this was hot, I came around to accepting that the "intention" of the design appeared to be to clamp the inner tubes together between the frame plates by using the spindle for the clamping force. This also required my accepting the opinion of folks more experienced than myself at the time (so they still would be), that the rubber can flex (without destruction) within the angle of arc generated by the movement of the swingarm. All fine, I suppose, for accepting the intention, however, there is a "however." I would be very surprised if the clamping force against the inner tubes held up very long, after which the pivoting is all around the spindle as an axle.  It is pretty common to see egg-shapped spindle holes in the frame flanges.

As for dimensions, my swingarm measures 234.2 mm and the distance between the flanges measures 236.5 mm. Both of these measurements are with the swingarm out of the frame.

Richard L.

[RDfella]

I agree with Richard re 'intention'. The idea that the rubber should flex is what the design engineers would have intended and is, indeed, the situation with many rubber suspension bushes in cars. If frame holes have enlarged, maybe that's because people are bolting the swing arm up with the suspension at full extension (ie with no weight on the bike). As with rear chain adjustment, some things are meant to be centralised in their working, as opposed to rest, position. In the case of these bushes, bolting them up tight with the bike unloaded (eg on the stand) would cause excessive flexation with the suspension at full compression.

[Peter in Aus]

Some were in the forum there is a bit about these bushes but here is copy of bush size's (length wise) that they should be.
Peter

[Guy Wilson]

thank you Peter, Richard and RD, Peter's diagram suggests the inner bushes are too long by about 2mm each and need adjusting/cutting. This has probably saved me a lot of pain, frustration  and disappointment if I'd just forged ahead and pushed the new bushes in...
more soon!
Guy

[mikeb]

Guy
i had mine off and measured them recently but.. cant find the number. so why am i telling you this? the real measurement is that end to end the two bushes need to fit snuggly between the frame plates on your bike (not my bike). so you can wriggle them into place and see if they fit. it should be snug. yes, the spindle clamps them tight between the frame plates.
i'd put these in place and carefully eyeball them before cutting off anything, and certainly before installing in the swing arm (coz they won't come out in one piece!)
also btw its the total width of the two that matters. if one is a few mm longer than the other its no problem

[Guy Wilson]

Thanks Mike, you make perfect sense! and I forget these bikes were made long before CAD and measurements often only a guide when you have the bit in hand, common sense should prevail..
I won't have time to test my common sense for a few weeks now...
Guy

[mikeb]

FWIW i just came across an old figure that the width between frame plates is 9.373 to 9.383" (copied from somewhre in the forum some time back)
i cant verify if this is accurate
in theory your two bushes should add to that. but i'd still test the fit.