MTB to motoped/Enduro/dirtbike thing

[Radar00]

Well today I started the butchering of my old MTB.

I was inspired by this thing.

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It had been sitting in the shed rotting for the past 5 years. Has front and back suspension and I actually have two of them. Rescued from the trash years ago.

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chopped the head tube off since i'm going to make a proper C type frame thing like the motoped in the above pic. I'm using a length of 36 in gas tube for that. and will basically just cut and re-weld the tube in the proper frame shape since bending isn't going to happen. The gas tube is like 1/8th thick.

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Chopped and cleaned up the front forks. The irony is- the handlebars are rusted in there so I have to bang em out (eventually. For now I'll keep em together just to get a better idea when I lay out the frame over the weekend.

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[Agreen]

Looks cool! You have my attention now

 

[Radar00]

Not much really done today. Only really got to chop up the rest of the bicycle to get the rear tire and suspension. I laid out the frame for what I'm thinking. Still haven't decided on the engine yet. So far overall wheel base is 51in which suits me just fine. The tires are the original 24in MTB tires which i'll keep until the frame is rolling. The seat tube will come off the down tube and I'll add some gussets which will help me mount that rear suspension. Additionally I'll put a fairly large gusset between the top and downtube and once i decide on an engine, i'll figure out how to close up the frame (depending on engine size)

Interestingly enough I have some old cranbrook front forks laying around. I'm tempted to put a large engine in this frame.

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[Radar00]

So after some grinding and welding and measuring I finally have something resembling a frame. It seems like nobody knows how to change a rear tire as this one's dropouts were bent so I have to torque them back into proper alignment. Next is welding the the frame together solidly. This is basically just tacked together at the moment to check for alignment. I'm going to also have to cut some 3/8 steel plate and begin making gussets for the joints for added strength and also figure out how im going to join the whole thing together. Also waiting on my engine which is just going to be a cheap chinagirl for now. maybe in the future i'll put something better in it.

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Originally this was going to be like a dirt bike but due to the length of the wheelbase, it's looking more like one of the sportsman frames. Atleast I think so. Even still with dirtbike plastics on it and a headlight it could still pass i think

 

[sbest]

I like the idea of a backbone frame.

Simple and rigid if the tubes are large enough.
If your going to make the tubes larger, you might as well use them to hold fuel:

 

[sbest]

Another nice design that only needs pedals:

 

[ezrider]

Looks cool! You have my attention now

And mine too !!

_

 

[WECSOG]

The motoped concept is of interest to me, too. That pedal/jackshaft arrangement looks like the weak link, though. Also I don't see any reason to use 26" MTB wheels and tires, as the Motoped does. Another weak link. Both problems can be solved, IMHO, by using a moped wheel and 17" trail bike tire (CT90, etc.) on the back. Probably on the front too.

Seems a shame not to use some kind of small motorcycle engine, complete with gears unless you are trying to remain legal in a state that doesn't allow gears on a motorized bike. Even then you can get a single speed, 50cc clone of the Honda horizontal engine.
Another option would be a small-motorcycle 2 stroke engine. Maybe an RM80 or similar.

 

[Radar00]

Since I posted this I've kind of scrapped the frame I originally built and am looking at buying a junked dirtbike/moped frame and then modifiying it. I'll post more as I find a frame thats suitable

 

[WECSOG]

You might consider starting with a Honda Z50, XR50 or CRF50 frame and building on that. It is a backbone frame that is designed for the horizontal Honda engines, and the clone engines from 50cc all the way to 160cc are designed specifically to fit it. Put long forks and swingarm on it, add some 17" wheels and you're almost there.

I built something similar using a Honda XL70 frame, longer forks and swingarm, and 17" rear/19" front wheels with a 125cc clone engine. I ride it everywhere and have several thousand miles on it. But a 50 frame might be an even better starting point.

Here's my little bike:

 

[ezrider]

Whats the specs on that ?

 

[WECSOG]

Whats the specs on that ?

It weighs about 200 lbs, I guess. 1974 Honda XL70 frame; early '80s XR100 swingarm and forks; tank, bar, seat and fenders from a 1981 XL80, front 19" wheel from an early '70s XL100, rear wheel (with overlay sprocket for a mountain gear) from a late '60s Trail 90. The engine is a Jialing/SSR 125cc with manual clutch and 4 speeds. It gets 80+ mpg if I run it like I'm trying to kill it, and over 100 mpg if I putt around at 25-35 mph. It is tagged and street legal.
I'm thinking about putting a 140cc Piranha engine in it.

 

[ezrider]

Must have been quite a scavenging hunt. Bikes reminds me of the CB-100 I once had:

https://www.youtube.com/watch?v=0Zy7uUNGNXc

_

 

[WECSOG]

CB100 was a neat bike. So was the CB125. Sym sells a 150cc clone of the CB125, called the Wolf Classic. I have more fun building my own stuff, though.

 

[ezrider]

NICE:

https://www.youtube.com/watch?v=vkXICxtJqqc

_

 

[WECSOG]

That is a nice bike. I thought it was pretty funny though when he said he had only put a couple hundred miles on it because he had only had it a couple months. In southern California where it's always nice riding weather? I'm pretty sure I would have a couple hundred miles on it before the first weekend!