Sprocket too large!

Discussion in 'DIY Home Built Motorized Bicycle (non kit)' started by TyrantTaren, May 17, 2019.

  1. TyrantTaren

    TyrantTaren New Member

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    So, this is my first official request for assistance (meaning I may forget a few details, please do ask. I actually have not built or modified anything, I've been collecting parts but the project is still a theoretical blueprint, mentally mapping gaps as I go) regarding what seems to be my most pressing issue so far... My sprocket is too large to fit in my frame!

    I'm going for a 'durability through simplicity' concept with this (fewer parts means fewer things to break) and of course, like always, 'cheap or free' is a must.

    First, my build details!

    I have a 29" Schwinn Ascension (profile pic), and a 79cc Predator Generator engine from Harbor Freight. As you can see in my pic, my bike has a rather slim frame gap... Even stripped and ported, I'm not certain I could properly fit and mount the engine within the frame (especially with the bike being made out of aluminum). Instead, I intend to build a heavy duty rear rack out of either welded rebar (can be found as scrap almost anywhere), or bolted Unistrut (I know a guy who will sneak me some :D)

    Pros:
    No pedal clearance issue
    Proper engineering will equal a more even weight distribution of the motor across the whole bike
    Engine/Driver gear closer to driven gear (check me on this?? It takes energy to transfer energy, less chain to move, that's just physics, but I don't know if it is a considerable factor or not)
    Customizability; I already want to add an alternator, and who knows what else. I think a custom rack will streamline that process.

    Cons:
    Added weight of frame
    (What else??? Even small/inconsiderable things, little pieces of information can be quickly extrapolated in to larger lessons)

    Okay next, a sort of open ended question regarding gear ratios... I understand the basics; torque is proportional while RPM is inversely proportion to the gear ratio. Torque equates to acceleration, while the RPM is indicative of current/max speed. Of course I can't manage to find the thread now, but I could have sworn I read someone saying that a gear ratio of 9ish would strip out the person's clutch, and that they should go higher... I don't understand this. I'm concerned because the gear ratio I settled on was 7.2 (10T driver and 72T driven), mainly because on top of my bicycle/rider/motor (~210ish lbs), I'm also hauling a trailer with gear and equipment, (~100 extra lbs). Now, I'm not at all opposed to upping my gear ratio... My threshold minimum is only to be able to travel 100 miles in a day's ride (even at 15 or 20 miles an hour, I'd have plenty of daylight, and time for lunch. 200 would make me happier but 100 is my minimum). I also want to be able to take hills (mountains, really) without too much struggle, so I'm all for the added torque. However, this leads me in to my main problem at this time.... (EDIT: My engine maxes at 3600 rpm, but I don't want to rely running it full throttle, so I do my calculations at 2500, at which the engine's box says I'll be putting out 3.5 ft•lbs of torque before the gear ratio, and my calculations say I'll run an average of just under 30mph w/ 29" tires and 7.2:1 GR, 43mph at full throttle)

    I don't have the clearance for my driven sprocket! Because of the nature of my treks, durability is a huge factor. Trying to adhere the sprocket to the spokes of the tire in any way sounds like a horrible idea for my situation. Fortunately, my rear tire has a disk break system on it, a perfect place to mount (and permit one to very easily swap out a new sprocket, changing the gear ratio as I change terrains... Want to be able to go coast to coast) but it sticks out a bit and a sprocket as large as I want simply will not fit.

    This is the part where I'm open to almost any suggestion... Please suggest EVERYTHING that's even remotely applicable, even if you're certain it won't quite apply, or that there are better options, I still might learn something. Like, I didn't even know what a jackshaft was until a few days ago, which I think will be my solution(ish), mounted on the engine's support frame... I don't need to actually transfer any momentum or energy, more act like a psuedo-transmission that doesn't actually shift gears... Can you tell I have no clue what I'm talking about??? XD

    The standard response on Google seemed to be "oh just bend the frame," like it's nothing, but especially with an aluminum bike, where durability is going to be tested to begin with, I don't want to do anything that might compromise the integrity of the frame itself... I would much rather have an added part. I've even thought about designating a 'weak point' that will break first, before anything else, something that can be easily repaired/replaced, because nearly all my repairs will be road repairs (I carry a nearly complete toolkit with me)... Opinions on this?

    Okay, okay, I know it was long-winded to begin with but now I'm officially rambling... Thanks for anyone who made it all the way through! I am really looking forward to picking ya'lls brains... I'll be on to check, at the least, daily.

    Thanks in advance!

    ||Tyrant
     
  2. curtisfox

    curtisfox Well-Known Member

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    Simple reduce the gearing with a jack shaft, and run a smaller sprocket...........Curt
     
  3. TyrantTaren

    TyrantTaren New Member

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    Thank you Curt. I'm juggling a few possibilities, but that is certainly one of them. Mentally blueprinting the jackshaft while I keep my eyes and ears open for any possibilities I may not have considered yet.
     
  4. curtisfox

    curtisfox Well-Known Member

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    Being you are cheep, do a good old fashion belt drive for primary, couple pulleys and a tension-er. Nice and quiet easy to do, just don't forget the belt guides. Study the home built DIY section back about 10 years ago or so............Curt
    [​IMG]
     
  5. curtisfox

    curtisfox Well-Known Member

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  6. indian22

    indian22 Well-Known Member

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    Tyrant distance trekking isn't exactly in my wheelhouse, but I recall a youtube video of a ongoing trek across the great western desert of Australia and it was fascinating and if I can spot it or if you run across it would be a good share. The gear he pulled on a trailer with his bike was incredible.

    Also don't bend aluminum (short version) of advice given me by a metallurgist working at NASA, Houston. Probably some wise guys know better...

    Dig into and really study the complete builds featured in this forum and or others. No series of individual posts in this thread can cover what you are looking to discover for your build. Photos, diagrams and descriptions galore are contained in the various categories. Both success and failures are recorded along the way. There's much great advice and it's mingled with a good deal of conjecture. Discern which is which & ask specific questions along the way. Discover and verify, but the basics learned from 120 years of motor biking design is a good foundation to build on.

    Since I seem to throw money at projects like a drunk sailor after a long deployment, I don't have a clue how to build on the cheap, but I'd like to learn some day and still build solid, safe and reliable transport.

    Rick C.
     
    #6 indian22, May 20, 2019
    Last edited: May 20, 2019

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