Originally Posted by
LarDasse74
Your explanation of why steel fails more quickly than aluminum is erroneous for two reasons - (1) steel does not fail more quickly - it usually deforms or bends before failure, and (2) the stiffness or 'springiness' of steel has nothing to do with failure.
I did not say that steel fails more quickly than aluminum. I said that it fails in a different
mode than aluminum does. The stiffness and/or brittleness of the material has everything to do with that failure mode
Originally Posted by
LarDasse74
1. Causes of failures of bike frames
In my experience, there are two main reasons why bike frames fail - either from a crash, accident, cased landing from a jump, etc. that exceeds the strength of the frame, or from a mistake in manufacturing that causes a joint or frame component to be weaker or more brittle. In the first scenario, steel has the advantage of bending considerably before breaking, and many accidents will result in a bent or mangled frame that may still be somewhat rideable - if only to get you home or to safety. Aluminum has far less ductility (the amount it can deform without actually snapping) so if you crash an aluminum frame hard enough to exceed the material's strength it will usually simply snap. Watch people riding bikes though any large city and you will see many who have had crashes on their bikes hard enough to bend the frame, but keep riding them anyways - usually the headtube is bent backwards, the front wheel is a little too close to the pedals, and there are 'wrinkles' on the bottom of the top- and down-tubes. I don't think I have ever seen an aluminum bike with similar damage - when you crash hard enough to bend the frame you almost always break the frame.
My experiences with aluminum frames and aluminum parts is that they almost never 'just snap'. They crack but they don't just snap as in 'I was riding along and the bike just snapped into two pieces'. I had a Specialized
Stumpjumper M2 frame that broke. It didn't snap, it cracked at the welds much like Geosammy's frame did. And that's for an aluminum matrix alloy that is far more brittle than other aluminum alloys.
I broke another frame above the seattube/toptube junction because I was using a seatpost with a huge setback like Geosammy's. Again it cracked and tore rather than fractured.
I have had numerous wheels fail because of cracks or because the spoke pulled out or because the sidewall thinned too much. I've had rims that have cracked completely around the rim...360 degrees... on the inner wall. There was no fracture and only some creaking of the wheel while riding.
I've had a crank arm shear off which is the only case I have ever experienced where an aluminum part actually fractured. However the crank arm creaked a lot before failure...which I ignored
After the part fractured, I could see dirt and grease in the leading edge of the crack which indicates to me that it took its time in breaking.
On the other hand, all of the steel parts and frames that have failed through breakage have been
fractures. No bending. No warning. And usually associated with a load pop or tink or pink or plink or whatever other onomatopoeia you'd like to use. I've broken pedal spindles...riding along and suddenly I've got a pedal attached to my foot and it's not attached to the crank. I've broken axles which let go with a loud tink and then one of the frame dropouts broke too. I hit a curb with a kid trailer, turned the trailer on it side and discovered a broken...not bent or deformed but broken...axle. I've broken another frame while climbing out of the saddle. I heard that familiar tink and the one of the dropouts was broken. The sound that fracturing steel makes is very familiar to anyone who has ever broken a spoke.
Aluminum just doesn't make that kind of sound nor does it just fracture like steel can. Even in crashes, aluminum don't shatter. It deforms and tears. I hit a curb with a Manitou II fork at about 25 mph. The fork's crown was bent, mangled and torn but it wasn't fractured.
As you can see, I've had lots of experience with parts failures of both kinds of materials. I have even had some experience with bending steel frames. Yes, they bend and deform more than aluminum. And they aren't likely to fracture in a crash (but then neither is aluminum).
Originally Posted by
LarDasse74
2. Material Properties
The other problem with your argument is that, while steel is 'stiffer' (higher modulus of elasticity than aluminum), aluminum frames almost always have much larger tubes and much thicker walls to make the frame acceptably stiff and strong. THe result of the larger tubes with thicker walls is that the actually deform less under load - not more... Aluminum bikes are often 'stiffer' than steel bikes, and they get this way by each of the individual frame components being made stiffer.
Aluminum frames use larger diameter tubes to make them stiffer because small diameter tubes would require thicker walls (and more weight) to achieve the same result. You could easily make steel bikes with larger diameter tubes but the frames would be so stiff as to be unrideable. But this really has nothing to do with the discussion at hand.