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DIY Home Built Motorized Bicycle (non kit) Post all about your home built rides here. Weedwacker motors, lawn mower engines ect. This area is for non kit builds


20 inch bmx

Post all about your home built rides here. Weedwacker motors, lawn mower engines ect. This area is for non kit builds


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  #1  
Old 02-13-2011, 10:32 AM
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Default 20 inch bmx

ok so i started out with a crappy bike some time ago and a Chinese engine i managed to fudge up , got blasted on here a lot for that one. i deal more with airplane engines than bikes though i have built some before. sometimes stupid things come out of my mouth before i think about it so bear with me. I was going to use a Schwinn point beach (red and white) for my build but i didn't want another cheap engine slapped on it so i decided a smaller more maneuverable 20 inch aluminum bike will do it took me 4 hours to find one that would work had to walk pretty far to get it too lol , since the engine i wanted to use is made for a pocket-bike it defiantly wouldn't fit in a 20 incher , not much would. so i used the rack from my Schwinn and cut the mounts down for the 20 inch wheel and mounted it to the frame originally it was to be just an electric brushed motor with pedal assist but the battarys vs the cost of an engine , i decided to go with a 54 cc big bore pocket-bike engine from a cag souped up to stage 3 the gearing will go through a small jack-shaft i made from a pocket-bike chain to the one that comes with the grubee (larger chain) both sides of the wheel have a free spin one is removable , which is the pedal side , the other is tack welded on , but it was press fitted so hard i dont think a full out acceleration could phase it. the motor comes with a 6 tooth sprocket im running to a 55 tooth then to a 10 tooth to a 16 tooth so a ratio of 19:17:1 from the engine to the jack-shaft then 1:60:1 to the 16 tooth sprocket (the 55 tooth is removable for different gearing) for a total ratio of 14:67:1 which is kind of high but it will keep the rpms down and i believe this engine can Handel it no problem blue is to the 6 tooth sprocket the red to the wheel . at 6000 rpm i should get an estimated 24.3 mph the engine has been clocked in at over 11,000 rpm (44.6 mph est)so she can haul if she wanted to but i will also test using gps the engine should be here in 2-3 days later on when i get more metal i will more than likely move this set up to my schwinn i ordered a manic mechanic hub adaptor just for this too , money spent so far 475$ lol nvm the big mess behind the bike , my room is my workshop my kitchen ect blah blah lol . most of the time its clean , till i start building ps the rack is bolted on , and the arms are bolted to the rack , but im going to weld the arms to the rack and reinforce them via underside then painted a simmple silver to match the bike and prevent rust , may put on some hand made fenders

(edit) now that i think about it i can use a 68 tooth where i have the 55 for a ratio of 18:13:1 closer to the 18:75:1 that i wanted a 72 will give me 19:20:1 i think ill stick with the 68
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Last edited by Vfalpha-1; 09-19-2013 at 08:47 PM.
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Old 02-13-2011, 12:05 PM
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Default Re: 20 inch bmx

I hope it works out for you.

I will be running a similar set up, I will have an engine mounted behind me as well with some sort of a jackshaft gear reduction.

Good luck with your build.
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Old 02-13-2011, 05:40 PM
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Default Re: 20 inch bmx

It should be a real wheelie machine with that motor on the rack.
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Old 02-14-2011, 12:55 AM
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Default Re: 20 inch bmx

Yes with the current sprockets i have it will either be a wheelie king or a speed demon. for now its going to be geared down until i can get a sprocket that will work or make one , I made a better jackshaft today then passed out , (still recovering from visit to hospital I.C.U.) my engine left the building today so it should be here in 2 days or so , tomorrow im going to home depot for oxygen, propane,welding rods and a 220 volt plug if my old lines cant take the arc welder ill be using a small gas welder , the new jackshaft reinforces the back tray as well , will post pics later ,gunna go crash again
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Old 02-14-2011, 01:03 AM
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Default Re: 20 inch bmx

I put a staton friction drive on a mongoose bmx bike for my nephew... It's simple and he loves it.

Don't know if you thought of that or if it helps, but good luck anyways.
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Old 02-14-2011, 01:11 AM
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Default Re: 20 inch bmx

I did think about a friction drive but i dont like the idea of the engine way out to one side , not to mention all of the parts for the jackshift im making by hand with a hacksaw a small dremel a few bits , all hand made. So im just using what i have , i have a 2 stroke engine but its insanly huge and overweight for its power output , the engine is about 40 years old. so i decided to use a pocketbike engine on a bmx bike vs a chinese engine on my schwinn , later i might put the pocketbike engine on the schwinn anyway
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Old 02-14-2011, 02:02 AM
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Default Re: 20 inch bmx

I understand... Very interesting, Please keep us posted with pics.
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Old 02-14-2011, 03:12 PM
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Default Re: 20 inch bmx

ok so here is the bike , i moved the pipe below so it would be easier to rout the exhaust to the engine port because its on the bottom of the engine. the back wheel isnt perfectly true , but i had my friend tow me to 30 mph and i really couldn't tell, still i remade the jack shaft so i can mount it higher allowing for up to an 80 tooth on the second sprocket from a 55 later on i will change the rear wheel to a brand new one compatible with the manic mechanic hub adapter. that way if i damage the wheel somehow i can just throw a new one in there , this wheel that is currently on if it gets messed up there is no way to re-spoke it due to the tack welded sprocket. the bike rides smooth with added weight to the rear , a little odd feeling to me but ill get used to it. tomorrow i will be getting new brakepads , the ones currently on it dont work to well. i cant wait to show this thing off , when i was getting towed a cop stopped and asked what we where doing and i told him. im only 17 but i look like im 15 or so sometimes i get stopped alot for things even though im almost 6 feet tall. anyway pictures for your enjoyment.
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Last edited by Vfalpha-1; 09-19-2013 at 08:47 PM.
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Old 02-15-2011, 10:33 AM
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Default Re: 20 inch bmx

This build was originally intended for a beach cruiser , specifically the one i had on hand , but i have seen a few with cracked frames from vibrations , but a balanced souped up pocket-bike engine isn't horridly out of balance. I have run into a snag though. the wheels on this bike where just a little off but i don't trust an un true wheel with an engine turning it. maybe if it was just pedaling i would tolerate it. the modus hub I ordered from piratecycles is for a 1.181"-1.187" hub or 29.99mm-30.14 the hub on my current tires is too small. but it fits my Schwinn perfectly. i have a hub that will work but i cant fork out the money to have it spoked and trued, though i could do it myself. I like the idea of a bmx bike because its smaller and more maneuverable if i need to i can have it inside of the mini van , my beach cruiser is too big for that. everything else is sorted out im just waiting for my engine and my dad to get better (migraines) so i can get welding supplies
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Old 02-17-2011, 07:18 PM
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Default Re: 20 inch bmx

ok so i got the MM hub adaptor im happy it got here faster than my engine which is shipping from a closer location as always im happy with piratecycles . only problem it doesnt fit my beach cruiser hub or any other rear wheels that i have , which i anticipated , now that I have it in hand i can look around for a wheel that this baby with fit i still want to keep this on a bmx bike and ill have to make a sprocket so i have a reasonable gear ratio . I already painted the gas tank to match the bike lol i dont want to have to repaint it again for the millionth time( got dropped dust got on the paint ect.) just so it will match my point beach , though the point beach would be more comfortable for longer rides , i dont really trust the thin aluminum frame. will keep you guys posted

Edit , the sprocket doesnt match the chain that goes to the engine its the standard chain that comes with the grubee but the teeth dont line up with the chain
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Last edited by Vfalpha-1; 02-17-2011 at 08:01 PM.
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