Tire Pressure & Odometer

On 25 Sep 2004 19:56:02 -0700, (John
McGraw) wrote:

(TBGibb) wrote in message ...
In article tkl3d.315613$sh.139545@fed1read06, "Bob Newman"
writes:

A friend claims that if your tires are underinflated it can make enough
difference in the circumference of the tire to make your odometer as much as
1/2 mile off in a 30 mile ride. Is this possible?

My observation:

I ride to work every day usually by the same (fastest) route. If my tires are
up to pressure the ride is 2.28 miles. If they are a little soft it can easily
get to the 2.30 stage. 0.02*15=.3, which isn't far off of that .5 per 30
figure.

If my computer is reading 2.31 upon arrival at work I'm not paying attention to
my tires and am at risk for a pinch flat. That would work out to .45 miles.

It's possible, but one would be doing a lot of extra work on that 30.

Tom Gibb

Hi All
I've always had hi accuracy in setting my Avocets, as evidenced by
riding for many miles on various paths, which for some reason usually
have miles painted on the pavement. Or by riding many miles around a ¼
mile track. When I ride I keep my eyes open for such markers to check
against. Also some states mark their highways w/ mile markers. I never
trust hiway markers that I ride less than 5 miles, & usually keep
track of 10 (@Least) to 35 miles. I used to be amazed how tiny my
cyclometer errors were, now I just take it for granted. I have
complete confidence if I say something is 17.34 mi, it is 17.34 mi.
More confidence than I have in my car spedo. Not that it really
matters.

My method to get the calibration #, is to have someone mark w/
industrial metal marker on my tire were it meets the road & then W/ me
sitting on the seat leaning forward putting pressure on the bars, w/
bike tire fairly tight up against a curb (to help the straightness of
the tire) I roll the bike out by pushing on the hi side of the curb w/
my foot. Do this 3 to 5 times. Throw out any unusual measurements,
pick the # that came up most often. Usually there are three identical
#s. Take one of my several beloved old HP Pocket calculators, convert
inches & fractions to mms. I have tried several times to use paint or
metal markers on tire & measure the spots on the pavement. Which seems
to me to be the best way to do it, but I've never had any success w/
that method, due to odd nonrepeating patterns or no repetition at all.
I'm definitely going to try the Q-Tip method next time.

But, Carl got me to thinking. If the tire diameter is less w/ a
rider's weight, where does the tread go as it touches the ground? I
mean I know it's minuscule, but what does it do? Does it make a lazy
(very lazy) sine wave or does it make a slight lateral arc? If
turning, it must make a slight arc w/ the apex toward the inside. Of
course the lower the air pressure, the larger the arc or sine wave. Or
does the tread just compress in all directions?
Interesting, John

Dear John,

Ou sont les neigedens d'antan?
--Villon

Whirr do them letters go when ya rub 'em off the blackboard?
--apocryphal student

Where does the tread go if bicycle tires actually do change
circumference according to load?
--Carl and John

I can't say that I've ever seen anyone post loaded versus
unloaded measurements for straight-line tire-rolling.
Perhaps someone else knows? Or can explain the claimed
difference, whatever it' supposed to amount to?

Carl Fogel

On Sat, 25 Sep 2004 12:51:33 -0600 wrote:

Yes, the bead diameter on on each flange should remain the
same, no matter how fat the extrusion becomes as the die
wears.

But wouldn't an enlarged cross-section of the rim
effectively narrow the rim, flange to flange?

Sounds reasonable to me. Still it would only amount to a few
thousandths of an inch, I would think.

What I'm wondering is what happens to the effective tire
circumference if it's mounted on a narrow rim versus a wider
rim.

I kinda-sorta think that a wider rim produces a smaller
effective tire circumference.

I understand your point, but I don't really know how the tire might
respond to this. My first guess is that there would be an effect at
the extremes, but that the effect would be nil in the range of
intended rim widths.

It would all depend on how the cord responds to this change. I'll have
to think about this some more.

-
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Jim Adney
Madison, WI 53711 USA
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On Sun, 26 Sep 2004 21:51:31 -0500, Jim Adney
wrote:

On Sat, 25 Sep 2004 12:51:33 -0600 wrote:

Yes, the bead diameter on on each flange should remain the
same, no matter how fat the extrusion becomes as the die
wears.

But wouldn't an enlarged cross-section of the rim
effectively narrow the rim, flange to flange?

Sounds reasonable to me. Still it would only amount to a few
thousandths of an inch, I would think.

What I'm wondering is what happens to the effective tire
circumference if it's mounted on a narrow rim versus a wider
rim.

I kinda-sorta think that a wider rim produces a smaller
effective tire circumference.

I understand your point, but I don't really know how the tire might
respond to this. My first guess is that there would be an effect at
the extremes, but that the effect would be nil in the range of
intended rim widths.

It would all depend on how the cord responds to this change. I'll have
to think about this some more.

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------

Dear Jim,

Here's a link to a Sheldon Brown page, where some
flange-to-flange rim widths are shown in a table near the
bottom:

http://www.sheldonbrown.com/tire-sizing.html

The table suggests that 23 mm tires fit on 13-15 mm interior
rim widths and that 25 mm tires fit on 13-15-17 mm rim
widths.

These 2-4 mm ranges amount to over 13-15% of the rim width
spacing for 23 mm tires and 23-30% for 25 mm tires.

It sounds as if the acceptable interior rim width can vary
far more than any die wear. Mount a 25 mm tire on a rim with
an interior width of 13 mm and then on a different rim with
an interior width of 17 mm, and the measured circumference
may vary significantly.

(Well, significantly to those of us who believe that
measured bicycle tire circumferences can be significant.)

Carl Fogel

On Sun, 26 Sep 2004 23:33:45 -0600 wrote:

It sounds as if the acceptable interior rim width can vary
far more than any die wear.

Yes, I agree.

Mount a 25 mm tire on a rim with
an interior width of 13 mm and then on a different rim with
an interior width of 17 mm, and the measured circumference
may vary significantly.

We might actually find that the effective circumference was smaller at
both extremes and largest somewhere in the middle....

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
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