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Weed Whacker-Powered Bicycle Answered

This is a bike I built a year ago. It uses a 30.5cc (about 1.5hp) weed whacker engine for propulsion.

A version of the same thing was on Instructables, and that's what partially inspired me to make this. ;)

The engine mounts are made out of wood, because I still have not learned to weld, but they hold up just fine.

The engine is an ten-year-old Echo weed whacker. It has 10,000 max RPMs. The bike peg attached is 2" in diameter and gives about 25mph top speed.

There is no clutch! The one that came with the engine was way too flimsy for a vehicle like this so I took it out. Saved a lot of space, too.

For the throttle, I disconnected the back brake line and connected it to the engine. The front brakes are still intact, and you can turn the engine off to slow the vehicle as well.

I added the extra heatsink to the engine to help keep it cooler, as it was never meant for this kind of exertion. It gets as hot as the engine, so I guess it conducts the heat away pretty effectively.

There is a 10-LED bargraph to show the engine temperature. It uses an LM3914, an op-amp, and a thermistor. There is also a 555 to drive a buzzer when it gets too hot.

The bike has head and taillights that are both driven from a joule thief. They are not very bright; they are more for being seen than to see.

Here's a youtube video, taken almost a year ago:

Some more photos and a little more information:

Comments/criticisms? :)


Dr Qui

8 years ago

Great build, how road legal is it where you live?

Under UK laws it would now need license plates, road tax, MOT test, insurance, a valid driving license and a dot approved safety helmet.

I've looked up the laws for California but it seems this thing would be in a bit of a gray zone. It meets some requirements for a motorized vehicle but not others.

Of course, I wear a helmet. It's illegal to not have one on a bike. But as for the legality of the bike itself, I am not sure.

why did you go for a friction-drive instead of a chain?


The gearing is just about right and its the most simple way of transferring power, a bit sore on tires though.

Totally illegal on UK roads of course, shame really.

I tried a 100watt electric scooter drive on my cruiser this week, resounding fail, well it worked and would be perfect for say Holland not the rolling hills of Co Down

The real problem is loss of friction if the friction drive gets wet.


Yes, friction drive is a lot easier and quicker, at the expense of the tire. I used a bike peg because it's what I had lying around. A small air-filled tire might do the trick.

A chain is, of course, a much more efficient way to transfer power. But I didn't spend a single thing on this bike besides the $10 mounting plate, and it's still working just fine a year later. I didn't have to replace the tire, either.
It seems that once the tread wore off, the smooth tire grips a lot better and does not wear any further.

That is simply awesome ! I always wanted to to an engine powered bike !

Why not make an Instructable, or at least a slideshow? Note the addendum to the Zeroth Law.

Very cool! I like the little electronic box o' fun.

Hmm, I guess I should. :P I'll do that now; thanks for the suggestion.


Reply 8 years ago

You know, I was about to post a slideshow, but thinking more about the law, I think I'll do an Instructable instead. That will not be coming immediately, but it should be done soon.


8 years ago


. Most excellent!