Next 2 stroke build idea.

ImpulseRocket

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The gist of this is, now with my E-bike being my main "conern" for riding my Phantom powered Kent Nazz is going to collect a little dust. Recently my girlfriends son - 15 years old and has an interest in bikes, cars, etc. expressed interest in my bike and was wanting to build one for himself to get to and from school, but being a 15 year old has a somewhat limited budget. He's a good kid and is trying to get a job near home so he can make some cash to build said bike. I talked to his mom and she is ok with him building a bike, so I am going to "sell" him my Phantom engine, fuel tank, and other parts that are on my Kent by trading his labor for them. Obviously nothing too crazy, but he is going to help me fix a retaining wall that is starting to buckle and do a little yard work for me as well. His mom already bought him a Hyper cruiser on my recommendation for his upcoming birthday. He has to figure out how to buy the wheels and a few other parts to upgrade it, and I have a bunch of surplus parts I can donate for things like the brakes.

I am going to convert my Kent Nazz back into a pedal bike for now, and I may look into converting it to an ebike, OR using it for a new build idea I have, which is the purpose of this post. Mainly, I am not sure if my Nazz would be a good bike for said idea since the frame space is a little limited.

I mainly want to get away from the standard bicycle engine kit 2 strokes, aka "China Dolls" and all of their variations and just get into something a little different. I really like the idea of using an AX100 clone engine to make a "shifter" bike, but those are a bit on the big side, especially for a road bike frame, and I want to avoid major frame modifications.

That leads me to the KTM 50SX style engines, aka the little 50cc 2 stroke pit bike motors. I know that others have used them in the past for motorized bike builds, so it's nothing ground breaking. Link to Aliexpress for an example. I specifically want to use the air cooled variety for simplicity and safety.

The upside to these little engines is, they make great power for their size, with the original KTM design making 7hp and most of these clones claming 8.5-ish HP at around 10k rpm. The downside is, they make zero power below about 7000rpm. They use an oil bathed centrifugal clutch that can be modified with different springs and other modifications to alter the engagement RPM to ensure they end up in the power curve, and they are designed to slip without eating themselves alive. I have found some great information on said clutch changes in order to ensure the engagement point allows the engine to get into it's power curve and not bog out.

Parts are also cheap for them, and there are 9-12 tooth sprockets that drive a 415 chain. Making a motor mount solution would also be relatively easy.

The only thing I ask people to understand is that this is just a tinker project. Just something fun to create and problem solve using a non-typical engine.

Until I know the primary reduction ratio on the engine, something I am actively looking for, I can't really plan out potential gearing combinations. The water cooled engine would be more fun, and the cooling system for them is about 50 bucks, but they also make quite a lot more power and even higher RPM (12-15hp at 12,000rpm) and I am not quite certain I want to try that on a bike. The goal isn't really speed.

I am considering combining one of those little buzzers with a 3 speed IGH jackshaft in order to be able to manipulate gearing for climbing vs. flat ground.
 
Just for an idea, this is an example of one on a bike build. He has a few other small videos out there if you are interested in how he mounted it (basically vertical) and it works out ok. You can see in the video when he gets a chance to open it up how it somewhat bogs until he manages to get it past about 32mph and then it really gets into the power curve. This is what I was talking about with the clutch engagement as well as gearing.

 
Does that engine shoed in the video have a transmission or is it just single speed?
No transmission, just a direct drive exactly the same as the engines in the nomal kits. The only difference is the clutch style, which is a wet bath centrifugal clutch and bell somewhat similar to your BT engine.

This guys video goes into some depth on the clutch springs and other mods he did to get the engagement he was after. A little bit of a long video, but some good info in there.
 
Does that engine shoed in the video have a transmission or is it just single speed?
There is a reduction from the crank to the clutch basket but, only one speed. Thus why this was written:
I am considering combining one of those little buzzers with a 3 speed IGH jackshaft in order to be able to manipulate gearing for climbing vs. flat ground.
 
No transmission, just a direct drive exactly the same as the engines in the nomal kits. The only difference is the clutch style, which is a wet bath centrifugal clutch and bell somewhat similar to your BT engine.

This guys video goes into some depth on the clutch springs and other mods he did to get the engagement he was after. A little bit of a long video, but some good info in there.

Seems neat, I think it would be better with a gearbox.
 
With the tunable centrifugal clutch and the power it has, it is perfect for riding around town and back roads. Should have no trouble keeping up with 35-40mph traffic and getting off the line and up hills.

Gearboxes are fun but it gets very expensive or very big, very quick.

The used and new bike markets are flooded, there should be no trouble finding a nice bike/frame for a fair price. It would be a huge upgrade to get a frame in the right rather than a one size fits all walmart bike.
 
With the tunable centrifugal clutch and the power it has, it is perfect for riding around town and back roads. Should have no trouble keeping up with 35-40mph traffic and getting off the line and up hills.

Gearboxes are fun but it gets very expensive or very big, very quick.

The used and new bike markets are flooded, there should be no trouble finding a nice bike/frame for a fair price. It would be a huge upgrade to get a frame in the right rather than a one size fits all walmart bike.
I would sacrifice speed for pulling power. Given the higher rpm range and somewhat narrow power curve of the engine I would be aiming to utilize a rather large rear sprocket in order to keep the top speed around 35mph. In the video of the bike I posted above, around the 5 minute mark you get to see that engne actually reach the powerband and it takes off pretty quick past 35-40mph.

The desire of the 3 speed IGH "transmission" is more of a way to get around gearing compromises. Second gear being the "normal" gear, which in my case would be that 35mph top speed, and then having a gear in either direction to help with hill climbs or drop the RPM down a little to help keep it cooler.
 
I would sacrifice speed for pulling power. Given the higher rpm range and somewhat narrow power curve of the engine I would be aiming to utilize a rather large rear sprocket in order to keep the top speed around 35mph. In the video of the bike I posted above, around the 5 minute mark you get to see that engne actually reach the powerband and it takes off pretty quick past 35-40mph.

The desire of the 3 speed IGH "transmission" is more of a way to get around gearing compromises. Second gear being the "normal" gear, which in my case would be that 35mph top speed, and then having a gear in either direction to help with hill climbs or drop the RPM down a little to help keep it cooler.
The benefit of a 3 spd IGH with that engine is that engine has a kick starter and won't need to be pedal started. Therefore you don't need to work around that issue.
 
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