cyclo-computer calibration - wheel/tire size variance?

B

Bob Palermo

Guest
Hi,

I have a Trek 2300 with Bontrager race X-lite wheels and tires. The tires
are 700 x 23. I also have a Cateye cyclo-computer. To calibrate, I referred
to the table in the instructions, and used the value for 700 x 23. From my
first rides, I had the feeling that the setting was off. I thought I was
going faster than what I saw on my computer. I know that there are methods
to measure the wheel circumfrence, but when I tried these, I couldn't get
consistent results. I've had good success using roads with mile markers for
calibration, but the road I used for my mountain bike had removed them
during road re-paving. (After setting my mountain bike computer based on the
table, I monitored my distance across 5 mile markers. The miles came in at
1.00, 1.01, 1.00, 1.00, 1.01 on my computer, leading me to conclude that my
mountain bike computer was calibrated accurately.)

On my road bike, rides with friends had my average speed between .5 and .7
mph slower than theirs. (I guess they're just faster than me . )
Anyway, a few days ago I was on a road that had 10 mile and 15 mile lines
for a triathlon that was held recently. Riding the loop twice, I took note
of the readings on my computer as I crossed these lines. Both times, the
distance differential on my computer worked out to 4.88 miles. That's about
2.5%, which would explain a .5 mph average time difference. So now I know

Finally to the questions.

1) Has anyone else who has Bontrager X-lite wheels and tires found that
using the "standard" setting for 700 x 23 resulted in the 2.5% below true
2) Is it the wheel or the tire that is the main cause of the discrepancy?
3) What are the expected tolerances for Bontrager xlite wheels and tires? In
(700 x 23), and tried them all out, using the standard 700 x 23 calibration
setting, how different would the results on my computer be, for trials on
my 5 mile course? Would they all be near 4.88 mi or would they vary
significantly?

This is by no means a critical issue but I am interested in others'
experiences or insights on this subject.

Thanks,

Bob P.

M

Mike Jacoubowsky

Guest
> Finally to the questions.
>
> 1) Has anyone else who has Bontrager X-lite wheels and tires found that
> using the "standard" setting for 700 x 23 resulted in the 2.5% below true
> readings that I have experienced?
> 2) Is it the wheel or the tire that is the main cause of the discrepancy?
> 3) What are the expected tolerances for Bontrager xlite wheels and tires?
> tires (700 x 23), and tried them all out, using the standard 700 x 23
> calibration setting, how different would the results on my computer be,
> for trials on my 5 mile course? Would they all be near 4.88 mi or would
> they vary significantly?

Bob: Input "2080" (or "208" if it asks for three digits) and you'll get
accurate distance & speed info out of your computer. We have part of the
floor of our shop measured out for on-the-fly rollout tests, and 208 is very
consistently the number that comes up. It's also the number I use for those
tires on my own bike, and works great.

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

"Bob Palermo" <[email protected]> wrote in message
news:[email protected]
> Hi,
>
> I have a Trek 2300 with Bontrager race X-lite wheels and tires. The tires
> are 700 x 23. I also have a Cateye cyclo-computer. To calibrate, I
> referred to the table in the instructions, and used the value for 700 x
> 23. From my first rides, I had the feeling that the setting was off. I
> thought I was going faster than what I saw on my computer. I know that
> there are methods to measure the wheel circumfrence, but when I tried
> these, I couldn't get consistent results. I've had good success using
> roads with mile markers for calibration, but the road I used for my
> mountain bike had removed them during road re-paving. (After setting my
> mountain bike computer based on the table, I monitored my distance across
> 5 mile markers. The miles came in at 1.00, 1.01, 1.00, 1.00, 1.01 on my
> computer, leading me to conclude that my mountain bike computer was
> calibrated accurately.)
>
> On my road bike, rides with friends had my average speed between .5 and .7
> mph slower than theirs. (I guess they're just faster than me . )
> Anyway, a few days ago I was on a road that had 10 mile and 15 mile lines
> for a triathlon that was held recently. Riding the loop twice, I took note
> of the readings on my computer as I crossed these lines. Both times, the
> distance differential on my computer worked out to 4.88 miles. That's
> about 2.5%, which would explain a .5 mph average time difference. So now I
> know how to adjust my computer.
>
> Finally to the questions.
>
> 1) Has anyone else who has Bontrager X-lite wheels and tires found that
> using the "standard" setting for 700 x 23 resulted in the 2.5% below true
> readings that I have experienced?
> 2) Is it the wheel or the tire that is the main cause of the discrepancy?
> 3) What are the expected tolerances for Bontrager xlite wheels and tires?
> tires (700 x 23), and tried them all out, using the standard 700 x 23
> calibration setting, how different would the results on my computer be,
> for trials on my 5 mile course? Would they all be near 4.88 mi or would
> they vary significantly?
>
> This is by no means a critical issue but I am interested in others'
> experiences or insights on this subject.
>
> Thanks,
>
> Bob P.
>

P

POHB

Guest
I always put a little chalk mark on the tyre and the floor, then sit on the
bike and scoot forward a couple of revolutions, make another chalk mark and
measure the distance with a tape.

B

Bob Wheeler

Guest
Bob Palermo wrote:
> Hi,
>
> I have a Trek 2300 with Bontrager race X-lite wheels and tires. The tires
> are 700 x 23. I also have a Cateye cyclo-computer. To calibrate, I referred
> to the table in the instructions, and used the value for 700 x 23. From my
> first rides, I had the feeling that the setting was off. I thought I was
> going faster than what I saw on my computer. I know that there are methods
> to measure the wheel circumfrence, but when I tried these, I couldn't get
> consistent results. I've had good success using roads with mile markers for
> calibration, but the road I used for my mountain bike had removed them
> during road re-paving. (After setting my mountain bike computer based on the
> table, I monitored my distance across 5 mile markers. The miles came in at
> 1.00, 1.01, 1.00, 1.00, 1.01 on my computer, leading me to conclude that my
> mountain bike computer was calibrated accurately.)
>
> On my road bike, rides with friends had my average speed between .5 and .7
> mph slower than theirs. (I guess they're just faster than me . )
> Anyway, a few days ago I was on a road that had 10 mile and 15 mile lines
> for a triathlon that was held recently. Riding the loop twice, I took note
> of the readings on my computer as I crossed these lines. Both times, the
> distance differential on my computer worked out to 4.88 miles. That's about
> 2.5%, which would explain a .5 mph average time difference. So now I know
> how to adjust my computer.
>
> Finally to the questions.
>
> 1) Has anyone else who has Bontrager X-lite wheels and tires found that
> using the "standard" setting for 700 x 23 resulted in the 2.5% below true
> readings that I have experienced?
> 2) Is it the wheel or the tire that is the main cause of the discrepancy?
> 3) What are the expected tolerances for Bontrager xlite wheels and tires? In
> (700 x 23), and tried them all out, using the standard 700 x 23 calibration
> setting, how different would the results on my computer be, for trials on
> my 5 mile course? Would they all be near 4.88 mi or would they vary
> significantly?
>
> This is by no means a critical issue but I am interested in others'
> experiences or insights on this subject.
>
> Thanks,
>
> Bob P.
>
>

Each wheel is different. The only way I have found is to carefully
measure the loaded circumference when the bike is ridden over a measured
course. See the following for instructions that work:
http://www.bobwheeler.com/bicycle/Circumference_/circumference_.html

--
Bob Wheeler --- http://www.bobwheeler.com/
ECHIP, Inc. ---
Randomness comes in bunches.

J

Jasper Janssen

Guest
>Bob Palermo wrote:

>> 2) Is it the wheel or the tire that is the main cause of the discrepancy?

The wheel has no effect whatsoever, except the rim width. Thinner rims
cause the tires to be a smaller effective diameter than broad rims. Other
than that, it's tyre size and more importantly tyre inflation and loading.
A sagging tyre makes for a smaller effective wheel diameter than one
that's pumped hard as a rock. Also, your riding style might have a slight
influence -- do you cut the corners or take them wide?

Jasper

J

John Everett

Guest
On Wed, 28 Sep 2005 20:40:15 -0500, "Bob Palermo"
<[email protected]> wrote:

>Hi,
>
>I have a Trek 2300 with Bontrager race X-lite wheels and tires. The tires
>are 700 x 23. I also have a Cateye cyclo-computer. To calibrate, I referred
>to the table in the instructions, and used the value for 700 x 23. From my
>first rides, I had the feeling that the setting was off. I thought I was
>going faster than what I saw on my computer. I know that there are methods
>to measure the wheel circumfrence, but when I tried these, I couldn't get
>consistent results. I've had good success using roads with mile markers for
>calibration, but the road I used for my mountain bike had removed them
>during road re-paving. (After setting my mountain bike computer based on the
>table, I monitored my distance across 5 mile markers. The miles came in at
>1.00, 1.01, 1.00, 1.00, 1.01 on my computer, leading me to conclude that my
>mountain bike computer was calibrated accurately.)
>
>On my road bike, rides with friends had my average speed between .5 and .7
>mph slower than theirs. (I guess they're just faster than me . )
>Anyway, a few days ago I was on a road that had 10 mile and 15 mile lines
>for a triathlon that was held recently. Riding the loop twice, I took note
>of the readings on my computer as I crossed these lines. Both times, the
>distance differential on my computer worked out to 4.88 miles. That's about
>2.5%, which would explain a .5 mph average time difference. So now I know
>
>Finally to the questions.
>
>1) Has anyone else who has Bontrager X-lite wheels and tires found that
>using the "standard" setting for 700 x 23 resulted in the 2.5% below true
>2) Is it the wheel or the tire that is the main cause of the discrepancy?
>3) What are the expected tolerances for Bontrager xlite wheels and tires? In
>(700 x 23), and tried them all out, using the standard 700 x 23 calibration
>setting, how different would the results on my computer be, for trials on
>my 5 mile course? Would they all be near 4.88 mi or would they vary
>significantly?
>
>This is by no means a critical issue but I am interested in others'
>experiences or insights on this subject.

Since you're already pretty happy with the setting on your MTB, why
don't you just ride that bike around the block, ride your new bike the
same distance, and compare results? Adjust as needed.

A

Andrew W

Guest
Cycle computers will always give approximate results. Diferences in
tyre inflation and weight carried will always lead to variations in
ditance covered per wheel revolution.

I have recently ridden the same 102 mile (as measured by GPS) route
twice. Once my cycle computer gave a 100 mile distance, the other 101
miles. As the GPS gives the same distance to within 0.2 miles the
variance must be in the cycle computer.

Some responses to the OPs query assume there is a "correct" calibration
that will give zero error for any particular bike setup. I do not
think this is the case.

Furthermore errors in speed are much more likely to be significant than
in distance. All speeds must be averaged over a distance and the
algorithm used may differ from one cycle computer to the next.

On the rides alluded to above the agreement on speed between cycle
computer and GPS was much worse than the distance agreements (indeed,
the max speed recorded differed by 5 mph). This is wholly attributable
to the longer averaging interval of the GPS. Similar differences are
to be expected between different computers.

Andrew Webster

J

John

Guest
> This is by no means a critical issue but I am interested in others'
> experiences or insights on this subject.
>
> Thanks,
>
> Bob P.

If you're looking for the right circumference in millimeters to enter
into your computer I would use two methods.
1. Take a string and rap it around the tire and measure it. You'll
have to use a little scotch tape to hold it in place.
2. Use this formula. Tire size in I.S.O + (width x 2) x 3.142 . So,
your 700x23 tire would be 622 + (23 x 2) x 3.142 = 622 + 46 x 3.142 =
2099mm. Which is the number you enter into your computer.
More on this formula at the bottom of Sheldon's page
http://www.sheldonbrown.com/cyclecomputer_calibration.html

When I measured my tire using both the string and the I.S.O formula
there was only 1mm difference between them.
I then took my bike to the LHST (local high school track) which I know
is exactly 400 meters measured from 8 inches into the first lane
http://www.philsport.com/narf/atrack.htm . and the distance came out
right on the money

I'm not a big fan of the "rollout method"
http://www.sheldonbrown.com/brandt/circumference.html because you can
get different measurements every time you do it. At least I do. Good
luck

John

P

POHB

Guest
"Jasper Janssen" <[email protected]> wrote

>... Also, your riding style might have a slight
> influence -- do you cut the corners or take them wide?
>

But that doesn't affect the accuracy of your measurement, it changes the
distance you travel.

I wouldn't over-estimate the accuracy of mile markers etc. either. I don't
suppose they're measured with laser accuracy and if there's any corners
involved are they assuming you take the shortest line, down the middle or
what?

J

John Everett

Guest
On 29 Sep 2005 16:38:02 -0700, "John" <[email protected]> wrote:
>
>I'm not a big fan of the "rollout method"
>http://www.sheldonbrown.com/brandt/circumference.html because you can
>get different measurements every time you do it. At least I do. Good
>luck

I also not a big fan or the "rollout method", but not because of
inconsistency. It's because it sounds like a lot of hard work. Here's
what I do, cut-and-pasted from a posting I did in May 2004:

"Here's the method I've used for years and years. I lay the wheel
horizontally on top of my kitchen waste paper basket, centering the
hub. I slide the basket over until the tire is firmly against a wall,
with either the stem or rim joint adjacent to the wall. I lay a metric
yardstick (oxymoron?) over the skewer end, the zero end of the
yardstick touching the wall and passing over both the stem and joint.
I hold a carpenter's level vertically against the tire at the point
furthest from the wall and adjust until the bubble shows it's
perfectly vertical. Where the level edge crosses the yardstick is the
outside diameter."

One might say, "But you have to take your wheel off the bike to do
this!" True, but when mounting new tires the wheel is already off and
this is the only time I'm inclined to change cyclocomputer
calibration.

When I lived in New Jersey we regularly rode past "measured mile"
hundredth to our inability to actually ride a straight line. ;-)

J

John Everett

Guest
On Fri, 30 Sep 2005 15:51:53 GMT, I
<[email protected]> wrote:

>On 29 Sep 2005 16:38:02 -0700, "John" <[email protected]> wrote:
>>
>>I'm not a big fan of the "rollout method"
>>http://www.sheldonbrown.com/brandt/circumference.html because you can
>>get different measurements every time you do it. At least I do. Good
>>luck

>
>I also not a big fan or the "rollout method", but not because of
>inconsistency. It's because it sounds like a lot of hard work. Here's
>what I do, cut-and-pasted from a posting I did in May 2004:
>
>"Here's the method I've used for years and years. I lay the wheel
>horizontally on top of my kitchen waste paper basket, centering the
>hub. I slide the basket over until the tire is firmly against a wall,
>with either the stem or rim joint adjacent to the wall. I lay a metric
>yardstick (oxymoron?) over the skewer end, the zero end of the
>yardstick touching the wall and passing over both the stem and joint.
>I hold a carpenter's level vertically against the tire at the point
>furthest from the wall and adjust until the bubble shows it's
>perfectly vertical. Where the level edge crosses the yardstick is the
>outside diameter."
>
>One might say, "But you have to take your wheel off the bike to do
>this!" True, but when mounting new tires the wheel is already off and
>this is the only time I'm inclined to change cyclocomputer
>calibration.
>
>When I lived in New Jersey we regularly rode past "measured mile"
>hundredth to our inability to actually ride a straight line. ;-)

The part I left out above is that you have to multiply the outside
diameter in millimeters by pi. This is the number to enter into your
cyclocomputer. Obvious to me but perhaps not so to everyone..

J

Jasper Janssen

Guest
On Fri, 30 Sep 2005 09:11:59 +0100, "POHB" <[email protected]>
wrote:
>"Jasper Janssen" <[email protected]> wrote
>
>>... Also, your riding style might have a slight
>> influence -- do you cut the corners or take them wide?

>
>But that doesn't affect the accuracy of your measurement, it changes the
>distance you travel.

Yes, but it influences the calibration with mile markers, GPS readings,
and map calculations. Also, your front wheel invariably travels further

>I wouldn't over-estimate the accuracy of mile markers etc. either. I don't
>suppose they're measured with laser accuracy and if there's any corners
>involved are they assuming you take the shortest line, down the middle or
>what?

Actually, mile markers *are* measured with laser accuracy -- at least ours
are. I would imagine UK and US ones are too. Surveying nowadays takes a
lot of equipment, hyper-accurate GPS receivers and lasers among them.
Which line they assume is a different thing. I would imagine they use a
line where the surveyors can stand decently without hindering traffic, ie,
off to the side.

Jasper

J

[email protected]

Guest
John who? writes:

>> This is by no means a critical issue but I am interested in others'
>> experiences or insights on this subject.

> If you're looking for the right circumference in millimeters to
> enter into your computer I would use two methods.

> 1. Take a string and rap it around the tire and measure it. You'll
> have to use a little scotch tape to hold it in place.

This method is inaccurate a far more difficult than the most common and
reasonable way of measuring rolling circumference.

> 2. Use this formula. Tire size in I.S.O + (width x 2) x 3.142 . So,
> your 700x23 tire would be 622 + (23 x 2) x 3.142 = 622 + 46 x
> 3.142 = 2099mm. Which is the number you enter into your computer.

> More on this formula at the bottom of Sheldon's page
> http://www.sheldonbrown.com/cyclecomputer_calibration.html

> When I measured my tire using both the string and the I.S.O formula
> there was only 1mm difference between them.

That is unusual because this has been tested many times and found to
be as much as 10mm off. That is why tire pressure and load are
essential parts of this measurement.

> I then took my bike to the LHST (local high school track) which I know
> is exactly 400 meters measured from 8 inches into the first lane
> http://www.philsport.com/narf/atrack.htm . and the distance came out
> right on the money

I take it this is an asphalt paved track. If not, I cannot imagine
comparing that lap to any standard besides which tracks are generally
measured around the inside boundary of the running surface where you

> I'm not a big fan of the "rollout method"

Why not? Is it to direct or precise. The reasons for using it are
explained clearly enough for someone unprejudiced about measuring.

> http://www.sheldonbrown.com/brandt/circumference.html because you
> can get different measurements every time you do it. At least I do.
> Good luck.

If that is the case, I suspect you are doing something different with
each of those measurements; measurements that are measuring exactly
that which you are trying to calibrate... the distance traveled per
revolution of the normally loaded wheel. I hope you didn't overlook
that inflation pressure and load are part of this calibration.

Jobst Brandt

H

Hank Wirtz

Guest
OK, here's how I set my computer:

I took a tape measurer, and wrapped it around the wheel. Came out at
exactly 85". Multiplied it by 25.4, which gave me 2159. It's a Sigma Sport,
so the M/Km indicator is just for show. You're supposed to divide by 1.61,
which gave me 1341.

I compared it to the manual. My tires are 700x28 Panaracer Paselas,which
I've heard are more like 26mm. The manual said 28s should be set to 1335
(so much for 1341). It gave 1333 for a 25 mm tire. I split the difference,
picked 1334 and rode.

There's a mile marker right outside my house on Highway 527. I zeroed the
computer there and rode. Every mile came out at N.00. Sounds good to me.

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