patch70 said:
You failed to get your point across because you just don't get it. If a Shimano Sora or Campy Mirage crank is not going to flex at all with the kind of stress that a mortal human puts on it, those tiny imperfections are not an issue. If there are major imperfections, then the crank might break (although it has to be pretty bad to break at less force than a chain will break). Making a crank (eg Dura Ace or Record) even stiffer than the above does not make it better or less likely to suffer due to those slight imperfections. None of them will flex at all anyway. Extra stiffness just gets used as a reason to upgrade and spend more money. Sure, buy them because you can afford to and you like the look, the weight, the wa-nk factor, but don't be sucked into paying lots more because they are 'stiffer'.
Several people on this forum have said that DA cranks are better purely because they are stiffer. They are kidding themselves if they think that other cranks are not stiff enough for them.
Perhaps we should start more simply and move a bit more slowly. Are you familiar with burnishing aluminum? Depending on the method it may be referred to as "ball burnishing". Do you know what the advantage to burnishing aluminum is? How about shot-peening steel? These procedures are designed to "smear" the metal on the surface to eliminate microscopic lines, crevices and contours. It's all about getting rid of the surface imperfections which, under the load of flex, lead to fractures which lead to failures. It
does happen! You can't simply make that fact go away by pretending you don't understand the point. I'm not talking about putting a brand new crank and chain on a bike and then applying enough force to break either one. I'm talking about repetative stress, applied through moderate forces which
will cause tiny fluctuations in the alignment of the metal grains.... something we call "flex".
You can't build a crank with absolutely zero flex. We're talking about microscopic changes in shape here. But the imprefections, whether they be tool marks, cold spots during casting or miniscule amounts of impurities in the metal, grow with every tiny shift in shape of the metal and this does occur with minimal force applied. It's like taking a piece of wire and bending it back and forth. If you do this enough times, it breaks. Each time you bend the wire you cause fractures, which usually start with tiny imperfections, to grow. The more distortion you impart to the wire, the fewer times it will take to break it. If you bend the wire less, then it will take far more repetitions before the wire breaks. It's commonly referred to as "metal fatigue" and it is most definitely a factor in bicycle cranks. Are you telling me you've never heard of a Dura-Ace crank breaking? It's not a common occurence but it absolutely does happen. Most of the time these cranks break after years and years of persistent use.
Every material reacts to forces applied to it. Some react differently that others. Steel happens to distort and "spring" back fairly well. Forged aluminum happens to be more brittle. In the case of forged aluminum, when you force a significant deformation to occur, any attempt to return the metal to it's former shape by force alone will usually result in breakage. Now take that same phenomenon and apply it at a miniscule fraction of that distortion. The metal appears not to flex to the naked eye. But it did. And in so doing, minute fractures begin at the microscopic level. And with every additional application of force, those fractures will grow. It will usually take years and millions of repetitions of such force but each one takes a toll on the material and it does so through microscopic flexing of the material.
If you could make a bicycle crank that had absolutely no flex and could assure that the crank never encountered damage from an accident, you'd have a crank that would never, ever break under its intended use. Such a crank does not exist.
I suggest you spend a bit of time studying chassis and suspension components for motor vehicles. You'll be surprised where flex shows up. Do you think that a straight box-section, aluminum swingarm of a 450 pound sport bike (motorcycle), exhibits zero flex? Do you think the 43mm stantion tubes in the telescopic front suspension of such a motorcycle exhibit zero flex from the moderate force of steering input? How about the telescopic front suspension of a cross-country mountain bike? Those are some pretty substantial bits of metal for just the weight of a bike and rider but the fact remains that when those parts are stiffened, the bike begins to handle better because they flex less under steering input and under forces imparted to the tire from pathway contours and obstacles. Indeed all of the above mentioned parts will flex slightly under normal use. All else being equal, a 43 mm stantion tube will flex less than a 39mm tube, and therefore provide slightly more stable handling because steering alignment remains more consistent. If materials didn't react this way, once the part became substantial enough that the force the material is intended to transfer failed to meet the yield strength of the material, the material would never fail under the intended use.
