Exact size of 700x38C
Leon C. Fu
Leon Fu
Ken Lee
|> Does anyone know the exact size of a 700x38C?(in mm)
It depends on the tire, air pressure, rider weight, etc. The most accurate
way to measure it is to ride a whole number of tire revolutions, then
divide to get circumfrence and/or diameter.
Ken
Jobst Brandt
What do you mean by "exact size"? I assume you want to know some
diametral dimension but you don't say which one. From the rest of
your question it seems you want to know the rollout distance of the
tire, that is a completely different value than those measured on the
tire. Rollout distance is dependent on inflation and load also, so
there is no single number for this dimension.
You can discover this difference from the Cyclometer calibration
method whereby you start the measurement with the valve stem directly
above a mark on the ground and move forward one full rotation (under
load) and make another mark when the stem is again straight down.
You'll see that load makes a big difference. It is about 30mm or more
between loaded and unloaded depending on inflation and load.
Jobst Brandt <jbr...@hpl.hp.com>
Alex Miller
Ken Lee <ken...@saimin.esd.sgi.com> wrote:
>In article <3gj6dm$g...@er7.rutgers.edu>, leo...@eden.rutgers.edu (Leon C. Fu) writes:
>|> Does anyone know the exact size of a 700x38C?(in mm)
>
>
When a tire has 700x38c on it, that is just a number that the
tire maker has put on the side of a tire. The actual size may
be a whole size larger or smaller (Avocet tires are often one
size smaller than they are actually marked.) Manufacturers do
this so that their 700x38c weights less than their competitor.
So, just measure the tire for yourself.
It depends on the tire, air pressure, rider weight, etc. The most accurate
>way to measure it is to ride a whole number of tire revolutions, then
>divide to get circumfrence and/or diameter.
>
>Ken
>
Alex Miller
a...@netcom.com
Trevor Bartleet
If I have got this correctly - 1 mile is 5280ft and you measured
5330ft instead. To correct for this you can simply reduce the
setting for the wheel circumference on the computer by
5280/5330 = 0.9906... times. This will only work if you have
an exact mile measured out.
So if you have a circumference of 2090mm then you need to reduce
it to 2090mm x 0.9906 = 2070mm. By doing this you should get the
value for the circumference of the wheel very accuately due to
averaging it out over 1 mile.
Trevor Bartleet <bart...@dip1.ee.uct.ac.za>
(Sorry if I am confused about the miles and feet but you can
adjust accordingly if I got them wrong - I am not used to them and
had to look them up)
Shane McRoberts
: Does anyone know the exact size of a 700x38C?(in mm) I need to know because I just got a cycling computer and it didn't have this wheel size in the look up table. I measured it myself using a ruler, but when I took the bike onto the track and road 1 mile, I was off by about 50 feet. When I used this exact same computer with 700x25C (and that was in the table), it was right on.(crossed the line at exactly 1.0 mile) So, what is the exact size of a 700x38C tire?
: Leon Fu
It depends on which (brand of) tire, how much pressure you use, etc.
You can measure it yourself, though a ruler is not a good way.
Put a piece of tape on the ground. Place the bike on the tape so that
the valve stem lines up with one edge of the tape. Sit on the bike
and push it (or have a friend push you) in as straight a line as
possible. Watch the valve stem and mark where it again meets the ground
with another piece of tape. For extra accuracy, use two revolutions
of the wheel (if your tape measure is long enough). Then use a tape
measure to measure the distance. This method provides more accuracy
than could possibly be useful.
=====================
| j.shane.mcroberts |
|mitek systems, inc.|
| |
| j...@miteksys.com |
=====================
Tony Bennett (...not the singer)
To what accuracy do you need to know your distances? An error of 50 feet in 1 mile is an error of less than 1%. That's pretty good, - I would doubt whether the average car mileometer would be any more accurate.
If you need more accuracy, you're going to have to take into account:
- tyre pressure,
- the load you're carrying,
- tyre tread wear,
and other factors.
Sounds like alot of work to me...
When I used this exact same computer with 700x25C (and that was in the table), it was right on.(crossed the line at exactly 1.0 mile) So, what is the exact size of a 700x38C tire?
|>
|> Leon Fu
tb
--
Tony Bennett
Al Knoll
Make a stripe of mustard on your measuring surface.
Ride through stripe.
Measure from center of first stripe to the center of the
mustard mark made by the tire after one revolution. This is the
loaded rollout distance.
Do try to ride in a straight line, helps the measurement accuracy.
To cut the error in half measure to the second mustard blot. It's twice
the loaded rollout distance.
Of course you should be riding with your usual environment re: tire pressure,
touring gear, water bottle etc etc if you're anal about the most accurate
measurement possible.
--
--
___ / __ __/ ____/ Al Knoll HP Performance Technology /
/ / / / Center Roseville, CA, USA 95747 /
_____/ / / 916.785.5317 (Telnet 785-5317) /
/ / / email: a...@hpptc44.rose.hp.com /
__/ __/ ______/ ___________________________________/
Richard Stallard
: and other factors.
The standard way to accurately measure the effective circumference of a
wheel is to measure the distance covered on the ground during one
revolution with normal weight on the bike and normal tyre pressure.
It only takes a few minutes but you will need a helper. Start with the
valve at the bottom of the wheel, and put a chalk mark on the ground in
line with the valve. Sit on the bike and roll the bike forward for one
wheel revolution. Ideally, you should balance on the bike so all you
weight is on the wheels and get the assistant to push you along. When
the valve is back at the bottom of the wheel, put another chalk mark on
the ground in line with the valve and measure the distance between the
chalk marks. If you want to measure it even more precisely, you could do
10 revolutions and divide by 10 but you would need a very long tape measure.
One of our local government authorities has accurately surveyed distances
of 1 km and 5 km along a straight stretch of road in a semi-rural area on
the outskirts of Perth, so that the local time-trial club can check their
bike computers. One of the council engineers is a TT enthusiast! One of
these days, I will pedal down there and check my bike computer.
--------------------------------------------------------------------
| Richard Stallard |
| Bicycle Transportation Alliance ,-_|\ |
| P.O. Box 8295 / OZ \ |
| Perth Business Centre WA 6849 PERTH --> *_,-._/ |
| AUSTRALIA v |
| e-mail: rich...@cougar.multiline.com.au |
| BTA is a non-profit organisation incorporated in Western Australia |
--------------------------------------------------------------------
ayo...@bfsec.bt.co.uk
>
>In article <3gj6dm$g...@er7.rutgers.edu>, leo...@eden.rutgers.edu (Leon C. Fu) writes:
>|> Does anyone know the exact size of a 700x38C?(in mm) I need to know because I just got a cycling computer and it didn't have this wheel size in the look up table. I measured it myself using a ruler, but when I took the bike onto the track and road 1 mil|> e, I was off by about 50 feet.
>
> To what accuracy do you need to know your distances? An error of 50 feet in 1 mile is an error of less than 1%. That's pretty good, - I would doubt whether the average car mileometer would be any more accurate.
>
> If you need more accuracy, you're going to have to take into account:
>
> - tyre pressure,
> - the load you're carrying,
> - tyre tread wear,
>
>and other factors.
>
> Sounds like alot of work to me...
>
>
> When I used this exact same computer with 700x25C (and that was in the table), it was right on.(crossed the line at exactly 1.0 mile) So, what is the exact size of a 700x38C tire?
>|>
>|> Leon Fu
>
>
Just got a great Idea !
Why don't you multiply what you have in your computer for the wheel circum' by the error factor
ie 1 +/- (50 feet / 1 mile) ,depending on wether you were short or long, and I bet it will give
you the correct measurement. As long as you did ride perficlty in the lane, havent put on or lost
any weight, the air temperature and atmospheric pressure haven't changed too much, and your tread
hasn't worn. Of course there are other things like going faster may cool your tires down causing
the presure to drop or if you use the brakes on a steep hill that might warm them up ;-)
Personaly my problem is I like lifting the front wheel of the ground too much, dose play havoc with
measuring your trip distance!
Andrew Young
Views are all mine,
Mine you hear me!
and you can't take them away!
Leon C. Fu
for me). First I switched the computer to km mode since the track (running) is
400 meters, not 1/4 mile. Based on my previous measurements, reduced it by
the appropriate amount. I rode 800 meters and was off by about1 foot, made
another adjustment. When I ride another 800 meters, it was right on. It might
have changed though since I just increased my tire pressure, but it's close
enough.
Leon Fu
bmurn...@gmail.com
The 38c is the tyre section diameter, so:
Circumference = (622 + 38 + 38) x Pi (or 3.142)
Circumference = 2193.116 or 2193 or nearest value you can set on your cycle computer. If you want it really exact then check out the following web site.
http://sheldonbrown.com/cyclecomputer-calibration.html#formula
bmurn...@gmail.com
AMuzi
In practice, with you on the bike and with tires inflated to
your preferred usual pressure, ride through a spot of paint
on a smooth floor or sidewalk. Measure from dot to dot in
mm, enter that number in your cyclometer.
--
Andrew Muzi
<www.yellowjersey.org/>
Open every day since 1 April, 1971
Sir Ridesalot
> The value is 2180 when taking into account other factors
Cheers
Frank Krygowski
> On 3/6/2014 7:00 AM, bmurn...@gmail.com wrote:
>
> > Don't go overboard on getting the exact value. Use the following for your tyre. Your tyre size of 700 is equivalent to 622mm (bead seat diameter).
> > The 38c is the tyre section diameter, so:
> > Circumference = (622 + 38 + 38) x Pi (or 3.142)
> > http://sheldonbrown.com/cyclecomputer-calibration.html#formula
>
> Nice theory.
>
> In practice, with you on the bike and with tires inflated to
> your preferred usual pressure, ride through a spot of paint
> on a smooth floor or sidewalk. Measure from dot to dot in
> mm, enter that number in your cyclometer.
- Frank Krygowski
avag...@gmail.com
avag...@gmail.com
> In practice, with you on the bike and with tires inflated to
>
> your preferred usual pressure, ride through a spot of paint
>
> on a smooth floor or sidewalk. Measure from dot to dot in
>
> mm, enter that number in your cyclometer.
>
>
>
> --
>
> Andrew Muzi
on a hard surface with a Cateye endure the accuracy over 6-7 miles is off @ 6 inches
AMuzi
>
>> In practice, with you on the bike and with tires inflated to
>>
>> your preferred usual pressure, ride through a spot of paint
>>
>> on a smooth floor or sidewalk. Measure from dot to dot in
>>
>> mm, enter that number in your cyclometer.
>
Cateye's claimed "99.9%"
--
Andrew Muzi
avag...@gmail.com
Radey Shouman
avag...@gmail.com
one tire revolution per 1000'
I remember clearly riding up riding up watching the display go to one mile an easing up to ONE MILE there was a white paint mark....I've forgotten an accurate recall of how many white mile marks were there....5 ? trip into town is 9-11 miles all on a smooth but sharply grained asphalt path.
The tire circumference method was pump to spec - mark tire -roll once - mark road - repeat 3 times
I'm looking at a CDT trip from lower CO to MX with off course GPS. Luv muh GPS.
There are folk disdain GPS going with paper maps and I assume cyclometer. A more involved game. I read Burch on kayak navigation...Burch teaches hand span measurement and kamal....a good quick read...then check the GPS.
With the trail wonder how wheel slip affects cyclometer readings esp in slippery road conditions or when to log mileage...I guess that's why the cyclo has more than one OD readout...one total, one from the last correct turn.