B40 rear shocks

[Greenfield]

Thanks Phil
.... so it appears that BSA made no changes to the frame or stand, but just fitted longer shocks to the later models.
Cheers everybody

[Phil C]

As I said earlier in this discussion, mine look like about 11.25 ins (in situ.). Any comments on this please?  Phil

[JulianS]

This from a Girling catalogue may provide some information.

[Phil C]

Thanks very much for this, Julian. So for my 1965 B40F the extended length should be 11.6 ins. I guess that is between centres. Mine look to be about 11.25 ins, measured in situ on the bike.  Any thoughts please?  Phil

[Phil C]

And I wonder why there are different figures around. 11.9?  11.6?

[Mike40M]

Reading the table Julian kindly provided, I see a slight increase in length .2" and a fairly large increase in spring rate 24%. I guess that BSA found that using same rear shocks on C15 and B40 wasn't optimal. Could be that heavier people bought the B40 or that they more frequently carried passengers. The increased spring rate gave a higher ride height or put it another way, less sag. The effect will be a slightly more agile steering and a little less stability. Most riders wouldn't notice.
Could be that BSA got customer complaints about too soft suspension with a passenger.

[Greenfield]

Thanks Julian. First hand info is far better than 2nd or 3rd hand interpretations. I had another look at my shocks on the bench. The nearside one is 10.9" which is therefore from a C15, whereas the offside one is 11.4" which is correct for the year. What's going on? I felt that the bike was a little down at the back and with the rear wheel off, I'd seen evidence of the tyre wearing the rear light cable inside the mudguard. The two look identical except that the black lower leg is 1/2" longer in one. I'd never noticed and they've certainly be like that for many years.
They'll definitely have to be replaced and the only ones now available are 11.9" with a softer spring than spec. When riding with weight on the seat, I expect they'll squash a bit more, and end up not far from where they should be.