Tire Pressure & Odometer

Bob Newman writes:

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

How about the fourth response:
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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

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Jobst Brandt

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

Now, what about the steering "wander" at these slower "driveway" speeds?
Wouldn't the total distance be slightly more at low speed than at higher
speeds?

Phil (stirring it up further)

On Mon, 20 Sep 2004 16:45:05 GMT,
wrote:

Carl Fogel writes:

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.

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


Dear Jobst,

I think that most printed manuals suggest 2124mm, which is
the industry default calibration for road rather than
mountain bikes.

Obviously, individual tires will vary slightly.

However, anyone who wants to delve into the details of
idealized bicycle circumferences can find them nicely done
(as usual) he

http://www.sheldonbrown.com/cyclecom...libration.html

Sheldon's table lists 2105mm for a 700c 25mmm cross section
with a jump to 2136mm for a 28mm cross section.

Oddly enough, midway between them would be something like
2121mm, so the industry 2124mm seems reasonable. In any
case, I use 120-125 psi, tires that claim a 26mm cross
section, and a model that includes a Kevlar belt that
slightly increases the circumference.

Of course, none of this affects the point that a 1%
calibration error would still require roughly a 21mm
variation on such tires.

Carl Fogel

Phil, Squid-in-Training wrote:
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


Now, what about the steering "wander" at these slower "driveway" speeds?
Wouldn't the total distance be slightly more at low speed than at higher
speeds?

Phil (stirring it up further)



Have to push the bike and rider in a
straight line, natch.

"Phil, Squid-in-Training" wrote in
message news
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

Now, what about the steering "wander" at these slower "driveway" speeds?
Wouldn't the total distance be slightly more at low speed than at higher
speeds?

Phil (stirring it up further)

Since you have the tape on the ground as a guide it can be done with almost
no wander. Just one-foot it with the bike weighted for five revs and your
done. I use a pen to make a mark on the sidewall but a piece of tape is
probably a better choice. You can get it to 1/5th of a 1/16th, convert to mm
and you have a reasonably accurate measurement. AND, if you want to resolve
the OP's question do it twice, take a 7x23 and go at it with 120 and 70 psi
and see what you get.
Bill

"Phil, Squid-in-Training" aszonygya:
:Now, what about the steering "wander" at these slower "driveway" speeds?
:Wouldn't the total distance be slightly more at low speed than at higher
:speeds?

An excellent and very valid point. Same about navigating around road
imperfections such as cracks and even puddles. /Paul

Ride at speed (makes path as straight as reasonably possible) thru an
aluminum foil 'pan' with a bit of salad-oil in it. Measure distance
difference between as many similar points marked on the pavement as you have
patience for.

I just did one run measured twice by eye, once between start points and once
between end points of the oil marks on the pavement.

Raw data: 9", 69", 128", 188", 247", 307" and 10", 70", 129 3/4", 189",
249" 308 3/4"

Intermediate step: Avg delta 59.6" and Avg delta 59.95" Average of deltas
= 59.775" = 1518 mm circumference and 9.43...E-4 mile or 1060 mile^-1.

Note that a mile is 5280 feet, a mile/10 is 528 feet, a mile/100 is 52' 9.6
", a mile/1000 is 63.4 " about the magnitude of my tire size (care to
guess?). Considering such as sinuosity, don't indulge in false
accuracy/precision. An occasional missed reed switch pulse seems pretty
likely too.

YMMV - literally and figuratively - 406 x 1.95

Paul Nevai wrote:
"Phil, Squid-in-Training"
aszonygya:
Now, what about the steering "wander" at these slower "driveway"
speeds? Wouldn't the total distance be slightly more at low speed
than at higher speeds?

An excellent and very valid point. Same about navigating around road
imperfections such as cracks and even puddles. /Paul

Yeah... I don't even have a bike computer on my bikes... I broke my first
one 5 times, resoldered it 4 times until the 5th resoldering attempt melted
the mount. I've given up on them since.

Phil

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

Phil Lee writes:

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.

Now, what about the steering "wander" at these slower "driveway"
speeds? Wouldn't the total distance be slightly more at low speed
than at higher speeds?

That is known as cosine error and alters straight line distance by
1/cosine of steering angle. You'll notice that even 5 degrees gives
a cosine of 0.996 or an error of 4/1000s of point to point distance.

Jobst Brandt