20" Mongoose BMX Race Bike

GoldenMotor.com

Wild Bill

New Member
Jan 29, 2013
478
5
0
Camarillo So. Cal.
This will be my 2nd build to be run in either the Amateur or Intermediate class but it will be my first single speed.
I started with an old Mongoose my son gave me for the project. I hacked off the top tube and built a perimeter top tube around the engine.

The thing was a little cramped for comfort so I added another bottom bracket to stretch the wheel base. I didn't have any tubing to replace the bottom tube with a longer one and lengthen the bike the right way and I wasn't gonna stub onto the old tube to lengthen it so adding the bottom bracket did the trick and made the wheel base 2 1/4" longer.



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Wild Bill

New Member
Jan 29, 2013
478
5
0
Camarillo So. Cal.
Fuel tank and fender mounts. One thing I can say is you are way down out of the wind on this thing!!! It actually is fairly comfortable to pedal and tuck down for a speed run. The overall rider height is very low and the engine is very low in the frame, it may handle very good but we will see.

I still have to build an engine for it, the one in it is a dud just for mockup. I wont build a pipe for it until after engine breakin and I can see that its gonna work ok.

Theres a local BMX track and after its all dialed in Im gonna see if I can do a few laps.



 
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Wild Bill

New Member
Jan 29, 2013
478
5
0
Camarillo So. Cal.
I was figuring on a jackshaft to get the gearing right but even if it never has the speed it will be fun to ride as a dirt bike, gear it lower and it will be good for trials.
 
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Wild Bill

New Member
Jan 29, 2013
478
5
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Camarillo So. Cal.
Actually I found aluminum sprocket blanks and I can run a rag joint if I have to since I don't think a 29-27 tooth will fit the sprocket hub, the chain would hit it. I just hope it doesn't come to a rag joint.....eeewwwwwwwwww!
 

16v4nrbrgr

Active Member
Mar 17, 2012
1,728
4
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North Bay
Awesome BMX Wild Bill! Motorized 20inchers are so much fun because they handle so well and you can really slide them around the tight stuff with ease!

BMX sprockets come in 14t and up, fixed gear bicycles also have a wide range of sprockets to choose from. You can weld a sprocket onto the hub, get a dual threaded tandem hub, a LHD BMX hub or wheel, or use a sprocket adapter and have Rebel or other sprocket manufacturers make you one for cheap.

I think a 20" bike would be more than competitive on a course where you have to accelerate and decelerate the rotational inertia of the wheels, smaller wheels mean less weight total and a shorter lever arm for overcoming the inertial effect.
 
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Venice Motor Bikes

Custom Builder / Dealer/Los Angeles
Mar 20, 2008
7,266
1,797
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Los Angeles, CA.
Your plan of using a shifter kit is good... another option is to use a fixie 'flip flop' hub with a freewheel on the pedal side & a 20T fixed sprocket on the engine side. ;)
 

Wild Bill

New Member
Jan 29, 2013
478
5
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Camarillo So. Cal.
I happen to have a flip flop hub, I will go that way instead. I couldn't find a fixie sprocket in a tooth count I wanted so I ordered an 18t fixie and a 25t front sprocket that I will weld to it. I hope the fixie sprocket lock nut will keep the sprocket from coming loose since its threaded in the wrong direction for power driving on that side, I would think it will but we will see. Thanks for the idea!
 

Venice Motor Bikes

Custom Builder / Dealer/Los Angeles
Mar 20, 2008
7,266
1,797
113
Los Angeles, CA.
I did this to a Fixie a few months ago... I welded a stock 44t to a Fixie sprocket.
The customer hasn't called back to say it broke yet, so that's a good sign! (^)



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16v4nrbrgr

Active Member
Mar 17, 2012
1,728
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38
North Bay
You gotta weld the fixed sprocket cog lockring in place though, even GutenTight with loctite it will spin off under engine power. I used a fixed cog LHD on my BMX and had to tack weld the steel lockring in place, had to do it again later with more weld because the cog managed to taco the steel ring. There are many different familiar ways to power a 20" wheel LHD, the hubs aren't anything special.

You don't need to weld the 9 hole sprocket to the fixie cog if you use an 18t cog and use the washer plates provided in the kit and some bolts.
 
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d_gizzle

Active Member
May 29, 2012
1,102
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ARDMORE,OK
20" disc brake rear wheel from cyclesus.com and a velosolo disc cog from benscycles.com. The cogs go from 22 teeth down to 15. 3/32" and 1/8". I'm using a 19t on a 12.5" electric scooter wheel and its great.
 

Wild Bill

New Member
Jan 29, 2013
478
5
0
Camarillo So. Cal.
I think a 22t would be too high geared, Im looking 25-26t but the 20" wheels with a disc mount hub could be used with an aluminum sprocket blank drilled to fit would be perfect, thanks for the link. I did a google search a couple weeks ago for just such a wheel but got nothing.
 

d_gizzle

Active Member
May 29, 2012
1,102
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ARDMORE,OK
Yeah I know what you mean wild bill. Cyclesus is the only seller I've found of 20" rear disc wheel. And I've only found one seller for 20" front disc wheels.

But..........isn't this a race bike? And norm said a 23 tooth sprocket was too big? Only good for 34 mph? And,these are steel and have a lifetime warranty. On a fixie with proper rotation that is.