BMX Build

Brains

New Member
Jul 21, 2008
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Adelaide Australia
Putting together a motorized BMX, powered by 50cc pit bike motor, put an oversize piston & rod in it to bring it up to 74cc, we got the piston ceramic coated as well, refitted a new pin to change timing, match ported the inlet runner & exhaust, rebuilt the rear wheel to incorporate disk hub & fitted sprocket direct to it, got rid of the multi speed & simply added a feewheeler as well, the exhaust is a custom build which we done in our home made pipe bender, havn't fitted the fuel tank as yet & unsure where we are going to mount it, either direct over the motor or open to suggestions if anyone wants to throw some our way, some pics below for all to have squiz, If some of you like the build I will add more pics as we go along

Cheers
 

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Brains

New Member
Jul 21, 2008
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Adelaide Australia
Thanks guys, will add more pics as we progress, tomorrow have to find the pin from the chain link which is hiding on the cement pavement somewhere beneath my bench, hopefully in a couple of days it should be together for a test run, Im sure there might be some bugs to iron out before we pull it apart and get it ready for paint, hopefully we wont rip the flippen spokes from the rim when we give it some hurry curry
 

Ilikeabikea

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Jan 27, 2008
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Good looking motorized bicycle. For sure keep us posted on the progress. Pictures are always welcome....................
 

Brains

New Member
Jul 21, 2008
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Adelaide Australia
Once Ive got this project finished, Im thinking of trying to incorporate a similar motor into a v Frame bike I have & see what its like, as I also purchased a water cooled motor as well, so thats the next custom project I will post for you all to have a squizz at
 
Jul 22, 2008
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Northglenn,Colorado
Wow! It looks sweet! Is that a freewheel sprocket? Can't see how it's mounted. Flip flop hub? Or is it fixed on a conventional freewheel?

Also just by looking at the pics is it possible to turn your pull starter 33 degrees so you can start it from the saddle? It looks like your three mounting bolts are spaced evenly allowing this.

Looking forward to the rest of your install. And skate park pics lol!
 

Brains

New Member
Jul 21, 2008
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Adelaide Australia
Its not a flip flop hub, I simply removed the original hub (seven gear cassette) which came with the bike & ditched it, I bought a new hub with disk break mount for ease of attaching the drive sprocket (standard pocket bike sprocket & theres heaps of different gearing available as they race these bikes on different tracks and they are always changing the gearing, so this makes it great for fine tuning) I had to machine out the center of the sprocket & drill the six mounting points thats all, I had to purchase a hub which gave me the same width as the original hub & could only do this buy purchasing one which was for a slide on gear cassette, I then simply machined the first spacer which went over the hub & threaded one end for the freewheler to screw onto & then machined another spacer to put in front of this to give me the width I was after, thats it. you could quite easily turn the pull start around so to be able to pull start while sitting on the seat, but I put it back to its original position as its got that much compression I could not crank it over, believe me you need to rest one hand on the seat & get an nice strong stance and yank, I am hoping that after its run in it might be a lot easier to pull start & then I will try starting it from the seated position, hope this makes sense
 

Brains

New Member
Jul 21, 2008
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Adelaide Australia
Once you start these little engines its got an auto choke, so you got to let it warm up first and after a few minutes you tap the throttle & the choke auto disengages, its got an auto clutch so you got time to have a fag while its warming up and your putting on your knee & elbow protectors & kidney belt lol...I will be pissed if I can't pop monno's down the street, Im pretty sure it will, I got to work out how to post a video clip on You Tube as never done it before & then I will post the link for all to have a laugh, hopefully you all won't see me go through one of my neighbors front fence
 

Brains

New Member
Jul 21, 2008
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Adelaide Australia
Some updates on todays build, finish welding in support bracket for the engine cradle, & after putting the motor back on and checking clearances with wheel properly mounted I noticed the clearance between tyre & bottom of carby was minimal , about 3mm, out with the grinding tool & cut back to allow a little bit more space between the two, have attached a pic on the free wheeler set up as I was asked if it was a flip flop hub, & you can see how I machined up 2 spacers to get the sprocket to line up and all, this was tricky but in the end the result was what I was after, I made up a temp fuel tank holder so we could give the little beast a kick in the guts & also a temporary throttle assembly as I have to make up a throttle cable to reach the handle bars, cranked it over slowly for 2 pulls with the choke on to get some fuel into its guts, & then it roared to life on the first quick pull I gave it, boy was it awesome, Its got some torque, just tapped the throttle from idle and the whole bikes rear end moved across on its own, the chain wasn't attached as yet though as still cant find the split pin, its actually scary just listening to it, it sounds sick, I'm going to borrow a mates video camera so I can post on you tube for you all to hear it, cant wait to finish the build and take it for a test ride, before we strip it and get it ready for the paint
 

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5-7HEAVEN

Well-Known Member
Aug 2, 2008
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Very nice build, Brains. I admire your work.

Unless I'm wrong, you need a jackshaft and more gear ratio to get the engine to work decently. Using stock 6-tooth and 68-tooth sprocket, that amounts to 11.33:1 ratio. You might want 28.33:1 gearing.
 

Radmanfly

New Member
Jul 28, 2008
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Oswego, NY
www.farleysradiator.com
Yeah, I was going to wait and see what happened, but I agree w/ 5-7Heaven. You'll burn up your clutch pretty quickly like that (your clutch springs only have to get real hot once and they will loose tension and not work right) and you may never see top speed, which would be about 90! Divide your larger gear tooth count by your smaller gear count and divide by the engine rpm....(about 265 rpm @3000) that's how many rpms your rear wheel will be turning if your centrifugal clutch ever engages. Convert your rear wheel circumference into feet and calculate how far you'd travel in a minute. Times by 60 to get MPH. You'll be scared. Shoot for above 18:1 to get any low end at all, and with that monster engine, I think 28:1 or so would give you best of both worlds, awesome torque and scary high end. Let us know if you want help crunching the numbers to select the right gears.
 
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Radmanfly

New Member
Jul 28, 2008
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Oswego, NY
www.farleysradiator.com
I don't know if I'm right....someone help me double check this, but using 5-7 H's ratio...."Using stock 6-tooth and 68-tooth sprocket, that amounts to 11.33:1 ratio" as an example, and a 20" tire with a circumference of 5.25', at 3000 rpm it would turn 265 times a minute which is 1391.25 feet. That is 83,475 feet an hour. Divide by 5280 ft and get 15.8 mph at 3000 rpm. (not my original guess of 90) That would be ok. Is this right?
 

Brains

New Member
Jul 21, 2008
132
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Adelaide Australia
Wow thanks guys, Im all confused at the moment with all the above gearing mathamatics, the front sprocket is 6 tooth & the rear sproket on the bike at the moment is 74 tooth, I can tell you guys that the engine revs to 14,000 rpm though!, I have also an 80 tooth rear sprokect which I can use, at the race track they use the same gearing I have installed at the moment & with the same diameter wheel, but now one there can tell me what speed they get as they reckon they havnt got time to look & they dont have speedos, but when Im standing there they go past mighty quick, one of the blokes there stated that I should use a 80 tooth rear sproket, so I was going to give this set up a go & see what happened as well, if you guys could help me with the calcs with these mentioned sproket sizes I would really appreciate the assistance, and it will teach me how to do it myself as well, my dad tought me how to weld & Ive spent a lot of time practicing, thanks for the comments on my welding I try my hardest to get a super clean weld as this helps out when I paint as I dont have to use bog.
 

Radmanfly

New Member
Jul 28, 2008
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Oswego, NY
www.farleysradiator.com
Wow thanks guys, Im all confused at the moment with all the above gearing mathamatics, the front sprocket is 6 tooth & the rear sproket on the bike at the moment is 74 tooth, I can tell you guys that the engine revs to 14,000 rpm though!, I have also an 80 tooth rear sprokect which I can use, at the race track they use the same gearing I have installed at the moment & with the same diameter wheel, but now one there can tell me what speed they get as they reckon they havnt got time to look & they dont have speedos, but when Im standing there they go past mighty quick, one of the blokes there stated that I should use a 80 tooth rear sproket, so I was going to give this set up a go & see what happened as well, if you guys could help me with the calcs with these mentioned sproket sizes I would really appreciate the assistance, and it will teach me how to do it myself as well, my dad tought me how to weld & Ive spent a lot of time practicing, thanks for the comments on my welding I try my hardest to get a super clean weld as this helps out when I paint as I dont have to use bog.
Wow, you're on the right track. Let's use the 74t sprocket. The ratio with the 6t primary will be 12.33:1, that is every time your crankshaft turns 12.33 times, your rear wheel turns once.

I don't know what the rpm will be at high speed under load, let's say 10,000. To get the rpm of your rear tire divide the rpm of the crank, 10,000 by 12.33. You get 811 rotations per minute.

This is where we need the circumference of the rear tire. Mark the tread and use a measuring tape or roll it on the ground to measure the outermost circumference around the tread back to the mark. (To estimate a 20" BMX tire, multiply pi times dia. which is 20".... stay with me here....a 20" tire will be 3.14 X 20" or 62.8" around. Now divide by 12" in a foot to get 5.23 feet that a 20 inch tire will roll on the ground from mark to mark, double check yours this way too.)

Ok, now we multiply 5.23 feet per rotation to 811 rotations per minute. You get 4241.53 feet traveled in one minute. In 60 minutes, you will go 254491.8 feet. Divide this by 5280 feet in a mile, you will be going 48.2 mph at 10,000 rpm w/ a 20" wheel.

Pretty cool huh? Are you seeing this? Is your tire 5.23 feet per rotation?

You can vary the rpm's and tire sizes and gearing to see what your speed will be if you were to twist it to 14000 under load.
 

Brains

New Member
Jul 21, 2008
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Adelaide Australia
Thanks Radmanfly, the steps you have given me are great, I dont understand all the logic behind it but from the steps you have given me I can get the end figure, when i get home tonight (Im at school & during my lunch break of course) I will check the tyre rotation & post it for you to double check my working out, really appreciate all the time you have taken in explaining this to me in such detail
 

Radmanfly

New Member
Jul 28, 2008
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Oswego, NY
www.farleysradiator.com
No problem, I invite others to double check as I may be going the long way around the cow!

But I think these steps are easy to follow logically anyways to learn how to do it. Just converting engine rotation into tire rotation then tire rotation into linear feet and spreading it over a mile to get mph.

If you're into racing and trying to get top speed, that's where it really gets fun because 14000 rpm is only top rpm. In there somewhere is a sweet spot (the power curve) of the engine. Gearing it to take full advantage of the high end of the power curve is what you are shooting for. Have fun but be careful. 45+ mph on a bicycle is fast! I hate to be a kill joy but if your chain falls off at that speed, and it will, lots of damage can occur to your engine, your bike and you or others around you. It was never meant to be pushed like that. Judging by your other projects, you know where to watch out. Sorry I get carried away sometimes. Have fun!