learn by destroying

goo.gl/CEmnS6

On 12/19/2017 4:05 PM, Mark J. wrote:
On 12/14/2017 5:45 PM, Tim McNamara wrote:
Huh.Â* CFRP rims strike me as a solution looking for a problem, creating
new problems along the way.Â* I suspect that I don't have the orginal
problem that is trying to be solved (I am not racing the TdF and large
sums of money aren't involved in the outcome of any of my bike rides).

The exploding rims were kind of cool, though.Â* Holy smokes.Â* That would
change change the outlook of your day.Â* But I wonder, in the real world,
how likely that would be.Â* Even wiht long descents, it's unusual to have
the brakes applied for minutes at a time (unless you don't know what
you're doing).


Trying to avoid the real work waiting for me, I did the following
quick-and-dirty analysis;
It's a bit garbled, but you can skip to the last three paragraphs,
starting at "Summarize":

Bottom line: it's going to be /really/ hard to duplicate this testing
abuse on a single fast, short descent.Â* Maybe you could still kill these
rims on an extended descent, that's not what the test is checking.
=================================================

1200W braking power:

See
https://www.bikerumor.com/2017/12/08...im-brake-test/


For me, at 200 lbs full weight bike+rider (91 kg=M), using power = f*v,
on a grade of R (as a decimal, e.g. 7%=.07)

Descending at velocity v, downward force on bike (neglecting air
resistance! and road friction)
Downward force is RMg, power is RMgv = R(91*9.8)v watts, with v in m/sec.

To get power=1200 (we need braking power to be 1200 W; this happens with
downward force = braking force, i.e. constant velocity)

Need 890Rv=1200; Rv=1.35.

Let's try R=15%; .15v=1.35; v=9 m/sec=20 mphÂ* Huh! seems attainable.Â* If
the road was straight enough.

But of course, to do this for the 184 seconds that killed a Bonty rim in
the test (see web link above), that's 184*9m distance = 1.656km, or
about a mile, and altitude loss is 15% of that, or 248m or about 820 feet.

Summarize: Go down a 15% grade at 20 mph for about a mile (with a 20mph
tailwind (!) so there are no aero losses)Â* Do it on good pavement so
friction (other than braking, which we are testing) is minimal.
This will take about 3 min (check), energy input is Mgh where h=altitude
loss
Â*Â*Â*Â*= 91kg*9.8m/sec^2*248m = 221kJ in 180 sec,
Â*Â*Â*Â*or average power input = 221kJ/180 sec = 1228 W (check, within
rounding error)
If you brake to hold speed constant (or nearly) for this 1 mile descent,
you will have put 1200W into your rims for 3 min.

Oh - but that's into *both* rims, so neither gets the full 1200W.Â* Do a
one-mile 15% descent at *30* mph with 2/3 of braking power in front rim,
with the *30* mph tailwind, that should kill the front rim.

Oh, but even this doesn't take into account heat dissipation (top of the
rim is going 60 mph and getting some cooling even with the atomic
tailwind).

Not the point of the test in the web link, but an amusing calculation.

Interesting, especially since I did a ride today that featured a very
steep descent. Google Earth calls it 14% overall, although I'm sure
parts of it are steeper. (About 250 feet drop in about 1800 feet
travel.) It's steep enough that my wife rode it once, years ago, and
refused to go down it again. She said she was braking so hard she hurt
her back.

Anyway, since the road was wet and I was riding solo and slow, I decided
to descend very slowly. I think I was going about 9 mph (or 13 feet per
second) all the way down. The slow speed means I was putting only about
500 Watts into the rims.

I was curious about the rim temperature, since I essentially never
descend that way, so I stopped and felt them at the bottom. I thought
"Hmm. A bit warm, but certainly nowhere near dangerous." My wild guess
would be about 100 degrees Fahrenheit. (Of course, these are aluminum
rims.) Ambient temperature was about 45 degrees today.

BTW, one nitpick: Even if you are descending at 20 mph with a 20 mph
tailwind, there are still aero losses. The wheel spokes, in particular,
chew their way through the air and really stir things up.

Long ago when IHPVA streamliners were new, I attended a workshop about
them at a bike convention. The guy giving the workshop had a typical
spoke wheel and another wheel whose spokes were covered with discs. When
cranked up to speed, the amount of air pushed out by the "bare spoke"
wheel was far greater than the wheel with covered spokes.

--
- Frank Krygowski