Bicycle Mechanics - Fragile Frames
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This is a truly elementary and uneducated question, so be nice please.
I have seen (and touched) a lot of top-end frames in the last two weeks. And one thing strikes me about many of them. To wit, the tubing on metallic ones - mostly the Alu ones - sounds like it is paper thin.
I know that is a bit of an exaggeration, but practically, doesn't one run a lot more risk with that kind of frame in everyday use? It seems to me that just dropping a helmet on the top tube, or virtually any kind of crash, would write them off.
How much unintended punishment can they take? Is it possible to have a single tube replaced? That would require a new paint job as well, but many of these frames cost in excess of $2K, so the repair price could pale by comparison to buying a new one.
Of course, if the team provided it ... ;)
Thin-wall construction, like on large-tube Cannondales and Kleins, is a method whereby the increase in diameter (with no increase in the amount of metal used) produces a tube which is much less likely to bend. Of course, this does nothing for side-impact strength. Be assured that the tube wall isn't going to buckle, except in unusaul situations, like a crash. If it did, the framebuilders would go out of business.
Having said that, if you do bend a thin-wall frame, you are probably S.O.L.. The cost of repairing such a frame is likely more than the replacement cost.
Most top-end frame builders have Crash Replacement policies. Which usually means you can get it replaced at cost, which is a little less than wholesale.
But as Alex said about Cannondale and Klein, the larger the tubing the stronger it is in aluminum. I've had a Klein since 1985 and have put it through a bit of torture during my tours. Nothing, that has ever happened to it, has damaged the frame in any way.
Actually, I have seen a couple of Colnagos whose top tubes seem to be in a diamond shape, corner up, and concavities on each side. That seemed intuitively to be a much stronger cross section that the large oval downtubes I see on many bikes.
Just to be sure, I did mean "unintended" incidents as I stated - like dropping a big wrench on the top tube, not everyday riding stresses. It seemed almost as though I could dent some of them by squeezing it in my hand.
After the "puns" thread earlier in the day, I am not taking any chances. :o
10-19-01, 08:56 AM
You CAN dent some "lightweight" Aluminum frames by squeezing them with your hand. Using car racks that clamp on the downtubes will negate the warranties on many models and brands of Aluminum Bikes.
Light weight is only PART of the equation
10-26-01, 10:18 AM
Tubes do get dented by mis-use, eg clamping heavily in a bike stand, dropping tools, airline baggage handlers.
Dented frames are still rideable. If the frame is expensive and steel lugged or fillet brazed construction, it may be worth replacing the tube, but for welded construction, its simply not cost effective.
For every-day utility bikes, a thicker touring grade of metal may be preferable to ultra-thin metal.
Aluminium bikes are often made with fatter tubes, for increased stiffness. Manufacturers protect them against sidewall collapse by makeing the walls thicker. An aluminium frame will have more metal than an equivelent steel frame, but being 1/3 the density, they can still be made lighter, just not 1/3 the weight of the steel frame.
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