conventional bicycle wheels now use 32 or 36 spokes front and rear, although the asymmetry of the rear wheel (to allow for the cluster of sprockets), and the additional weight it carries, means it benefits from having more spokes than the front. Commonly used models vary from 18 spokes for racing bikes to 36 for cross-country touring bikes to 48 spokes on tandems and heavily abused BMX bikes. The minimum number of spokes allowed for competition is 12. Some lowrider bicycles use as many as 144 brightly chromed spokes per wheel, although these are not meant for serious riding. Wheels can be built by machine instead of by hand. However, machine-built wheels are rarely as satisfactory as handbuilt wheels, partly because it is uneconomical to allow the machine to spend long enough on each wheel for a perfect result but also because most machines leave spokes with some residual twist. Machine-built wheels can be identified by their lacing pattern (if it is not radial), as the spokes are laced the same on each side, rather than mirrored as on hand-built wheels. More modern "factory built" wheels such as Mavic's Ksyrium series are of quite different construction from that of a conventional wheel, trading a deeper and stronger rim for fewer spokes. They are popular, and quite light (in the more expensive models) but not as durable, readily repairable or maintainable as a conventional wheel. Stainless steel is the most common material for spokes, although most mass-produced budget wheels use galvanized steel spokes. Other materials such as titanium or aluminum are often used to reduce weight. Some wheels are designed around carbon fiber spokes, which are often completely integrated with the hub and rim and bladed in shape. Non-steel spokes are normally reserved for racing bikes and other specialist applications where weight, aerodynamics and performance are valued over durability and cost. Rims were traditionally made of steel but currently aluminum is by far most common and best choice due to its light weight, high durability and stiffness. Wood is also used. Composite materials such as carbon fiber are sometimes used, typically for racing competitions such as time trial, triathlon and track cycling, although carbon fiber is becoming more common for recreational uses such as road cycling or mountain biking due to its looks, strength and feel. Motorcycle wheels Motorcycle wire wheels typically use 36 or 40 spokes, of much heavier gauge