Accurate Odometer

On 13 Feb 2005 05:37:25 -0800, "jms"
wrote:


Dan wrote:
wrote in message
...

As a cross-check, my $15 Nashbar odometer varies perhpas
0.03 miles on either side of 15.25 miles on my daily ride,
which may say more about how much I swerve and how tight I
cut my corners than it says about the accuracy of the
speedometer.


Carl Fogel

Or variation in tire pressure

Maybe you could rent something like this which is made for measuring
distances:

http://www.engineersupply.com/Produc...=ES181&affid=7

Dear JMS,

Actually, the smaller hand wheel is probably less accurate
than the larger bicycle tire.

That is, both are likely to be calibrated to the same level
of absolute accuracy, +/- so many millimeters or fractions
of millimeters.

But the smaller wheel's variation is larger compared to each
revolution. Think of measuring a football field with a foot
ruler and then with a yardstick, both accurate to within a
tenth of an inch.

The foot-ruler will accumulate 300 inaccuracies, while the
yard stick will accumulate only 100.

The 36" hand wheel will be less than half as accurate as a
well-measured ~80" bicycle tire--and a lot slower.

The smaller hand wheel will also wobble a good deal more,
due to both its smaller size and its lower speed.

And the hand wheel stops at only 100,000 feet, so it won't
even handle a 20-mile ride.

The 36-inch hand wheel is, however, more accurate for
shorter distances--unless you fit three magnets on a
36-spoke 700c bicycle wheel and begin measuring in roughly
27-inch increments.

For shorter direct distances like wheat and football fields,
I think that laser rangefinders are now popular:

http://www.opticsplanet.net/buyapro600co.html

Carl Fogel

Unlike the distance, the scenery varies quite a bit. So does
the wildlife--squirrels...

OK, let's stop right there. On some rides, trying to avoid squirrels alone
might add up to a considerable number of feet!

[snip]

"You talk back."
--Calvin Coolidge on how he finished with White House
visitors by 5pm when his successor as governor of
Massachusetts was still seeing people at 9pm

"You swerve for squirrels."
--Carl Fogel on why some riders see more variation on their
odometers

In truth, I don't actually swerve for squirrels anymore, at least not
intentionally. It's been my experience that they (squirrels) are so
unpredictable that swerving is as likely to bring you into contact with them
as not, and if you hold a straight line instead, at least you have a better
chance of not going down in the event you make contact.

And I *have* made contact in the past. Oh my have I. And thus the infamous
"Killer Squirrel" section on our website, to serve as a warning to others.

--Mike-- Chain Reaction Bicycles
www.ChainReactionBicycles.com

Carl Fogel writes:

As a cross-check, my $15 Nashbar odometer varies perhaps 0.03
miles on either side of 15.25 miles on my daily ride, which may
say more about how much I swerve and how tight I cut my corners
than it says about the accuracy of the speedometer.

Or variation in tire pressure

Possibly tire pressure is involved, but unlikely, since the tiny
odometer variation doesn't seem to go in one direction.

That is, I don't see my distance slowly increasing (or decreasing)
until I remember to pump my tires up again.

The variation could also be due to weight changes, since the
standard Fogel often varies by several chocolate doughnuts.

Don't rely on others reports if you don't believe them. Just measure
the rollout distance with different inflation pressures on your own
bicycle. I have done it and it isn't just a millimeter. I'm
satisfied that 2093mm is good enough for my Avocet 700-25 tires at
average inflation. I don't care if it changes a little with inflation
because the tires hold air to my satisfaction for more than a month.

Many posters here on rec.bicycles.tech warn of the dangerous
inaccuracy of measuring an unloaded wheel, but I've never seen a
post that mentioned any measured difference in tire circumference
between a loaded and an unloaded bike. (Indeed, the warnings often
fail to mention which way they think the loading affects the
outcome.)

What dangers have been reported? As inflation pressure decreases, so
does the rollout distance for one revolution. As I pointed out, when
driving a car over Botts Dots on roads you'll notice the slamming
effect get harsher with increasing speed. That is because the change
in rolling radius causes a momentary acceleration of the wheel that
through inertia cannot occur at higher speeds as easily. Therefore,
the slamming effect bends wheel suspension. The bumps do not get
larger nor does wheel bounce but the the rolling radius changes almost
instantaneously. Since most car tires are radials (with a constant
circumference belt), I find that a dramatic demonstration of rolling
radius.

I wouldn't be surprised if the difference was more theoretical than
measurable, but it would fun if someone posted the results of some
careful loaded and unload tire measurements.

Be surprised!

Er, you need to read more carefully. Your unreported "dangers" are
not the same as my rather obviously tongue-in-cheek "dangerous
inaccuracy."

I'd be more surprised if you were to include the figures that you
say are dramatic. I can't even tell from your post whether you are
saying that the loaded tire produces a larger or smaller
circumference. I'd be willing to believe your figures if you'd
simply post them.

You find 2093mm a good average figure for your tires, presumably
measured loaded. What was the unloaded measurement? It "isn't just
a millimeter."

As I corrected in a followup. That was a typo. It is 2096mm.

If you reply with a 4-character post, it will be the first time that
I know of an actual difference appearing in this newsgroup.

I hope it's not a secret.

As I said, you should be aware of this difference if you measure
rollout distance. The difference per psi varies with weight of rider,
riding position and tire cross section. With my tires in my most
common position, hands on the bar tops (with drop bars), the
difference is 20mm reducing inflation pressure from my usual 100psi to
80psi. I took those pressures because they represent the limit with
which I ride and because it gives a repeatable and easily measured
distance.

The Botts Dots experiment should be obvious the next time you are on a
road that allows speeds up to 65mph. That these accelerations are
large are shown by the tire rubber that leaves skid marks in the
process. It is also why Elbert Botts invented the significant part of
these lane markers, the adhesive. Previously any glued devices were
readily knocked loose. It was his research in the adhesive that made
them possible.

http://www.dot.ca.gov/hq/paffairs/about/botts.htm

The first significant application of these lane markers was on
"Hospital Curve" on HWY101 in San Francisco, a long relatively sharp
curve for an 8-lane freeway around San Francisco General Hospital that
in wet weather, especially at night, was a challenge to not drift into
other lanes, the road stripes being invisible in the wet glare. There
were many serious crashes at that curve that did not have a median nor
a dividing crash-wall. Today, it is hard to visualize how drives
managed before Botts Dots.

http://tinyurl.com/5brzg

Jobst Brandt

Arthur Harris writes:

I don't care if it changes a little with inflation because the
tires hold air to my satisfaction for more than a month.

I'm curious what inner tubes you're using that maintain air pressure
that well. Also, how much pressure do you lose over a month?

Any of the Specialized, Trek, Avocet, etc tubes that are labeled
700-28 or thereabouts. These are regular black butyl rubber and look
like they are made by the same supplier. I don't buy ultra light
tubes or ones that are several sizes smaller than the tire inside
cross section.

It all depends on what you find important for your ride.

Jobst Brandt

On Mon, 14 Feb 2005 05:47:45 GMT,
wrote:

[snip]

With my tires in my most
common position, hands on the bar tops (with drop bars), the
difference is 20mm reducing inflation pressure from my usual 100psi to
80psi.

[snip]

Dear Jobst,

So with you sitting on the bike, your front 700c x 25 Avocet
rolls a measured 2096 mm in one spin at 80 psi, right?

Just to be sure that I'm following you, this is 20 mm less
than the same tire's single spin of 2116 mm with no rider,
just the bike rolling along a smooth floor, and the same 80
psi, a roughly 1% difference?

I've never seen any details, so I'm quite interested. If I
missed any details or have them wrong, please let us know.

Thanks,

Carl Fogel

On Mon, 14 Feb 2005 02:59:42 GMT, "Mike Jacoubowsky"
wrote:

Unlike the distance, the scenery varies quite a bit. So does
the wildlife--squirrels...

OK, let's stop right there. On some rides, trying to avoid squirrels alone
might add up to a considerable number of feet!

[snip]

"You talk back."
--Calvin Coolidge on how he finished with White House
visitors by 5pm when his successor as governor of
Massachusetts was still seeing people at 9pm

"You swerve for squirrels."
--Carl Fogel on why some riders see more variation on their
odometers

In truth, I don't actually swerve for squirrels anymore, at least not
intentionally. It's been my experience that they (squirrels) are so
unpredictable that swerving is as likely to bring you into contact with them
as not, and if you hold a straight line instead, at least you have a better
chance of not going down in the event you make contact.

I've learned to more or less aim for the squirrel. That little dance they do is
too unpredictable to anticipate, as it should be given the many millions of
years they've been working on it. On the other hand I feel I can trust them to
get out of my way, a thing they wish even more ardently than I. So I aim for
the point where he starts his juke maneauver. My thinking is that the squirrel
is going to act on the assumption that you are trying to get him, reasonable
enough at his end of the food chain, therefor he will act to avoid that, so if I
aim for him then we are both working from the same playbook.

And I *have* made contact in the past. Oh my have I. And thus the infamous
"Killer Squirrel" section on our website, to serve as a warning to others.

I've never had the misfortune. I'll have to check out your site.

Now armadillos and possum are an entirely different thing.


Ron

--Mike-- Chain Reaction Bicycles
www.ChainReactionBicycles.com

Carl Fogel writes:


With my tires in my most common position, hands on the bar tops
(with drop bars), the difference is 20mm reducing inflation
pressure from my usual 100psi to 80psi.

So with you sitting on the bike, your front 700c x 25 Avocet rolls a
measured 2096 mm in one spin at 80 psi, right?

Wrong. I said it measures 2096mm at 100psi and 20mm shorter (=2076mm)
at 80psi. I thought it was obvious that wheel rolling radius is
smaller under lower inflation pressure or for that matter, greater
load with constant inflation pressure rather than greater.

Just to be sure that I'm following you, this is 20 mm less than the
same tire's single spin of 2116 mm with no rider, just the bike
rolling along a smooth floor, and the same 80 psi, a roughly 1%
difference?

I've never seen any details, so I'm quite interested. If I
missed any details or have them wrong, please let us know.

You travel a shorter distance per wheel revolution, the closer
the axle is to the road (wheel radius).

Have you measured rollout distance to calibrate a Cyclometer?

Jobst Brandt

RonSonic wrote:


I've learned to more or less aim for the squirrel. That little dance they do is
too unpredictable to anticipate, as it should be given the many millions of
years they've been working on it. On the other hand I feel I can trust them to
get out of my way, a thing they wish even more ardently than I. So I aim for
the point where he starts his juke maneauver. My thinking is that the squirrel
is going to act on the assumption that you are trying to get him, reasonable
enough at his end of the food chain, therefor he will act to avoid that, so if I
aim for him then we are both working from the same playbook.

Pretty much my philosophy as well, but I look at them like railroad
tracks - you want to be vertical and run over the things perpendicularly
with a good grip on the bars but not tense. I've rolled over two
squirrels and although they made some funny sounds, they both ran away
afterward with little sign of injury.

Much more likely to go down if you run over a snake in my vicarious
experience.
--
My bike blog:
http://diabloscott.blogspot.com/

On Mon, 14 Feb 2005 19:06:51 GMT,
wrote:

[snip]

Dear Jobst,

Let's try one more time.

You have an Avocet 700c x 25 front tire, right?

What does it measure at 100 psi with you on the bike?
a) 2116 mm
b) 2113 mm
c) 2096 mm
d) 2093 mm
e) 2076 mm
f) 2073 mm
g) other

What does it measure at 100 psi with no rider?
a) 2116 mm
b) 2113 mm
c) 2096 mm
d) 2093 mm
e) 2076 mm
f) 2073 mm
g) other

What does it measure at 80 psi with you on the bike?
a) 2116 mm
b) 2113 mm
c) 2096 mm
d) 2093 mm
e) 2076 mm
f) 2073 mm
g) other

What does it measure at 80 psi with no rider?
a) 2116 mm
b) 2113 mm
c) 2096 mm
d) 2093 mm
e) 2076 mm
f) 2073 mm
g) other

Carl Fogel