building a transmission

Discussion in 'General Questions' started by darrylportelli, Jun 18, 2015.

  1. darrylportelli

    darrylportelli New Member

    Hi, Im new to the forum :D been wanting a motorized bike since I was a kid and now I finally have a weed eater engine and Im going for it ...I want a chain drive and I'm looking at something of about a 16:1 reduction according to my calculations on a 20'' wheel, now I know this needs to be done in a 2 stage reduction of 2 4:1 steps ... I dont want to use the bike's crank as a jackshaft as I want to leave the pedals intact so I could pedal as well so I thought it would be a great idea to build a transmission similar to this : lot of the 4 stroke kits come with similar transmissions to this one and the 2 strokes have a similar one with spur gears... so I got to thinking on how to build this , it's simple enough , find a steel or thick aluminum plate, cut it to size and bolt it to the engine over the clutch and attach a 10 tooth sprocket to the clutch ... my question is how do I attach the second sprocket to the plate and the jack shaft?? what I was thinking is drill a hole in the plate for the shaft to pass through and then attach 2 flange bearings (like the one in the pic below) one on each side of the plate: run the shaft through the bearings and attach a sprocket on each side..will this work well??thanks, sorry if Im not clear enough

  2. darrylportelli

    darrylportelli New Member

    sorry the links to the pics are not working , I downloaded the pics and will upload them here

    Attached Files:

  3. HeadSmess

    HeadSmess Well-Known Member

    get the "then" out of the link.... ;)

    clever. hate forums full of these dead links or "user has removed picture" nonsense. :)

    post the pics right here for all to see :)


    23... yeah, thats old enough...

    unanswered (may seem irrelevant even...) questions...

    previous experience with silly things like these? equipment available and/or funds available?

    things to be wary of...chain pitches. finer is better, just for space and weight and smoothness and reasonably small sprockets...

    your 4:1 X2 is just on the money.

    covers... chains do serious damage when snapped... something a bit heavier than the tin foil used on that pic you linked ;) especially on the primary redux. (the fast one!)

    the engine will always try to pull itself to the jack axle,twsit its mounts off, vibrate them off, etc etc. there is a LOT of force. mount everything securely, and dont think because its rigid when you try bending it by hand, that it wont break... ie, if you can isolate the whole thing from the frame BUT still have it held firmly and squarely...(3point clamping, etc) thats ideal. hose clamps and tek screws are not the way! nor are welders, if you can avoid them...

    an outrunner bearing for the primary pinion on the crankshaft is almost mandatory... huge distance between the crank bearing and the actual clutch/sprocket makes for huge leverage on a small shaft... remember that on a grass trimmer the engine is NOT loaded like a sprocket will do, to the side... they only are designed for torsional(twisting) loads really...

    personally, i was churning over the idea of running a flat pulley off the flywheel, removing the clutch and starter so i could "just" squeeze the engine in between the cranks, and using a slip-belt clutch to start and idle etc... (to get really fancy, one would remove the flywheel as well, and make a new one as like rc planes... but to suit this application)

    if i was to run solely chain and a centri clutch, it would be on the jackshaft, with some fiddling to get the spring tension right for the lower speed.
    (just like in your pic i now notice! which, to my mind, would buzz like a tuning fork being 1/4 inch(? if not its close!) alloy plate...any support up near the sprockets? and what exactly is being used for bearings because it looks like they are very close together! the sprocket facing us gets pulled towards the engine under load... on the top side. the clutch pinion gets reefed down towards the wheel...also on the top side... what happens if you get a bar pivoted in the middle and apply equal but opposite forces to the ends? yep. it, a tap handle? and if the bearings arent going to flex...the mount will...that flimsy alloy plate? the more i look at that design the more i see wrong with it...
    you want two bearings with a fair distance between them and as close to the sprockets as possible.)

    that takes me to the last real thought... only my opinion...

    sticking an engine between the cranks makes for a nice ride...IF the cranks stay standard. any oddly bent or offset cranks etc make it awkward to pedal. most engines are simply too wide for anything except the schwinn stingray type bottom brackets... once you factor in a clutch and sprocket and all that, the things are 300mm wide (long?) at least!

    rack mounts may ride nice and make it easy to build but really are simply frustrating at all other talking about leaving them against fences and stuff like that... too top heavy and at some point they will always fall over and scrape your shin despite not even riding the freaking things! by the third time its gone beyond a nuisance and you would rather stomp on the things once its down.

    anyways, boil it all down... there will always be compromises and steep learning curves... cant have everything. especially on a bike!

    yep. flange bearings are fine. ideal even. find the pressed steel carriers...much cheaper ;)
    darrylportelli likes this.
  4. darrylportelli

    darrylportelli New Member

    Thank you VERY much !!!! very good info ... I am learning as I go along ... the crank thing that you mentioned came to my mind + as my bike is a bmx and I have a 4 stroke which is wider than a 2 sstroke of the same displacement, I opted for a rear mount.. I am going to use a jack shaft as you mentioned. I am going to use a wheel hub as a jackshaft since it already has a shaft mounted on bearings.... since I posted I have manufactured a bike rack which is very solid made out of 6mm iron flatbar and angle iron ... I will post my progress as I go

    to reply for your questions, I do not have much experience with motorized bikes, but I have good experience with machining and have a good amount of tools including a lathe ....
    as you said I'm 23 ... I hope Im old enough :p ... I really take safety as a big priority as Im a final year medical student and I KNOW what damage power tools or these motorized bikes can inflict... I will NOT attempt to take it out for a spin until Im sure I have everything in check including lights, kill switch, helmet, highlighter vest ecc ...

    again, thank you for your reply!!!
  5. darrylportelli

    darrylportelli New Member

    I have previous racing kart experience and i know how dangerous chains can be, they ARE projectiles, I remember an accident where a kart chain snapped and it shot through the arm of who was driving!!!! In fact for this project im using a high quality kart chain and i have chain lube at hand as it is the only thing i would put on any of my chains, no oils or wd40....
    regarding money, im not looking to spend a lot of money on this project as it is a hobby of mine and im still a student, HOWEVER, i will spend money on things that make it safe
  6. HeadSmess

    HeadSmess Well-Known Member

    you sound like you dont really need to worry about asking questions ;)

    the lathe is a bonus. very important if you want it to last or find certain ideas arent so clever after all...

    the bike hub is far cheaper than flanges... better again. and allows for attaching pullbacks to tension one chain, and a simple tensioner on the other one.

    a bmx does suit rack mounts, theres really no other way. i always kept my tank up on the bars, pull as many parts off the engine as possible... every gram you get off stops it vibrating to death just that bit more.

    just as an idea, to (possibly) simplify things...

    friction drive done on a steel roller of the required 3/4 " or so really suck. but an FD setup using one of those solid rubber scooter wheels, the 6 inch types... calculate your redux based on the smaller tyre. saves a chain? makes the centri clutch unnecesary, as well as the pull starter. use an over-centre toggle clamp for the clutch action (these things with an adjuster, let you lock the roller down in a fixed spot, rather than rely on springs etc...) rubber to rubber will have far more grip when wet combined with a larger surface area. only an idea. havent actually bothered doing it :) shame i dont have any pics of my early FD builds...

    they been banned where i live so all i can do is think now :(

    nah! you gotta ride it BEFORE its completely finished! nikhedonistic behaviour is a curse, i know....
  7. darrylportelli

    darrylportelli New Member

    Thanks for the reply.... you mentioned a very good point in your first post about the longitudinal force and that the crank does not handle this force as these engines are designed for rotational forces not longitudinal,,,, i was thinking about hooking a shaft to the clutch and then passing the shaft through a bearing (a simple pillow block bearing mounted to the carrier) that way this external bearing takes all the longitudinal force from the chain ... do you know of any pics i can take ideas from??
    Thanks again for the discussion and your input!!!
  8. HeadSmess

    HeadSmess Well-Known Member

    not really a good example but you get the idea :) no, it wont fit.

    keeping the bearing central is the biggest part.

    main issue is which clutch, or where to mount the clutch.

    ie, make a carrier on your hub/shaft to utilize the brushcutter clutch bell. then you only need to make a plate with a bearing using the four shaft mounting holes to support the sprocket.
    assuming its a standard engine, ie, starter one end, shaft and clutch etc at the other? not those el cheapo things that are all plastic and the starter is on the shaft?
    keeps the shroud for make it (the support bearing housing) flush and chop away the sections in the shroud needed to pass a chain through.

    pillow block will be too hard to align. and if its not bolted directly to the engine...

    what size kart chain? with only 1-2 hp, you only need good #35H... and chain load rating increases as speed increases ;)

    me...use 25 or 35 on primary, then go HD bmx chain (1/2") to the wheel... its the second one that cops the torque/strain/call it what you want...

    that clutch i picced is 25...silly little 49cc pocket rocket stuff. but you can get various sprocket sizes that are interchangeable ;)
    using too heavy a chain for the chain speed (sprocket diameter x speed) it flings off from centrifugal force.

    interchangeable...this is a big thing. while some parts DO require machining... its sensible to use certain items that follow standards.

    say if you do use 1/2 chain on the secondary... pit bike sprockets come in from 12 to 17, and im sure even 11 to 19... they might require a splined shaft, but at these simply mount them on a spigot the ID of spline, and use the two handy tapped holes for attaching the keeper plate to drive/secure them. or a keyway thats easy to do on the lathe.

    do a few drawings. google sketchup isnt too bad if you dont like paper...

    and somewhere... ah...

    this guys build ;) bit over board for you but hey.... worth a look.