Primary Chain

[cus]

G'day Everyone,
Have noticed on my '63 s/r, that I have to adjust the primary chain
a bit. After adjusting, tightening adjuster nuts etc. all is good, but after
a few weeks when I check again, the chain is tight again!
Have adjusted twice this year, maybe things are still settling in after
re-build? Has anyone else had similar,

regards, cus

[Rocket Racer]

What typically happens is that the rear chain also loosens. The rear chain ratios effectively pulls the gearbox backwards.
Making sure the box is bolted up nice and tight helps, but make sure your gearbox adjuster is set not to tighen the primary but at the point of backing the primary chain off looser. Not sure if I'm explaining this well but given the choice the gearbox will always tighten the primary, so your gearbox adjuster should be set to so it has no slack to allow this.

[beezermacc]

Make sure your rear chain isn't too tight. When the suspension is compressed a tight rear chain will put extra stress on the primary chain. When adjusting the rear chain it is sometimes worth removing one rear shock so you can compress the suspension easily to check chain tension easily through the compression range.

[Greybeard]

Interesting tip about primary vs secondary chain adjustment.
Does this happen with plunger suspension as well?

Neil Ives

[muskrat]

 With s/a bikes I use ratchet tie downs to compress the rear suspension till the axle, s/a spindle and c/l of front sprocket are in line. I adjust chain to 1/2" play. Once done put the bike up on center stand and note the amount of play. That's your bench mark.
Cheers
Saw Neils post, No. The box is bolted to the motor.

[iansoady]

What typically happens is that the rear chain also loosens. The rear chain ratios effectively pulls the gearbox backwards.
Making sure the box is bolted up nice and tight helps, but make sure your gearbox adjuster is set not to tighen the primary but at the point of backing the primary chain off looser. Not sure if I'm explaining this well but given the choice the gearbox will always tighten the primary, so your gearbox adjuster should be set to so it has no slack to allow this.

Absolutely right. There's enough slop in the mountings etc to pull the gearbox back appreciably. Always overtighten the primary slightly then finish off by pushing the gearbox forward to slacken it thus taking up all the play.

[duTch]

And also if both chains are too tight and fighting against each other will contribute to extra wear on the dreaded mainshaft fourth gear sleeve bush??

     Supposedly(according to what I read in a factory BSA manual somewhere) on a plunger, the final drive chain maintains constant tension throughout it's travel. I meant to check that before I put my springs in but got excited and forgot

[iansoady]

And also if both chains are too tight and fighting against each other will contribute to extra wear on the dreaded mainshaft fourth gear sleeve bush??

     Supposedly(according to what I read in a factory BSA manual somewhere) on a plunger, the final drive chain maintains constant tension throughout it's travel. I meant to check that before I put my springs in but got excited and forgot

I think you're thinking of the famous (infamous?) Anstey Link which some Ariels used. Very complex mechanism to try to keep the chain tension constant but had so many pivots that it just resulted in the back wheel flopping all over the place. At least according to contemporary riders - I've never had the pleasure.

[cus]

Thanks,
Must be the slack in the mounts being pulled back, couldn't be anything else I guess.
Muskrat, could you please check how much slack is in your final chain in the middle of the run
at the tightest spot on the centre stand, also, do you measure the slack by pushing the chain down & up, or by
just from resting to up or down, I never really knew which way is correct,

regards, cus

[muskrat]

G'day cus,
              mine (the cafe) will be a bit different than yours in that my rear shocks are 13 3/4" long. But with it all in line I have 1/2", sitting on the wheels I have 1 1/2" and on the center stand I have 2"  . I measure at the pillion peg and from rest push up.
 So you can see that the arc of movement makes a big difference on chain play. Modern bikes have the swing arm pivot as close as possible to the front sprocket. You could put her up on the center stand and remove the shocks, then chock the wheel up till its all level and set the chain at 1/2". Then you have your own bench mark.
Cheers

[shabashow]

Interesting tip about primary vs secondary chain adjustment.
Does this happen with plunger suspension as well?

Neil Ives

I think it won't happen in a plunger as the gearbox is bolted to the engine, fixing the distance between both primary chain sprockets, where as in the swinging arm models (I believe) the distance between the primary sprockets could change if the gearbox were to be moved slightly backwards in the frame. Isn't the primary chain tensioned by moving the gearbox relative to the engine? (I maybe wrong in this deatil, as I'm a plunger man). Perhaps your rear chain is pulling the gearbox back a wee bit.

John

[cus]

Thanks Everyone,
Cus