20hp? BTR build

GoldenMotor.com
Jun 4, 2024
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Just about finished up with my modernized BTR build. Modeled after a sportsman flyer 200 with a predator 212. Frame is a worksman, motor is a well.... it's mostly aftermarket. Block, crank, and head are a predator 212, head is ported and port matched, decked .045, pocketed valve springs, 32lb dual valve springs, 1.3 ratio rockers, ec retainers, 308 cam, arc rod, arc flywheel with the fins removed, arc side cover, flat top piston, stainless valves, chromoly pushrods, nibbi pz24f carb. Lots of custom parts made on my cnc like the sprockets, flanges, motor mounts, bar drops, brackets ect. Tank and oil tank are from Pat Dolan of Sportsman Flyer. Brake drums are from a motobecane moped. Getting ready to tear it all down and powdercoat the frame and parts.

The thing is wicked quick. Gearing is 11-36 to 17-54 double reduction. Clutch is bully 2 disc with a 2800rpm engagement. I wanted something that could cruise at 20mph for my neighborhood but still be capable of doing 60+.

Have some other things to finish like mounting the lights, RPM and speedo, and setting up the electrical. Will be machining the arc flywheel further and installing magnets for charging system. South Carolina considers motorized bicycles mopeds so I can get it registered since it has a 49cc engine :)...

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Sidewinder Jerry

Well-Known Member
Dec 19, 2011
2,026
997
113
62
Rockwood, TN
Just about finished up with my modernized BTR build. Modeled after a sportsman flyer 200 with a predator 212. Frame is a worksman, motor is a well.... it's mostly aftermarket. Block, crank, and head are a predator 212, head is ported and port matched, decked .045, pocketed valve springs, 32lb dual valve springs, 1.3 ratio rockers, ec retainers, 308 cam, arc rod, arc flywheel with the fins removed, arc side cover, flat top piston, stainless valves, chromoly pushrods, nibbi pz24f carb. Lots of custom parts made on my cnc like the sprockets, flanges, motor mounts, bar drops, brackets ect. Tank and oil tank are from Pat Dolan of Sportsman Flyer. Brake drums are from a motobecane moped. Getting ready to tear it all down and powdercoat the frame and parts.

The thing is wicked quick. Gearing is 11-36 to 17-54 double reduction. Clutch is bully 2 disc with a 2800rpm engagement. I wanted something that could cruise at 20mph for my neighborhood but still be capable of doing 60+.

Have some other things to finish like mounting the lights, RPM and speedo, and setting up the electrical. Will be machining the arc flywheel further and installing magnets for charging system. South Carolina considers motorized bicycles mopeds so I can get it registered since it has a 49cc engine :)...

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What diameter size tires are you using? To do 60 mph using a 10.395721925\1 reduction would require the following engine rpm for these size tires.

26"~8064 rpm
27.5"~7625 rpm
29"~7230

Certainly isn't a stock Predator 212cc engine (3600 rpm). Seems to be top of the line mods that's been done to it.

Nice looking bike. Though I live in Tennessee now, I'm originally from Rock Hill SC. Got to love those Carolina Reapers
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Venice Motor Bikes

Custom Builder / Dealer/Los Angeles
Mar 20, 2008
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1,865
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Los Angeles, CA.
Jun 4, 2024
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Sounds good, Im just going off what RBG does with his engine dyno and the parts he adds on. I have been searching on the internet for some good info as for building 212 kart engines but everyone seems to be pretty tight lipped about everything. Rather disappointing.

I would love to get access to a dyno if I could but its not like people advertise small engine dynos. lol.
 

Venice Motor Bikes

Custom Builder / Dealer/Los Angeles
Mar 20, 2008
7,288
1,865
113
Los Angeles, CA.
Sounds good, Im just going off what RBG does with his engine dyno and the parts he adds on. I have been searching on the internet for some good info as for building 212 kart engines but everyone seems to be pretty tight lipped about everything. Rather disappointing.

I would love to get access to a dyno if I could but its not like people advertise small engine dynos. lol.
I was able to dyno mine because there was a small dyno at a minibike show that I participated in.
Check it out here... @jmbrofficial • Instagram photos and videos
 

sportscarpat

Bonneville Bomber the Salt Flat record breaker
Jun 25, 2009
1,841
478
83
california
Haven't really found a 3" tire I trust for a 212 build. I have run 26 x 2.125 tires. The Maxxis is good and there is a solid pedal cab tire I have used as well. I prefer the downhill MTB tubes, too. Tubes are a big part of the package. Same with rim strips and I buy 21" motorcycle rim strips for my 26" bicycle applications. They are wider and thicker than bicycle rim strips.
 

Tony01

Well-Known Member
Nov 28, 2012
1,789
1,848
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sf bay area
Nice build!! It appears you have the motor sitting on some rubber mounts. I was schooled early on that hard mounting is the way to go. Something about how mc manufacturers like Harley spent hundreds of thousands of dollars and hours trying to make rubber mounts work for bikes in the days before computer simulations and it was all for nothing.

Although I have put over 30k miles going WOT late to work 60+ on 26” tires I would never do it again. I did it just cause I didn’t know any better. You should really consider switching to 21” moto size. The sidewalls are way stronger and won’t be prone to pinch flats. I’m just lucky I never got a front flat cause I used to ride with just a t-shirt many summer days. In this pic I’ve got a cyclops/hookworm on the front and the 3” beach bum on the back. Those 3” lasted only 6-700 miles for me. They were super soft and a little less so with thicker tubes. The cyclops/hookworm at 2.4/2.5 are the best IMO. There’s also this Kenda tire with the raised tread edges, a 1.95, which was very good.
 

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Jun 4, 2024
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Nice build!! It appears you have the motor sitting on some rubber mounts. I was schooled early on that hard mounting is the way to go. Something about how mc manufacturers like Harley spent hundreds of thousands of dollars and hours trying to make rubber mounts work for bikes in the days before computer simulations and it was all for nothing.

Although I have put over 30k miles going WOT late to work 60+ on 26” tires I would never do it again. I did it just cause I didn’t know any better. You should really consider switching to 21” moto size. The sidewalls are way stronger and won’t be prone to pinch flats. I’m just lucky I never got a front flat cause I used to ride with just a t-shirt many summer days. In this pic I’ve got a cyclops/hookworm on the front and the 3” beach bum on the back. Those 3” lasted only 6-700 miles for me. They were super soft and a little less so with thicker tubes. The cyclops/hookworm at 2.4/2.5 are the best IMO. There’s also this Kenda tire with the raised tread edges, a 1.95, which was very good.
Yeah , tried the no rubber mounts and the thing had a severe harmonic that shook like no other at a specific rpm range. Put the rubber mounts in and it's smooth as can be expected. I may try some 10mm thick ones for more rigidity eventually.

I would like motorcycle tires but they won't fit the fork. Eventually I would like to build my own frame out of .12 Dom chromoly and set it up as a true motorcycle. That's for the future though.

Thanks for sharing your build, looks fantastic!
 

Tony01

Well-Known Member
Nov 28, 2012
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sf bay area
Yeah , tried the no rubber mounts and the thing had a severe harmonic that shook like no other at a specific rpm range. Put the rubber mounts in and it's smooth as can be expected. I may try some 10mm thick ones for more rigidity eventually.

I would like motorcycle tires but they won't fit the fork. Eventually I would like to build my own frame out of .12 Dom chromoly and set it up as a true motorcycle. That's for the future though.

Thanks for sharing your build, looks fantastic!
Yes Harmonics is the word I forgot. What’s the max tire width for that fork? I know pirelli had a nice 21 tire but I can’t remember the width. You could check in 19” as well, I know Mitas made skinny tires in that size for mopeds.
 
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Yes Harmonics is the word I forgot. What’s the max tire width for that fork? I know pirelli had a nice 21 tire but I can’t remember the width. You could check in 19” as well, I know Mitas made skinny tires in that size for mopeds.
Unfortunately without making a whole new fork and extending the rear triangle, motorcycle tires won't fit. Requires all new wheels and everything. Pat knows more about it than anyone. If anything I would do the simplex servi cycle tire for this build. From what I know it's Dot rated and fits a 26 bike rim.
 

Tony01

Well-Known Member
Nov 28, 2012
1,789
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sf bay area
Unfortunately without making a whole new fork and extending the rear triangle, motorcycle tires won't fit. Requires all new wheels and everything. Pat knows more about it than anyone. If anything I would do the simplex servi cycle tire for this build. From what I know it's Dot rated and fits a 26 bike rim.
No, you don’t need to do all that to make moto tires fit. There’s nothing magical about them, they come in different sizes. Yeah you’d need new wheels or rebuild existing ones. You said you run the beach bum tires which are 26x3. If 26 bicycle was a moto size it would be approx a 22” rim. So a moto 21x3 or 19x3 would fit fine. An 80/90-21 tire is 3.15” wide and has a 26.5”OD so only a quarter inch taller than a standard 26”. I’ve been running a 120mm motorcycle drum hub I laced into a 17” rim. Tried those moped drum hubs and the performance is lackluster for a 60mph 212 bike (had a 90mm on the rear of that black bike). I can’t recommend bicycle tires any more for motor builds. Only gotten rear flats and with the bicycle tires they left the rim right away and the bike would fishtail. Scary as ****. With the moto tire it was scary too but it stayed on there at least.

Also a 10.4 gear ratio like you have is super low and it’ll be screaming at 60mph. I’ve built for 6k max with a ratio of 7-7.4 and that put me around 64mph top. I’d go no lower than 9 or so, maybe 8.5. Hit around 70.
 
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