B40 rear shocks

[Greenfield]

Idle thoughts whilst confined to barracks. My early B40 has 10.9" long rear shocks. Looking at various suppliers' websites, it appears that from around 1963 onwards 11.9" shocks were fitted. If this is the case is anyone aware of any changes being made to the frame to accommodate the longer shocks, or does it mean that the back end is a little higher?

[Phil C]

How are they measured?

[Greenfield]

Measure from centre of top bolt to centre of bottom bolt assuming 'at rest'.

[chaz]

for clarity eye to eye is how they will be advertised on length. I bought Wassells W60077

   SHOCK ABSORBERS - BSA/Norton

Pair of BSA B40/C15 (1958-67), Norton ES400 Electra (1965-)shocks with Open Chromed springs 11.9"

Fully Shrouded: NO
Chrome Springs: YES
Extended Length: 11.9"
Spring Rate: 90Lbs
Top Bush : 9.5mm
Bottom bush : 12.7mm

https://www.classicmotorcyclespares.com/index.php/default/shock-absorbers-bsa-11-9-unshrouded.html#Shock_Absorbers

[idie]

As I see it, if BSA changed the length of the shockers and didn't change the frame the back being raised will steepen up the fork angle. This in turn will quicken up the steering. Maybe this was what they were after. A cheap way of doing it without frame alterations.

[Phil C]

Whilst not wishing to distract people from Greenfield's original query, I notice mine (1965 B40F), measured between centres,  appear to be neither 10.9ins nor 11.9 ins. (see photo.)  They look more like 11.25 ins or something like that. I wonder why?  Phil.

[Phil C]

I was just thinking about my question. I guess it's because the shock is compressed to an extent when being  fitted? In which case mine are maybe 11.9 ins, according to what Greenfield was saying?  Phil

[Greenfield]

By increasing the length of the rear shocks, if no changes were made to the frame, it increases the distance between the rear subframe and swinging arm which will raise the back of the bike. I think that there were some frame changes for some C15 models but haven't found anything for B40s. Was it that BSA was just using what was on the shelves in the early days of the B40 and that when all the 10.9" shocks were used, it then changed to 11.9"? - or was it a positive decision to change the ride height and handling of the bike?

[AdrianS]

What it does do is alter the centre stand security. Shorter shocks makes the bike a lot more stable as I found out when I fitted the longer shocks to my B40!

[chaz]

but does help when rider is taller , every little helps

[Greenfield]

Thanks to everyone for your input. This started when cleaning and polishing as an alternative to riding. A poor substitute I know. My rear shocks have the old type split chrome lower shrouds which I'd chrome painted in the past as they were a bit scabby, and was thinking about fitting new shrouds. These were available some years ago but can't remember who produced them. I had a look on Feked and they sell the complete 11.9" shocks but the 10.9" aren't available. I'm still a bit puzzled as to why the shorter ones were fitted for the first couple of years of production. Were these fitted to the C15s?
To add to this mystery, according to the instruction manual, the seat height of the B40 is supposed to be 32". This can't be too precise and there are many factors that can change this, but mine is clearly no more than 31". Also its always been a pig to get it on the centre stand, so by fitting the longer shocks should theoretically raise the seat to around 32" and also make putting the bike on the centre stand easier.

[AdrianS]

It may well be easier to get the bike onto the centre stand, but both wheels will probably remain on the ground and on some slightly uneven surfaces the bike may be unstable.

[Marshall]

Hi.
10.9" shocks were fitted to C15's with 17" wheel rims. The B40 with 18" rims has the 11.9" version, I think?. Its almost impossible to find decent 10.9" shocks as I found out when restoring my C15 (I ended up stripping and overhauling mine, when I say overhauling I mean cleaning, painting and having the chrome replated!)  Hagons will make them for you, rather pricey though.

[Greenfield]

I was looking through parts books yesterday. The part number of the '61 to '64 shocks is 40-4222. I also found a reference to a rating of 136lb which seams pretty excessive, the later ones being 90lb.
I removed my shocks and fitted a couple of pieces of timber with holes at 11.9" to have a look at things. Both wheels touched the ground on the centre stand and the side stand worked fine. Apart from some slack in the chain, it all looks fine.
I would have liked to be a fly on the office wall to listen in to why the length of the shock absorbers was changed.
The parts numbers for the frame were the same through the life of the B40, but can someone confirm the measurement from top bolt centre to top of the rear frame on a later model, in case they shortened this bracket - its 4" on mine.

[Phil C]

Here's mine. 1965 B40F.
(But see my previous two posts in this thread dated 17 April.)

Phil