Tire Pressure & Odometer

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?

Bob

"Bob Newman" wrote: (clip)1/2 mile off in a 30 mile ride. Is this
possible?
^^^^^^^^^^^^^^
That's only about 1.6%. I'd be happy to have mine that close in the first
place. Unless you went to special pains to calibrate it, chances are it
will be off one or two percent. A 26" wheel, measured with a tape measure
to within 1% has to be down to within 1/4" on the diameter, or 1/8" on the
radius.

"Bob Newman" wrote:

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?

I used to measure my tire pressure by riding a known route and reading
the odometer. As I recall, the difference between normal inflation and
under-inflation (to the point where I probably shouldn't have been
riding it) was on the order of 1%. So a half mile out of 30 (1.7%)
doesn't seem impossible, especially for fat tires.

--
Ray Heindl
(remove the Xs to reply to: )

On Sun, 19 Sep 2004 15:44:44 -0400, "Bob Newman"
wrote:

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?

Bob

Dear Bob,

A half mile error in 30 miles is an error of about 1.67%.

A typical 700c tire is 2124mm in circumference

So to read a half mile low, you'd need a speedometer set to
2124mm and a tire whose effective circumference when
underinflated was about 2089mm (1.67% smaller).

One theory is that the tire is a mis-shaped figure, a circle
flattened at the contact patch.

So some people would theorize that to lose 35mm in
circumference, you need to reduce the effective radius from
axle to contact patch from 338mm to 332.5mm--only 5.5mm,
about a quarter inch. (Again, that's about 1.67%.)

A conflicting theory is that this is well-meant but
mistaken. The speedometer counts each revolution of the
tire's complete circumference, whether it follows a
perfectly spherical path or is twisted into a figure 8. That
is, 2124mm of rubber must engage 2124mm of road, regardless
of inflation.

A middle-of-the-road theory is that the truth lies somewhere
in between, with the error amounting not to the full 1.67%
difference, but to the difference between the "shortcut"
taken inside the arc of a normal circle by a flat contact
patch. That is, draw a small line or chord inside a circle
(normal tire pressure) and the path is a tiny bit shorter
than the full circle. Draw a longer line or chord inside a
circle (lower tire pressure) and the path shortens.

A simple test would be to ride out with normal pressure and
then ride back after letting some air out and compare the
distances. I suspect that the differences would be
noticeably less than half a mile per 30 miles.

I look forward to someone working out the chord differences
between 100 psi and 75 psi on a 700c tire, after first
measuring actual contact patches at those pressures.

Carl Fogel

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?

Yes, but more important is how you set the calibration number on your
instrument. This is done as follows and should answer your question
since I don't know what tire you are riding and how underinflated it
is.

http://draco.acs.uci.edu/rbfaq/FAQ/8b.24.html

Jobst Brandt

On Sun, 19 Sep 2004 20:09:37 GMT, "Leo Lichtman"
wrote:


"Bob Newman" wrote: (clip)1/2 mile off in a 30 mile ride. Is this
possible?
^^^^^^^^^^^^^^
That's only about 1.6%. I'd be happy to have mine that close in the first
place. Unless you went to special pains to calibrate it, chances are it
will be off one or two percent. A 26" wheel, measured with a tape measure
to within 1% has to be down to within 1/4" on the diameter, or 1/8" on the
radius.

Dear Leo,

Another trick is to measure the circumference of the tire by
rolling it a few times on a smooth garage floor along a tape
measure. This gives readings to within 1/8th" without much
trouble on a distance of around 80 inches--much more
accuracy.

In any case, I think that the original poster is asking
whether his odometer reading would change up to half a mile
in 30 miles just because of tire inflation.

I expect the odometer reading for my daily 15 mile ride to
vary less than 0.03 miles either way--I'm getting 15.18 to
15.22 miles with monotonous regularity lately. (Lower speeds
usually yield higher readings because the tires swerve back
and forth more times per mile--this is a matter of about 200
feet in 15 miles.)

So I expect my speedometer to vary less than a tenth of a
mile in thirty miles.

The speedometer is quite accurate about how many times the
magnet passed the sensor. I think that the original poster
is really asking whether this will be affected by tire
inflation.

Carl Fogel

wrote:

On Sun, 19 Sep 2004 20:09:37 GMT, "Leo Lichtman"
wrote:


"Bob Newman" wrote: (clip)1/2 mile off in a 30 mile ride. Is this
possible?
^^^^^^^^^^^^^^
That's only about 1.6%. I'd be happy to have mine that close in the first
place. Unless you went to special pains to calibrate it, chances are it
will be off one or two percent. A 26" wheel, measured with a tape measure
to within 1% has to be down to within 1/4" on the diameter, or 1/8" on the
radius.


Dear Leo,

Another trick is to measure the circumference of the tire by
rolling it a few times on a smooth garage floor along a tape
measure. This gives readings to within 1/8th" without much
trouble on a distance of around 80 inches--much more
accuracy.

snip
Carl Fogel

Here's my calibration method: Properly inflated tires of
course. Cut the end off of a Q-Tip, tape it to the center of
the tread. Dab it with black ink or paint, roll it in a
straight line across a smooth garage floor with the rider in
the saddle. Measure between the two marks left on the floor.

"Richard Tack" schreef in bericht
news:nxm3d.4251$Ii2.3445@trnddc09...
wrote:

On Sun, 19 Sep 2004 20:09:37 GMT, "Leo Lichtman"
wrote:


"Bob Newman" wrote: (clip)1/2 mile off in a 30 mile ride. Is this
possible?
^^^^^^^^^^^^^^
That's only about 1.6%. I'd be happy to have mine that close in the
first place. Unless you went to special pains to calibrate it, chances
are it will be off one or two percent. A 26" wheel, measured with a tape
measure to within 1% has to be down to within 1/4" on the diameter, or
1/8" on the radius.


Dear Leo,

Another trick is to measure the circumference of the tire by
rolling it a few times on a smooth garage floor along a tape
measure. This gives readings to within 1/8th" without much
trouble on a distance of around 80 inches--much more
accuracy.

snip
Carl Fogel

Here's my calibration method: Properly inflated tires of course. Cut the
end off of a Q-Tip, tape it to the center of the tread. Dab it with black
ink or paint, roll it in a straight line across a smooth garage floor with
the rider in the saddle. Measure between the two marks left on the floor.

Why the Q-tip? Why not apply the inkt or a little dot of paint directly on
the tire.
I did a test once. I rode exactly the same course the first time the front
tire (Michelin Pro Race 23 mm) inflated to 8 bar and the second time
inflated to 4 bar. I measured both times a distance of 27.6 km. Since then I
don't bother that much setting the wheeldiameter.

Lou

Why the Q-tip? Why not apply the inkt or a little dot of paint
directly on the tire.

Does the paint method take into account the spread of the paint? Or would
one just measure at the centers of the dots after the first rotation?

Phil

"Phil, Squid-in-Training" schreef in
bericht ...
Why the Q-tip? Why not apply the inkt or a little dot of paint
directly on the tire.

Does the paint method take into account the spread of the paint? Or would
one just measure at the centers of the dots after the first rotation?

Phil

You get two alike shaped paintspots on the floor and you just pick a
characteristic point.

Lou