Gear Ratio and Jack Shaft Confusion......

Discussion in 'Frame Mounted Engines' started by BrettMavriK, Oct 25, 2008.

  1. BrettMavriK

    BrettMavriK Member

    K, my brain is starting to smoke.
    I've been trying to calculate numbers for my homemade intermediate jackshaft
    plans, so I can purchase the sprockets. I know gear reduction is needed, but how much for these motors? 4:1? 3:1? 6:1?

    And I know I'm not calculating it correctly. (wish I knew the formula)
    I know the motor has a 10 tooth on it, and I'm using a Sturmey Archer 3 Speed Hub on the 20" Stingray rear wheel.... it's just the in between gears on the jack shaft I'm having trouble with.

    I was thinking of the first sprocket from the motor on the jackshaft would be an 18t. Then from there, I get lost and don't know if I should be going down in reduction or up in multiplication.

    Anyone have a good equation to figure this out???

    Thanks in advance,

  2. HI,

    Pablo and Ghost (of Sick bike parts to the left (sponsor)) make a shift kit for the HT might try to contact them and ask them about the gearing in their kits (I have a kit of theirs but it is at the bike shop being installed so I can't tell you the sprocket sizes you seek).

    Hope this helps you.

  3. BrettMavriK

    BrettMavriK Member


    I have spoken to them a little about it. The problem I'm having is I hope to use only 4 sprockets in my setup from the engine sprocket to the wheel sprocket, and they are using 6 in theirs. Plus, I'm using a 20 inch wheel and not a 26.

    Here are my points where sprockets are:

    Engine - 10T
    Left Jack Shaft- ??T
    Right Jack Shaft- ??T
    Rear Wheel Freewheel 19T (Sturmey Archer 3 Speed)

    I'd like to cruise at about 36-38 miles an hour if possible with motor mods.

    I was thinking of plugging in the values for the right and left side jack shaft as follows:

    Engine - 10T
    Left Jack Shaft- 18T
    Right Jack Shaft- 42T
    Rear Wheel Freewheel 19T (Sturmey Archer 3 Speed)

    Am I way off???

    The other sprocket which the pedals turn is out of the mix and separate from the drive system, although another sprocket with a freewheel will go on the right side of the jack shaft for it.

    I'm definitely missin' something here...
    Last edited: Oct 25, 2008
  4. BrettMavriK

    BrettMavriK Member

    K, the fog is lifting a bit...

    I see the direct drive stock kits are running a 44t or a 36t sprocket
    So it goes to show anywhere from a 3.60-4.40:1 ratio is needed.


    Since I'm using only 4 total sprockets from the engine to the rear wheel.
    How about this:
    1. Engine 10T
    2. Left Jackshaft 18T
    3. Right Jackshaft 9T
    4. Rear Wheel 19T

    This gives a 3.8:1 ratio on a 20 inch rear wheel, but, I also have 3 speeds built into the rear hub. So, anywhere from 3.6-4.4:1 is achieved with gears.......or am I completely way of course???
  5. Hmmm....My brain is somewhat dead tonight BUT what are the ratios of the hub?...Is it something like .75 / 1/ 1.33 or something?....I think the under and over drive were roughly 25% under and or 33% or so over if I recall correctly?.

  6. BrettMavriK

    BrettMavriK Member


    I am running a direct jackshaft to the hub. Front crank is out of the loop and on a separate chain.

  7. Hmmmm again....We must be gluttons for punishment but I'm with ya! ;-O

    This would make a persons brain burn!.....FWIW I know a 27T sprocket on a 20" wheel is similar in performance to a 35 T sprocket on a 26" wheel....and I know that several folks in FL are using 27T sprockets for high speed cruising on the flat lands on a 26" wheel....this would be like running a 21 T on a 20" wheel.

    Of course all this assumes direct gearing....the jackshaft and internal gears compounds problems that have to be thought out....

    OK...looking at it more I believe you want the equivalent of a 32T sprocket on your rear wheel..that way OD makes it act like a 21T sprocket on a 20" wheel which is like a 27T on a 26" wheel which I know is doable on the flats...

    Since you have a 19T cog on the hub we need to manipulate the jackshaft gearing so that it makes the 19T on the hub "act" like a 32T sprocket...thus the jack shaft ratios must ultimately reduce the gearing by XX % roughly...

    engine driven 10T to a 13T on one end of the jackshaft would reduce gearing by XX %

    Soooo I am thinking that one end of the jack shaft needs to be 13T (the engine drives this 13T), and the other end of the jackshaft needs to be a XXX make the rear an "effective" 32 T Rear sprocket.

    OK....Engine 10T drives 13T at one end of the jackshaft, yielding a % reduction in gearing, other end of the jackshaft XX T drives the 19T on the rear hub yielding the other reduction needed.

    I think that does it....what do you think? We jyst have to figure out the XX parts... BUT at least we know where we need to get to...The effective 32T....

    Last edited by a moderator: Oct 28, 2008
  8. OK....ate some dinner so back to it.....

    I think gearing needs to be reduced by 68.4% (that is to make the 19T sprocket you have on the hub "act" like a rear 32T sprocket).

    Since you have an internal hub with an Overdrive of 1.33 that would make a 32T act like a 21T which is roughly the same as a 27T on a 26" wheel

    The direct gear would make a 32T act like a 42T sprocket on a 26" wheel (close to "normal" stock).

    First gear being 25% under drive, the 32T would act like a 52T on a 26" wheel....definitely pretty low gearing!

    So the only figuring left to do is determine what the 2 sprockets on the jack shaft should be.....KNOWING we need a 68.4% reduction (to make the 19T THINK it is a 32T rear sprocket).....

    We know the engine drives with a 10T sprocket and we want the ratio to NOT be even to reduce wear on components....we can arbitrarily choose 13T....this is about 23% reduction here....thus we need 45% more reduction....(I hope my math is right)

    So, we know the rear driven sprocket is 19T.....thus the question becomes : What sprocket driving this 19T sprocket yields a 45% reduction in gear ratio?....The answer being 8T ( 8 divided by 19....actually yields 42%) or 9T ( 9 divided by 19...yields 47%)...

    Does this seem reasonable?....My percentage math is only so so BUT I know you would like to make the rear hub THINK it has a 32T sprocket on it....(then all the gears would be usable).

    Hope this helps you.

    Last edited by a moderator: Oct 28, 2008
  9. Hi Brett,

    I think your last set of sprockets would yield too much gear reduction....I figure it at 102% overall reduction VS. what I calculated as needed being about 68.4% reduction....

    My math is subject to errors tho....

    There is one way to find out tho....Are you going to build it?


    PS - I vote "YES" ;-)
  10. BrettMavriK

    BrettMavriK Member

    Here's where I'm at so far.....

    Attached Files:

    Last edited: Oct 29, 2008
  11. Nice....Just curious tho BUT shouldn't the shock go up top?....Springs generally like to compress rather thank be stretched but I suppose if the spring is attached ( the ends of the spring held firmly or welded to) the shock body it could act in tension rather than looks unconventional but it may work...

    Hmmmm buy hey aren't motorized bikes for the most part unconventional too? I'm glad you are a tinkerer too!

  12. BrettMavriK

    BrettMavriK Member

    Yes, they normally should...
    This is going to be a reverse cantilever set up. Meaning the main frame is bolted to the rearward bolt hole of the shock, and the rear tire cage is bolted to the forward part of the shock. Kinda hard to explain. It's an old motorcycle chopper trick. Picture 2 chain links enclosed and linked to each other. One is 90 degrees rotated to the other. The links can move freely, but cannot separate. Now put a shock between the area that the links both share. The brackets I'm building will be decorative, and be an artistic focal point of the bike.
    I thought Flames would be the way to go.... =-]'


    Attached Files:

    Last edited: Nov 2, 2008
  13. Gear ratios, and other boring tech stuff

    Hello Brett, well I got to tell you Amigo, without giving away any trade secrets here are the ratios of the lil Chinese 2 strokes.

    There is a 4.1 to 1 ratio in the clutch area of the engine. NOW take 10 to 44 and get 4.4 to 1. NOW take 4.1 TIMES 4.4 and get?

    My calculator tells me 18.04, with admittedly a 26" wheel and tire, netting 6997 rpm, at 30 mph.

    Your speed goals are unrealistic, without shifting, so it would seem.

    Remember, Shifting is Illegal in ALMOST every state in Moped, and Motorized bicycle classes.

  14. hill climber

    hill climber Member

    my setup, ht motor 10 tooth to a 21 tooth on jack shaft, other side is a 10 tooth on jack shaft to a 16 tooth on the wheel(20" wheel). top speed of 25mph and climb about any hill.
  15. Multiple ratios

    HI Hillclimber, you have 4.1x2.1x1.6=10.08 BUT you have 20 inch wheel, very different and so much smaller than a 26, try the rolling distance of 1 rev and see.

  16. BrettMavriK

    BrettMavriK Member

    Sorry, man, but you didn't read the beginning of this post. You are calculating gears that I'm not even using. I am setting this bike up with a jackshaft that will serve as an intermediary between the cranks and rear wheel. I am also using a 3 speed internal hub. The gears inside the HT motor are 20/82, or 4.10:1; the countershaft off the engine is a 10t, and My rear hub is a 20t. There will be 3 gears on this jackshaft. One on the left connected to the 10t on the motor, one on the right connected to the rear hub, and another on the right with a freewheel going to the front crank. I have to not only find the right rpm range for my engine, but also for my legs, as both inputs share the last two gears to the rear wheel.
    I think I have it ironed out on the 3 gears I need to have on the jackshaft:
    a 24t, to a 22t, and then a 16t for the front crank.

    This yields 33.3mph @ 5000rpm in 2nd gear (1:1) with 3rd gear to push into the high 30's, and lets me cruise at two cranks per second at 11mph in 2nd gear with again, 3rd as an overdrive.

    Now the only thing that concerns me is if my rpm's are too low at that speed of 33.3mph and the engine won't have the torque to do it. If not, I'll put on a 28t on the left side of the jackshaft.

  17. hill climber

    hill climber Member

    brettmavrik, on your jack shaft, pedal chain side, free wheel to crank chainring and solid sprocket to back wheel should be the same size, number of teeth, that way you keep your current pedaling gear ratio. i have a 14tooth freewheel to the cranks,thats a 44 tooth, and a 10 tooth solid to the rear tire, witch has 16tooth freewheel, i can only pedal to 20mph max, my legs cant go no faster, but who wants to pedal anyway.
  18. BrettMavriK

    BrettMavriK Member

    My rear tire has 5 speeds. I've nailed down the gears on all points that serve the motor as well as the pedals. I've got the perfect powerband gear for every rpm of the motor and every ppm (pedals per minute)


  19. LocustsOfSteel

    LocustsOfSteel New Member

    Wow, i cant wait to see how this turns out.
  20. BrettMavriK

    BrettMavriK Member