Bike designers can made a bike "as strong" as anyone would want, but the trade-off is weight. Designers of 16 pound and 17 pound bikes are always trying to go to the limit of the material: a bike strong enough to last for one or two hard summers of riding, yet light enough to brag about.

The word "strong" can be applied to bikes in different ways. One of the standard tests of a bike was to place it under stress, and repeat the stress thousands of times. At a certain level of stress, a light aluminum frame will fail after around 200,000 cycles of stress, while a light steel or light carbon frame will withstand that same stress for 10 million cycles, or more...a lifespan fifty times longer than aluminum. But, up to the cycle that broke the frame, the aluminum frame is just as strong as steel or carbon. It just isn't strong for very long.

If someone wants an ultra-light frame for one or two years of hard riding, aluminum will do the job. If you want that ultra-light frame to last ten or twenty years, you would need carbon, steel, or Ti.