On 12/14/2017 5:45 PM, Tim McNamara wrote:
Huh. CFRP rims strike me as a solution looking for a problem, creating
new problems along the way. 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. Holy smokes. That would
change change the outlook of your day. But I wonder, in the real world,
how likely that would be. 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. 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 mph Huh! seems attainable. 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) 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
= 91kg*9.8m/sec^2*248m = 221kJ in 180 sec,
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. 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.

Mark J.