On Tue, 26 Sep 2006 21:17:00 -0700, Mark Hickey <
[email protected]>
wrote:
>[email protected] wrote:
>
>>If you plug in 80-lb bike and 150-lb rider, hands on tops, and 50
>>miles into the distance at the bottom, it predicts a speed of 17.5 mph
>>and a time of 2:51:25.7. (Wind drag would increase with panniers, but
>>we'll ignore that.)
>>
>>Change from "narrow racing" tires to the hideous "robust touring"
>>tires and try again. The calculator predicts 15.8 mph and 3:09:52.4,
>>about 18 minutes longer on a 3-hour trip.
>
>That seems like an awful lot of penalty just for going with fatter
>tires - what are the specs on those "hideous robust touring" tires?
>Inflation?
>
>For example, I ride typical 23 or 25mm high-pressure (110-120psi)
>tires on my road bike, and 35mm semi-slicks (85-90psi) on my 'cross
>bike, and would be surprised if there's a 1mph delta, even with the
>more upright position on the 'cross bike.
>
>OTOH, I've never actually measured the relative speed (don't bother
>with a computer on anything other than the tandem, and then only to
>make sure the wife is working hard enough). ;-)
>
>Mark Hickey
>Habanero Cycles
>http://www.habcycles.com
>Home of the $795 ti frame
Dear Mark,
Consider your sincere but extremely dubious theory that a 1 mph
difference between "more upright" and presumably "on the drops"
positions would be surprising.
Take the defaults here:
http://www.kreuzotter.de/english/espeed.htm
Choose on-the-drops, and the prediction is 19.4 mph.
Choose hands-on-tops, and the prediction drops to 17.3 mph, 2.1 mph
slower.
That's without the wider, squashier cyclocross tires.
Go to 1.75" off-road tire, and the predicted speed drops to 16.2 mph.
For a cross-check, try this calculator, which is the MPH version that
suggests tire types instead of allowing specific RR values:
http://w3.iac.net/~curta/bp/velocity/velocity.html
Reduce the 300 watt default to 160 watts, choose clincher tires, and
use "drops"--the prediction is 19.46 mph.
Switch to "hoods"--the predicted speed drops to 17.98 mph.
Put on a pair of MTB tires--the predicted speed drops to 16.05 mph.
This is why Tour riders aren't using 38 mm tires--the extra rolling
resistance of the heavier construction and the increased wind drag of
the wider tires is subject to the laws of physics. To reduce sidewall
flexing enough to improve the RR of the wider tires would require an
unreasonably harsh ride.
If you're interested in the details of the calculations and the RR
values used, here's the source code for the mph version of the Austin
calculator:
String words[] = {"hoods", "bartops", "bar ends", "drops", "aerobars",
"drafting"};
double values[] = {0.388, 0.445, 0.420, 0.300, 0.233, 0.200};
String words[] = {"clincher", "tubular", "MTB"};
double values[] = {0.005, 0.004, 0.012};
http://w3.iac.net/~curta/bp/velocity/Velocity.java
Obviously, individual riders will have different actual wind drag
coefficents, just as individual tires will vary (as will inflation and
road surface). Plenty of clinchers are over 0.0060, and lots of mtb
tires are under 0.0120. Heavy touring tires tend to be around 0.0100.
Here's a table that suggests such averages, but remember that it's
mostly for small versions of the named tires:
http://www.legslarry.beerdrinkers.co.uk/tech/JL.htm
And here's the IHPVA's calculator, harder to use, but with some tire
RR and drag coefficient values for comparison:
http://www.recumbents.com/wisil/simul/HPV_Simul.asp
These raw differences are very small--19 vs 17 mph.
And we're bad at noticing them--try guessing your speed for a while
and then comparing it to the cyclocomputer--can you reliably
distinguish 17 from 18 mph?
It takes a cyclocomputer, a stopwatch, and a lot of rides around the
same route to find out that we were averaging 10% slower. Indeed,
that's why the calculators and computers are useful--they take out the
variables of traffic, how lively you felt that day, and what the
almost imperceptible wind was doing.
Three mph is a gentle walk down a long hallway. The wind on your face
is practically imperceptible. But plug a 3 mph headwind into one of
those calculators and see what happens:
http://www.kreuzotter.de/english/espeed.htm
Take the defaults, choose hands-on-tops, and it predicts. 19.4 mph.
Add an almost unnoticeable 3 mph headwind, and the predicted speed
drops almost 9% to 17.7 mph.
Turn it around to a -3 mph tailwind. Predicted speed rises to 21.2
mph, a little more than a 9% increase.
Cheers,
Carl Fogel
