The Indian flat-track engine has gear-driven primary balancers. Years ago, when four-stroke engines were much heavier (try putting a Z1 engine in the frame by yourself!) piston weight was small compared with engine weight. As a result, the up-and-down motions of the pistons imparted a moderate down-and-up shaking force to the engine. As design experience, improved casting techniques, and market forces produced lighter engines, this shaking force became too much—especially at higher revs (shaking force increases as the square of rpm). The result has been wide use of balancers, resulting in light but smooth engines. Freed of vibrational stress, cycle parts can in many cases be made lighter, more than making up for any added balancer weight.