computer averge speed algorithm

Allan Leedy said:

Anybody know what formula is used by cycle computers to calculate average speed? It doesn't seem
to be d/t=r on mine or any of the others I've checked. Average speed, based on recorded distance
and riding time, is underreported. I'm wondering how & why.

It appears to depend on your unit.

On my Cateye, the average is determined from the running time and distance since last reset, up to
24:00 hours, because this is the max of the unit.

--
Linux Registered User # 302622 <counter.li.orgcounter.li.orgOpen ↗

Allan Leedy said:

Anybody know what formula is used by cycle computers to calculate average speed? It doesn't seem
to be d/t=r on mine or any of the others I've checked. Average speed, based on recorded distance
and riding time, is underreported. I'm wondering how & why.

Are you saying that if you ride 18 miles in one hour (actual elapsed time) the computer displays
less than 18 mph? Or are you factoring in times when the bike is not moving? How much of a
difference are you seeing?

Most computers don't count times when the bike is stopped into the calculation. But if you're say
stopped at a traffic light, and you rock the bike ever so slightly back and forth, it will lower the
average. And walking the bike even a short distance will have an effect.

I use an Avocet 40, and the average speed seems to be quite accurate for non-stop rides such as
time trials.

Art Harris

Harris said:

Are you saying that if you ride 18 miles in one hour (actual elapsed time) the computer displays
less than 18 mph? Or are you factoring in times when the bike is not moving? How much of a
difference are you seeing?

Don't know what the OP is saying, but certainly they use some funny rounding method (or something)
which produces odd results sometimes. Eg I can be riding along at 20mph+, the average was 18.0 and
then it jumps to 17.9 for a while.

I think I remember someone explaining this some time ago, but I don't recall the details.

James

Quoted message said:

Quoted message said:

Anybody know what formula is used by cycle computers to calculate

average

Quoted message said:

Quoted message said:

speed? It doesn't seem to be d/t=r on mine or any of the others I've checked. Average speed,
based on recorded distance and riding time, is underreported. I'm wondering how & why.

It appears to depend on your unit.

On my Cateye, the average is determined from the running time and

distance

Quoted message said:

since last reset, up to 24:00 hours, because this is the max of the unit.

I've noticed this on a couple of computers I've used. Divide trip distance by trip time and the
"average" speed it quotes often does not agree this.

The "computer" used probably doesn't have the ability to divide - so instead it's using some sort of
mathematical "trick", effectively adding the current speed onto the accumulated average, using a
weighting factor. Or sumfink like that :-)

AndyK

"Harris" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

Are you saying that if you ride 18 miles in one hour (actual elapsed time) the computer displays
less than 18 mph?

Exactly that, based on the computer's displayed data for time and distance.

Or are you factoring in times when

Quoted message said:

the bike is not moving?

As you suggest, the settings are for time in motion only, ignoring time stopped and total
elapsed time.

How much of a difference are you seeing?

3-5% (e.g., 15.4 instead of 16)

Quoted message said:

Most computers don't count times when the bike is stopped into the calculation. But if you're say
stopped at a traffic light, and you rock

the

Quoted message said:

bike ever so slightly back and forth, it will lower the average. And

walking

Quoted message said:

the bike even a short distance will have an effect.

Why should it matter? Presumably, nothing registers unless the magnet passes the sensor. If it does,
won't it record time and distance?

"Allan Leedy" <[email hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

"Harris" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

Are you saying that if you ride 18 miles in one hour (actual elapsed

time)

Quoted message said:

Quoted message said:

the computer displays less than 18 mph?

Exactly that, based on the computer's displayed data for time and

distance.

Quoted message said:

Or are you factoring in times when

Quoted message said:

the bike is not moving?

As you suggest, the settings are for time in motion only, ignoring time stopped and total
elapsed time.

How much of a difference are you seeing?

3-5% (e.g., 15.4 instead of 16)

Quoted message said:

Most computers don't count times when the bike is stopped into the calculation. But if you're
say stopped at a traffic light, and you rock

the

Quoted message said:

bike ever so slightly back and forth, it will lower the average. And

walking

Quoted message said:

the bike even a short distance will have an effect.

Why should it matter? Presumably, nothing registers unless the magnet passes the sensor. If it
does, won't it record time and distance?

You probably need to query users of the brand of cyclo-computer you use, as brands will differ.
FWIW, I just checked the numbers for my last ride on my Sigma Sport BC800 and the average speed
readout is accurate to the limit of its display, or .01mph.

"AndyK" <[email hidden]> wrote in message news:<[email hidden]>... <cut>

Quoted message said:

The "computer" used probably doesn't have the ability to divide - so instead it's using some sort
of mathematical "trick", effectively adding the current speed onto the accumulated average, using
a weighting factor. Or sumfink like that :-)

AndyK

My cateye unit certainly does something odd, I can be riding along steadily at below my current
average speed and see the average speed go up.

When average is increasing it often goes up by 0.2 more a minute or so, then down by 0.2 then up
0.1 again.

I wonder if this is what Intel did with all those reject Pentiums that couldn't divide properly.

Andrew Webster

Allan Leedy said:

Anybody know what formula is used by cycle computers to calculate average speed? It doesn't seem
to be d/t=r on mine or any of the others I've checked. Average speed, based on recorded distance
and riding time, is underreported. I'm wondering how & why.

It probably is correct. It knows the distance, since it counts the pulses from the magnet. If the
rolling speeds and distances tend to be right (check the odometer against a calibrated roadway
sometime), then the average is going to be as well. They all basically have the same chip in them.
There may be a setting to include/exclude time spent standing still, but let's presume that is set
correctly.

Averages may be deceiving. Consider the following Gedankenexperiment: you climb a really, really
tough hill. It's so steep that you climb it at 1 mph. It's also a mile long, so it takes you an
hour to get to the top. You then turn around and zoom down, say at 100 mph. What is your average
speed? As unfair as it may seem, your average speed is about 2mph since it took you over an hour
just to go 2 miles.

We tend to think in terms of averaging over distance. You know, the headwind part of the time-trial
course was tough, so you went only 15mph, but you were able to fly on the tailwind part, going
30mph. You check your average and are disappointed that you did not get an average speed of 22.5
mph. But if the course were, say, 30 miles long, that trip would have taken you 1.5 hours (one hour
headwind, 30 minutes tailwind). So your average comes out as only 20.

--

David L. Johnson

__o | Accept risk. Accept responsibility. Put a lawyer out of _`\(,_ | business. (_)/ (_) |

Your formula is correct. But many computers "stop the clock' after a few seconds of inactivity. That
way things like rest stops and stop lights are not taken into account. The avarage speed is the
avarage speed when you're actually moving.

If you want to include minor stops, use the stop watch function, if it has one, and calculate the
result with a pocket calculator.

May you have the wind at your back. And a really low gear for the hills! Chris

Chris'Z Corner "The Website for the Common Bicyclist": geocities.comczcornerOpen ↗

"Chris Zacho"

Quoted message said:

Your formula is correct. But many computers "stop the clock' after a few seconds of inactivity.
That way things like rest stops and stop lights are not taken into account. The avarage speed is
the avarage speed when you're actually moving.

But he's saying the displayed average speed is LESS than the actual miles covered in an hour of
elapsed time. "Stopping the clock" would make the displayed average speed higher, not lower.

It can't be an intermittent sensor problem either because that would cause the distance to be
inaccurate.

I think it's possible the algorithm IS the culprit.

Art Harris

Allan Leedy said:

Anybody know what formula is used by cycle computers to calculate average speed? It doesn't seem
to be d/t=r on mine or any of the others I've checked. Average speed, based on recorded distance
and riding time, is underreported. I'm wondering how & why.

In re-reading this I think I misunderstood you. So, you look at reported riding time, reported
distance, and reported average speed and see a discrepency. Hmm. I have not noticed that, but I can
check. Just checked my computer. It's accurate to within rounding error.

--

David L. Johnson

__o | Some people used to claim that, if enough monkeys sat in front _`\(,_ | of enough
typewriters and typed long enough, eventually one of (_)/ (_) | them would reproduce the
collected works of Shakespeare. The internet has proven this not to be the case.

In article <[email hidden]>, [email hidden] (Andrew

Webster) said:

"AndyK" <[email hidden]> wrote in message news:<[email hidden]>... <cut>

Quoted message said:

The "computer" used probably doesn't have the ability to divide - so instead it's using some sort
of mathematical "trick", effectively adding the current speed onto the accumulated average, using
a weighting factor. Or sumfink like that :-)

AndyK

My cateye unit certainly does something odd, I can be riding along steadily at below my current
average speed and see the average speed go up.

When average is increasing it often goes up by 0.2 more a minute or so, then down by 0.2 then up
0.1 again.

I suspect that it use only the hour and minute fields when calculating the average speed. I have
seen the same thing, but if you do a looooong ride, the behaviour less "visible".

Jørn Dahl-Stamnes, EDB Teamco AS e-mail: [email hidden] (remove nospam first)
web: spiderman.novit.nodahlsOpen ↗

Quoted message said:

Quoted message said:

The "computer" used probably doesn't have the ability to divide - so instead it's using some sort
of mathematical "trick", effectively adding the current speed onto the accumulated average, using
a weighting factor. Or sumfink like that :-)

Interesting. My Cateye Astrale 8 (new Astrale model) correctly averaged my last ride of just
over 2 hours.

If a small computer processor doesn't calculate math, it uses what's called a "lookup table."

x y
-------
1 6 2 9 3 13 4 19 5 28 6 40

So given a value 3, simple lookup in ROM gives a quick 13 as an answer. The y values are of course
precalculated by the manufacturer, allowing any level of equation difficulty to be handled by the
simplest on-board processor.

Quoted message said:

My cateye unit certainly does something odd, I can be riding along steadily at below my current
average speed and see the average speed go up.

Weird. I can see how this can happen in a lookup table situation, but...

Quoted message said:

When average is increasing it often goes up by 0.2 more a minute or so, then down by 0.2 then up
0.1 again.

this is a large discrepancy.

Quoted message said:

I wonder if this is what Intel did with all those reject Pentiums that couldn't divide properly.

I wonder what's inside these pups. Small embedded processors are quite small and powerful now.
It's possible it does its own math. Even without explicit math ops in the instruction set, even
the simplest processors can be programmed to do math. Cateye and others surely developed such
code long ago.

Doug

Quoted message said:

I wonder what's inside these pups. Small embedded processors are quite small and powerful now.
It's possible it does its own math. Even without explicit math ops in the instruction set, even
the simplest processors can be programmed to do math. Cateye and others surely developed such code
long ago.

Especially modern bike computers with like several hundred functions need a fair bit of integer
processing capability just to run menu systems and cetera. All bike computers almost certainly use
specially designed ASICs, probably most often fliptop physcial packaging for cost and room reasons,
and they probably use the really simple embedded processors by now for easy of developing different
models. I mean, hell, for a couple of bucks nowadays you can buy an atmel AtTiny 8pin-SOIC processor
that runs at several megahertz with several k of program rom and half a k or so of ram integrated.
Those and similar processors are usually available for integrating into a custom chip of your own,
if necessary, as well.

Jasper

Hey, David, I understand everything you said here, and I don't disagree with any of it, except that
all you've told me about the problem I posed is that the facts are probably not as I stated them.
And yet, they are!

"David L. Johnson" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

Allan Leedy said:

Anybody know what formula is used by cycle computers to calculate

average

Quoted message said:

Quoted message said:

speed? It doesn't seem to be d/t=r on mine or any of the others I've checked. Average speed,
based on recorded distance and riding time, is underreported. I'm wondering how & why.

It probably is correct. It knows the distance, since it counts the pulses from the magnet. If the
rolling speeds and distances tend to be right

(check the

Quoted message said:

odometer against a calibrated roadway sometime), then the average is going to be as well. They all
basically have the same chip in them. There may be a setting to include/exclude time spent
standing still, but let's presume that is set correctly.

Averages may be deceiving. Consider the following Gedankenexperiment: you climb a really, really
tough hill. It's so steep that you climb it at 1 mph. It's also a mile long, so it takes you an
hour to get to the top. You then turn around and zoom down, say at 100 mph. What is your average
speed? As unfair as it may seem, your average speed is about 2mph since it took you over an hour
just to go 2 miles.

We tend to think in terms of averaging over distance. You know, the headwind part of the
time-trial course was tough, so you went only 15mph,

but you

Quoted message said:

were able to fly on the tailwind part, going 30mph. You check your average and are disappointed
that you did not get an average speed of 22.5 mph. But if the course were, say, 30 miles long,
that trip would have taken you 1.5 hours (one hour headwind, 30 minutes tailwind). So your average
comes out as only 20.

--

David L. Johnson

__o | Accept risk. Accept responsibility. Put a lawyer out of _`\(,_ | business. (_)/ (_) |

David, I "flamed" your last post before reading this one. (Well, maybe just "embered".) Still,
thanks for getting back to the point I wanted help on. Allan "David L. Johnson"
<[email hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

Allan Leedy said:

Anybody know what formula is used by cycle computers to calculate

average

Quoted message said:

Quoted message said:

speed? It doesn't seem to be d/t=r on mine or any of the others I've checked. Average speed,
based on recorded distance and riding time, is underreported. I'm wondering how & why.

In re-reading this I think I misunderstood you. So, you look at reported riding time, reported
distance, and reported average speed and see a discrepency. Hmm. I have not noticed that, but I
can check. Just checked my computer. It's accurate to within rounding error.

--

David L. Johnson

__o | Some people used to claim that, if enough monkeys sat in front _`\(,_ | of enough
typewriters and typed long enough, eventually one of (_)/ (_) | them would reproduce the
collected works of Shakespeare. The internet has proven this not to be the case.

Allan Leedy said:

David, I "flamed" your last post before reading this one. (Well, maybe just "embered".) Still,
thanks for getting back to the point I wanted help

on.

No problem.

It occurs to me that the issue for some of the disrepencies that have come up on this thread -- not
yours, though, which still baffles me -- is that the computation of current speed must do some sort
of short-time averaging:; say computing the time between sets of 4 or 5 pulses. This would open up
possible discrepencies -- lower speed readouts than the real speed due to missed pulses, and that
effect may be dependent on fault-inducing conditions like bumpy roads. It may be that over time the
effect of missed pulses becomes less important, but while you are looking at the speedometer they
accumulate. Or, the software sucks.

--

David L. Johnson

__o | Enron's slogan: Respect, Communication, Integrity, and _`\(,_ | Excellence. (_)/ (_) |

"Allan Leedy" <[email hidden]> wrote in :"]news:[email hidden]:

Quoted message said:

Anybody know what formula is used by cycle computers to calculate average speed? It doesn't seem
to be d/t=r on mine or any of the others I've checked. Average speed, based on recorded distance
and riding time, is underreported. I'm wondering how & why.

If your reporting is consistently underreported (or, for that matter, overreported), you may have
set the wrong wheel size. The computers I have used need to be told the rolling circumference of the
wheel, and use that in the computation, some by direct entry in mm, some via a number taken from a
lookup table provided with the computer documentation. Even a small difference can be noticeable in
the result.

--
Rik Old enough to be an adult

Rik said:

If your reporting is consistently underreported (or, for that matter, overreported), you may have
set the wrong wheel size. The computers I have used need to be told the rolling circumference of
the wheel, and use that in the computation, some by direct entry in mm, some via a number taken
from a lookup table provided with the computer documentation. Even a small difference can be
noticeable in the result.

No, in this case the average speed should agree with computing distance/time, using the distance on
the computer. It doesn't know you cheated by underreporting wheel size; but it still should be
internally consistent.

--

David L. Johnson

__o | "Business!" cried the Ghost. "Mankind was my business. The _`\(,_ | common welfare was my
business; charity, mercy, forbearance, (_)/ (_) | and benevolence, were, all, my business. The
dealings of my trade were but a drop of water in the comprehensive ocean of my business!"
--Dickens, "A Christmas Carol"