h squared said:Tom Kunich wrote:
Quoted message said:Quoted message said:You may not think that the additional friction of worn bearings is much
but consider: A typical 35 mile ride might take two hours of riding
time. That's 120 minutes. At 80 rpm that's say 10,000 revolutions or
20,000 pedal strokes. If you add just one ounce to each pedal stroke
with worn bearings (not an unusual number) that is 20,000 ounces or
more than half a ton you've moved.
But as a percentage of the amount of work you've done it's
still very small.
Quoted message said:when i was skating i had 20 bearings to deal with. since they're
rotating only an inch off the ground, they get quite dirty and worn. so
i was constantly changing and cleaning bearings- there were little piles
of them all over the house, separated into various grades of "fast".
(just so you know that i'm not thinking bearings don't matter, etc.)
Bearing friction is much more important for skates than
for bicycles. The reason for this is basically that the
wheel bearings are about the same radius for both (1-2cm),
but the skate has small wheels, about 3cm radius, while the
bike wheels are about 33cm radius. So if the bike and
skater are traveling at the same speed, the skater's wheels
are rotating 11x as many rpm, which means the speed of the
bearings is 11x as fast and 11x as much power is lost to
friction. Even if the bike is going twice as fast as the
skater, bearing friction is still 5x as big for the skater,
and the bike rider is losing way more power to air drag than
bearings.
Which is just a longwinded way of saying that you were right
to worry about fast and slow bearings for skates but it would
be hard to ever measure a practical difference between fast and
slow in bicycle bearings.
Quoted message said:the bike still rides ok, though. i'm not in a hurry when i ride it, so i
don't mind extra work, as long as there's no danger of the bearings
seizing or otherwise failing (i've had them "literally explode" when
skating). but considering that i've had my bike about 4 years, it's
probably time to do *something*. i'll ask the shop to change the
bearings when i go there, thanks tom.
Bike wheel bearings hardly ever seize completely - they can
either get too tight, in which case the wheel won't spin freely
by hand, or too loose, in which case the wheel can wobble side
to side a little and the bearing surfaces will wear out. It's
not a safety issue, but it is a routine-maintenance item.