motorized mountain bike build

[pdub42]

Hey guys, I am just starting work on building my first motorized mountain bike. The bike is a vertical pk7

I have an engine from a weedeater lawn mower it is a 4.5 hp briggs and stratton 450 series motor.

I also have a 70 tooth #40 gear for the bike and a 11 tooth gear for the motor. I am not quite sure if that is geared low enough with just that but I have been looking into jack shafts and have not found a cheap option to lower it more, and I am thinking I may try it like that and see how it works. I think it was coming out to about 10mph at a little over 1000 rpms. I am planning on trying to mount the gear on the opposite side of the bike as the main gears so that I can leave all the other gearing alone and it will not make the pedals turn when powering the back wheel from the other side. Also this will allow me to have the motor somewhat centered over the wheel wheras I would have to have it way over to the side to get the rotation in the correct direction for the other side. I am still looking for a good method to mount the main gear on the back wheel, I found some adapters on pirate cycles but $50 seems expensive does anyone else have any other options they have used? I also am planning to build some sort of rack over the back wheel to mount the motor on. I will post pictures as I work on this, hopefully it will be a success.

 

[NormO]

Welcome to the forum. I'm sure other members will help out here but i'll start. How do you plan to mount the engine ? I hope you don't plan to mount it with the shaft horizontal. This can cause a lot of problems and you're engine won't last very long. Unless it's a 2-stroke, But it doesn't appear to be.
There is a gear ratio calculator listed in the DIY section. I suggest you download it. It makes life so much easier when figuring out ratios. It's been said a good gear ratio is 20:1, the gear ratio you plan on is 6.36:1.
Nice looking bike.

 

[biknut]

That's going to be a difficult bike to motorize with a gas engine. It would probably be easier to go electric. Good luck.

 

[fishyfish777]

I hear that a worm gearbox from somewhere like staton will reduce your output significantly and orient your power in the correct direction - but I'vent got much clue in terms of vertical shaft engines.

 

[NormO]

The only thing coming to mind as far as vertical shaft gear boxes are concerned are from older ride-on lawnmowers and lawn tractors. May take up way too much space and add too much weight.

 

[pdub42]

Hey guys, thanks for the responses, I already have a plan for the engine. I am going to run it horizontally above the rear tire. I have taken the engine apart and check the way the oil system is setup and all that is needed is to adjust the level so that the oil splasher is still getting oil to throw up on everything. I'm not sure how many of you have seen the insides of these things but they literally just use a plastic gear with paddles that just splashes oil up on everything to keep it lubed, its crazy haha. I remounted the carburetor in the correct orientation as well and its good to go as a horizontal engine. It still may not be as good as a true horizontal engine but it appears that it will work for the time being. I have salvaged some steel bracing that I am going to try and build a frame for above the rear tire to mount the engine on. Hope to get that done this weekend but you know how that goes.....

 

[pdub42]

I hear that a worm gearbox from somewhere like staton will reduce your output significantly and orient your power in the correct direction - but I'vent got much clue in terms of vertical shaft engines.

I did look into worm gearboxes as an alternative to converting the motor to a horizontal but they were very expensive for any that would handle even a few horsepower and required lots of lubrication because of all the friction in those things. Plus I would have to incorporate a clutch of some kind and would have to pull start the engine as I would be unable to turn the motor over by peddling because the worm drives only transfer power in one direction. I did find some shaft driven rear parts from a motorcycle that may have worked but they would be big and heavy and probably not as efficient as a chain drive system in this case.