Sick Bike Parts New 21T Jackshaft Sprocket

Oh, thanks. :D

According to bicycle forums, FDs can handle up to a 24 tooth difference if well set up and gear ranges are changed very carefully. Shimano info states 22, but that's a conservative figure. So if keeping within that 24 tooth difference, the smallest low range chainring you could pair up with a 50T high range would be 26T; but that would still give slightly more pulling power if used with the 21T, than you currently get with the 17T and a 24T low range chainring.

I haven't seen any technology currently available that tackles this chain slack take-up problem specifically. Plenty on managing/stabilising long chains but not specifically maintaining a high degree of tension with large changes in chain length.
 
I didn't know there was a difference between old and new FDs, thought it was just MTB or Road, with the latter being suited to larger chainrings. Unless you mean really old ones, that just had completely flat plate sides. I suppose those could be more limited in range. I will have to have a look at what styles of FD are available..
 
Yes, there would need to be another tensioning device in the chain return side if using the 21T left hand side sprocket and using a 48T or 50T final drive sprocket powering a 36T rear cassette sprocket, as well as being able to operate the front 24T chainring with a front derailleur.
I have thought about this problem, but as always, the problem has already been solved, and in this case, by the designers of recumbent bicycles, which use extraordinary chain lengths in their drive system.

I've spent a while looking for usable tensioners tonight but not found anything that looks right. :/ In fact the only recumbent specific (because you mentioned that), adjustable, non-sprung tensioner I've found is the boom adjustment tensioner.. I was thinking along different lines for an adjustable tensioner, but is that the type of thing you meant?
 
I didn't know there was a difference between old and new FDs, thought it was just MTB or Road, with the latter being suited to larger chainrings.

I am also aware that there are only two types, being MTB and Road


Unless you mean really old ones, that just had completely flat plate sides. I suppose those could be more limited in range

That's exactly what i have on my bike because i need the front derailleur to almost rub up against the 48 tooth chainwheell; in fact when i had the tetra chainwheel setup, i had to modify an old style front derailleur to work within the narrow confines of the triple final drive and the inner face of the 48 tooth chainwheel.

After some research, new front MTB derailleurs are available with completely "flat" sides, giving more FD options that can work with the SickBikeParts shift kit.
 
I see what you mean. Using a 50T final drive high range chainring would solve that clearance issue though. ;)

That's very interesting. I must admit to not being totally up to date on what's available since I moved to a small town with one little bike shop that mostly sells kiddy's pushchairs. :rolleyes: I will have to have a good look at/chat about what's available when I'm next at the bike shop in the city. :)
 
Having a final drive chainwheel diameter greater than that of the jackshaft chainwheel diameter would solve three problems in one go when using the 21 tooth left hand side jackshaft sprocket: it would get back the lost high speed gearing as well as having much slower pedal rpm, and also allowing the use of "any" front derailleur system, not to mention giving the added benefit of the extra low range gearing afforded by the 21 tooth sprocket.

The best of "all" worlds.


Although you can't properly see the mechanical action, it becomes clear why you need the front derailleur to have a flat outer face when using a final drive chainwheel diameter smaller than that of the jackshaft chainwheel diameter.


 
Last edited by a moderator:
Having a final drive chainwheel diameter greater than that of the jackshaft chainwheel diameter would solve three problems in one go when using the 21 tooth left hand side jackshaft sprocket: it would get back the lost high speed gearing as well as having much slower pedal rpm, and also allowing the use of "any" front derailleur system, not to mention giving the added benefit of the extra low range gearing afforded by the 21 tooth sprocket.

The best of "all" worlds.


Although you can't properly see the mechanical action, it becomes clear why you need the front derailleur to have a flat outer face when using a final drive chainwheel diameter smaller than that of the jackshaft chainwheel diameter.




I agree. That's just what I was getting at in my first post when I said the new 21T is a giant leap forward! Except I forgot to even mention the ability to use a larger diameter final drive chainwheel than that of the outer chainwheel!
It's a win, win, win, win situation! (y)

And I'm perplexed as to why we're the only two people posting on this thread!! :confused:


What I can't really see in your video is that to upshift into the high range requires a very slight overshift before releasing the lever and the FD settling back to the running position, but I am familiar enough with it. FDs are such a simple way of derailling and I'm sure they all work exactly the same way.

I've decided what FD I'm going to get now (Shimano R443), and it turns out it's one of the cheapest ones.. but it doesn't really matter when using the new S.B.P. 21T jackshaft sprocket, as I'll have a considerably larger diameter final drive chainring than that of the outer chainring. :) I just have to find a good used set of chainrings in my perfect sizes now, so I don't have to buy a crankset (with the crank arms). I should be able to pick some up in the city though I might have to wait if there's none around right when I'm ready to buy them. It's no problem though- I can use any chainset built for the 8/9 speed chain and front derailleur and just go slower til I find a true road racing triple (or double) chainset. :)
 
Last edited by a moderator:
Back
Top