2 speed 79cc predator build

TheNecromancer13

TheNecromancer13

Active Member
After building a few 79cc predator bikes for friends of mine and having them turn out far better than the first one I built for myself, I want to have the best bike again, so I'm starting a new build on an old trek 820 with a 21" frame that I got off craigslist for $150. I figure I'll keep my first bike around as a backup bike so I can still get to work if my other one is being tinkered with.

Here is the bike when I bought it... http://imgur.com/nuCdsYm

And here it is now... http://imgur.com/5TirMqs

I have replaced the junky front fork with a nice new one, stripped it down to the frame and painted the bike black and bright red, added a high end front disc brake with a 200mm rotor, replaced the seat, converted it to single speed, replaced the brake levers, added grip shifts (left grip shift will become a selector for my 2 speed transmission, right shift is being converted into a twist throttle), replaced the wheels with really sturdy ones (36 spokes, double walled rims, puncture resistant tires and inserts, etc), ground the teeth off the largest front sprocket and turned it into a chain guard (no more pants getting caught in the gears for me), cleaned and serviced everything, replaced the cables and cable housings, and a few other things I can't remember.

The stuff I still need to do...
Finish modifying my new predator engine. (mostly governor removal, and a few other little things)
Mount the engine to the frame. (I have figured out an adjustable mounting system for these engines that eliminates the need for a chain tensioner)
Add aftermarket exhaust and air filter.
Mount the rear sprocket to the disc brake hub.
Throttle linkage.
Build a 2 speed dog clutch transmission from scratch. (I could probably buy one, but what fun is that?)
Extend the pedals.
Buy more stuff.
Other stuff I can't remember right now.

I'm going to try running gear ratios of about 20:1 for the low gear and 10:1 for the high gear. Hopefully I'll be able to hit 50mph and still have enough low end power to take off from a standing start without pedal assist. Should be fun.

I will post more pics as I continue building.

On another note, there will be an instructable on this bike!
 
In the dawn ages of motorcycling, Scott made a 2 speed transmission using chains, I'll see if I can find details of how it was done.
 
Well, I've got it as a single speed bike for now, too impatient to wait for myself to finish building the transmission, had to get it running, but I think it turned out really well so far.
HSXHZ9Q.jpg
 
Wow! Great job of shoe horning the engine in to the frame!

Looks like it should have a pretty decent top end judging by sprocket size.
 
Nice! Very nice! I hope it works with those bolts. Replacing them might be a bit of an issue though. As they are used the threads will start to deform at the interference points, so removing the nut will be difficult. Other than that I think it will be good. Also a 70% jump will be good too, for a manual shift 2-speed. Consider making the sliding clutch holes into slots as there will be a very high rpm differential between the 2-speeds and lockup will be rough.
 
Turn the bolts around and saw off the heads and use the unthreaded shoulder. If you file lead in ramps into the hole it might engage a bit easier.

This makes me think of using outboard shift dogs, the shaft and the slider.
 
beginner01 said:
Nice! Very nice! I hope it works with those bolts. Replacing them might be a bit of an issue though. As they are used the threads will start to deform at the interference points, so removing the nut will be difficult. Other than that I think it will be good. Also a 70% jump will be good too, for a manual shift 2-speed. Consider making the sliding clutch holes into slots as there will be a very high rpm differential between the 2-speeds and lockup will be rough.
Click to expand...

I will be grinding the threads off, turning the bolts into smooth nubs with rounded ends in order to help with engagement.
 
So what exactly was the problem with this transmission? What needs to be done for it to work? I would like to build a closer ratio 3-speed similar to this someday (keep the rpm differential between clutches lower).

You should talk to member Thud on endless-sphere. He built and sold around 10 of these... For something like $500 a pop I think....
 
beginner01 said:
So what exactly was the problem with this transmission? What needs to be done for it to work? I would like to build a closer ratio 3-speed similar to this someday (keep the rpm differential between clutches lower).

You should talk to member Thud on endless-sphere. He built and sold around 10 of these... For something like $500 a pop I think....
Click to expand...

It was simply a matter of me not being able to machine parts accurately enough in my garage. If I had a good drill press I could do it, and I may be getting one soon, but for now a handheld drill isn't cutting it. I made 4 plates and none lined up well enough to be used together. If I get a drill press I'll try again and let you know how it goes.
 
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