weed eater chopper bike build

T

thewikkidrider

New Member
I have a kids schwinn chopper bike, with a 31 cc weed wacker engine. In the process of building it. This is the official build thread. 4.wee.
 
this is my lil brothers build...im helping but hes doing all the footwork...any and all advice is needed

specs
31cc Ryobi weedeater
16" Schwinn kids chopper
old donkey kong skateboard (seatpan)

our first order of business should be to figure out the best mounting spot for this

currently its "in frame" but may be moved...

any and all advice welcome

7/16ths shaft with square port imbedded into the shaft...

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thinking belt drive for the current setup
but may end up chain driven if we rear mount it...

thanks for looking
 
I doubt you will like a belt drive on this engine. They sap too much power. Go with a chain drive.
 
Friction is absolutely the easiest way, but I dislike the tire wear. Just my opinion.
 
Kioshk said:
Why not "Wickedest2". All I can say is "UH OH! Not another one!".
Click to expand...


Haha...jerk...lol...sup kioshk...got any parts layin around?

After a tank of gas in my van we couldnt find a key thats 3/16x3/16x3

Chain drive might work...but we dont have plans for any kind of clutch...direct drive
belt was best choice with an idler pulley for tension

The shaft picturedd below is aluminum...bummer
Was gonna weld a shaft sleeve to increase shaft diameter

But now we r stuck

Options?
 
I built an FD with one of these. Its in the DIY section some where. Seems I threaded a knurled BMX rear axle peg right to the shaft for the driver. Was just about the perfect diameter for an ideal ratio for 31ccs.
 
I can't really tell from the pic, but that square hole might actually be in an adapter a little over an inch long that screws onto the threaded crankshaft. A lot of weed wackers are built that way, although my McCulloch's crankshaft is about 2 1/2 inches long with a square hole in the end.

If that is an adapter, you might want to remove it and see if a bmx peg will thread onto the shaft (like cannonball2 says).

That adapter is usually what's holding the flywheel down, so however you rig it, you gotta make sure the flywheel is secured.

You say a kid's 16 inch bike. Does that mean a 16 inch diameter rear tire? To get usable torque from a wacker motor, you need at least 20:1 reduction on a 26" bike. If your rear tire is 16", that translates to a 12:1 ratio. That's still too much reduction to do without a gearbox or jackshaft setup. That's why almost all wacker motor DIYs are friction drive.
 
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hey cannonball, how in heck did ya get the shaft adapter off? I don't have a piston stop. ive tried rope..no go...I tried the under piston block trick and no go...im afraid to heat it, don't know whats behind there that may suffer from that.

I cannot find a service manual for the motor...id assume because its Ryobi and cheap enuff to just buy a new one rather than fix it..

maybe im wrong...

maybe one of you diy guys have a secret source

model no: 700R
Ryobi item no: 41CD700G034

Ive googled and came up empty...

thanks in advance
 
I wouldn't try blocking the piston to remove the adapter. You'd be putting a lot of stress on the roller bearings of the crank/piston. I've always got mine off by finding a way to keep the flywheel from turning (without breaking fins off). Heating the adapter could certainly help, especially if there is some sort of threadlocker on there. You won't hurt anything if you keep the heat to maybe 300 F. or so. Maybe put some water in the square hole and stop heating when it boils out. Or some low-melting-point solder for a heat gauge.

But I don't think I've ever had to heat mine to remove. Usually a couple small wood pieces between flywheel fins and some immovable spots. Sometimes the ignition module has some sturdy mounting points cast into the motor housing. You may have to get creative with C-clamps or the like.

Good luck.
 
After having another look at your motor, since you have the starter pawls on the flywheel, then you must have the typical simple 4-bolt crankcase cover on the other side. You can remove that cover and block the crank from turning with a block of wood. But don't block it so there is force on the bearings. If you have to, pull the jug, remove the piston, then block the bare crank arm.

But I always preferred blocking from the flywheel side.
 
I have the advantage of air tools. Find some one with an impact. It will remove the nut easily with out blocking anything. Most small shops would probably knock it off for you for nothing. I would have when I ran mine.
 
Got the shaft adapter off....

Days of staring at it untouched and a brain that wont stop got me messin with it...

Took the coil off and stuck a 8" long wrench between fins and one of the cast stud hole thingys...16 mm deep socket and a 24" breaker and popped loose in like a quarter turn...

I got to messing with the adapter a bit and drilled and tapped it obly to have my tap snap off inside...

Bummer right?

But then i remembered seeing a how its made about bridges and those giant threaded couplers ...i thought i may have seen smaller sized ones...

Heres my idea...thread the coupler onto the shaft...get a long enough bolt and washers and flanged hex nuts and stop nuts...make the homemade diy roller that cb2 wrote about...
Probably gonna need a 2 3/4" or bigger roller...put it onto the bolt and lock it down with those nylocks...

Oh and i had boom moment too...harvard frames equal angle iron...

I tripped over a section in my shed and thought damn i just saves myself 15% off my build by kicking that thing...

Ok im done. .pics coming soonish
 
Glad to hear you got that off.

So how long is your threaded shaft now? If you can find a threaded coupler that fits, you can figure out some way to make it work with bolts, nuts, washers, etc.. Last summer at a garage sale, I bought two string trimmers, a Stihl and an Echo, $25 for the pair. Both run good (now). I found out that the Stihl is only 20cc. Probably too small for me to even try on a bike. The Echo is 31cc. Echo is supposed to be top of the line. I found threaded couplers at Menards that fit the shaft - 8 x 1.25 metric about an inch long.

You said "Probably gonna need a 2 3/4" or bigger roller". You're referring to the length, right?
 
No...im speaking of the roller diameter...roller will probably be 3 1/4" long x 2 3/4" dia.

There is about 3/4" of threads sticking out..

The good thing is that, as the motor spins, its trying to tighten the coupler/roller, so no worries about it backing off...

Got a few things to do today...then back to building...

Gonna hack up that harvard frame to make my mounts and pivots...
gonna use a spring to keep the roller down and a handle to lift the motor/roller off the wheel...

I messed with wikkidrider and flipped the frame upside down...

The bike looks hilarious...ill take a pic in a bit...
 
A 2.75" roller is pretty ambitious for 31ccs. You need to let the little engine spin to around 7k rpm. A 1.25" calculates to 26mph@ 7K. Probably a good point to start. If the 31cc could spin the 2.75@7k you would make 57mph!
 
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