it's misguided to generalize final weight with frame material. Frame material does influence a bike's final weight but it's not the end all, be all. There are alloy bikes heavier than steel and the opposite with steel bikes lighter than some alloy bikes. I might even go as far as say there are low end carbon bikes heavier than some alloy bikes. What's truly important is the planned load of a build. If durability and stiffness is not an important factor then by all means make a bike using the lightest skinny tubes possible but durability and stiffness are very important.
Different materials exhibit more differences than strength to weight but strength to weight play into tubing size. Steel is incredibly dense but also incredibly strong, allowing for much skinnier tubing. If aluminum is about 3 times less dense than steel, then why aren't aluminum bikes 3 times lighter? That's because aluminum is not as strong as steel, so thicker tubing is required to match the load it was designed to handle. If material density alone told the whole story, a carbon bike would be 6 times lighter than a steel bike but we know that's not reality.
Titanium is about half as dense as steel. It's strength is in it's elasticity which is higher than aluminum and steel. I haven't had a chance to ride a quality titanium frame but from what I've heard, it's like riding a well tuned spring that jumps forward with every pedal stroke. Couple that with it's high fatigue limit, then it's understandable to be a Fred's 'final bike.' It's largest drawback is difficulty in manufacturing. Any oxidation during welding weakens the joints greatly, so I wouldn't buy a titanium bike unless it was built by someone with A LOT of titanium bike building experience.>>1209757
the problem about carbon is the quality of the resin binding the composite. It is a plastic after all and sunlight degrades plastic.
In the end, frame material is like every aspect of bicycles. Compromise.