Sick Bike Parts New 21T Jackshaft Sprocket

Discussion in 'Transmission / Drivetrain' started by FurryOnTheInside, Mar 15, 2014.

  1. FurryOnTheInside

    FurryOnTheInside Active Member

    I thought it'd be good to have a thread discussing the effects of changing the stock S.B.P. Shift Kit 17T sprocket for the new 21T sprocket that Sick Bike Parts just released. Perhaps we can collectively come up with suggestions or answers to the question of what sprockets/chainrings are needed to make best use of the new 21T.
    Or just enthuse about it nerdily. :goofy:

    I am very excited about it and can hardly contain myself. :grin5: It seems it will (well it IS intended to) solve the issue of matching optimum or at least reasonable engine RPMs with optimum or at least reasonable pedalling RPMs, for those riders who like to pedal while being motor-assisted by their engine, both staying happy and working together as one!
    It will (and is intended to) also slow the RPMs of the jackshaft itself, which will (if the RHS jackshaft sprocket remains unchanged) slow the speed of the RHS/secondary/intermediate chain (to approx 80% of what it was with the 17T); that should help enormously to stabilise it and make the RHS chain tensioner's job easier (less chance of chain coming off the tensioner(idler) wheels), and extend the life of the tensioner(idler) wheels.
    All in all, I think the 21T sprocket from Sick Bike Parts is a GIANT leap forward.

    Now, correct me if I'm wrong....

    An increase in tooth count from 17T to 21T is 123.5%

    If the outer/input chainring at the pedal cranks is left the same, it means the final/combined drive sprocket at the cranks would need to be enlarged also, by 123.5%, assuming you want to keep the road speed unchanged at the same engine RPMs.
    If you were previously using a 36T for the final/combined drive chainring, then a 44T would give (near enough) the same road speed, because 36 x 123.5% = 44.46
    If you were previously using a 44T for the final/combined drive chainring, then a 54T would give (near enough) the same road speed, because 44 x 123.5% = 54.34

    If the final/combined drive chainring is left the same, it means the outer/input chainring would need to be reduced in size/tooth count, assuming you want to keep the road speed unchanged at the same engine RPMs.
    If you were previously using a 48T for the outer/input chainring, then a 39T would give (near enough) the same road speed, because (48 / 123.5) x 100 = 38.86 or 48 x 80.97% = 38.86
    If you were previously using a 44T for the outer/input chainring, then a 36T would give (near enough) the same road speed, because (44 / 123.5) x 100 = 35.62 or 44 x 80.97% = 35.62

    I'd like to get a 54T final/combined drive chainring personally. Only road bike frames will accept such a large chainring in the normal position (distance from bike's centre line) but that is fine for me. 54T chainrings are included on road bike triple chainsets, sometimes on doubles if intended to run in the outer position. To use a road bike chainset with a S.B.P. Shift Kit requires the optional S.B.P. spider, probably the 5-arm 130mm BCD version. I'll have to find out what issues there are with combining the road bike chainset with the S.B.P. spider and whichever outer/input chainring I end up using. I think it won't be a problem, but I know I'm going to have a lot of fun though. :grin5:

    Which chainrings/sprockets will you use when you make the leap from 17T to 21T? :D

  2. Fabian

    Fabian Well-Known Member

    I thought it was a 23.5% gear reduction.

    Now someone correct me if i am completely wrong, but i crunched the numbers (with the left hand side 21T sprocket) and to remain with the same final drive speed for the same engine rpms in top gear (using an 11 tooth sprocket) on the rear cassette, you will need a 48 tooth chainwheel driving the rear cassette, if you are currently using a secondary drive chain jackshaft gearing combination of 9T - 48T, for maximum low speed pulling power or crawling ability.

    The problem with using a 48T is that a conventional front derailleur won't span the distance to grab the chain from the 24T sprocket and drag it onto the 48T sprocket.
    A 40T sprocket is about as far as you can go (in a dual range derailleur operated mechanism) if you are using the 24T as a low range gearing option.

    Naturally i want the lowest gearing i can get, so i want to keep the 9T - 48T jackshaft gearing, and i want to use the new SickBikeParts left hand side 21T sprocket, but, the problem then exists in how to get back my high range gearing so i can travel at the same speed with the same rpms that i am used to, which is currently a 38T sprocket as my high range gearing.

    Another problem exists in the fact that you will need a lot more chain links than 114 if using a final drive chain that works with a 48 tooth high range sprocket and a 36 tooth rear cassette sprocket.
    It becomes a serious problem for the rear derailleur to take up all of the extra chain links when the front derailleur moves the chain onto the 24 tooth sprocket, making the difference 24 teeth, and effectively 24 chain links that need to be kept under derailleur tension.

    Another option exists, but it would need to be a custom made enhancement to the Shift Kit which would work in perfectly with the right hand side chain tensioner: a quick change sprocket mechanism on the right hand side of the jackshaft, where you can run the 9 tooth sprocket for hill climbing/towing or slower speed pedal assist, and, then the ability to pull a circlip or "R" clip and slip out the 9 tooth sprocket for an 11 tooth right hand side jackshaft sprocket.
    Such a mechanism would allow you to have good road speed on flat ground (with the 11T right hand side jackshaft sprocket), whilst enabling good pedal assist (with a no-fuss 9T right hand side sprocket change) when the going gets ludicrously steep (requiring you to constantly stop/start; balancing the bike on the pedals whilst rock-hopping over obstacles) or just simply requires massive pulling power/hill climbing ability.

    The extra two links between the 9T and 11T sprockets would easily be accommodated by the right hand side chain tensioner, and, it also allows you to keep the 36T as a final drive sprocket; thereby eliminating a 40t sprocket requirement.

    It should be easily achievable if splining the right hand side of the jackshaft to accommodate a motorcycle style splined sprocket, in 9T and 11T options.
    Incidentally, the gearing difference between 9T and 11T is 22.2% which effectively gets back the 23.5% difference between the 17T and 21T.

    To increase durability of the jackshaft spline, it could undergo a nitride surface treatment.



    Now if i could only hire The A-Team to work with the boys in the SickBikeParts skunkworks, using motivation that only the A-Team can provide !!!

    Cue the music, because we've got a problem that needs a serious solution, even if gratuitous use of incendiary devices are part of the problem solving process.

    Last edited by a moderator: Dec 15, 2015
  3. FurryOnTheInside

    FurryOnTheInside Active Member

    I'm so unsure of my maths, lol. That's really why I'm interested in discussing! I may well be right but I'm just not sure.
    When I said "An increase in tooth count from 17T to 21T is 123.5%" I meant a 21T has 123.5% of the tooth count of a 17T. AFAIK that's 80.9% of the jackshaft RPM that you'd get with the 17T for the same engine RPMs.

    Yes, you need a 48T if you're currently using a 39T for the final/combined drive high range chainring, for the same road speeds, if you'd like to keep your outer/input chainring the same.
    I suppose there's nothing wrong with changing BOTH the outer/input chainring AND the final/combined drive chainring.. if other factors allow. i.e. chain takeup by the rear derailleur and clearance (you probably can't run identical tooth-count input AND output because the FD would probably rub against the RHS/intermediate chain).

    So, yes there's a limit to how much of a spread in your front chainrings you can have, as AFAIK the front derailleur (FD) has the ability to span a 24 tooth spread at the most, on a triple range, if gears are changed carefully under pedal power (they are listed as 22 tooth spread). Normally on bicycles there is a maximum step of only 14 teeth between adjacent front chainrings, though I have read of a 18 tooth step being used this would possibly require getting off the bike and pedalling the chain around by hand to shift between ranges. The 24 tooth total spread (difference) between ranges would still apply.

    Also as you said there's a limit on total gear spread.
    IF the rear derailleur (RD) is the only component that takes up the slack in your chain then it is limited to the max (stated on the cage or on the sales blurb) of the long cage rear MTB derailleur (RD). From MTBR Forum
    "Manufacturer stated derailleur capacities are as follows:
    Shimano long = 45T; medium = 33T
    SRAM long = 43T; medium = 37T; short = 30T
    A difference of 24 teeth, as in the case of using a 24T low range chainring with a 48T high range chainring would be inside the limitations of a long cage derailleur as long as you don't intend to use excessively small-small combinations. Chain length must be sufficient to allow the big-big combination for safety (i.e. accidental shifts into big-big should not snap the chain or bend your frame or sprockets); but chain tension does not need to be kept in the small-small combination as it is not really a risk, and you won't ever want to use anything smaller than 2nd or 3rd gear on the rear cassette when you're in the low range on the front.

    Perhaps it could be possible to add extra chain slack takeup somewhere in the chain stabilisation system?

    But I'm not sure you will have a great deal of traction using the 24T front chainring when in the 36T rear cassette sprocket. With the reduction of the new 21T jackshaft sprocket you'll be putting 23.5% more torque through the rear wheel AND being able to pedal in harmony with your engine you'll be producing even more torque! It might be that a 30T low range chainring is adequate for your needs. If the large spread of gear ranges you're after is not possible at all, then I guess you'd have only one option which would be disassembly- swapping out your front chainrings and running the 24T front low range ring only on those days when you know you'll require ultra-low gearing.

    Although I agree that changing the 9T RHS JS sprocket for an 11T on days you want high road speed is an option, and a quick change system would make that easier... Using an 11T on the RHS would cancel the beneficial effects of the 21T LHS, making your pedalling and engine RPMs out of harmony again, and bringing RHS chain speed (actual links-per-second speed) back up to the rate it used to be. I really don't like this idea for those reasons.
    Last edited: Mar 17, 2014
  4. Fabian

    Fabian Well-Known Member

    You make valid points.

    I just would like a little less engine rpm for the same road speed as my gearing currently gives me, being a 9T -48T on the JS and a 38T driving an 11T cassette sprocket.
    Ideally i would like a 40T driving the 11T cassette sprocket, and ideally i would want to retain that engine rpm when in top gear and when using the 21T left hand side JS sprocket.

    To make that happen, i would really need to swap out the 48T for a 44T and switch the 9T JS sprocket for an 11T JS sprocket when running the 21T left hand side JS sprocket.


    i would need to switch to a 9T right hand side JS sprocket when doing serious hill climbing, even if i am running on the 24T front crank sprocket.

    How i would love to have a dual speed jackshaft system, using a Schlumpf Innovations Gearing System incorporated in to the jackshaft itself.
  5. FurryOnTheInside

    FurryOnTheInside Active Member

    Glad I make some sense to someone. :jester:

    I don't wanna sound like I'm telling you how YOU should build YOUR bike; but I would concentrate on the front final/combined output personally. Any other alterations further back up the drivetrain will mess up the new-found harmony of engine and pedalling RPMs that you will gain with 21T and 9T JS sprockets going to a 48T outer/input chainring, and would never be easy to alter during a ride. Okay except the Schlumpf, but that's a planetary gear therefore questionable in terms of reliability (or at least self-reliance during expedition biking) and it's not available incorporated into a jackshaft at this time.
    You had a triple range gear system that was close enough to perfect that you used it for quite a while. Why not have another go at it? :) You said there was not quite enough room for the standard indexing to work with the three chainrings in such a small space.. What length bottom bracket axle did/do you have? How much wider would it need to be for there to be room for three chainrings at the standard spacing?
  6. Fabian

    Fabian Well-Known Member

    The triple chainring system didn't increase the ratio spread, it just increased the number of total gear ratios within the maximum and minimum available ratio, which didn't turn out to be of any benefit.
    A 2 speed front chainring system was more effective in practice.

    I agree with you on the unknown strength and durability of the Schlumpf two speed planetary drive, but it would be a great option that i would pay serious money for if it were to be offered as an accessory option by SickBikeParts.
  7. FurryOnTheInside

    FurryOnTheInside Active Member

    Ah.. So you actually want a variable engine : pedal ratio. And that's simply because you want to keep lots of tension on your chain even when using the low range gears and you found that a larger than 14 tooth spread on the front gear ranges produced too little chain tension for the very uneven surfaces you ride on (even though you only need to use the largest two or three cassette sprockets), which would cause excessive chain slap. Now I get it! ... I'm still "sure" there is a better solution to that problem though.
    Yes, you had a 24T-30T-38T and now you have a 24T-38T, eliminating a redundant middle range. I think there's a way to extend the range beyond 14 teeth while still keeping sufficient chain tension.

    Just the fact that it's not a technology currently available or even in the pipeline rules the Schlumpf out for me. I would consider a planetary gear in the future in a position in the crankset or in the rear hub or even in between (but not above), if it was reliable and inexpensive enough, if it ever comes into existence and if I can afford it at the time. At the moment all we have to work with is chains and sprockets and tensioners, which are cheap, easy to get, easy to fix, and easy to play around with and experiment with (fun!).
    Last edited: Mar 17, 2014
  8. Fabian

    Fabian Well-Known Member

    Yes, i "want" it all.

    I want a good engine/pedal ratio and i want good road speed at lower engine rpms than what i currently have now, and i want even more hill climbing ability/rock hopping ability and even greater pulling power to haul firewood out of the forest; along narrow bush walking tracks.
  9. FurryOnTheInside

    FurryOnTheInside Active Member

    Do you "want" the variable engine : pedal ratio only as a fix for the chain tension issue, or is there more to this?

    Assuming a gear reduction of 4:1 before the drive sprocket (which I'm not sure is correct for all engines as most posts quote 4.1:1)
    A 10T : 17T ~ 9T : 48T combination gives an engine : pedal ratio of 36.26:1, or 2175:60 , 2901:80 , 3626:100 , 4351:120
    The 10T : 21T ~ 9T : 48T combination gives (IMO a great) ratio of 44.8:1 , which is 1792:40 , 2688:60 , 3584:80 , 4480:100
    Or a 10T : 21T ~ 9T : 44T combination gives a good ratio of 41.07:1 , which is 1642:40 , 2464:60 , 3286:80 , 4107:100 ideal if you want to keep very low engine RPMs and you like to spin your pedals fast, which some cyclists do.

    The equivalent of using a 10T : 17T ~ 9T : 48T ~ 40T : 11T but with the optional 21T would be 10T : 21T ~ 9T : 48T ~ 49or50T : 11T

    The 21T gives you greater pulling power if you keep the other sprockets including the front low range chainring the same size as they are now.
    Is rock hopping, (and insufficient chain tension when using the low range on seriously rough terrain) the problem that is stopping you from increasing the size of the high range chainring? Chain tension is currently maintained only by the RD cage and the spring which rotates/tensions it rearward.
    Either strengthening the spring (if that's actually possible, there isn't much room) or adding another tensioning device somwhere along the return side of the chain would possibly be a way to increase chain tension in the low range and permit the use of a larger spread of gear ranges, but if it is constantly (not adjustably) under spring tension that would add parasitic drag when using the high range, which is a drawback.. so perhaps there could be a cable actuated extra tension device which you only engage when you shift into your low "log haulling" range?

    I hope the lurkers are enjoying this, lol. Lots of views but only Fabian and I are posting our ideas.. I suppose I'll see the cable actuated extra tension device on sale soon then, eh? :thinking: I've just done a very quick sketch of a device, but I don't feel like posting it if this thread is just a two-way convo in public, I might as well PM it to Fabian and say "screw all you lurkers!" lol. :rolleyes7:
  10. Fabian

    Fabian Well-Known Member

    Now that's a good reply post.

    I don't have any more problems or issues with the return side of the chain on the final drive, as it works in it's current form.

    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.
  11. FurryOnTheInside

    FurryOnTheInside Active Member

    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.
  12. Fabian

    Fabian Well-Known Member

    I might need to update to a new style front derailleur, that's able to handle a larger chainring diameter.
  13. FurryOnTheInside

    FurryOnTheInside Active Member

    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..
  14. FurryOnTheInside

    FurryOnTheInside Active Member

    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?
  15. Fabian

    Fabian Well-Known Member

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

    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.
  16. FurryOnTheInside

    FurryOnTheInside Active Member

    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. :rolleyes7: 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. :)
  17. Fabian

    Fabian Well-Known Member

    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: Dec 15, 2015
  18. FurryOnTheInside

    FurryOnTheInside Active Member

    :iagree: 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! :clap:

    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: Dec 15, 2015
  19. Fabian

    Fabian Well-Known Member

    It's got nothing to do with "the quantity of content" in a thread", but everything to do with "the quality of content" in a thread.
  20. Pablo

    Pablo Motored Bikes Sponsor

    You guys are just deeper than your average bear.