Brompton chain tensioner design

[Brucey]

the tensioner can either have long arms and small pulleys or large pulleys and shorter arms. The extant brompton tensioner is the first sort but actually the second sort is arguably better because it usually has less drag. Packaging-wise you would expect one form to be a clear winner but it appears not to be the case; the overall length is about the same for both types.

If mounting a tensioner on the chainstay of a brompton, you only really need to worry about the fixed pulley; the other one always gets itself out of the way during the fold. This being the case, I suspect the optimum solution will use larger pulleys (~12-14T) and a shorter arm, moving through about 135 degrees. This could leave the hub sprocket as the lowest point on the bottom chain run of a standard brompton, but if the wheel size is increased slightly this may no longer be the case.

If using a chainstay-mounted tensioner, it is less easy to see how it could be used for a 2s derailleur but I don't think you need to. I think you can use something like a FD instead, which (because it is working on the slack run of the chain) is expected to work better than normal. However, if using a derailleur of any kind, it is expected to be advantageous to have a floating fixed pulley, which should not be a big deal for a talented designer.

[Cyclothesist]

Thanks Brucey. Neat idea - bigger pulley to wrap more chain when folded. I knew we had missed something!

[2_i]

On a full-size bike, I have a derailleur with 3 pulleys that gather far more slack than a large tensioner pulley. However, due to the tensioner on Brompton's independence from the derailleur and the pulley lateral adjustment, this would need to be transcribed onto two independent tensioner arms on a Brompton.

Going back to a larger pulley, I think it might cause lock-up problems for the wheel moving laterally due to a larger torque tilting the pulley laterally. I think a pin catching the chain during folding is a better solution than either a larger or extra pulley for gathering the slack while folding, allowing for a shorter tensioner arm. Incidentally, I need to gather far more slack on my Brompton than on any standard one, so the longer Advance Tensioner arm turns out to be a blessing for me.

[SA_SA_SA]

.... I think a pin catching the chain during folding is a better solution than either a larger or extra pulley ...

I always disliked the convoluted constrained chain path over the inner jockey/pulley where it is sandwiched between my 15T sprocket and said inner jockey....and wondered if that inner jockey wheel could be replaced by a correctly positioned* pin, so that when riding said pin does not contact the chain so only one jockey idle loss but, on folding, the pin does same job as current inner jockey?

*but where?

[2_i]

I always disliked the convoluted constrained chain path over the inner jockey/pulley where it is sandwiched between my 15T sprocket and said inner jockey....and wondered if that inner jockey wheel could be replaced by a correctly positioned* pin, so that when riding said pin does not contact the chain so only one jockey idle loss but, on folding, the pin does same job as current inner jockey?

*but where?

I am unsure where you ultimately want to go, but below is where I put the pin to remove the slack due to my triple. It hangs off a clamp around the seatpost. Different options are available here and elsewhere, but this worked for me.

[SA_SA_SA]

(replying to SA_SA_SA)
I am unsure where you ultimately want to go, but below is where I put the pin to remove the slack due to my triple. It hangs off a clamp around the seatpost. Different options are available here and elsewhere, but this worked for me.

Interesting but
I presumed the pin would still be behind the lower jockey (ie still part of the tensioner) but moved back and/or down so that chain misses it in unfolded use.

[2_i]

Interesting but
I presumed the pin would still be behind the lower jockey (ie still part of the tensioner) but moved back and/or down so that chain misses it in unfolded use.

I have a pin there, too, but this is to prevent chain drops off the tensioner arm, especially when the arm dives into snow that fills the pulley.