SBP Shift Kit

Discussion in 'Hubs, Gearboxes, Sprockets and Chains' started by Rusty_S85, May 14, 2017.

  1. Rusty_S85

    Rusty_S85 Member

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    Today I was on youtube and came across a few videos of some Grubee motorized bike frames with shift kits on them and after watching a few of them I got hooked on them.

    Thing is I am doing reading right now but just trying to figure out how I could do this without having a bicycle style derailleur.

    I probably don't need a shift kit but I really like the freedom it gives you to have a few different tooth gears to choose from for road conditions.

    Ive read through the SBP installation instructions a few times now how ever and I am just left with how can I do something like this to make it look quality and not like someone just slapped on a mountain bike derailleur for the rear.

    I also am now curious about the crank free wheel sprocket, I read on here someone not wanting it cause he doesn't like the thought of a failiure where the freewheeling clutch locks up and breaks a leg.



    I am looking at the Grubee GT2A frame and will be running a 66cc 2 stroke engine. I will be upgrading eventually to a port and polished cylinder/piston kit as well as the billet diamond high compression cylinder head along with the tuned exhaust.
     
  2. scratchbuilder

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    Nix the derailer..run a 3speed nexus hub.
     
  3. Rusty_S85

    Rusty_S85 Member

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    Thanks for the reply. Ive read about some internal shift hubs but been reading old topics where people were talking about they were not able to handle power and some could.

    I will do some more searching on the 3 speed nexus hub. I saw one that was listed as automatic.
     
  4. 5-7HEAVEN

    5-7HEAVEN Well-Known Member

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    Which sprockets would you recommend with the 3-speed Nexus hub? What would be your optimum gear ratio in each gear?
     
  5. scratchbuilder

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    I did my own Js with SBP wide bb,freewheel& adapter, with 11t to 36 over to 11t 60#35 chain.inside the 60t I ran a 24t #410 back to an 18 3sp. Ratios?? It was a just right setup. The advantage with a hand clutch engine, is just that! I ran mt centrifugal, which meant, rolling off throttle, shift, roll back on. No shifting under acceleration! Ran derailer before geared hub both worked fine.
     
  6. Rusty_S85

    Rusty_S85 Member

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    On the kits sold by sbp, what are the teeth on the gears supplied?

    On their website i cant really find any specs on it.

    Right now im contemplating going with a 7 speed shimano hub but theres a bunch of different tooth combos for the rear hub.
     
  7. DRK

    DRK Member

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    .....17t & 10t.....
     
  8. Rusty_S85

    Rusty_S85 Member

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    Thanks for the reply. Im trying to figure out what gearing would work for my goals.

    Im looking at either the Shimano Hg37 7 speed 13 to 28 tooth or the Shimano touring Mf-TZ31 7 speed 14 to 34 tooth.

    If my under standing is right the 44 tooth crank sprocket going back to a 34 tooth would be lower than 1:1 since its a larger gear turning a smaller gear. Should give me a better top end with lower rpm.
     
    #8 Rusty_S85, May 15, 2017
    Last edited: May 15, 2017
  9. 5-7HEAVEN

    5-7HEAVEN Well-Known Member

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    What I try to do is formulate a usable final drive close to15:1ratio. That's like a 36-tooth rear wheel sprocket on a China Girl. If I had an HT engine, I might shoot for 13:1 gears.

    I like using 8-speed/11-34cassettes and 24t chainring drive sprockets. Working backwards, 15:1/(11÷24=32.72:1.....
    That's your final drive with 11t.

    Now, to get to 32.73:1 is another story.
     
  10. Rusty_S85

    Rusty_S85 Member

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    This is where im getting thrown for a loop is the kit im looking at uses a 10t to 17t from the china girl engine to the jackshaft. Then gear on the right side of the jack shaft is a 10t gear down to a 36t on the pedal assembly. Then from the pedal back is a 44t gear.

    These gear changing back and forth i guess is confusing me. From what i am seeing the jack shaft spins slower than the engine due to the slightly larger gear. But the other side has a gear the same as on the engine so in my view it is now spinning back like if it was connected straight to the engine. Then that is spinning a 36t sprocket that turns a 44t to the rear. That is where i am stumped.

    I will be using a china girl 2 stroke on a gt2a grubee frame. I will also be getting a ported cylinder and piston kit along with a high compression head and a cheetha cdi. Im also looking at upgraded carb and exhaust.

    My main goal is to have the bike capable of 40 mph as i want to have a useable speed of 25 to 35 mph. I probably dont need a shift kit but the videos i seen i do like the lower rpm the engines turn at speed.
     
  11. scratchbuilder

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    Contact 'kc's cruisers (KC Vale) he has done quite a few 'kits' with shifters.
     
  12. 5-7HEAVEN

    5-7HEAVEN Well-Known Member

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    Rusty, it's junior high school math.

    Determine which sprocket drives the chain. That's the denominator. Driven sprockets are numerators. Work out each equation separately, then multiply them to each other.

    The only odd ball where the driving sprocket is larger is the relationship between chain ring sprockets and rear wheel sprockets or cassettes.(Like a 24t chain ring and an 11-tooth cassette gear).

    To solve your gear ratio, divide jackshaft sprocket by engine sprocket .
    17/10=1.7:1.....
    Divide 36t chainring by 10t jackshaft:
    36/10=3.6:1.....
    There's a built-in engine gear reduction of4.1:1.
    Multiply 4.1 x 1.7. X3.6. =25.092:1
    If you have an 11-34t rear cassette and a driving 24t chainring sprocket,
    Multiply 25.092 by (11/24) = 25.092 x .45833333 = 11.50....
    Divide that number by 4.1 to determine perspective rear wheel sprocket of 2.805:1, like a 28t in 8th gear.
    Using the same formulae, you'll find that 7th gear is likened to a 33t sprocket. First gear is like a stump-pulling 87-tooth rear sprocket.

    If you want 8th and 7th gear ratios to be like 31t and 37t rear wheel sprockets, change the10t JACKSHAFT sprocket to a 9-tooth one.

    This should get you in the ballpark. The ratios in my shift kit are similar, and allow me pedal without engine assist.

    Good luck in your gear selections.
     
  13. Rusty_S85

    Rusty_S85 Member

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    So in this case since the drive sprocket driving the rear cassette is a 36t, that means in this case it would be like a 18t gear in 8th correct?

    I see how the math is supposed to be done. Looks like I will need to play around with different chain ring`s since all the 7 speed cassettes ive seen has either an 10 to 12t gear.

    Cause from what I am thinking, an 18t gear in 8th would be unusable unless you are at a fairly high rate of speed. Or do like you said and swap the 10T jackshaft sprocket to a smaller one.
     
  14. 5-7HEAVEN

    5-7HEAVEN Well-Known Member

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    There are at least 2 chainring sprockets on the bottom bracket(BB). The outer one with the freewheel is driven by the jackshaft sprocket. In this case, it's 36t. You could install a 24t-30t or 32t as the drive sprocket on the inside of the BB. Unsure if a 36t would fit on the inside as the drive sprocket.
    If you use a 36t as a drive gear, 8th gear would be unusable ad comparable to a 19t sprocket. 7th, 6th and 5th gear unusable at 22t, 26t, 29t, 34t.
     
  15. Rusty_S85

    Rusty_S85 Member

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    The math i just did as follows.

    Using a 9T on the chain ring driver.

    Jackshaft 1.7:1
    Chain ring 4.0:1
    Built in reduction 4.1:1

    4.1x1.7x4.0= 27.88:1

    Switching the 44T drive sprocket for the cassette for a 36T version gives me the following.

    27.88x(14/36)= 10.84/4.1 = 2.64:1 or 26T in 7th gear
    27.88x(34/36)= 26.33/4.1 = 6.42:1 or 64T in 1st gear.

    It seems i need to raise the tooth cound for 7th gear.
     
    #15 Rusty_S85, May 16, 2017
    Last edited: May 16, 2017
  16. Rusty_S85

    Rusty_S85 Member

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    Well ive been doing my math wrong.

    Looking at the installation instructions the 10T jackshaft drive connects to the 44T sprocket and the 34T sprokcet is the drive for the cassette.

    With that said if i buy the optional 30T drive i get the following with a 10T jackshaft drive.

    30.668 x (14/30) = 14.31:1/4.1:1 = 3.49:1 or 34T in 7th
    30.668 x (34/30) = 34.76:1/4.1:1 = 8.48:1 or 84T in 1st.

    If i further shrink the cassette drive to their optional 24T i get the following.

    30.668 x (14/24) = 17.89:1/4.1:1 = 4.36:1 or 43T in 7th
    30.668 x (34/24) = 43.45:1/4.1:1 = 10.60:1 or 106T in 1st.
     
    #16 Rusty_S85, May 16, 2017
    Last edited: May 16, 2017
  17. 5-7HEAVEN

    5-7HEAVEN Well-Known Member

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    Use 44t chainring and 32t driving chainring.
    4.1x 1.7 x (44÷10) = 13.42:1 or 33t in 7th gear.....
    4.1 x 1.7 x (44÷9) = 14.91:1 or 36t in 7th gear.
    The 9t/44t or 10t/44t combination would determine 7th gear' lowest gear ratio.
     
  18. Rusty_S85

    Rusty_S85 Member

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    Thanks for the reply. That's what I was looking at on my math was a 7th gear around 30T to 40T.

    I really been leaning towards the 30T cassette drive sprocket which gives me a 34T in 7th gear.

    I did think of the 24T but a 106T 1st gear I thought would be just way too much.
     
  19. 5-7HEAVEN

    5-7HEAVEN Well-Known Member

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    Actually, my 113t 1st gear comes in handy for crawling on sidewalks amongst pedestrians. It's also good for climbing over berms, logs and boulders.
    My 86t 2nd gear zooms up short, steep hills and parking lot ramps.
     
    #19 5-7HEAVEN, May 16, 2017
    Last edited: May 16, 2017
  20. Rusty_S85

    Rusty_S85 Member

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    Interesting. Might be something I will need to keep in mind.

    Right now I am just making my drive train list for parts I will be sourcing out myself. Such as the derailleur, shifter, and rear cassette.

    I originally was looking at the Shimano Tourney MF-TZ31 7 speed but I found for more money I can get the HG50 7 Speed Shimano that has a more balanced gear separation.

    The derailleur I need to stop for a moment and focus on frame mounting for theses as well as the difference between the trigger shifter I'm looking at and the ESP shifter.
     

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