On Thu, 6 Dec 2007 17:16:55 -0600, "Pat" <[email hidden]>
Quoted message said:
Quoted message said:
That would be pompous [censored] on their part, Paul. If the bike's
aluminum, they're a good idea, if steel, they aren't needed. Simple as
that. Geez, you'd think people would just go all the way and start
selling us vertically compliant versions....
I saw a new bike at the LBS and when I asked about what's so great about it,
the salesman said it was vertically compliant. What the heck does that mean?
Dear Pat,
By "vertically compliant" the saleman meant that the bicycle's main
frame bends or compresses a little as it hits bumps in the road,
complying vertically with the bumps.
The sarcastic phrase on RBT is "stiff but compliant," the point being
that you can't make a frame that's both stiff and soft.
With a double-diamond frame, the compliance for typical forces is
usually impossible to measure without an impressive micrometer--the
metal tubes of a bicycle's main frame triangles just don't bend up and
down enough to be detected. They're in straight compression and
tension, not bending sideways, so they tend to "comply" about as much
as an anvil from an up-and-down point of view.
The same main frame that's so stiff vertically can bend visibly
_sideways_ as you pedal, but that's horizontal, not vertical. The
chain doesn't have much leverage, but it does pull the drive side of
the rear triangle sideways. Meanwhile, your weight on the pedal bends
the slightly tilted frame from side to side. In other words, jumping
up and down on an upright bicycle frame won't produce any noticeable
bending, but you don't want to lay the same frame flat across two
chairs and jump up and down on it.
Inflated tires, handlebars, seats, and even wheels "comply vertically"
far more than the main frame when you hit bumps. Sit on a bike, and
you can see the tires flatten. Pull or push on the handlebar, and you
can see it flex a little. Look up the calculations, and you'll find
that the round metal rims flatten ever so slightly (about the
thickness of a sheet of paper). But the double triangle doesn't bend
even that much under a vertical load.
One counter argument is that different frame materials can damp some
vibrations (road buzz) just as a wooden baseball bat doesn't sting
your hands as much as a metal bat.
Another counter argument is that there can be much more "vertical
compliance" (bending) when the fork is added to the main frame.
But lowering tire pressure 5 psi would probably have a greater effect
than anything claimed for magical forks and wonder frames.
Cheers,
Carl Fogel
