by Grahame » Thu Feb 10, 2005 5:48 pm
Rob, I'm afraid you are thinking of children's bikes where low cost is all.
For "grown-ups" bikes:
(Here's the science bit)
Steel has a high Young's modulus (stiffness), high density (weight per size) and good fatigue properties (can be flexed quite a lot before it snaps). So a thin walled steel frame can be designed to flex and absorb some shock loadings without fatigueing to death. Vibration is repeated, small shock loadings. Unfortunately, steel rusts when wet.
Aluminium has a Young's modulus about 1/3 of that of steel, and a density about 1/3 of steel, but VERY poor (read zero) fatigue strength. So an Aluminium frame has to be made stiff to stop it wearing out and snapping fairly quickly - that is what happened to the early aluminium frames (the 1970's Alans, etc) that were made to the same designs as the steel frames of the time with small diameter tubes. Result - aluminium frames are not generally good at absorbing vibration, and can feel "harsh" but they do transmit pedal power a bit more efficiently by not flexing, so they feel "fast"
Titanium is between steel and aluminium in modulus and density, and has a good fatigue life. So titanium frames can be made to have the best of both worlds - long life (no rust, good fatigue), vibration absorbtion (some flex), and power transmission (but not too flexy)
Carbon fibre reinforced plastic can be almost infinetely engineered by varying the number of layers, and orientation for the fibres. Unfortunately this is expensive, and not necessarily very "worker friendly"
Aluminium lighter than steel? not necessarily. In 1993, my road (training for MTB racing) bike was a steel Breezer Venturi that weighed 19lbs for the complete bike.
Most frames are now aluminium 'cos the fashion (in mountainbikes) makes aluminium tubes cheaper to put together into frames than steel or titanium ones, this volume production spills over into road bikes.