On Tue, 29 Apr 2008 09:48:42 -0700 (PDT), Kastnerd
<
[email protected]> wrote:
>I know there has been many discussion on rolling resistance. But i was
>wondering how it would be best to test tires for real world
>conditions.
>
>I would like to test it with the mavic ksyrium wheels.
>I would test the difference of tubed vs tubeless.
>Hutchinson makes the Fusion 2 tire for tubes (220g) and tubeless
>(295g) both are triple compound.
>I would like to test tubeless vs tubed at the same psi at the same
>weight.
>Test it at 80psi, 100psi and 120psi. i weight 140lbs.
>The biggest factor will be the real world road conditions. Every small
>little bump that is not absorbed by the tire losses from forward
>momentum, also loss of traction and if the ride is painful you hurt
>more.
>
>What would be the best way to test these conditions?
Dear K,
It's darned hard to do a credible practical test, unless there's a
large difference.
You could start by coasting down the same very long hill, day after
day, and checking the max speed on your cyclometer.
Unfortunately, several details besides tires and pressures may affect
the maximum speed display and confuse things.
First, you have to calibrate the cyclometer for each tire and
pressure. You're looking for a tiny difference in top speed, so tiny
that 2096 mm versus 2124 mm for the effective roll-out might skew the
results.
(You could avoid this by timing between two distant points, but it
turns out that starting and stopping stopwatches introduces a fair
amount of variation--the max speed function is automatic.)
Next, you have to duplicate the temperature and barometric pressure as
closely as possible. Peek at a few online speed calculators, and
you'll find that these affect top speed enough to goof things up.
(One detail not often mentioned is that temperature will change not
just the wind drag, but how easily the tire rubber flexes.)
Then there's the wind--a head or tail wind of half a mile per hour is
undetectable except by watching smoke, but it's going to change your
results significantly. Early morning is when the wind dies down, but
anyone who's watched smoke pots at a grass air strip knows how rare
truly still conditions are.
Of course, the rider's aerodynamics make a huge difference--even
slight changes in your posture show up in wind tunnels, which is why
top riders spend money on such testing. Tuck your shirt in
differently, and you may see a difference.
And the rider's weight varies, too. It's normal to vary a pound or two
either way from the weight on your driver's license, depending on what
you ate and drank last and when you last visited the bathroom. Again,
the calculators will show speed differences for 139 versus 141 pounds.
Here's a few calculator that shows the predicted differences for even
small changes, including rolling resistance:
http://bikecalculator.com/veloMetricNum.html
If you reduce the default 0.0050 RR 10% to 0.0045, holding everything
else the same, the calculator predicts a 0.28 km/h difference for the
default values, a little less than 0.2 mph.
Set the watts to 0 and roll the two bikes down a -6% grade, and the
predicted speeds are 51.43 and 51.20 km/h, a 0.23 km/h difference for
a 10% improvement in RR.
An undetectable 0.23 km/h headwind will erase that difference. (It's
not a straight loss of exactly 0.23 km/h top speed for 0.23 km/h
headwind, but the difference is less than 0.01 km/h.)
A temperature drop from 25 C to 22.5 C (77.00 F to 72.50 F) will also
erase the tiny effect of 10% less rolling resistance.
Changing frontal area from 0.40000 m^2 to 0.40355 m^2 wipes out the
speed gain from a 10% RR reduction.
Raising the altitude from 100 meters above sea level to 163 meters has
the same effect on speed as a 10% RR reduction.
Adding 0.73 kg (1.6 pounds) to the rider will let reach the same top
speed with the 10% worse RR tires.
In other words, real world variation in wind, temperature, air
pressure, rider posture, and rider weight are all likely to have more
effect on top speed down a 6% grade than the rolling resistance of
roughly similar road tires.
Plus you'll roll down the same road faster if rain or street sweepers
have cleaned the winter sand off the surface, or if you pick a
slightly different line to avoid the rougher patches and hit the
cracks where they're smallest.
You can get larger absolute differences at lower speeds, where rolling
resistance is greater. On a -1% grade, the theoretical speeds drop to
16.19 and 15.44 km/h, a whopping 0.75 km/h (just under 0.5 mph)
difference for 10% less rolling resistance. But few people are willing
to roll repeatedly down 1% grades at 10 mph--and holding the same tuck
for long enough to reach top speed at such a snail's pace (without
swerving, which will goof things up) is awfully hard.
That's why spin-down tests on drums are generally considered to be
more reliable than real-world efforts for RR, even though they factor
out such things as the increased drag of wider tires and the effect of
higher pressures causing bouncing.
Cheers,
Carl Fogel
