Bumps and efficiency

SYJ wrote:
Do bumpy road surfaces typically exact costs in riding efficiency?

Yes... but it is difficult to tell how much.

I've seen no testing that addresses this, but Bicycling Science
discusses it briefly. They show an old chart developed by the army (for
tank crew) that gives power absorbed by a person's body versus
frequency and amplitude. An amplitude of only .1 inches corresponds to
~50W power loss at 5hz (10.7m/sec; 23.9mph... if the bump occurs once
per wheel rev). Some approximate values from the chart are tubulated
below:

Amplitude (inches), Frequency (hz), Speed (mph), Power Absorbed (W)

..1, 4, 19.0, 3
..1, 5, 23.9, 50
..1, 6, 28.6, 100

..25, 4, 19.0, 290
..25, 5, 23.9, 1200
..25, 6, 28.6, 1900

Clearly, the losses go up a lot with increasing frequency and
amplitude. In a field test this would tend to mimic aero losses more
than rolling resistance. In fact it would increase much faster with
speed than even the aero losses.

The difficult thing is relating this to a bicycle. I don't think I
typically ride on any roads that are smoother than .1 inches every
wheel revolution... so am I lossing 50-100W when riding at a good
speed? Somehow I doubt it. Probably, considerable attenuation of
vibration occurs before it reaches the rider's body. I would expect
elastic parts like tires, stems, frames, seats, etc, to absorb little
energy (ie they would flex with little loss). Still, .1 inches is not a
lot of motion, and I'm sure I experience that much fairly often... on
my hands and butt.

If someone wanted to test for this, a large roller with a rough surface
could be constructed, and the tire could be weighted by an actual
person... ie have a bicycle with rider on top supplying the force. Or
maybe someone with a good power meter could do a field test on
different surfaces.

I'm curious if it is more beneficial in a long ride/race
where crappy roads are the norm to use 25mm tires vs. 23s.

I'd think so... even 28s... and drop the pressure as low as you dare. A
good quality tire suffers little degradation in Crr going from
120-80psi even on an extremely smooth surface, so I think you'd be
faster... and that doesn't even consider the comfort issue!

wrote:
of discomfort reported by the rider. So the more it hurts, the more
its slowing you down

.... which is probably as good a gauge as any. If the vibration is
enough to be noticable and annoying, you could go faster (as well as
more comfortably!) by lowering the pressure.

wrote:
Effect of chip seal is easily seen if one rider coasts down the
shoulder of the road that wasn't chip-sealed while another coasts on
the rough surface. Switching back and forth shows a noticeable
relative difference in speed.

Do you know the approximate speed and difference in speed when you
experienced this?

On 29 Jun 2006 12:02:23 -0700, "SYJ" wrote:

Do bumpy road surfaces typically exact costs in riding efficiency?
While doing a loop that I've ridden dozens of times, I noticed that my
speed had dropped almost two MPH from my norm on a section that had
recently been chip sealed (as opposed to the baby's butt smooth
pavement that existed previously). I didn't feel especially cooked (at
least, no more than usual), there didn't seem to be any wind, and I was
able to ramp the speed back up to normal after turning off the choppy
surface, all of which led me to question whether it was indeed the
bumpiness that was affecting me.

Does anyone have any similar anecdotes? Any studies of this? Was I
dreaming? I'm curious if it is more beneficial in a long ride/race
where crappy roads are the norm to use 25mm tires vs. 23s.

It's more a matter of the vibration affecting the engine than any increase in
rolling resistance. I think the fatter tires at lower pressures and tubular
tires are advantageous.

I have had a hard time going fast on bad buzzy road and cross riding on clumpy
uneven grass. Familiarity helped a lot. I still work on that, and it was work
getting used to pushing for speed on bad surfaces.

Ron

From: (Werehatrack)

Here in sunny, parched[1] Houston,
there is an additional reason to favor
tires of greater width, with my preference
being not to drop below 30mm; we have
*lots* of wide, open linear seams in our
streets, and wider tires traverse these
much more safely and predictably than
narrow ones.

Not much problems with road cuts, but we do have some RR crossings that
would send shivers up your spine, and I don't mean from road vibration
either!

- -
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My web Site:
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From: (BigBen)

Assuming we're talking about same
model of tyre, same tyre pressure, how
would that be any better than decreasing
the 23mm tyre pressure a bit - say, from
120 to 110 psi, for instance??

cheers,
jbr

Well, it would make for an even rougher ride for one, but the problem is
not so much road vibration as it is control. A 23mm tire wouldn't be any
wider that the gravel it's rolling over, and narrower tires tend to e
harder to keep on a line, as the road texture constantly tries to direct
it's direction of travel and the rider has to struggle more to keep the
bike riding straight.

- -
Comments and opinions compliments of,
"Your Friendly Neighborhood Wheelman"

My web Site:
http://geocities.com/czcorner

To E-mail me:
ChrisZCorner "at" webtv "dot" net

On Sun, 2 Jul 2006 15:38:25 -0400,
(Chris Z The Wheelman) wrote:

Well, it would make for an even rougher ride for one, but the problem is
not so much road vibration as it is control. A 23mm tire wouldn't be any
wider that the gravel it's rolling over, and narrower tires tend to e
harder to keep on a line, as the road texture constantly tries to direct
it's direction of travel and the rider has to struggle more to keep the
bike riding straight.

You don't think that droping tyre pressure a bit - 10 or 15 psi -
would make the contact surface of the tyre with the ground any wider??

And you think a 23mm tyre with less air pressure would be harsher? I
could swear I had the diferent feeling.

jbr