Tire Pressure & Odometer

"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 said:

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: [email hidden])

Bob Newman said:

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 said:

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
[email hidden]

Leo Lichtman said:

"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

Quoted message said:

Leo Lichtman said:

"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" <[email hidden]> schreef in bericht
news:nxm3d.4251$Ii2.3445@trnddc09...

Quoted message said:

Quoted message said:

Leo Lichtman said:

"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

Quoted message said:

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" <[email hidden]> schreef in
bericht news:[email hidden]...

Quoted message said:

Quoted message said:

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

Lou Holtman said:

"Phil, Squid-in-Training" <[email hidden]> schreef in
bericht news:[email hidden]...

Quoted message said:

Quoted message said:

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

Exactly. And to answer the OP's question of why use the
Q-Tip, it is for that reason as well as getting two good
spots. Without the swap to retain some of the marking
substance, all the paint sometimes will adhere at the first
instance of rolling on the cement and/or the second spot is
very faint.

Carl Fogel wrote: (clip)Another trick is to measure the circumference of the
tire by
rolling it a few times on a smooth garage floor(clip)

^^^^^^^^^^^^^^^^^^

Actually, I am aware of the various methods of calibrating a cyclometer for
maximum accuracy. What I was getting at was that if you just enter the
numbers that come printed with the cyclometer, or take a tape measure
reading of the tire radius, you are likely to be off a couple percent. And,
the second point was, "Who cares?" Or am I just too casual about this? When
I put my bike away, if my cyclometer reads 28.307 miles, I come in the house
and say, "I rode about thirty miles this afternoon."

^^^^^^^^^^^^^^^^^^
And, Carl wrote: (clip) (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.)(clip)

^^^^^^^^^^^^^^^^^^

Back in the days when I drove in T & D sportscar rallies, we used to
calibrate the odometer at the start of each event. The more serious drivers
used Michelin X tires (the only radials available in those days,) while many
of the novices like me ran those old expandable tires. The main factor
affecting tire size vs speed was tire expansion due to centrifugal force. I
am sure this is less of a factor on bicycle tires, because the diameter is
larger, and the cross section is smaller, but it could also play a role,
along with the effect of weaving you mentioned.

Thanks for your interesting comments.

Leo Lichtman said:

Carl Fogel wrote: (clip)Another trick is to measure the circumference of the
tire by
rolling it a few times on a smooth garage floor(clip)

^^^^^^^^^^^^^^^^^^

Actually, I am aware of the various methods of calibrating a cyclometer for
maximum accuracy. What I was getting at was that if you just enter the
numbers that come printed with the cyclometer, or take a tape measure
reading of the tire radius, you are likely to be off a couple percent. And,
the second point was, "Who cares?" Or am I just too casual about this? When
I put my bike away, if my cyclometer reads 28.307 miles, I come in the house
and say, "I rode about thirty miles this afternoon."

^^^^^^^^^^^^^^^^^^
And, Carl wrote: (clip) (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.)(clip)

^^^^^^^^^^^^^^^^^^

Back in the days when I drove in T & D sportscar rallies, we used to
calibrate the odometer at the start of each event. The more serious drivers
used Michelin X tires (the only radials available in those days,) while many
of the novices like me ran those old expandable tires. The main factor
affecting tire size vs speed was tire expansion due to centrifugal force. I
am sure this is less of a factor on bicycle tires, because the diameter is
larger, and the cross section is smaller, but it could also play a role,
along with the effect of weaving you mentioned.

Thanks for your interesting comments.

Dear Leo,

I see your point about the speedometer being off in terms of
true mileage, but I think that it's unlikely that you'd be
off even 1% with the basic recommendations.

A typical 700c tire is expected to have a 2124mm
circumference. I've measured various 700c tires from 2109mm
to 2136mm. To be just 1% off, high or low, they'd have to be
2145mm and higher, or 2103mm and lower.

To use your example, if a bicycle speedometer calibrated
just to the 700c default showed 28.307 miles, I'd expect the
true mileage to be between 28.000 and 28.600--about a 1%
error either way, or plus or minus 0.300 miles. I'd be
astonished if the error was 1.700 miles, almost six times
that much.

Indeed, I suspect that the error would tend to reflect quite
accurately slightly different lines through corners. When I
first began my four-mile ride on the Fury RoadMaster, the
distance came up 4.06 miles for a week. But as I grew more
comfortable with the corners and started shading them, the
distance dropped to 4.01 to 4.03 miles. Off-hand, I can
think of 31 corners where a better line saves swinging out
and back in a few feet, easily enough to account for the 150
to 250 foot reduction in measured distance.

Carl Fogel

Richard Tack said:

Lou Holtman said:

"Phil, Squid-in-Training" <[email hidden]>
schreef in bericht news:[email hidden]...

Quoted message said:

> 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

Exactly. And to answer the OP's question of why use the
Q-Tip, it is for that reason as well as getting two good
spots. Without the swap to retain some of the marking
substance, all the paint sometimes will adhere at the first
instance of rolling on the cement and/or the second spot is
very faint.

You would also have to be riding the bike in order to compensate for tire
smush, right?

Bob Newman said:

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, absolutely.

--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971

Phil said:

Richard Tack said:

Lou Holtman said:

"Phil, Squid-in-Training" <[email hidden]>
schreef in bericht news:[email hidden]...

>>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

Exactly. And to answer the OP's question of why use the
Q-Tip, it is for that reason as well as getting two good
spots. Without the swap to retain some of the marking
substance, all the paint sometimes will adhere at the first
instance of rolling on the cement and/or the second spot is
very faint.

You would also have to be riding the bike in order to compensate for tire
smush, right?

Or sitting on it while someone pushed you.

Thanks you Andrew! 300 technical responses and I think yours is the first
one with a yes or no.

Bob

"A Muzi" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Bob Newman said:

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

Quoted message said:

Quoted message said:

1/2 mile off in a 30 mile ride. Is this possible?

Yes, absolutely.

--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971

Is it possible? Yes.
Is it likely? No.

How is that?

Lou

"Bob Newman" <[email hidden]> wrote in message
news:0xz3d.323815$sh.194569@fed1read06...

Quoted message said:

Thanks you Andrew! 300 technical responses and I think yours is the first
one with a yes or no.

Bob

"A Muzi" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Bob Newman said:

A friend claims that if your tires are underinflated it can make

enough

Quoted message said:

Quoted message said:

Quoted message said:

difference in the circumference of the tire to make your odometer as

much as

Quoted message said:

Quoted message said:

1/2 mile off in a 30 mile ride. Is this possible?

Yes, absolutely.

--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971

"Richard Tack" <[email hidden]> wrote in message
news:zuv3d.2220$Co1.1669@trnddc02...

Quoted message said:

Phil said:

Richard Tack said:

Lou Holtman wrote:

>"Phil, Squid-in-Training" <[email hidden]>
>schreef in bericht news:[email hidden]...
>
>
>>>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

Exactly. And to answer the OP's question of why use the
Q-Tip, it is for that reason as well as getting two good
spots. Without the swap to retain some of the marking
substance, all the paint sometimes will adhere at the first
instance of rolling on the cement and/or the second spot is
very faint.

You would also have to be riding the bike in order to compensate for

tire

Quoted message said:

Quoted message said:

smush, right?

Or sitting on it while someone pushed you.

My system is to stretch out a 50 ft tape on the ground, sit on the bike and
roll out 5 revolutions of a marked location on the tire. Measure and divide.
I figure that has to be more consistent than trying to get an accurate
reading on one revolution and not any more difficult to do. With a
cyclometer that calibrates in mm I think it is good to within a few tenths
per hundred miles.
Bill

Carl Fogel said:

I see your point about the speedometer being off in terms of true
mileage, but I think that it's unlikely that you'd be off even 1%
with the basic recommendations.

Quoted message said:

A typical 700c tire is expected to have a 2124mm circumference. I've
measured various 700c tires from 2109mm to 2136mm. To be just 1%
off, high or low, they'd have to be 2145mm and higher, or 2103mm and
lower.

A 25mm cross section 700 tire has a 2096mm rollout distance from
any measurements I have made at 100psi. I think you need to specify
more parameters when offering calibration numbers.

Jobst Brandt
[email hidden]