A chain is an exposed, dirty component that needs regular attention, yet it remains the standard final drive on most motorcycles. The reasons are mechanical rather than traditional.
Efficiency is the first argument
A clean, correctly tensioned chain transmits a very high proportion of the engine's output to the rear wheel. Little energy is lost in the roller and sprocket engagement.
A shaft drive must turn power through ninety degrees twice, and each bevel gear pair loses a measurable amount. On a machine where power-to-weight is the point, that loss is unwelcome.
Belt drives sit between the two but need wide pulleys to carry high torque, which becomes awkward to package on a sports machine with tight clearances.
Weight sits in the worst possible place
Final drive components hang at the rear of the bike, largely below the suspension. Mass in that position is unsprung, meaning the suspension must control it directly.
Unsprung mass resists rapid changes in direction, so a heavier rear end follows bumps less faithfully and reduces grip over uneven surfaces.
A chain and two sprockets weigh far less than a shaft, its housing, bevel gears and the oil they run in, so the chain wins on the metric that matters most.
Gearing becomes a five-minute decision
Changing a sprocket alters the overall gearing of the machine. A rider can trade top speed for acceleration, or the reverse, for the cost of a single part.
That flexibility matters for track use, for touring with luggage, and for adapting a machine to terrain, and it is exploited constantly by owners.
Shaft drive fixes the ratio inside a sealed housing. Changing it is a workshop job involving the removal of the drive unit, so almost nobody does it.
Suspension design stays free
A shaft drive reacts against the swingarm as torque is applied, which tends to make the rear of the bike rise or squat depending on the layout.
Designers can counteract this with additional linkages, but each addition brings weight, cost and complexity to an area already crowded with suspension hardware.
A chain applies its load along a line that engineers can position deliberately, giving them a straightforward tool for tuning how the bike behaves under power.
Where the alternatives win
Shaft drive dominates on large touring machines, where the rider covers long distances in poor weather and values a sealed system that needs almost no attention.
Belts appear on cruisers, which run moderate power through relaxed gearing and have the space for wide pulleys, and they run quietly and cleanly.
Modern sealed chains have narrowed the maintenance gap considerably, needing lubrication and adjustment rather than the constant attention their predecessors demanded, which has kept them competitive.