Cycle computer: wheel size

[Oaf]

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]

> 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. ;-)

Oaf

[clodhopper]

"Oaf" > wrote in message
ups.com...
> 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

[Steve Watkin]

Try knocking off a 0. Is 150 nearer the figures in the instruction book ?
Some are calibrated in cm and some are mm.

[Al C-F]

clodhopper wrote:
> "Oaf" > 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

[Tom Crispin]

On Sat, 06 Oct 2007 08:55:22 -0700, 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,

The roll out distance should be very accurate. Mind you, the diameter
of the tyre height * 3.14 should give a very similar result.

[Robin]

"Oaf" > wrote in message
ups.com...
> 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 :)

[Simon Brooke]

in message >, mb
') 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.

--
(Simon Brooke) http://www.jasmine.org.uk/~simon/

;; part time troll.

[Danny Colyer]

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

[Andrew Price]

On Sat, 06 Oct 2007 18:51:27 +0100, Al C-F
> 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.

[Rob Morley]

In article om>, Oaf
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.

[Tom Crispin]

On Sat, 06 Oct 2007 09:00:57 -0700, Oaf > 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.

[clodhopper]

"Al C-F" > wrote in
message ...
> clodhopper wrote:
>> "Oaf" > 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

[Jeremy Parker]

"Andrew Price" > wrote in message
...
> On Sat, 06 Oct 2007 18:51:27 +0100, Al C-F
> > 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

[Danny Colyer]

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

[dkahn400]

On Oct 7, 4:31 am, Tom Crispin >
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...

[dkahn400]

On Oct 6, 7:30 pm, "Robin" > 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...

[Don Whybrow]

dkahn400 wrote:
> On Oct 6, 7:30 pm, "Robin" > 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.

[David Damerell]

Quoting Don Whybrow >:
>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 > Distortion Field!
Today is First Potmos, October.

[Danny Colyer]

On 09/10/2007 20:06, Don Whybrow wrote:
> 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 guess it depends on how autistic you are.

It matters to me. Nearly as much as putting a question mark at the end
of a question. :-P

> 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.

If I've ridden 100 miles, I really want to know that I've ridden 100
miles and not 98. If I'm hoping for a PB on a course that I ride
regularly I go by average speed, rather than time, so I want to know
that my computer is showing the same speed as it would have shown with
whatever tyre I was using when I set the previous PB.

I also find the accuracy to be a useful prompt to top up my tyres. If I
get to work and my trip distance is shown as 5.38 miles, I'll top up my
front tyre when I get home if I think about it and if I can be arsed.
If the trip distance is shown as 5.39, I'll make sure I top up the tyre
when I get home.

And I like knowing that my computer is more accurate than car odometers.
Years ago I was passed far too close on a downhill bend by WVM. I was
doing 32mph in a 30mph (for motor vehicles) limit. When I caught him up
at the back of the traffic jam 50 yards later and had words, he insisted
that I had been doing 29mph and was therefore an obstruction to be
passed. He further insisted that his speedo was more accurate than
mine, so he must be right. It was only later that it occurred to me
that I should have asked him to explain how his antiquated analogue
device could possibly be more accurate than my digital computer,
calibrated for my tyres pumped up to my riding pressure and carrying my
weight.

--
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

[dkahn400]

On Oct 9, 8:06 pm, Don Whybrow > wrote:

> 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.

That is a different question from the one that was asked. And the
answer is: it depends on what you want to get out of your computer.

--
Dave...

[Oaf]

Thanks to all who replied. I shall use the "multiple markings with
chalk" method and see how I get on.

Thanks again

Oaf