Originally Posted by
digger531
This could get to be a heated discussion. I prefer steel. My steel frames weighs 4.2lbs and was reasonably priced at $450. Alluminum is very stiff so it has no give, none. So rather than it flexing, it will break. Now thats not to say that an alluminum frame is going to break just that it will break before it flexes. Alluminum also, because it is so stiff, tends to vibrate. It can and does actually develop a hum. Some people such as myself are sensitive to this. I can feel it in my teeth. There is also the off chance you could be biking across outer mongolia and run into an ox and break your frame. There is someone everywhere that can repair steel not so with alluminum. The biggest difference is ride quality. Go ride a salsa casseroll you will believe in the feel of steel. Plus you get to use all the ryhmes.
So much, so wrong.
Steel is about 3 times the density of aluminum. So an aluminum bike built using the same volume of material as a steel frame would weigh 3 times less. For your example, that would make your 4.2 lb frame a 1.4 lb frame.
But aluminum isn't stiff at all. It's actually about 3 times less stiff then steel. It's also about 3 times less strong. Because it's not stiff nor strong, it can't be built using the same volume of as a steel frame and thus more material has to be used to make the frame. An aluminum frame of comparable strength to your steel frame would need more material which would drive the frame weight closer to 3 lbs. By a trick of geometry that is similar to what is used for I-beams, you can make tubes of aluminum stronger by increasing their diameter. That's where the stiffness of an aluminum
frame comes from. You can make the tube thinner and able to withstand the forces necessary for a bike frame and end up with a frame that isn't a noodle. You could do the same with steel but you'd have a bike so stiff it wouldn't be comfortable.
Aluminum
does flex. It just has limits on how much it can flex. Aluminum also doesn't vibrate all that well. Because it's not very stiff as a material, it doesn't propagate vibrations as well as steel does. Think of a tuning fork. A steel on will vibrate for a very long time. An aluminum one is rather 'dead', i.e. it doesn't vibrate. Aluminum bikes don't vibrate as well as a steel bike of similar dimensions. The issue is the differences in dimension. As above, if you made a steel bike the same as an aluminum bike it would vibrate like...well...a tuning fork. On the other hand, unless you are riding steel wheels, any road vibration that reaches the rider is being transmitted through the tires (which deaden the vibration), aluminum rims and aluminum hubs. Why don't their vibrational qualities bother you?
Finally there's the repair issue. You can go look around for a Bike Forum member's actual experience with trying to get a damage frame repaired in far off lands...China, actually...on the touring forums. Modern steel frames...and by modern I mean anything made since around 1975...are thin metal frames that require specialized skill to repair. Some guy with a welder in Outer Mongolia will be far more likely to burn through the material than repair it...which is exactly the experience that the poster on the touring forums ran across.