Cycle computer: wheel size



O

Oaf

Guest
Hi,

Just got a Sigma computer for my excellent new folding bike. Am trying
to set the computer up so that it's accurate with the small wheels,
but there's no corresponding value to put into the computer for my 20
x 1.5 (40-406) wheels in the manual.

I've tried the other 2 ways of working out the value. One is to
multiply the diameter of the wheel in mm by 3.14; t'other is to
measure the distance of one full revolution of the wheel. I've tried
both of these, but the values I've got out aren't anywhere near alike.
They're both near 1500, but not close enough for me to be convinced
that it'll be anything remotely accurate.

Just wondered whether there's any of you out there with mathematical
leaning who might be able to work out what the correct value should
be. As I say, the tyres say 20 x 1.5 (40-406).

Thanks for any assistance,

Oaf
 
"Oaf" <[email protected]> wrote in message
news:[email protected]...
> Hi,
>
> Just got a Sigma computer for my excellent new folding bike. Am trying
> to set the computer up so that it's accurate with the small wheels,
> but there's no corresponding value to put into the computer for my 20
> x 1.5 (40-406) wheels in the manual.
>
> I've tried the other 2 ways of working out the value. One is to
> multiply the diameter of the wheel in mm by 3.14; t'other is to
> measure the distance of one full revolution of the wheel. I've tried
> both of these, but the values I've got out aren't anywhere near alike.
> They're both near 1500, but not close enough for me to be convinced
> that it'll be anything remotely accurate.
>
> Just wondered whether there's any of you out there with mathematical
> leaning who might be able to work out what the correct value should
> be. As I say, the tyres say 20 x 1.5 (40-406).
>
> Thanks for any assistance,
>
> Oaf
>

Should be using the tyre diameter not the wheel rim. So therefore 406 x
3.14...

= 1275
 
Try knocking off a 0. Is 150 nearer the figures in the instruction book ?
Some are calibrated in cm and some are mm.
 
clodhopper wrote:
> "Oaf" <[email protected]> wrote in message

>> Just wondered whether there's any of you out there with mathematical
>> leaning who might be able to work out what the correct value should
>> be. As I say, the tyres say 20 x 1.5 (40-406).
>>
>> Thanks for any assistance,
>>
>> Oaf
>>
>
> Should be using the tyre diameter not the wheel rim. So therefore 406 x
> 3.14...
>
> = 1275
>
>

Are you sure about that? I would have thought the sum was (Bead Seat
Diameter + 2x the tyre height)x pi, or (406 + 2x40)x3.142
 
On Sat, 06 Oct 2007 08:55:22 -0700, Oaf <[email protected]> wrote:

>Hi,
>
>Just got a Sigma computer for my excellent new folding bike. Am trying
>to set the computer up so that it's accurate with the small wheels,
>but there's no corresponding value to put into the computer for my 20
>x 1.5 (40-406) wheels in the manual.
>
>I've tried the other 2 ways of working out the value. One is to
>multiply the diameter of the wheel in mm by 3.14; t'other is to
>measure the distance of one full revolution of the wheel. I've tried
>both of these, but the values I've got out aren't anywhere near alike.
>They're both near 1500, but not close enough for me to be convinced
>that it'll be anything remotely accurate.
>
>Just wondered whether there's any of you out there with mathematical
>leaning who might be able to work out what the correct value should
>be. As I say, the tyres say 20 x 1.5 (40-406).
>
>Thanks for any assistance,

The roll out distance should be very accurate. Mind you, the diameter
of the tyre height * 3.14 should give a very similar result.
 
"Oaf" <[email protected]> wrote in message
news:[email protected]...
> Hi,
>
> Just got a Sigma computer for my excellent new folding bike. Am trying
> to set the computer up so that it's accurate with the small wheels,
> but there's no corresponding value to put into the computer for my 20
> x 1.5 (40-406) wheels in the manual.
>
> I've tried the other 2 ways of working out the value. One is to
> multiply the diameter of the wheel in mm by 3.14; t'other is to
> measure the distance of one full revolution of the wheel. I've tried
> both of these, but the values I've got out aren't anywhere near alike.
> They're both near 1500, but not close enough for me to be convinced
> that it'll be anything remotely accurate.
>
> Just wondered whether there's any of you out there with mathematical
> leaning who might be able to work out what the correct value should
> be. As I say, the tyres say 20 x 1.5 (40-406).
>
> Thanks for any assistance,
>
> Oaf

use a tape measure to measure the outside diameter of the tyre, then mult by
3.1415927 :)
 
Oaf wrote:

> Hi,
>
> Just got a Sigma computer for my excellent new folding bike. Am trying
> to set the computer up so that it's accurate with the small wheels,
> but there's no corresponding value to put into the computer for my 20
> x 1.5 (40-406) wheels in the manual.
>
> I've tried the other 2 ways of working out the value. One is to
> multiply the diameter of the wheel in mm by 3.14; t'other is to
> measure the distance of one full revolution of the wheel. I've tried
> both of these, but the values I've got out aren't anywhere near alike.
> They're both near 1500, but not close enough for me to be convinced
> that it'll be anything remotely accurate.
>
> Just wondered whether there's any of you out there with mathematical
> leaning who might be able to work out what the correct value should
> be. As I say, the tyres say 20 x 1.5 (40-406).
>
> Thanks for any assistance,
>
> Oaf

Check Sheldons website and get several wildly different figures:
http://sheldonbrown.com/cyclecomputer-print.html


--
Mike
Van Tuyl titanium Dura ace 10
Fausto Coppi aluminium Ultegra 10
Raleigh Record sprint mongrel
Huissoon project bike (in bits, natch)
 
in message <[email protected]>, mb
('[email protected]') wrote:

> Oaf wrote:
>
>> Just got a Sigma computer for my excellent new folding bike. Am trying
>> to set the computer up so that it's accurate with the small wheels,
>> but there's no corresponding value to put into the computer for my 20
>> x 1.5 (40-406) wheels in the manual.
>>
>> I've tried the other 2 ways of working out the value. One is to
>> multiply the diameter of the wheel in mm by 3.14; t'other is to
>> measure the distance of one full revolution of the wheel. I've tried
>> both of these, but the values I've got out aren't anywhere near alike.
>> They're both near 1500, but not close enough for me to be convinced
>> that it'll be anything remotely accurate.
>>
>> Just wondered whether there's any of you out there with mathematical
>> leaning who might be able to work out what the correct value should
>> be. As I say, the tyres say 20 x 1.5 (40-406).
>
> Check Sheldons website and get several wildly different figures:
> http://sheldonbrown.com/cyclecomputer-print.html

And tyres vary. I've just done this exercise for 700c x 28, which Sheldon
quotes as 2136, but I got 2115. The roll-out measurement is best; average
over three (or, still better, five or ten) full revolutions for more
accuracy.

--
[email protected] (Simon Brooke) http://www.jasmine.org.uk/~simon/

;; part time troll.
 
On 06/10/2007 16:55, Oaf wrote:
> Just wondered whether there's any of you out there with mathematical
> leaning who might be able to work out what the correct value should
> be. As I say, the tyres say 20 x 1.5 (40-406).

The diameter of the unladen wheel will not be the same as the distance
travelled per wheel revolution. You want the latter, which will be
different for different tyres of nominally the same size and will vary
with tyre pressure and weight carried.

Best to do a roll-out. First make sure that the tyres are pumped up to
your preferred riding pressure and that the bike is carrying a typical
load. Make a mark on your patio, or on the pavement or the road, or at
your local park. Align the valve stem of the wheel that you plan to
mount the magnet to with the mark. Sit on the bike and ride forwards 2
or 3 revolutions. Make another mark on the patio alongside your valve
stem. Measure the distance travelled. Divide by the number of
revolutions to get your wheel circumference.

Do this multiple times to get an average. I like to do it 5 times,
discard the highest and lowest measurements, then use the mean of the
remaining measurements.

--
Danny Colyer <http://www.redpedals.co.uk>
Reply address is valid, but that on my website is checked more often
"The plural of anecdote is not data" - Frank Kotsonis
 
On Sat, 06 Oct 2007 18:51:27 +0100, Al C-F
<[email protected]> wrote:

>Are you sure about that? I would have thought the sum was (Bead Seat
>Diameter + 2x the tyre height)x pi, or (406 + 2x40)x3.142

That's also what I'd use.
 
In article <[email protected]>, Oaf
[email protected] says...
> Hi,
>
> Just got a Sigma computer for my excellent new folding bike. Am trying
> to set the computer up so that it's accurate with the small wheels,
> but there's no corresponding value to put into the computer for my 20
> x 1.5 (40-406) wheels in the manual.
>
> I've tried the other 2 ways of working out the value. One is to
> multiply the diameter of the wheel in mm by 3.14; t'other is to
> measure the distance of one full revolution of the wheel. I've tried
> both of these, but the values I've got out aren't anywhere near alike.
> They're both near 1500, but not close enough for me to be convinced
> that it'll be anything remotely accurate.
>
> Just wondered whether there's any of you out there with mathematical
> leaning who might be able to work out what the correct value should
> be. As I say, the tyres say 20 x 1.5 (40-406).
>
The diameter of a 40-406 tyre is about 406 + 40 + 40 = 486mm so the
circumference is 1.53m, but measuring the rolling circumference (with
you on the bike) is more accurate because it allows for the deflection
of the tyre.
 
On Sat, 06 Oct 2007 09:00:57 -0700, Oaf <[email protected]> wrote:

>> Just got a Sigma computer for my excellent new folding bike.
>
>Oh, by the way, here it is:
>
>http://farm2.static.flickr.com/1332/1464624234_830d7efa4d.jpg
>
>In the living room, not the kitchen though. ;-)

Perhaps if you posted a full size image of one of the wheels we could
help you. Using your photo, my tape measure makes those tyres a
diameter of 57mm giving a circumference of 179mm.
 
"Al C-F" <[email protected]> wrote in
message news:[email protected]...
> clodhopper wrote:
>> "Oaf" <[email protected]> wrote in message
>
>>> Just wondered whether there's any of you out there with mathematical
>>> leaning who might be able to work out what the correct value should
>>> be. As I say, the tyres say 20 x 1.5 (40-406).
>>>
>>> Thanks for any assistance,
>>>
>>> Oaf
>>>
>>
>> Should be using the tyre diameter not the wheel rim. So therefore 406 x
>> 3.14...
>>
>> = 1275
>
> Are you sure about that? I would have thought the sum was (Bead Seat
> Diameter + 2x the tyre height)x pi, or (406 + 2x40)x3.142

Oops sorry you're right. 406 is bead to bead not outside diameter
 
"Andrew Price" <[email protected]> wrote in message
news:[email protected]...
> On Sat, 06 Oct 2007 18:51:27 +0100, Al C-F
> <[email protected]> wrote:
>
>>Are you sure about that? I would have thought the sum was (Bead
>>Seat
>>Diameter + 2x the tyre height)x pi, or (406 + 2x40)x3.142
>
> That's also what I'd use.

When you have got the circumference correctly set, as you believe,
why not check it against some mileposts on your local roads.
Mileposts seem to have been very accurately set, originally, so,
provided that the road hasn't been redesigned since the milestones
were installed, you will accurately check your computer over a
distance more than two thousand times the circumference of your
wheel, with weight on the tyre, inflation pressures, likely deviation
from straight line riding, etc. as in the real world.

If your computer shows the distance as less than a mile, when in
reality you have gone exactly a mile, then the computer thinks the
wheel is bigger than is actually the case, so reduce the
circumference by the percentage that the computer is off. If the
computer thinks that you have gone more than a mile, increase the
circumference.

150 years ago distances were accurate. Stagecoaches charged by the
mile, so such things mattered. Be more sceptical on modern bike
paths, though, unless the original railway mileposts are still there.
Modern mileposts may just have used a maintenance vehicle's odometer
to set up any mileposts, likely not very accurate.

Does Sustrans have standards about such things?

OS maps have left some things unchanged since the days of
stagecoaches. They still show mileposts. Some mileposts still exist
even though the OS hasn't noticed them.

Jeremy Parker
 
On 07/10/2007 17:52, Jeremy Parker wrote:
> 150 years ago distances were accurate. Stagecoaches charged by the
> mile, so such things mattered. Be more sceptical on modern bike
> paths, though, unless the original railway mileposts are still there.
> Modern mileposts may just have used a maintenance vehicle's odometer
> to set up any mileposts, likely not very accurate.
>
> Does Sustrans have standards about such things?

The Millennium Mileposts on the Bath-Bristol path seem to have been
placed where they look prettiest. They're certainly not a consistent
distance apart.

Fortunately long stretches of the path have paint markings at 100m
intervals, which *are* consistently spaced and I therefore assume that
they're correct. I used them a few years ago to calibrate my computer,
having done so whenever I now fit a new front tyre I adjust my computer
until it shows a door-to-door morning commute distance of 5.37 miles.

--
Danny Colyer <http://www.redpedals.co.uk>
Reply address is valid, but that on my website is checked more often
"The plural of anecdote is not data" - Frank Kotsonis
 
On Oct 7, 4:31 am, Tom Crispin <[email protected]>
wrote:

> Perhaps if you posted a full size image of one of the wheels we could
> help you. Using your photo, my tape measure makes those tyres a
> diameter of 57mm giving a circumference of 179mm.

Perhaps you need a larger screen, Tom?

--
Dave...
 
On Oct 6, 7:30 pm, "Robin" <[email protected]> wrote:

> use a tape measure to measure the outside diameter of the tyre, then mult by
> 3.1415927 :)

I note the smiley, but no matter how many decimal places of pi you use
in the calculation this method will be inaccurate, firstly because of
the physical difficulty of measuring a wheel and tyre with a tape
measure, and secondly because it doesn't take into account tyre
deformation under load.

Use the roll-out method described by Danny to get a very good first
approximation. A variation is to make a chalk or paint mark on the
tyre that will transfer to the road so that you can measure between
marks.

If you feel the need you can then use the known-distance method to
fine tune the setting. Divide the true distance by the distance shown
on your computer. Multiply that by the original calibration number to
give the new calibration number.

--
Dave...
 
dkahn400 wrote:
> On Oct 6, 7:30 pm, "Robin" <[email protected]> wrote:
>
>> use a tape measure to measure the outside diameter of the tyre, then mult by
>> 3.1415927 :)
>
> I note the smiley, but no matter how many decimal places of pi you use
> in the calculation this method will be inaccurate, firstly because of
> the physical difficulty of measuring a wheel and tyre with a tape
> measure, and secondly because it doesn't take into account tyre
> deformation under load.
>
> Use the roll-out method described by Danny to get a very good first
> approximation. A variation is to make a chalk or paint mark on the
> tyre that will transfer to the road so that you can measure between
> marks.
>
> If you feel the need you can then use the known-distance method to
> fine tune the setting. Divide the true distance by the distance shown
> on your computer. Multiply that by the original calibration number to
> give the new calibration number.

Not having a pop at you Dave, or anyone for that matter, but does it
really matter if your computer is out by a small amount. I just used the
factory settings for the wheel/tyre combination I am using. When I am
riding, I don't look at the decimal place on the speed, just the
approximate speed and when I arrive at a destination, I am happy to know
that I cycled about that distance. The only time I have checked it has
been going past one of those signs that tell you how fast you are going.
Close enough was my thought.

--
Don Whybrow

Sequi Bonum Non Time

One tentacle, one vote.
 
Quoting Don Whybrow <[email protected]>:
>Not having a pop at you Dave, or anyone for that matter, but does it
>really matter if your computer is out by a small amount. I just used the
>factory settings for the wheel/tyre combination I am using.

The original poster specifically stated a desire for accuracy _and_ that
the manual contains no suggested setting for their wheel/tyre combo, so...
--
David Damerell <[email protected]> Distortion Field!
Today is First Potmos, October.
 
You must log in or register to reply here.

Similar threads

M
Cycle computer for repetitive course
Replies
7
Views
301
UK and Europe
Ben C
B
E
Lidl Cycle Computer
Replies
14
Views
891
UK and Europe
Ed_Zep
E
S
HRM and/or cycle computer with Bluetooth
Replies
6
Views
653
UK and Europe
Tooraj
Tooraj
H
Cycle Computer Instruction for Giant Revive Series
Replies
2
Views
471
UK and Europe
Hoi Polloi
H
D
Cycle Computer with metric converter?
Replies
3
Views
344
UK and Europe
POHB
P
Top Bottom Back