Chad Waldman said:I just bought my first computer (Ascent Delta V) and I am confused on
how to calibrate it for my tires. I am riding a mtn bike with slick
tires (26 x 1.25).
The chart in the instructions does not include this tire size. I have
read other web sites to figure out what to enter in my computer (which
appears to take circumference in mm) however the numbers given on my
chart (and my friend's, who has a Bell) differs from the numbers given
on web sites and my other friend's computer (which is a Cateye). All
appear to use circumference in mm.
Example: Take the 26 x 1.5 tire size. I have listed on my chart: ATB 26
x 1.5 = 2030.
On http://www.sheldonbrown.com/cyclecomputer_calibration.html, it lists
for a 26 x 1.5, 1985. Why are they so far off? This concerns me.
Sheldon's site's chart also seems inaccurate. For example, for the 26 x
1.5 size tire, Group A (which is listed as circumference in inches)
lists 77.71in and for Group F (circumference in mm) lists 1985mm. Last
time I checked however, 77.71in = 1974 mm. Why the discrepancy?
After measuring and trying the rolling test to the best of my ability, I
got roughly 1975mm. On Sheldon's site for a 26 x 1.25 size tire, he
lists 77.44 in = 1967mm, or he lists (in Group F) 1953mm.
What should I enter? How much do these numbers make a difference in the
great scheme of things? I usually ride between 30 and 50 miles at a
time. How much could I possibly expect to be off if I am off by say
20mm?
Thanks,
Chad
A private email complained that RBT was debating the
effects of inflation and load on tire roll-out without much
actual data and asked me to post my data again.
Again?
I was tempted to reply that I never posted any such data,
but . . .
I bored a hole through the middle of a 2x4, banged a
short piece of pipe through it, roped it to the handlebars
of a spare bicycle lurking in my dungeon, found some old
weights, armed myself with a nicely calibrated tire pump,
and played tailor to the front tire with a measuring tape.
The tire was an old 700x25C on a 622 rim, inflated first to
90 and then to 125 psi.
"No load" means the weight of the bicycle with no rider, a
few pounds of 2x4 and rope, and the slight pressure of the
experimenter's hands pushing the bike around.
The 60-lb test load on the handlebars is less than a typical
front-wheel load, but stacking too many weights on a badly
designed rig turns out to be a bad idea.
(It was determined that an ordinary front wheel and tire can
withstand without damage the impact of five 15-lb weights
toppling forward off the handlebars. Watch your toes.)
On a smooth concrete floor, rollout was measured to 1/16th
of an inch (~1.6mm) three times for each value.
The tests averaged to suspiciously round values:
psi load #1 #2 #3 average
--- ------- -- -- -- -------
125 no load 83 & 08-07-06/16ths = 83 & 07/16ths
125 60 lb 82 & 15-15-15/16ths = 82 & 15/16ths
90 no load 83 & 02-04-03/16ths = 83 & 03/16ths
90 60 lb 82 & 09-10-11/16ths = 82 & 10/16ths
There appears to be no pattern of bias in the variation,
which is either a tribute to the experimenter's impartiality
and objectivity, or else evidence of a well-camouflaged
conspiracy whose goals are too terrible to contemplate.
125psi 125psi 90psi 90 psi
roll-out no load 60 lbs no load 60 lbs
---------- ---------- ---------- ----------
16ths 83 & 07/16 82 & 15/16 83 & 03/16 82 & 10/16
decimal 83.4375 82.9375 83.1875 82.6250
mm 2119.3 2106.6 2113.0 2098.7
mm change 0.0 -12.7 -6.3 -20.6
Roll-out shrank a small but measurable amount with increased
load. Adding a modest load of 60 lbs reduced roll-out roughly
13-14mm, about 0.6%.
Lowering tire pressure also reduced roll-out, roughly 6-8mm
for the drop from 125 to 90 psi, about 0.3%.
Cheers,
Carl Fogel
