New Build Question With SBP/5:1 Gearbox

Discussion in 'Transmission / Drivetrain' started by Mike St, Nov 28, 2011.

  1. Mike St

    Mike St Member

    I am thinking about building a shift mb using the dax 5:1 gearbox connected to a Tanaka 32cc 2 stroke engine. It's a simple idea of turning the motor and gearbox to face the front sprocket which will drive seven gears on the rear wheel. The question I would like to know is about shifting: Would shifting be smooth and would I be able to shift down and up skipping some gears? Could I shift smoothly from a high gear to a low gear without running through all the gears? I would like to have definite answers before doing all the work. Other wise, I'll just build a single gear rack mount, which I know will work. Mike
     

  2. 5-7HEAVEN

    5-7HEAVEN Member

    Yes to all your answers. I have a Tanaka 47R engine connected to a pocket bike 5:1 transmission. With a series of other sprockets, first gear is 46.32:1 and eighth gear is 15:1. The reason for extreme gears is because there is very little pedal-assisting you can physically do with a shift kit system. Shifting is silky smooth from any gear to any other. I always pedal and release the throttle when shifting. One thing you must remember is that the bike has to be in motion before you can shift gears, just like a normal multi-speed bike with rear cassette.

    Remember 5:1 gear ratio is only the beginning; more gears and sprockets need to be installed to achieve super-low gear ratios.
     
  3. Mike St

    Mike St Member

    What gear would I use on the motor and front sprocket? Maybe 10 tooth on the gearbox and 44 on the front sprocket? I need to learn how to select these sprockets. What do you have on your bike?
     
  4. 5-7HEAVEN

    5-7HEAVEN Member

    Since 8mm pocket bike sprockets are available in larger sizes than bicycle sprockets, I decided to use those. An 11-tooth drive sprocket slips onto the 5:1 gearbox spline, and chains up to a 72t chainring driven sprocket. Multiply the tranny's 5:1 to get.32.7272:1.
    A 24t bicycle chainring sprocket chains to an 11t-34t rear cassette. That calculates to 46.36:1 granny gear and highway gear of 15:1.

    These gears are good for steep hills or ramps. That's when I use every gear. On the flats, I skip every two gears, like from 1st to 4th, then to 7th gear, which is a responsive 17.72:1.

    If you like bike sprockets, you could run 10t/48t engine drive, then 22t @ the chainring to get 37.09:1 first and 12:1 in final gear.
     
  5. Mike St

    Mike St Member

    Thanks much.I too prefer 8mm and have built my rackmount cvt with 8mm. Now to get started.
     
  6. 5-7HEAVEN

    5-7HEAVEN Member

    The best thing is that there is a vast assortment of pocket bike components available. When you split the gearbox, you'll be surprised how tiny the gears are. Some boxes have helical gears, others have straight-cut gears. I have never had a tranny fail me, although they are quite noisy at times. I use general-purpose grease instead of oil in there.

    The Achilles' heel is the $12 4-bolt freewheel which screws onto the crank arm. Runout is extreme, but I've never had one fail. If that bothers you, a HD $70 5-bolt freewheel is available from SBP. You just have to redrill the chainring sprocket's bolt pattern.

    Good luck.
     
    Last edited: Nov 29, 2011
  7. Mike St

    Mike St Member

    This has to be the lightest system ever: The combination

    of the Tanaka Purefire 32cc and the Dax 5:1 gearbox weighs

    in at 8 pounds, the motor 6 pounds and the gearbox 2 lbs.

    The total width is 9 inches, great for a frame mount. I'm

    very excited about either a shift frame mount or a single

    gear rackmount using treewks double rightsided chain drive.

    Better then a GEBE system, cheaper, more reliable, and

    better than a SBP shift kit which is far more complicated

    because it requires a jackshaft. Using Treewks double

    rightsided drive for a rackmount, the engine is centered

    over the rear wheel for better balance and a better

    look.
     
    Last edited: Nov 29, 2011
  8. Mike St

    Mike St Member

    5-7, there's just one thing I don't understand about your drive system. You're mixing 8mm and bicycle sprockets on the front but these take different freewheels with different hole patterns. What freewheel are you using that can take both types? You must be drilling new pattern holes in the 24t bicycle
    drive sprocket so it can fit on the 4 hole 8mm freewheel. Right?
     
  9. 5-7HEAVEN

    5-7HEAVEN Member

    Yes I DID drill the bike chainring sprockets to match the 4-bolt freewheel and the 70t 4-bolt pocket bike chainring sprocket. My original bike chainring sprocket was 22t. That cracked, so it was replaced by a 30t. When I needed lower gearing, I had a 24t sprocket.

    I like the 24t the best.:detective:

    The system is light; don't forget to add some for the motor mount, and the freewheel, and the extra chain, maybe the thicker crank arms and the HD wheels.

    One nicety is that OEM-size spokes can be used. They can take the abuse. Cassette sprockets wear prematurely from abusive powershifting. Chains break from chain jumping, then jam between inner sprocket and bottom bracket. Normal motorized bicycling is possible without constant breakage. I've only replaced one SS spoke, which broke at the head tip.
     
    Last edited: Nov 29, 2011
  10. Mike St

    Mike St Member

    I am a little concerned about chain problems. With my cvt rackmount, the build was relatively easy, no drilling of sprockets necessary, and the final result is a very reliable system, smooth acceleration. I don't think the shifting can match a cvt, it's the ultimate in a motorized bicycle, totally automatic continously variable transmission.
     
  11. Mike St

    Mike St Member

    5-7: If I wanted to go with all bicycle sprockets, where would I get the 10T drive sprocket that fits over the 5:1 gearbox? I don't see it on the dax part pages. Apparently, the 10T 6 cog drive sprocket is not commonly available and Dax only has a flat version (no neck), so using the Dax 5:1 gearbox would require drilling new hole patterns in the sprockets to fit the 8mm freewheel. The cvt option is looking far superior as a transmission, totally reliable, safe, and automatic.
     
    Last edited: Nov 30, 2011
  12. 5-7HEAVEN

    5-7HEAVEN Member

    Having never tried CVT, I can't really say drive which is superior.

    10t drive sprockets are available at several scooter websites. Don't EVER use flat sprockets; there's not enough meat to keep the sprocket from wobbling on the spline.
     
  13. Mike St

    Mike St Member

    I haven't found one so far. Scooter sites don't feature spline sprockets that fit a bicycle chain. None that I know of. They don't use bicycle chain, mostly 8mm. Got any links or sources?
     
    Last edited: Nov 30, 2011
  14. Mike St

    Mike St Member

    This 5:1 gearbox is very cool. I opened it up to check it and it was filled with gear grease, good to go. I'm leaning strongly to use it for a light weight single speed chain drive using the double rightsided drive which brings the motor over the rear wheel for better balance and look. Don't believe this has been done before. Very simple and cost effective.
     
  15. 5-7HEAVEN

    5-7HEAVEN Member

    Yeah, I used to see a few 10t-6 spline sprockets for bike chains. Dax had both the junk flat one and the good ones.
     
  16. Mike St

    Mike St Member

    Dax tells me they won't have anymore bicycle 10T drive sprockets for the 5:1 gearbox. They are switching entirely to 8mm sprockets which is great because they are more available.
     
  17. 5-7HEAVEN

    5-7HEAVEN Member

    Yes, that's great. Prices for 8mm T8F chain and sprockets are inexpensive and readily available. Since the links are more closely spaced than bicycle chain, the sprockets are smaller in diameter that bike or #35 drive components. My engine chainring is 72t, but it looks the same size as a 44t bike sprocket.
     
  18. Mike St

    Mike St Member

  19. lowracer

    lowracer Member

    5-7 & Mike St,
    Sorry if this thread is a bit old.
    I recently received a 5:1 pocket bike gearbox w/10T sprocket which got my wheels turning.
    I'm thinking of spreading an old MTB (100mm spacing) Cromoly fork to 135mm's & using a rear wheel as my front wheel. I have a 11-34 cassette and would just use the 34T. A track bicycle (fixie) chain fits the 10T & 34T. Correct me if I'm wrong on my math, if my engine turns 8500 rpm's, I divide it by 5 (5:1 transmission)= 1700 rpm's at the 10T. With a 26" wheel/tire I get 38-39 mph?
    I like this idea for the freewheeling aspect. I have 2 MB's now (v-belt & friction drive) & neither freewheels. I think the freewheel will give better fuel economy & less wear on these wimpy clutches (not really designed to haul a man up to speed & engine brake).
    -Lowracer-
     
  20. lowracer

    lowracer Member

    Here is a pic of what I'm thinking.
    I spread the front fork blades slightly. The wheel isn't 135mm, but 120mm w/ a screw on 7 spd freewheel.
    Will be pretty easy to make a rack for this setup using the long 3/8 axle to mount the upright struts & the cantilever bosses for a 2nd mounting point.
    I think I have an spare Sram pc-1 chain around here somewhere...
    Might even have to bust out one of those 3 pocketbike cags from storage since I'll need a bottom mounting point to mount the engine to the rack & I dont want to spend any money for this project.
    -Lowracer-
     

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