Well, using chains, sprockets and a freewheel seem like a simple way of connecting a small engine to the bike's bottom bracket(BB). Engineering two engines to hook up seems do-able, especially since no components need be custom made. Just drill matching holes between freewheels and sprockets, then assemble.
And don't forget to fab chainguards!!
This morning, I awakened with an ingenious manner of using timing belts and pulleys to connect twin engines with the bike's BB(Single engine installs can benefit too)!
1. Install a go kart's driven pulley to the freewheel on SBP's pedal arm. Instead of serpentine belt system, karters proved to use timing belts/cogs with 5mm/8mm/10mm pitch. Then install the driving pulley onto the jackshaft or engine's PTO.
Kart pulleys range from 9t(.90" overall diameter{OD}) on the driver pulley to 143 teeth(8.96" OD) w/5mm pitch teeth. They have a 5.25" bolt circle diameter(BCD). SBP's pocket bike freewheels' BCD is about 54mm, or 2.12" BCD. Obviously the freewheel won't bolt onto the kart pulley, so an adaptor has to be fabricated. Find any 6" OD or larger spare chainring sprocket with a solid core and a 2.12" BCD. Drill the sprocket with six holes in a 5.25" BCD, to accept 1/4-20 bolts. Then machine off the unwanted sprocket teeth.
Now you've made the adaptor to connect the large driven timing pulley to the BB's freewheel of your bicycle.
Using correctly-sized pulleys, a gear ratio as high as 15.89:1 can be attained.
You can attach the small driver sprocket to your jackshaft. If its inside diameter(ID) is too large, use a larger jack shaft diameter or an adaptor reducing bushing. Measure for appropriate length and buy the belt.
You'll probably need to use an idler system.
Now to fabricate an adaptor to install a timing pulley onto a small engine, using the 6-spline/13mm shaft on the pocket bike's 5:1 clockwise-spinning(CW) gearbox.
Using the 6-splined 5:1 gearbox produces spectacular gear ratio combinations. The 11t/72t T8F combo w/5:1 gearbox yields 32.73:1 gearing. Linked to a 34t/12t cassette, that's a stump-pulling 46.36:1 and cruising in final drive at 16.36:1.
Now with the go kart pulleys, gear ratios could be as low as 112.55:1 low and 39.72:1 cruising.
Wayy too low? Ok, choose bigger drive sprockets and smaller driven ones.
A 14t driver with a 90t driven pulley gets 45.54:1 First and 16.07:1 final drive.
Perfect for a high-winding/stump-pulling/hill climbing bike and city speed limits with a heavy rider.
The biggest hurdle of using serpentine or timing pulley systems is attaching the driver pulley to the 6-spline/13mm shaft on the 5:1 gearbox. After that, it's a simple matter of choosing the corresponding pulleys, belt and idler system.
Enter the go kart's driver/driven timing belt and pulleys.....race-proven, easy to find used and new, light and relatively inexpensive, with the correct gear ratio combinations!
Parts needed to adapt the go kart's driver/timing pulley onto the 5:1 pb gear box:
A $4 14t/T8F pocket bike sprocket from Ebay.
A $10 used 14t x 20mm x 8mm go kart drive pulley from Ebay.
Weld the two together; the go kart pulley now has a 6-splined center.
However, the machined groove and the snap ring on the gearbox's PTO(power take off), which secures the 14t sprocket is now inaccessible.
Disassemble the gearbox and remove the gear box's 6-spline PTO shaft.
See how delicate the gearbox's internal gears are!
Weld a 11mm or 7/16" shaft 3" long onto the splined end of the PTO. Be careful not to damage the splines.
Using 3 mounting bolts of the gearbox, fabricate a bearing support for a 1 5/8" OD bearing with a 7/16" ID.
Welding on the 11mm shaft and fabbing the bearing support are the only fabrication needed. Everything else is simple bolt on stuff.
Reinstall the PTO into the gearbox.
Bolt on the bearing support.
Trim the shaft and machine a groove for the new snap ring( or thread the shaft for an 11mm or 7/16 nut).
Voila! Using the proper length/width timing belt and idler pulley, the go kart pulley system can now bolt on and provide the correct gear ratio and connect to the BB of your bicycle's drive system.
Disclaimer: FWIW, this is all theoretical.
I haven't created it yet, because it's simpler for me to chain together TWO separate 11t 6-splined drivers to TWO separate 72t sprockets, then bolt it to TWO separate SBP freewheels and ONE 24T chainring sprocket, for a "V-twin" engine project.
It was MUCH simpler using a single engine and chaining ONE 11t cog, ONE 72t driven sprocket, ONE freewheel and a 24t chainring sprocket.
If all these chains and sprockets fail into one hot mess, then I'll try go kart timing belts and pulleys on my "V-twin" project.
But I'll still try to do the go kart pulleys on my Predator 212/shift kit project.
And that's another one on the burner.
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