# Sigma Computer Wheel Size Chart is wrong

#### Spider1977

##### New Member
If you own a Sigma Cycle Computer and have relied on the Wheel Size Chart to calibrate you will be underestimating your speed and distance travelled by a reasonable margin (approximately 2%).

I have been calibrating my Sigma BC1600 Computer after a battery change. As I have 22-622 Conti Force tyres, the size was not in the Wheel Size chart in the Manual, so I thought I'd go to the fantastic Sheldon Brown site http://www.sheldonbrown.com/cyclecomputer-calibration.html to find out how to calculate the Wheel size.

This says:

The I.S.O. tire size consists of a tire width and a bead seat diameter. Both of these numbers are in millimeters. For example, a 28-622 (700 x 28C) tire has a nominal width of 28mm on a rim with a bead seat diameter of 622 mm
To get an approximate diameter (in mm) add the bead seat diameter to twice the tire width (since the tire comes into the diameter twice: 622 + (28 X 2) = 678. Multiply this by pi (3.142) to get the circumference in mm = 2130.

Well the Sigma Maunual had the wheel size at 2149 for a 28-622, a difference of 19mm. I got suspicious, so I checked calculations for a 23-622 which is a pretty standard road bike tyre size. The Sheldon Brown calculation was 2098mm against the Sigma calculation of 2133mm a difference of 35mm. This is an error of 1.6%, which is quite a bit really . I checked the calculation by doing the roll out test to actually measure the wheel circumference and hey presto, the Sheldon Brown calculation is right. The Sigma Manual also suggests this roll out test, so they agree with the Sheldon Brown formula (which isn't rocket science, its simply the calculation of the circumference of a circle).

So, I'd suggest you check the Wheel Size you have entered into your computer. The roll out test is probably the most reliable, but the Sheldon Brown formula is also pretty easy.

P

#### Patrick Keogh

##### Guest
Spider1977 wrote:
> If you own a Sigma Cycle Computer and have relied on the Wheel Size
> Chart to calibrate you will be underestimating your speed and distance
> travelled by a reasonable margin (approximately 2%).
>
> I have been calibrating my Sigma BC1600 Computer after a battery
> change. As I have 22-622 Conti Force tyres, the size was not in the
> Wheel Size chart in the Manual, so I thought I'd go to the fantastic
> Sheldon Brown site
> http://www.sheldonbrown.com/cyclecomputer-calibration.html to find out
> how to calculate the Wheel size.
>
> This says:
>
> The I.S.O. tire size consists of a tire width and a bead seat diameter.
> Both of these numbers are in millimeters. For example, a 28-622 (700 x
> 28C) tire has a nominal width of 28mm on a rim with a bead seat
> diameter of 622 mm
> To get an approximate diameter (in mm) add the bead seat diameter to
> twice the tire width (since the tire comes into the diameter twice: 622
> + (28 X 2) = 678. Multiply this by pi (3.142) to get the circumference
> in mm = 2130.
>
> Well the Sigma Maunual had the wheel size at 2149 for a 28-622, a
> difference of 19mm. I got suspicious, so I checked
> calculations for a 23-622 which is a pretty standard road bike tyre
> size. The Sheldon Brown calculation was 2098mm against the Sigma
> calculation of 2133mm a difference of 35mm. This is an error of 1.6%,
> which is quite a bit really . I checked the calculation by doing
> the roll out test to actually measure the wheel circumference and hey
> presto, the Sheldon Brown calculation is right. The Sigma Manual also
> suggests this roll out test, so they agree with the Sheldon Brown
> formula (which isn't rocket science, its simply the calculation of the
> circumference of a circle).
>
> So, I'd suggest you check the Wheel Size you have entered into your
> computer. The roll out test is probably the most reliable, but the
> Sheldon Brown formula is also pretty easy.
>
>

I had similar experience with a Sigma on a previous bike.
My solution was to go for a ride with both the cycle computer and my GPS
turned on. The course I chose was almost dead straight out and back.
Since the positional error in the GPS is less than 5m, a 50K ride gives
you a recording of distance accurate to better than 0.1%. Then I
calculated the ratio between the distance recorded by the GPS and the
distance recorded by the Sigma and applied this as a correction to the
wheel size. After that, perfecto!

As an aside, the GPS is handlebar mounted, and to within a very small
margin for error it travels the same distance as the front wheel. This
error can be eliminated by travelling in a straight line. But ask
yourself, for more "normal" riding with twists and turns, avoiding
potholes and glass etc., is the distance travelled as recorded by the
front wheel the same as the distance travelled by the bike? What about
if you put a cycle computer on the rear wheel? Which wheel gives the
closest approximation? I'll leave this one as a class exercise

B

#### BrettS

##### Guest
Spider1977 wrote:

> If you own a Sigma Cycle Computer and have relied on the Wheel Size
> Chart to calibrate you will be underestimating your speed and distance
> travelled by a reasonable margin (approximately 2%).

<snip>

> The I.S.O. tire size consists of a tire width and a bead seat diameter.
> Both of these numbers are in millimeters. For example, a 28-622 (700 x
> 28C) tire has a nominal width of 28mm on a rim with a bead seat
> diameter of 622 mm
> To get an approximate diameter (in mm) add the bead seat diameter to
> twice the tire width (since the tire comes into the diameter twice: 622
> + (28 X 2) = 678. Multiply this by pi (3.142) to get the circumference
> in mm = 2130.
>
> Well the Sigma Maunual had the wheel size at 2149 for a 28-622, a
> difference of 19mm. I got suspicious, so I checked
> calculations for a 23-622 which is a pretty standard road bike tyre
> size. The Sheldon Brown calculation was 2098mm against the Sigma
> calculation of 2133mm a difference of 35mm. This is an error of 1.6%,
> which is quite a bit really . I checked the calculation by doing
> the roll out test to actually measure the wheel circumference and hey
> presto, the Sheldon Brown calculation is right. The Sigma Manual also
> suggests this roll out test, so they agree with the Sheldon Brown
> formula (which isn't rocket science, its simply the calculation of the
> circumference of a circle).

Spider,

I hate to break it to you, but you would have actually been
OVERestimating your speed. The wheel size value is the distance
travelled for every revolution of the wheel. If you put in 2133, the
computer thinks you are travelling 2133mm for every revolution. If the
actual distance is only 2098, then you are tavelling less per revolution
and hence less distance per hour than you thought you were...

--
BrettS

#### mtbnewbie

##### New Member
I have a Sigma sport and a CatEye too. Wasn't until I got the CatEye that I noticed something was up. The Sigam wheel chart said the tire should be 2114 the CatEye 2055 and my roll out measurement came up at 1980.

I think the roll out test is the best way to calibrate.

K

#### K.A. Moylan

##### Guest
In article <>,
Patrick Keogh <[email protected]> wrote:

> Spider1977 wrote:
> > If you own a Sigma Cycle Computer and have relied on the Wheel Size
> > Chart to calibrate you will be underestimating your speed and distance
> > travelled by a reasonable margin (approximately 2%).
> > ...
> > So, I'd suggest you check the Wheel Size you have entered into your
> > computer. The roll out test is probably the most reliable, but the
> > Sheldon Brown formula is also pretty easy.
> >

> I had similar experience with a Sigma on a previous bike.
> My solution was to go for a ride with both the cycle computer and my GPS
> turned on. ...

For those of us without GPSs (such as myself), I use mileposts on
country roads (well, actually 5km posts). I'll reset the bike computer
at one milepost & compare the computer's distance to what the next
milepost says. A little bit of calculating & by the 3rd ride I've
calibrated my bike computer exactly.
(Note: milepost pairs can be inaccurate, but over 50km their errors
average out.)

--
K.A. Moylan
Canberra, Australia
Ski Club: http://www.cccsc.asn.au
kamoylan at ozemail dot com dot au

S

#### Spiny Norman

##### Guest
On Fri, 14 Apr 2006 10:02:10 +1000, Spider1977
<[email protected]> wrote in
aus.bicycle:

>
>So, I'd suggest you check the Wheel Size you have entered into your
>computer. The roll out test is probably the most reliable, but the
>Sheldon Brown formula is also pretty easy.

I can't see why anyone would do it any other way than rolling the bike
and measuring the distance travelled for one or two revolutions (with
a bit of weight on the bike too).

Simple

Regards
Prickles

Timendi causa est nescire
This message only uses recycled electrons

W

#### Wilfred Kazoks

##### Guest
Perhaps someone can tell me why it is so important to calibrate the computer
to such accuracy?

On my computer (Sigma BC500) the speed is not going to be displayed with
any greater precision than the whole numbers allows, no decimal places, and
the distance only gets updated in 10m increments.

Now if I go out on a limb and attempt some math here.
If you had a perfectly calibrated computer on a 20km ride and at the end of
the ride the best the computer could display was 19.99 - 20.01km an error of
0.05%.
If the circumference of your wheel was 2000mm and error of 0.05% is +/-1mm
which is the limit of most peoples tape measures. You could roll out
multiple revolutions of the wheel to improve the calibration accuracy but
unless you have a computer that can display greater precision (not accuracy)
what is the point?

Perhaps not being a super fit athletic cyclist i'll never understand. To me
the computer is just a means to help motivate me to get out and get
exercise. It is the circumference of my waistline that is of primary
interest to me.

And to stay on topic. I too found the chart with my computer totally off.

Wilfred

B

#### BrettS

##### Guest
Wilfred Kazoks wrote:
> Perhaps someone can tell me why it is so important to calibrate the computer
> to such accuracy?
>

Wilfed,

This is USENET. What sort of self respecting geek wouldn't nedd/want
that sort of accuarcy. I'm disappointed that my bike computer doesn't
compensate for changes in rolling circumference due to heat related
pressure change...

--
Brett"Need more significant figures"S

R

#### Random Data

##### Guest
On Sun, 16 Apr 2006 00:39:18 +0000, Wilfred Kazoks wrote:

> Perhaps someone can tell me why it is so important to calibrate the computer
> to such accuracy?

You don't actually need to, but it's handy if you're doing stuff where
navigation is important, or long rides. Having a fair idea that there're
15km to go on a 100km ride means you know you can pick up the pace a touch
and probably be safe. That's even more the case in the bush where buying
an emergency Mars Bar and Coke from the servo is a bit tricky.

If you're going to fart about to get ~1% accuracy, you may as well do as
good a job as possible.

Oh, and the sigmonster's telling me it knows the date, and wants an easter
egg.

--
Dave Hughes | [email protected]
Y'know, I think Jesus had one hell of a lag problem. It
took him three days to re-spawn... - Chalybeous, /.

W

#### Wilfred Kazoks

##### Guest
> This is USENET. What sort of self respecting geek wouldn't nedd/want that
> sort of accuarcy. I'm disappointed that my bike computer doesn't
> compensate for changes in rolling circumference due to heat related
> pressure change...
>
> --
> Brett"Need more significant figures"S

Ha ha ha ha ha. Geez I'm a dopey bugger

Wilfred

K

#### Kingsley

##### Guest
On Sun, 16 Apr 2006 00:39:18 +0000, Wilfred Kazoks wrote:

> Perhaps someone can tell me why it is so important to calibrate the computer
> to such accuracy?

Well, what's the point of having a computer at all if
it's non-accurate?! Just use a piece of string on a map.

Do you wear a watch? I bet it's like this one:
http://www.boingboing.net/2006/01/27/watch_displays_cheek.html

If your wheel measurement is out, every single revolution
is throwing out the total distance travelled. How do you really
*know* you deserve that first-1000km-on-tour beer and pizza bender?

You might eat all that beer an pizza, and only have gone 960km!
*shame*

--
Kingsley Turner,
(mailto: [email protected])

T

#### TimC

##### Guest
On 2006-04-17, Kingsley (aka Bruce)
was almost, but not quite, entirely unlike tea:
> On Sun, 16 Apr 2006 00:39:18 +0000, Wilfred Kazoks wrote:
>
>> Perhaps someone can tell me why it is so important to calibrate the computer
>> to such accuracy?

>
> Well, what's the point of having a computer at all if
> it's non-accurate?! Just use a piece of string on a map.
>
> Do you wear a watch? I bet it's like this one:
> http://www.boingboing.net/2006/01/27/watch_displays_cheek.html

I WANT!

It would be most satisfying when someone asks me for the time. I tell
them, and then they ask, "no really, I need to calibrate my speedo".

--
TimC
Er, RFC 882 put the dot in .com.

V

#### Vincent Patrick

##### Guest
TimC wrote:

You might want to use the fuzzy clock on your K Desktop then, and
right-click to configure its degree of fuzziness to your liking. See,
there is a reason to sometimes use KDE as well as the CLI...

Cheers,

Vince

S

#### Spiny Norman

##### Guest
On Sun, 16 Apr 2006 00:39:18 GMT, "Wilfred Kazoks"
<[email protected]> wrote in aus.bicycle:

>Perhaps someone can tell me why it is so important to calibrate the computer
>to such accuracy?

It probably isn't that important, certainly not for commuting cyclist
like me (with the odd Sydney to Gong thrown in) BUT on the other hand
how time consuming is it to mark the tyre tread with a chalk line or a
spot of paint, leap on the bike, cycle in a straight line for a couple
of metres and then measure the distance between the marks left on the

It probably takes longer to read the directions and enter the
information in the speedo. Though to be honest once I have done it I
rarely recheck it when I change tyres

Regards
Prickles

Timendi causa est nescire
This message only uses recycled electrons

T

#### Tamyka Bell

##### Guest
Spiny Norman wrote:
>
> On Sun, 16 Apr 2006 00:39:18 GMT, "Wilfred Kazoks"
> <[email protected]> wrote in aus.bicycle:
>
> >Perhaps someone can tell me why it is so important to calibrate the computer
> >to such accuracy?

>
> It probably isn't that important, certainly not for commuting cyclist
> like me (with the odd Sydney to Gong thrown in) BUT on the other hand
> how time consuming is it to mark the tyre tread with a chalk line or a
> spot of paint, leap on the bike, cycle in a straight line for a couple
> of metres and then measure the distance between the marks left on the

<snip>
If that was me, on the MTB, cycling in a straight line for a couple of
metres could take me a week of practice.

Tam

Z

#### Zebee Johnstone

##### Guest
In aus.bicycle on Tue, 18 Apr 2006 18:01:27 +1000
Spiny Norman <[email protected]> wrote:
>
> It probably isn't that important, certainly not for commuting cyclist
> like me (with the odd Sydney to Gong thrown in) BUT on the other hand
> how time consuming is it to mark the tyre tread with a chalk line or a
> spot of paint, leap on the bike, cycle in a straight line for a couple
> of metres and then measure the distance between the marks left on the
>

I don't know, but I think the therapy needed for someone who can't
bear to walk a couple of metres but has to ride their bike that
distance could be time consuming and expensive.

Zebee

#### KEN SY

##### New Member
This is my second sigma and both have been way out . The first one i ended up finding a straight stretch of road and marked out a 1 km stretch in a new car and rode my bike back and forth adjusting the BC each time untill i got it to match the marked out 1 km on the road and found that to be accurate after about 5 kms back and forth . With my new BC i thought they may have got i right but no , I did the roll out measurement of the wheel and went for a known 50km ride the BC showed that i had travelled 93 kms . When i set the old one up my top speed was a beleivable 43 kmh . Not the 63 that the wheel roll out method comes up with . I also rechecked the measurement incase i had miss calculated it was the same . The way iam going to set it this time is take a GPS and keep adjusting the BC untill i get the speed spot on . Will get back with results ...

#### pyscho

##### New Member
The rollout test, in my opinion, is the greatest approach to calibrate. I have a Sigma sport as well as a CatEye. I didn't see anything was wrong until I got the CatEye. The tire should be 2114, the CatEye 2055, according to the Sigma wheel chart, and my rollout measurement came up in 1980. cookie clicker

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