deXtrous' Journal Of His First Project!

Discussion in 'Rack Mounted Engines' started by deXtrous, Mar 8, 2010.

  1. deXtrous

    deXtrous New Member

    Hey folks, so I've decided to put togeather a few words with pictures of what I've done thus far.

    It's far from finished and I actually could use some help with the gearing and hooking it up.

    So far I've got a 49cc cag pocket bike motor, with a front hub from the pocket bike to be used as a jackshaft, and the frame of the pocket bike used as a rack mount. I've also torn apart an old bike to make the forks that lead to the rear wheel hub on the bike.

    I'm still cutting and welding the rack togeather so no pictures yet.... oh, wait, it seems I'm very good at balancing, so there is some pictures!


    So what do you think?

    I don't exactly know how I'm going to get the jackshaft in there.

    I have a few questions, I will be running the rear pocket bike sprocket to the jackshaft, but what second sprocket should I use for the jackshaft and bike rim? I know the ratio needs to be around 2:1, but I don't know where to find sprockets or anything.. I mean, do I just use bicycle sprockets? I wouldn't think their chains would be strong enough to support the rpms?!

    That's the only question I really have at the moment. I'm sure more will pop up when I get further into the project.

    Hope you enjoy and I'll be sure to update soon!

    Attached Files:

    Last edited: Mar 9, 2010

  2. augidog

    augidog Banned

    cool! i liked the cag when i tried it, it was pretty tough. can you give us side-shot mockups of the "jackshaft" in front of and behind the engine? need to see how much stuff we gotta cram into how much space.

    (btw-my forum-readers's opinion is that i'm fine with just the clickable thumbnails at the bottom or "attached")

    proper ratios is better left to others more adept, but why not get a HT sprocket and good bicycle chain for your secondary? i could see that happening.

    in case you're not aware: if you want more positioning options, and more gastank-placement options, get a diaphragm carby and intake from ada-racing...i know i know, it's easy to spend someone else's money, eh? ;)

    i'll watch this one, it could turn out pretty neat.
    Last edited: Mar 10, 2010
  3. deXtrous

    deXtrous New Member

    Okay so after some thinking I have decided it may be best (well, easiest) to mount the motor off to the right side, so the jackshaft can sit in between the modded pocket bike frame/rack.

    I will have to mount it as far back as possible so the carby doesn't interfere with the frame [thanks for the suggestion about the diaphram carby but this project is a really low budget type thing - will consider for the future though! :)].

    Pictures attatched.

    I still need to know what kind of secondary sprockets I should use (thanks again for the terminology augidog :cool:). Not too sure what a HT sprocket is.


    Attached Files:

    Last edited: Mar 10, 2010
  4. augidog

    augidog Banned

    progress...hmmm the hub is a bit bulky but it looks i said you'll need some help from folks who know how to work ratios before you can move on, but...

    sorry about the abbreviation...HT refers to the chinese integrated-engine...they, and the new inframe 4-strokes too, utilize a universal rear sprocket assembly that i think would work out just fine for you...and with some shopping you could snag one at a decent price.
  5. Alaskavan

    Alaskavan Guest

    I'm not a "number cruncher", I'm a trial and error kind of guy. But let's look at it this way. How fast did the pocket bike (PB) go? If it was geared in a way that you feel was appropriate, then you just need to figure the difference between the size of the wheel on the PB and the wheel on the motorized bike (MB). Roughly (just as an example) if the circumference of the PB wheel was 20", and the circumference of the MB wheel is 40", you'll want about a 2:1 reduction at the jackshaft (assuming the sprocket on the wheel of the MB is the same size as the rear sprocket was on the PB). Hope that helps.
  6. loquin

    loquin Active Member

    Look's workable.

    Here's a link to some gear ratio calculators. Crunch the numbers to get an idea of the top end.

    The only thing I would add is, before you ride it too much, either get a rear fender mounted, or add a mud guard under the motor, esp. the carb & spark plug area. On a wet day, I could see you choking off the engine otherwise, by clogging the air filter. Or by grounding out the plug...

    What's the max RPM on that engine?
  7. deXtrous

    deXtrous New Member

    Thanks for the replies folks. 8000 is the motors max RPM. I'll make sure I get a mud guard too :D

    With the nice tip from Alaskavan I've worked out the secondary gear ratio needs to be 2.5:1, but for a bit more low end range I may slip it down to a flat 2:1.

    Hopefully I can find some HT sprockets that come in different sizes. If not then.. uh oh.
  8. augidog

    augidog Banned

    HT sprockets are available in a good range of sizes.

    assuming your math is correct or close:

    have a look at a coaster-brake sprocket, available range 17-22T i believe...maybe you can 4-bolt "sandwich" it to the sprocket already on the hub, washers for proper chain-clearance...then down to the HT sprocket :helmet:
  9. loquin

    loquin Active Member

    Are you trying to get to 70 MPH?

    You took the gear ratio the wrong way to get more low end.
    8000/2.5 = 3200 RPM (lower speed/higher torque)
    8000/2 = 4000 RPM (higher speed/lower torque)

    You need to get the RPM into the 388 RPM range with a 26 inch tire to run at 30 MPH. To run at 40 MPH, the wheel rpm needs to be 517.

    With a 2:1 motor-jackshaft reduction:
    Assume a 10 tooth sprocket on the output of the jackshaft. You would need a 103 tooth sprocket at the axle to get to 30 MPH, or a 77 tooth sprocket to get to 40 MPH.

    If you have a 2.5:1 first stage reduction, those two sprockets would then need to be be 82 or 62, respectively.

    You would have more rangebility if you drop the first stage reduction to at least 3:1, and better yet, 4:1.

    With a 4:1 first stage, the jackshaft output would be 2000 RPM. Then, you could have a 10t:52t rear sprocket to get to 30 mph, or a 10t:39t sprocket to get to 40 MPH.
  10. loquin

    loquin Active Member

    Took a closer look at your jackshaft. It looks as if you have about 40 teeth on the input sprocket on the jackshaft, and 8 on the motor? The first stage would then be about 5:1

    The total gear ratio, to get 30MPH at 8000 RPM with a 26 inch tire is 20.6:1

    Since you already have approximately 5 to 1, you would need another 4.1:1 reduction between the jackshaft and the rear axle.

    In order to be able to coast downhill, you really should have a freewheel on either the rear axle (LH threads) or on the output of the jackshaft (RH threads.)

    If it's on the jackshaft, about the smallest standard freewheel you can get is 16T, so, the axle sprocket would need to be about 66 teeth.

    Ref the attached screen shot.

    Attached Files:

    • Cag.JPG
      File size:
      36.7 KB
    Last edited: Mar 10, 2010
  11. Alaskavan

    Alaskavan Guest

    DeX, I'd advise listening to Lou. I've never found an error in his calcs.
  12. deXtrous

    deXtrous New Member

    Thanks loquin! The pocket bike sprocket is a 54t and the pinnion is a 6t. This gives me a 9:1 ratio. The pocket bike wheel is about 7 inches in diameter. My bike wheel is 22. Lets say 21". This means it has to travels 21" instead of 7". This would mean a secondary ratio of 3:1, if I wanted to keep the same speeds as the pocket bike. Your measurements ended up being quite similar in terms of ratios, which is good!

    So, even with the accurate measurments, do you suggest I increase the gearing to 4:1 to allow more low end, or keep 3:1, the same as the pocket bike?

    You've been a great help and I really hate asking stupid questions, but how the heck does a freewheel engage and disengage? Say I were to put one on my jackshaft, to, as you said, allow myself to coast, how does it all work? I tried to search but I don't think anyone has asked such a silly question! :goofy:

    Once again thanks folks, you guys are much more help than the crew over at (but please don't tell them that! :tongue3:)
  13. deXtrous

    deXtrous New Member

    Ok scrap the previous question about the freewheel. I did a bit of reading and from what I understand it's not the freewheel sprocket that disconnects, it's the clutch, when you don't want to use it. Correct?

    Therefore I could kill the motor when going down hill or when I just want to pedal, then when I need the motor just give a pull on the pull start and get going again? Is this what you're suggesting?
  14. deXtrous

    deXtrous New Member

    Scrap that question again. Figured it all out.

    I just need to know where to get a couple sprockets now, one being a freewheel.

    I got a pair on a BMX bike. a 16t freewheel/44t sprocket. Don't know if they're any good for the high rpm load though.

    By the way, check this out, what do you think?


    (cool program btw).

    Attached Files:

  15. augidog

    augidog Banned

    20" or 26" ?

    i'm confused now...what size is that bicycle?
  16. deXtrous

    deXtrous New Member

    I measured the diameter of the rear wheel and it was 21" [or maybe 22" if they're more common and I just measured wrong (couldn't get it flat due to hub and forks)].
    Last edited: Mar 12, 2010
  17. augidog

    augidog Banned

    ah. the calculator should say "tire" diameter for clarity, because that's the dimension you need to work with. "26" is close enuff...third time's a charm? ;)
    Last edited: Mar 12, 2010
  18. deXtrous

    deXtrous New Member

    Ah ok! 26 inches it is then.


    Attached Files:

    Last edited: Mar 12, 2010
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