Motorized Bicycle Product

Overview

A motorized bicycle is the cheapest form of motorized personal transport that exists: a standard pedal bicycle with a small gasoline engine clamped into the frame triangle, driving the rear wheel through its own chain. The format is a century old — Smith Motor Wheels and cyclemotors predate the moped — but its modern incarnation is the Chinese-pattern two-stroke kit: a 66 cc engine, tank, carburetor, exhaust, controls and drive hardware in one box for one to two hundred dollars, fitted to a donor bike in an afternoon with hand tools. Millions have been sold, and the build remains a standard first project for backyard mechanics.

The engine

The kit engine, universally sold as "80 cc" but actually 66 cc (47 x 38 mm), is about the simplest internal-combustion engine in production. The Cylinder and Head is piston-ported: the Piston skirt itself opens and closes the intake, transfer and exhaust ports as it moves, so the engine has no valvetrain, no cam, no reed block on base versions. Lubrication is premix — oil mixed into the petrol at 25:1 — so there is no oil system either. The Crankshaft runs on two Ball Bearing mains behind Oil Seal closures, and the Flywheel Magneto on the crank nose powers a CDI Box with no battery anywhere on the vehicle. Output is 2-3 hp at around 5,000 rpm, delivered with the characteristic two-stroke ring through the baffled Muffler Body.

The engine clamps into the frame with two U-bolt Engine Mount Clamp fittings against the down tube and seat tube, which is why the Steel Diamond Frame must be steel and conventionally proportioned — oversized aluminum tubing and interrupted triangles are the most common fitment failures.

Two separate chains

The drive layout is the kit's defining trick. The bicycle's original Pedal Drivetrain stays untouched on the right side: crank, chain, single-speed Rear Freewheel. The engine drives the left side through its own heavy 415-pitch Drive Chain, from a 10-tooth Engine Drive Sprocket to a 44-tooth Wheel Sprocket bolted straight through the rear spokes between rubber clamp discs. The freewheel on the pedal side lets the wheel overrun the pedals when the engine drives; the engine side has no freewheel, so the motor turns whenever the rear wheel does and the clutch is engaged — which is exactly how the engine is started.

There is no kick or pull starter. The rider pulls the Clutch Lever (a lock button holds it out), pedals up to jogging speed, and drops the clutch; the rear wheel spins the engine through the chain and the Helical Gear Pair primary reduction until it fires. The Manual Clutch is a small multi-plate stack of cork-faced Clutch Plate discs squeezed by a single central Coil Spring, unloaded by a pushrod when the lever is pulled. With one fixed gear ratio of about 4.4:1 at the chain and 4:1 in the primary, the engine lugs below 25 km/h and buzzes hard past 45; the machine has a clear cruising sweet spot around 35-40 km/h.

Fuel and controls

Fuel falls by gravity from the 2 L Teardrop Fuel Tank on the top tube, through a Fuel Petcock with sediment bowl, into a 14 mm NT-pattern NT Slide Carburetor — float bowl, slide, one jet, a choke lever for cold starts. The donor Handlebar gains a Twist Throttle, the clutch lever and a Kill Switch that simply grounds the magneto. Consumption is around 2 L/100 km, giving roughly 100 km per tank.

What the kit does not upgrade

The donor bicycle's running gear was designed for half the speed. The Rim Brake calipers, the spoke tension, the tire ratings and the headset bearings all see loads outside their design envelope at 45 km/h with 11 kg of added drivetrain, and the experienced builder's first money goes to brakes and tires, not engine tuning. Vibration is the second chronic issue: a single-cylinder two-stroke bolted rigidly to a thin-wall steel frame loosens every fastener it can reach, which is why thread-locker on the Wheel Sprocket bolts and engine mounts is treated as part of assembly, not maintenance.

Legal position

Most jurisdictions class a 66 cc gasoline bicycle as a moped or motorcycle, requiring registration, licensing and insurance that almost no kit bike carries; some US states allow them under 50 cc/30 mph definitions the 66 cc kit exceeds. The kits persist anyway, occupying the same niche they did in 1915: minimum-cost motorization, one clamp at a time.