Rims are critical components of your bike wheel. Though most rims are made of aluminum alloy, other rims are available, including those made of steel. Technically, steel rims are also made of an alloy – iron alloyed with other metals. Steel rims are often plated with chrome to discourage rusting. Though available at shops and online, steel rims are rarely used on modern bikes. They are usually found on older bikes from the 70's and 80's. The nearly universal migration to the use of aluminum alloy rims has occurred because of several advantages alloy rims offer.
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The first bike, invented in 1817 by Baron von Drais, had no pedals and had wheels made of wood. The rider propelled it the same way Fred Flinstone moved his car: by pushing on the ground with his feet. As time rolled along, bikes and wheels improved. By the 1870's, bike wheels had steel rims and rubber tires. A great advance to bike rims came in the mid 1980's, when the quest for lightness and speed lead to the development of aluminum alloy rims. By the mid-eighties, most bikes were sold with alloy rims. To this day, alloy-rims rule the market place.
The first and most important distinction between alloy and steel is the weight. Specific gravity refers to the amount of weight for a given volume. Aluminum alloy has a specific gravity of 168.5 while steel has a specific gravity of 490. Aluminum weighs about a third of what steel does. Since aluminum alloy is lighter than steel it provides the wheel with less rotational mass, meaning less energy is necessary to accelerate the wheel.
Strength and Stiffness
Strength and stiffness are different qualities. Strength refers to how much force it takes to bend a material to the point where it yields and does not return to its original shape. Stiffness refers to how much the material can flex or bend before it breaks. Some material can have great strength but little flexibility while another material could have the same strength but greater flexibility. In this situation it takes the same amount of force to break it, but it bends more. Strength is essential in rims. First, rims must hold up under the weight of the bike and the rider. Second, though the tires absorb and distribute some of the shock transmitted by pot holes, stones, roots and other hazards, the rim is the first structural component of the bike in the line of fire.
Steel has greater strength than aluminum per given volume, but it weighs much more. When the volume of aluminum is increased so steel and aluminum are of the same strength, steel has greater flexibility, but aluminum has less weight. In rims, and in bikes for that matter, lightness is valued more than flexibility, so you see more rims and frames made from alloy than steel.
Steel rims do not work as effectively as aluminum rims with rim brakes. The shoes on rim brakes do not grip well on steel rims, especially when the rims are wet. Unless you have disk brakes, this reason alone provides sufficient rationale to go with alloy rims.