DIY jackshaft shift kit

Discussion in 'Frame Mounted Engines' started by jaguar, Dec 2, 2010.

  1. jaguar

    jaguar Well-Known Member

    This design directly drives the rear gear cluster of multi-speed bikes. It bypasses the pedal crank altogether so is not to be incorporated on non-reliable bikes unless you modify it to drive a crank freewheel. The cost of parts is $53 (+shipping) w/o the $40 cost of the pull starter accessory (which is needed since you can't pedal drive the bike to start it). If I did this to my bike I would always carry a chain breaker and extra chain that I could connect between the pedal crank and rear gear cluster if the engine ever quit and I was far from home. Total cost is $93 + around $25 shipping + cost of making the jackshaft support. So maybe it isn't a great saving but it's good info that others with specialized modification projects may make use of.

    Homemade jackshaft
    9 tooth sprocket with 3/16" keyway, 1/2" pitch x 1/8" width (standard BMX non-derailer chain width but will work on rear gear cluster) $14

    17 tooth sprocket 5/8" Bore, 3/16" keyway, for 415 chain $18

    Jackshaft Bearing 5/8" ID x 1 3/8" OD $2.49 (buy two)

    Jackshaft 5/8" OD x 4.5" long x 3/16" keyway. $4

    #415 half link $1.39 (or

    3/16" x 3/16" x 1-1/4" Key Stock $.32 (buy two)

    Lock collar 5/8" $1.25 (buy four)

    Bearing hanger, jackshaft weldment. 1-3/8" ID x 3" OD, 1/4" thick. $2.29 (buy two)

    Centrifugal Clutch and Pull Starter Kit $40

    Chain Breaker #35, 40, 41, 42, 43 or 50 $13

    Attached Files:

    Last edited: Dec 3, 2010

  2. skrufryder

    skrufryder Member

    Have you seen the one I made and posted under the picture section?
  3. jaguar

    jaguar Well-Known Member

    No I haven't. Can you post a link to it? Or attach the picture with a reply?
  4. 5-7HEAVEN

    5-7HEAVEN Guest

    what's the gear ratio on the engine"s tranny?:detective:
  5. spad4me

    spad4me Member

    Nice idea .
    Which way does the jackshaft rotate???

    It reminds me of thats dax's setup except he uses the motor mount plate as a jackshaft holder.and better drive ratios.

    As shown the tiny amount of reduction is going to net a speed of over 70 mph at around 6000 rpm. This is not possible.

    With a ten tooth engine sprocket feeding a 48 tooth jack shaft sprocket.
    Then on the other side of the jackshaft a nine tooth sprocket feeding a 34 tooth megarange cassette
    You will achieve a speed of around 30 mph at around 6000 rpm.

    I prefer the simplicity of the peddleable jackshaft like sickbike parts and the scooterguy's stuff.
  6. jaguar

    jaguar Well-Known Member

    Stock to DIY jackshaft:
    Stock sprocket teeth: 10 front to 44 rear = 4.4:1 ratio
    DIY 10 to 17 = 1.7:1 and 9 to 22 (third gear) = 2.44:1
    1.7 x 2.44 = 4.15:1 (close to standard 4.4:1)
    first gear with 28 tooth rear sprocket = 5.29:1
    seventh gear with 14 tooth sprocket = 2.64:1
    If stock gearing is 30 mph top then with this DIY jackshaft the top speed in first gear is 25 mph, and in 7th gear is 51 mph.
    Of course this depends on how many teeth your gear cluster has for 1st and 7th gear.
  7. skrufryder

    skrufryder Member

  8. 5-7HEAVEN

    5-7HEAVEN Guest

    Where are you getting that 10 front to 44 rear combination? Standard 1st gear might be 32 tooth or 34 tooth. An uncommon 1st gear is 38 tooth. Never heard of 44-tooth.

    If your pocketbike tranny is 2.19:1...
    10t to 17t = 1.7:1
    9t to 34t(first gear) is 3.78:1...
    Your starting gear is 2.19 x 1.7 x 3.78 = 14.07:1...not good
    2nd gear is 9t to 26t = 2.89.....
    Your 2nd gear is 2.19 x 1.7 x 2.89 = 10.76:1.....very bad.....
    3rd gear is 9t to 22t is 2.44:1.....
    Your 3rd gear is 2.19 x 1.7 x 2.44 = 9.08:1.....unusable.....
    The rest of the cassette's gearing is totally unusable...

    No disrespect meant, just trying to help. :)
    Last edited by a moderator: Dec 3, 2010
  9. jaguar

    jaguar Well-Known Member

    By "stock" 10 to 44 I mean the standard setup of an HT which has a 10 tooth front sprocket (at engine) and a 44 tooth sprocket that clamps to the rear wheel.

    skrufryder, can you explain about your foot shifter or provide an up-close picture?
  10. skrufryder

    skrufryder Member

    Oh yeah,

    I can get a pic for you later, The toe shifter is off an old specalized globe. Its a single paddle shifter( on lever forward is up shift back towards you is down shift/or visa versa) anyways i welded a peice of handle bar to the frame in front of the foot pegs(mid you there is no pedals) then used a combination brake with the single paddle up/down shifter. ran the cable under the frame and around.... ( I am tearing the bike apart this weekend-will show you the shifte/brakes without motor in frame)If you look at the picture on the deralure side youl notice that the "jack shaft" is at a weird angl..The weld on the mount I made has broken. I didnt do a good job preping the frame for this as this frame is junk and is only for trial and error. the gear ratio is i think the first bevel on the crank is 44 then the cluthc i think is 88 then 10-10 two stock drive sprokets the deralure side is17 drive to 32/28/24?/and so on standard mountain bike. this gear ration finder i have says at the final drive top speed should be 132.somthing mph's. We all know its never gona go that fast, and this ration finder dont take into acount weight of bike/rider,tires ect.....Well half way threw 2nd gear I gestomate to be around 50-55 as I was esialy passing traffic that was aprx.40. until it mucked up as you see in the pic.mind you the motor is heavily modified.
    Last edited: Dec 3, 2010
  11. 5-7HEAVEN

    5-7HEAVEN Guest

    I See...

    Okay, I see it now.....

    10t/17t = 1.7.....
    9t/34t = 3.78:1.....
    Engine built-in gearing = 4.1:1.....
    1.7 x 3.78 x 4.1 = first gear, similar to 64-tooth rear sprocket.....
    2nd gear is 9t/26t = 2.89.....
    1.7 x 2.89 x 4.1 = second gear, like a 49-tooth rear sprocket...
    6th gear might be 9t/14t = 1.56:1.....
    1.7 x 1.56 x 4.1 = 10.87:1 in 6th gear, similar to a 26t rear sprocket.....

    If you can keep the chain from jumping off the 9t sprocket, the ratios will work VERY well with your Happy Time engine.

    In comparison, an SBP kit with 10t/44t = 4.4.....
    First gear is 34t/30t = 1.13.....
    First gear ratio = 1.13 x 4.4 x 1.7 x 4.1 = an 85t rear sprocket
    Second gear is 26t/30t = .87 x 4.4 x 1.7 x 4.1 = a 65t sprocket
    6th gear is 14t/30t = .47:1 = .47 x 4.4 x 1.7 x 4.1 = a 35t sprocket

  12. I need the Retainer that spaces it out..Mine seems to be missing that..