My First build, The Monstosity

[Vecken]

I was just doing some research on some other motorized bikes and that inspired me to build my own. it is still in progress and maybe you guys can give me some input on what i can do with the drive train.

I have a list of sprockets i already have from junk bikes that i can use. for the rear end i have 2 speed 48t/38t off a front crankset that will be going on the opposite side of the wheel, but im trying to find the best way to mount it solid with enough clearance between the spokes and chain stay.

A rear hub cassette that ranges from 14t and 28t. and a coaster sprocket 16T. im aiming for maybe a 18 to 24:1 ratio as i want to be able to get a good speed on slight inclines.

so here she is: http://i.imgur.com/Yj7YX.jpg

Its a gary fisher tarpon 5 series frame i got in a trade, cromoly steel frame. im using a rear 7 speed wheel off an old trek.

The engine is from a Husqvarna 350, 52CC that puts out almost 3.2HP. the housing partially melted from the exhaust so its useless as a chainsaw. its only 8lbs and has a small enough profile the crank arms can pass right by it.

Im mounting the engine backwards because i want to have a gear attached to the clutch to do the gear ratio for me and greatly increase reliability, but need to figure out how im going to attach it, and then have another sprocket going from that to the rear wheel. has anyone done something similar to this before?

I'd really appreciate some comments or suggestions. thanks!

ps. im in maine and if there is anyone nearby with a mig welder that can do just a couple of spot welds that would make this project much easier.

 

[5-7HEAVEN]

It is not simple to gear a chainsaw engine. The main reason is its lack of a heavy-duty clutch, a lack of a gear reduction gearbox hookup and a penchant for low gearing.

Get the lowest cassette gearset, like 11t-34t. If you can get the 16t to work, that's 2.125:1 for starters. You need to create 13.176:1, which will require two more jackshafts. If you can use an 8mm T8F 72t chainring sprocket and 11t jackshaft sprocket, that's 6.545:1 and 2:1 to go. Run a 10t engine sprocket chained to a 20t jackshaft sprocket.

There you go.....

Methinks that your chainsaw engine will want MUCH stiffer gearing. I'm using 46.32:1 first, and my engine SCREAMS! Final drive is 15:1, an excellent highway gear.

 

[Vecken]

i was hoping to use the 2 speed crankset attatched to the wheel because its fairly heavy duty and is perfectly flush with the wheel. and as an added bonus i keep both gears that i can change simply by adjusting the tensioner for the type of terrain i will be using.

the scrap cassette from 14 to 28t i was going to use as a chain tensioner/gear reduction jack shaft (chain from engine goes to 28t, and then 14t to 48t) so what i need is a gearbox off the shaft of the engine that i attatch another sprocket to go to the 28t. is this feasable to have 2 chains? or just spend some $$ on a gearbox so i reduce it to one chain for reliability.

it would be awesome to have 2 options for gears that i can change simply by adjusting a tensioner. one for smooth mostly flat road, and a second for steep hills/ light trail use.

 

[wheelbender6]

If all this gear reduction doesn't sound appealing, you could add a roller and friction mount it.
On the other hand, it will not live up to the name of Monstrosity without plenty of mechanical complication like gear reduction.

 

[Vecken]

the plan of this build is to actually use it to commute and to make it completely modular, have it clamped to the frame in 4 places so i can put it on any 26" mountain bike. it fits just too well in the frame and i have the rear wheel built now with the 38/48T sprocket i was able to make clamps directly to the hub without putting strain on the spokes. i will take apart the clutch and see what size shaft i have to work with. what would be the best gear ratio to run in a crank case mounted right to the engine? im aiming to not even engage the engine until 10mph, and then the second rear gear able to start from a stop. ive found the engine idles around 2700rpm and will go all they way to 11000.

 

[5-7HEAVEN]

Is your 38t/48t wheel sprockets on the cassette-side or left-side?

Your 2-speed crankset on the wheel...is it an internal wheel hub?

Are you going to use the 14t-28t cassette as a jackshaft, directly above the chainrings? Keep in mind that rear cassettes run clockwise when observed. If your engine spins counterclockwise, it will not turn the cassette.

What do you mean by changing gears by adjusting the tensioner?

 

[Vecken]

sorry i was not very clear, the 38/48t chainring is on the opposite side of the cassette and is mounted solid to the hub. the engine shaft is on the left side with the driveshaft spinning counter clockwise opposite the wheel, so i have to have at least one gear to reverse its direction. with that one gear i would like to mount a 14t on the outside so there is a single chain running to the rear wheel, working much like a regular motorcycle. ill take pics of the setup that i am trying to do. thanks

 

[5-7HEAVEN]

Ok, the picture's clearing up. 38t/48t on the left side, engineshaft running CCW on the left side.