An inline four cylinder, or even a V6 engine with a crank-speed balance shaft, will experience significant secondary dynamic imbalance, resulting in engine vibration. As a general rule, the forces arising from any dynamic imbalance increase as the square of the engine speed – for example, if the speed doubles, vibration will increase by a factor of four. In contrast, inline six engines have no primary or (significant) secondary imbalances, and with carefully designed crankshaft vibration dampers to absorb torsional vibration, will run more smoothly at the same crankshaft speed (rpm). This characteristic has made the straight-six popular in some European sports-luxury cars, where smooth high-speed performance is very desirable. As engine reciprocating forces increase with the cube of piston bore, the straight-six is a preferred configuration for large truck engines.