Accuracy of cycle computers

Following on from FranklynMint's message. . .

Quoted message said:

thought something was amiss. I reckon that the computer is about 1 mile
out every 15. Is that par for the course with these gadgets? I'm not

In themselves extremely accurate. If you tell it 2031 pulses then it
registers exactly 2031 pulses. (May not apply to cordless - I wouldn't
know.)

Quite simply you have to sit on the bike and measure one or more wheel
revolutions on the ground. You should get 1%-2% accuracy.

--
PETER FOX Not the same since the deckchair business folded
[email hidden]
www.eminent.demon.co.uk/wcc.htm Witham Cycling Campaign
www.eminent.demon.co.uk/rides East Anglian Pub cycle rides

FranklynMint said:

Greetings fellow cyclists I have a mid-price cycle computer, which is
set up according to instructions (double-checked and triple checked).
On a long ride, recently (recommended by a friend, who said it was
about 40+ miles, he hadn't measured it) i was quietly impressed by
the distace i rode, which was longer than he'd said. But looking at a
map later, I thought something was amiss. I reckon that the computer
is about 1 mile out every 15. Is that par for the course with these
gadgets? I'm not interested in measuring to the millimetre, nor do I
have more than a passing interest in the average speed, etc, but it's
nice to have an idea of how far you've done. Arrivederci FM

Modern cycle computers tend to be as accurate as each other so don't worry
about the model. Just make sure it's working and that the tyre
circumference setting is correct. The result will be true.

Bike ride distances will be greater than map measurements because you
can't measure all the bends properly on a normal map, and all the little
micro-turns and wobbles add up.

~PB

FranklynMint said:

Greetings fellow cyclists I have a mid-price cycle computer, which is
set up according to instructions (double-checked and triple checked). On
a long ride, recently (recommended by a friend, who said it was about
40+ miles, he hadn't measured it) i was quietly impressed by the distace
i rode, which was longer than he'd said. But looking at a map later, I
thought something was amiss. I reckon that the computer is about 1 mile
out every 15. Is that par for the course with these gadgets? I'm not

That comes out at nearly 7% error, which I would think is quite large.
There are two main sources of error for these things that I see:

1) The accuracy of the wheel circumference measurement
2) Sensitivity of the magnet + sensor (ie does one revolution of the
wheel register as one revolution in the sensor)

The wheel circumference (C) is something that you should be able to get
quite accurately with a roll-test (where you sit normally on the bike,
keeping the front wheel straight, mark the start and end of one (or
more) wheel revolutions while a friend pushes you and measure).

This should get it to within 0.5% at the worst (5mm error on a 2000mm C
wheel is 0.25% error). As C varies with tyre pressure, ride weight etc
this is probably more like 1-2% in practice. Generally this bit is as
accurate as you can make it (within setting resolution of mostly 1mm on
the computer, ie about 0.001%).

What could be causing you trouble, assuming your C is set as accurately
as you can get it is the magnet sensor set up. You want it as near to
the hub as possible so that the magnet sweeps past the sensor as slowly
as possible. This gives the sensor longer to react. If you've got it
nearer the rim and you're going at a decent pace the sensor may be
saturated and thus record a different number of revolutions than there
have actually been.

If your sensor works properly your distance should simply be

revolutions x C

so your only significant error lies in C (measurement error and how it
varies as you ride).

Quoted message said:

interested in measuring to the millimetre, nor do I have more than a
passing interest in the average speed, etc, but it's nice to have an
idea of how far you've done. Arrivederci FM

Speed and distance are related, so getting one accurate means the other
should be as well. If you get your C to within a few mm and get your
sensor properly set up you should easily achieve 1%, which is really
quite accurate when you think about it (1 mile in a hundred). Don't
forget that something that's 10miles in a straight line might not be
10miles on a bike that weaves a bit (as a front wheel tends to do a bit)
so your distance reading on the bike may well be accurate even if it
doesn't quite match what the map says.

As we're generally not looking for scientific precision it doesn't
matter overly much. My gf's computer was giving a few % different to my
computer so we tweaked hers to give the same results. This isn't because
mine was any more accurate but just to make things fair (and hers was
more tweakable).

Frink

--
Doctor J. Frink : 'Rampant Ribald Ringtail'
See his mind here : http://www.cmp.liv.ac.uk/frink/
Annoy his mind here : pjf at cmp dot liv dot ack dot ook
"No sir, I didn't like it!" - Mr Horse

FranklynMint said:

But looking at a map later, I thought something was amiss. I reckon
that the computer is about 1 mile out every 15. Is that par for the
course with these gadgets?

You need to set it to measure the rolling circumference of your wheel. I
don't know what your instructions say, but the easiest way is to ride
over a line of something like washing-up liquid and keep going, then
measure the distance between as many lines as you can.

Depending on your computer, divide by the appropriate combination of 2
and pi if you need the diameter or radius.

1 part in 15 is just under 7%: you should be able to measure your wheel
better than that. There are three sources of inaccuracy:

1) Inaccurate wheel measurement
2) Missed or duplicate wheel counts
3) Precision (rounding) errors

These are in order of importance for typical computers: 2) and 3) are
practically negligible if set up properly.

Don't forget that the computer measures the distance travelled by your
front wheel - if you weave, it'll measure longer distances than the road
actually travelled.

--
Mark.

Mark Tranchant said:

Don't forget that the computer measures the distance travelled by your
front wheel - if you weave, it'll measure longer distances than the
road actually travelled.

Not just front wheel weaving, but many or most roads include slight kinks
that won't show up on anything other than very large-scale maps. All that
will add up considerably on a ride.

~PB

Mark Tranchant said:

FranklynMint said:

But looking at a map later, I thought something was amiss. I reckon
that the computer is about 1 mile out every 15. Is that par for the
course with these gadgets?

You need to set it to measure the rolling circumference of your wheel. I
don't know what your instructions say, but the easiest way is to ride
over a line of something like washing-up liquid and keep going, then
measure the distance between as many lines as you can.

Depending on your computer, divide by the appropriate combination of 2
and pi if you need the diameter or radius.

1 part in 15 is just under 7%: you should be able to measure your wheel
better than that. There are three sources of inaccuracy:

1) Inaccurate wheel measurement
2) Missed or duplicate wheel counts
3) Precision (rounding) errors

These are in order of importance for typical computers: 2) and 3) are
practically negligible if set up properly.

Don't forget that the computer measures the distance travelled by your
front wheel - if you weave, it'll measure longer distances than the road
actually travelled.

If you have a car drive a fixed distance and then ride the same, and
then adjust your cycle computer by some proportion to make the
two read the same. You would need top do it over a fair distance,
some miles I guess, but it's likely that in a modern car with
correctly inflated tyres the odometer will be pretty accurate.

IE if your car reads 10.0 miles and your bike 11.0, multiply the wheel
circumference in your computer by 10 then divide by 11 and reenter it.

--
Trevor Barton

Thanks for the practical advice, folks. I'll measure the circumference
accurately and move the sensor nearer to the hub. Cheers.

--

Trevor Barton said:

If you have a car drive a fixed distance and then ride the same, and
then adjust your cycle computer by some proportion to make the
two read the same. You would need top do it over a fair distance,
some miles I guess, but it's likely that in a modern car with
correctly inflated tyres the odometer will be pretty accurate.

Here you're introducing an unknown error (ie just how accurate is your
car's odometer, both in the absolute measurement, its resolution and
under what car conditions is it accurate) along with the fact that a car
covers ground in a different way to a bike (the normal wobble of the
bike, and differing suspension putting different loads on the wheels
which affects wheel circumference). All you're getting then is the
distance you would have covered in your car, which you haven't done, cos
you're on your bike.

Measuring the wheel circumference directly using a rolling measurement
(either directly or on a known length track) should give you much better
accuracy, well under 1% if done properly which surely puts most car
odometers to shame. It's also quicker and doesn't require an unnecessary
long drive for both car and bike.

I would be more inclined to set the car odometer to the *bike's* but
then that might be illegal ;0).

Frink

--
Doctor J. Frink : 'Rampant Ribald Ringtail'
See his mind here : http://www.cmp.liv.ac.uk/frink/
Annoy his mind here : pjf at cmp dot liv dot ack dot ook
"No sir, I didn't like it!" - Mr Horse

FranklynMint said:

Thanks for the practical advice, folks. I'll measure the circumference
accurately and move the sensor nearer to the hub. Cheers.

Oh, one final thing about moving closer to the hub, if it's a wireless
sensor this may reduce battery life (further away it is the more power
it has ot use to reach the computer), although it's hard to say by how
much (probably not much if it's only a few cms). I expect if it's well
within the limits of the sensor it shouldn't affect the accuracy.

Frink

--
Doctor J. Frink : 'Rampant Ribald Ringtail'
See his mind here : http://www.cmp.liv.ac.uk/frink/
Annoy his mind here : pjf at cmp dot liv dot ack dot ook
"No sir, I didn't like it!" - Mr Horse

In article <[email hidden]>,

FranklynMint said:

Greetings fellow cyclists I have a mid-price cycle computer, which is
set up according to instructions (double-checked and triple checked). On
a long ride, recently (recommended by a friend, who said it was about

Over ten miles this morning mine agreed to within about 0.05 miles (~100
yards) with the GPS box I had clipped to the handlebars. Given they're
measuring different things in different ways, given I was running the
GPS at a slow update rate and given the fact that the computer had
recorded 100 yards _more_ I'd say that's about bang on. The more is
reassuring, as the GPS box will be measuring a succession of short
chords to the route.

Quoted message said:

But looking at a map later, I
thought something was amiss. I reckon that the computer is about 1 mile
out every 15.

Unless you're talking about riding in a dead straight line along 15
miles of salt flats, the front wheel will go further than you think.
Once you factor in bends, it goes a _lot_ further. I don't know id ~6%
is typical, but the difference between GPS tracks and GPS point-to-point
bearings is usually at least that.

ian

In article <[email hidden]>,

Trevor Barton said:

some miles I guess, but it's likely that in a modern car with
correctly inflated tyres the odometer will be pretty accurate.

Well...

Take a car running on 175/60R15s (a tyre size I've made up, but is the
sort of thing that a family car might run on). The circumference of the
tyre is 3.14 * ((15*25.4)+(175*0.6)*2) = 1.855m (oh, I love mixed metric
and imperial sizing). But tyres ship with 10mm of tread, and are safe
down to 3mm, so that's 3.14 * ((15*25.4)+(175*0.6-7)*2) = 1.812m at the
other extreme. So there's potentially +/-1% of error just in the wear in
your tyres.

ian

Doctor J. Frink said:

Trevor Barton said:

If you have a car drive a fixed distance and then ride the same, and
then adjust your cycle computer by some proportion to make the
two read the same. You would need top do it over a fair distance,
some miles I guess, but it's likely that in a modern car with
correctly inflated tyres the odometer will be pretty accurate.

Here you're introducing an unknown error

agreed, my car odo is about 5% accurate. Bike computers can suffer from
contact bounce of the reed switches at higher speeds so will tend to
over read all things being equal.

The council should have an accurate 'measured mile' somewhere which can
be used to calibrate odometers, try the highways department for information.

Ian G Batten said:

The circumference of
the tyre is 3.14 * ((15*25.4)+(175*0.6)*2) = 1.855m (oh, I love mixed
metric and imperial sizing). But tyres ship with 10mm of tread, and
are safe down to 3mm, so that's 3.14 * ((15*25.4)+(175*0.6-7)*2) =
1.812m at the other extreme. So there's potentially +/-1% of error
just in the wear in your tyres.

And that's before you've driven them for a year without checking the
pressures ;-)

Guy
--
May contain traces of irony. Contents liable to settle after posting.
http://www.chapmancentral.co.uk

"Doctor J. Frink" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

As C varies with tyre pressure, ride weight etc
this is probably more like 1-2% in practice.

This is interesting.

So the outside circumference of an inflated tyre is larger than that of
a flat tyre is it?

Curious

Quoted message said:

What could be causing you trouble, assuming your C is set as accurately
as you can get it is the magnet sensor set up. You want it as near to
the hub as possible so that the magnet sweeps past the sensor as slowly
as possible. This gives the sensor longer to react. If you've got it
nearer the rim and you're going at a decent pace the sensor may be
saturated and thus record a different number of revolutions than there
have actually been.

If your sensor works properly your distance should simply be

revolutions x C

so your only significant error lies in C (measurement error and how it
varies as you ride).

Quoted message said:

interested in measuring to the millimetre, nor do I have more than a
passing interest in the average speed, etc, but it's nice to have an
idea of how far you've done. Arrivederci FM

Speed and distance are related, so getting one accurate means the other
should be as well. If you get your C to within a few mm and get your
sensor properly set up you should easily achieve 1%, which is really
quite accurate when you think about it (1 mile in a hundred). Don't
forget that something that's 10miles in a straight line might not be
10miles on a bike that weaves a bit (as a front wheel tends to do a bit)
so your distance reading on the bike may well be accurate even if it
doesn't quite match what the map says.

As we're generally not looking for scientific precision it doesn't
matter overly much. My gf's computer was giving a few % different to my
computer so we tweaked hers to give the same results. This isn't because
mine was any more accurate but just to make things fair (and hers was
more tweakable).

Frink

--
Doctor J. Frink : 'Rampant Ribald Ringtail'
See his mind here : http://www.cmp.liv.ac.uk/frink/
Annoy his mind here : pjf at cmp dot liv dot ack dot ook
"No sir, I didn't like it!" - Mr Horse

On Tue, 15 Jun 2004 13:45:16 +0100, Dr Curious <[email hidden]>

Quoted message said:

"Doctor J. Frink" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

As C varies with tyre pressure, ride weight etc
this is probably more like 1-2% in practice.

This is interesting.

So the outside circumference of an inflated tyre is larger than that of
a flat tyre is it?

Curious

Take bike.

Deflate front tyre.

Wheel descends to floor to rest on rim.

Note distance r_flat from hub to floor (ie the radius of the circle that
we would be rolling on).

Pump up tyre.

Wheel rises from floor as tyre inflates.

Note distance r_inflated from hub to floor (ie the radius of the circle
that we would be rolling on).

You'll find that r_inflated > r_flat.

The actual circumference of the tyre itself may not change but the
effective wheel radius does, and that's what we're really measuring (ok,
labelling it Circumference wasn't totally accurate but it's close
enough).

This is also affected by how you ride the bike, how much you weigh etc,
as under normal pressures where the tyre meets the ground the tyre
deforms perceptibly, meaning r_loaded < r_unloaded. This is why you must
be on the bike when measuring C.

To summarise: measure how far one wheel revolution takes you
(effectively its circumference) under the conditions you normally ride.

Frink

--
Doctor J. Frink : 'Rampant Ribald Ringtail'
See his mind here : http://www.cmp.liv.ac.uk/frink/
Annoy his mind here : pjf at cmp dot liv dot ack dot ook
"No sir, I didn't like it!" - Mr Horse

Quoted message said:

Quoted message said:

Quoted message said:

As C varies with tyre pressure, ride weight etc
this is probably more like 1-2% in practice.

All of which goes to explain something for me. I took the Cateye Astrale 8
off my tourer and put it onto a new (second hand road bike) and put an old
Halfords 12 function on my tourer. Both were set with the wheel setting
(circumferance) to 2180 for a 700 x 38. I then did my usual morning commute
and found that instead of 6.35 miles the distance shown was 6.27 miles or
abotu 100 yards less.

That must entirely down to my weight bearing down on the rear tyre (assuming
both computers can count and calculate correctly. The Astale 8 magnet is on
the rear wheel, but the Halfords magnet is on the front wheel.

"Doctor J. Frink" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

On Tue, 15 Jun 2004 13:45:16 +0100, Dr Curious <[email hidden]>

Quoted message said:

"Doctor J. Frink" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

As C varies with tyre pressure, ride weight etc
this is probably more like 1-2% in practice.

This is interesting.

So the outside circumference of an inflated tyre is larger than that of
a flat tyre is it?

Curious

Take bike.

Deflate front tyre.

Wheel descends to floor to rest on rim.

Note distance r_flat from hub to floor (ie the radius of the circle that
we would be rolling on).

Pump up tyre.

Wheel rises from floor as tyre inflates.

Note distance r_inflated from hub to floor (ie the radius of the circle
that we would be rolling on).

You'll find that r_inflated > r_flat.

The actual circumference of the tyre itself may not change but the
effective wheel radius does, and that's what we're really measuring (ok,
labelling it Circumference wasn't totally accurate but it's close
enough).

This is also affected by how you ride the bike, how much you weigh etc,
as under normal pressures where the tyre meets the ground the tyre
deforms perceptibly, meaning r_loaded < r_unloaded. This is why you must
be on the bike when measuring C.

To summarise: measure how far one wheel revolution takes you
(effectively its circumference) under the conditions you normally ride.

....

I'm sorry but this all totally irrelevant.

The radius of the wheel is irrelevant.

When a tire is flat, all this means is that the wheel/tyre is no longer
circular. It's now a part circle with a flat at the bottom. And so there's
more surface area in contact with the ground. But the actual outside
circumference remains exactly the same. It wouldn't even matter if you
were riding on oval wheels. All you ever need measure are the number
of revolutions multiplied by the circumference of the tyre.

Curious

....

Quoted message said:

Frink

--
Doctor J. Frink : 'Rampant Ribald Ringtail'
See his mind here : http://www.cmp.liv.ac.uk/frink/
Annoy his mind here : pjf at cmp dot liv dot ack dot ook
"No sir, I didn't like it!" - Mr Horse

Mike Dodds said:

All of which goes to explain something for me. I took the Cateye
Astrale 8 off my tourer and put it onto a new (second hand road bike)
and put an old Halfords 12 function on my tourer. Both were set with
the wheel setting (circumferance) to 2180 for a 700 x 38. I then did
my usual morning commute and found that instead of 6.35 miles the
distance shown was 6.27 miles or abotu 100 yards less.

That must entirely down to my weight bearing down on the rear tyre
(assuming both computers can count and calculate correctly. The
Astale 8 magnet is on the rear wheel, but the Halfords magnet is on
the front wheel.

I, however, have a Mity3 and an Astrale, both set to circumference = 1440 mm
and both looking at the same magnet. Distance and average speed are
sometimes considerably different...

--

Dave Larrington - http://www.legslarry.beerdrinkers.co.uk/
===========================================================
Editor - British Human Power Club Newsletter
http://www.bhpc.org.uk/
===========================================================

Mike Dodds said:

All of which goes to explain something for me. I took the Cateye Astrale 8
off my tourer and put it onto a new (second hand road bike) and put an old
Halfords 12 function on my tourer. Both were set with the wheel setting
(circumferance) to 2180 for a 700 x 38. I then did my usual morning commute
and found that instead of 6.35 miles the distance shown was 6.27 miles or
abotu 100 yards less.

That must entirely down to my weight bearing down on the rear tyre (assuming
both computers can count and calculate correctly. The Astale 8 magnet is on
the rear wheel, but the Halfords magnet is on the front wheel.

As has already been mentioned (I think), the front wheel actually does
go slightly further than the rear. Ride through a puddle and it will be
very obvious. Weight distribution may also make a difference, I have not
thought about their relative magnitudes.

James