Cycle Computer Accuracy ?

TJ said:

Maybe I am missing something or is the computer really getting it wrong.

I have a SigmaSport BC1200 comp which is seemingly recording my average
speeds incorrectly.

example 5.03 Miles in 19m47s returns an Average Speed of 15.39 Mph

My own calcs make this 15.26 Mph

If the comp is right can someone explain why ?

Might it be factoring out the periods spent stationary? Thus the time
spent actually moving will be a little shorter.

Colin

TJ said:

Maybe I am missing something or is the computer really getting it wrong.

I have a SigmaSport BC1200 comp which is seemingly recording my average
speeds incorrectly.

example 5.03 Miles in 19m47s returns an Average Speed of 15.39 Mph

My own calcs make this 15.26 Mph

If the comp is right can someone explain why ?

Thanks

TJ

I wouldn't expect these devices to be very accurate, TBH. There is no requirement for them to be calibrated. Perhaps internally the calculation uses more decimal places than the display shows you.

TJ said:

Maybe I am missing something or is the computer really getting it wrong.

I have a SigmaSport BC1200 comp which is seemingly recording my average
speeds incorrectly.

example 5.03 Miles in 19m47s returns an Average Speed of 15.39 Mph

My own calcs make this 15.26 Mph

If the comp is right can someone explain why ?

If your computer is like mine, it stops counting the trip time if you
stop for more than 4 seconds (at junctions, traffic lights etc).

So if you time your journey with your watch, and compare this to
the trip time on your computer, the trip time will usually be
shorter. Less time = higher average speed.

--
jc

Remove the -not from email

TJ said:

Maybe I am missing something or is the computer really getting it wrong.

I have a SigmaSport BC1200 comp which is seemingly recording my average
speeds incorrectly.

example 5.03 Miles in 19m47s returns an Average Speed of 15.39 Mph

My own calcs make this 15.26 Mph

If the comp is right can someone explain why ?

Assuming both the distance and the trip time come from the computer,
at a guess I would say that the computer calculates distance and speed
in km and kph and converts by using a look-up table into miles and
mph. The look-up table is not sufficiently accurate to give a reading
to two decimal places.

1 kph = 0.621371192237 mph (approx 5/8)
1 km = 0.621371192237 miles (approx 5/8)

Converting 5.03 miles into kms, you cycled 8.05 km.

You cycled 8.05 km in 1187 seconds. That's a speed of 24.41 kph.

Converting 24.41 kph into mph, you cycled at... 15.26 mph.

Hurrummph - your cycle computer's calculations are [censored]!!!

My computer does feature a dist timer as you mention which does stop within
a second or two whenever the bike becomes stationary so I dont think that
this this is an issue.

I really don't see why it isn't calculating the Average Speed from it's own
figures in the same way I am ?

I'm using excel to calculate the A/Spd by simply dividing the Distance
covered by the time taken to cover it.

(Because a minute is only 60 secs and excel works in Decimal I multiply the
seconds by 1.665).

The entire result is then multiplied by 60 to give the MPH.

e.g.

This mornings Distance / Time was 5.00 miles in 15m 06s

The 6 seconds are multiplied by 1.665 (.06 * 1.665) to give .10 this is
done to give the base 10 representation of the number for the calculation

so

( 5.00miles / 15.1 ) * 60 = 19.08 Mph

I'm sure i'm right - Aren't I ?

when I use other test data in my formula i.e. miles 10.5, Dist 10.3 10.5
miles in 10 mins 30 seconds for example the result is 60mph as expected.

Even if I skip the corrective adjustment I mention the result is still only
19.92 yet the Sigma BC1200 is reporting the average speed this morning as
20.12 Mph

TJ

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

Quoted message said:

Maybe I am missing something or is the computer really getting it wrong.

I have a SigmaSport BC1200 comp which is seemingly recording my average
speeds incorrectly.

example 5.03 Miles in 19m47s returns an Average Speed of 15.39 Mph

My own calcs make this 15.26 Mph

If the comp is right can someone explain why ?

Thanks

TJ

Quoted message said:

Assuming both the distance and the trip time come from the computer,
at a guess I would say that the computer calculates distance and speed
in km and kph and converts by using a look-up table into miles and
mph. The look-up table is not sufficiently accurate to give a reading
to two decimal places.
...
Hurrummph - your cycle computer's calculations are [censored]!!!

Allowing for the more sensible answers that have already been...
Would it make sense to do anything to four significant figures?

How finely are you able to calibrate the device in the first
place? And any sensor that measures a single point on a 700mm
wheel is bound to be anything up to about 0.25% of a mile wrong
on any journey, however long or short, anyway. (A hundredth of
a mile is only about seven rotations of the wheel.) How much
further does the front wheel of a bike go than the back one?

One of my grandfathers used insist on sitting in the front seat
upstairs on a bus because he "went further" that way.

TJ said:

Maybe I am missing something or is the computer really getting it wrong.

I have a SigmaSport BC1200 comp which is seemingly recording my average
speeds incorrectly.

example 5.03 Miles in 19m47s returns an Average Speed of 15.39 Mph

My own calcs make this 15.26 Mph

If the comp is right can someone explain why ?

The computer is using low resolution arithmetic to calculate the average
speed, based presumably on the time and distance it has recorded. In
principal, and certainly if you do it on your PC in excel, or write a
program to do it, you will get an answer that is as accurate as the
time and distance measurments up to several significant figures.

However, the cycle computer uses not very many significant figures in
its calculations, and so it looses accuracy as it calculates. As an
example, suppose it uses only 2 decimal digits throughout the calculation,
and simply throws away less significant digits, the answer you calculate
will be 15.00. If it uses 3 you get 15.20, and to 4 you get 15.25 (if
you use the same algorithm I did). Your trip computer will use either
binary or binary coded decimal arithmetic, and so its calculation
accuracy will not be the same as I have above.

My own computer is always between 0.00 and 0.20 mph below the actual
(excel calculated) average speed.

--
Trevor Barton

Surely

average speed = distance/((seconds+(minutes*60)+(hours*60*60))/3600)

by using number of seconds * 1.665 you are introducing an error, it may only
be 1% error but that greater than the error in the original question which I
believe was 0.85%.

-----------
LeeG
-----------
"TJ" <[email hidden]> wrote in message
news:[email hidden]...
My computer does feature a dist timer as you mention which does stop within
a second or two whenever the bike becomes stationary so I dont think that
this this is an issue.

I really don't see why it isn't calculating the Average Speed from it's own
figures in the same way I am ?

I'm using excel to calculate the A/Spd by simply dividing the Distance
covered by the time taken to cover it.

(Because a minute is only 60 secs and excel works in Decimal I multiply the
seconds by 1.665).

The entire result is then multiplied by 60 to give the MPH.

e.g.

This mornings Distance / Time was 5.00 miles in 15m 06s

The 6 seconds are multiplied by 1.665 (.06 * 1.665) to give .10 this is
done to give the base 10 representation of the number for the calculation

so

( 5.00miles / 15.1 ) * 60 = 19.08 Mph

I'm sure i'm right - Aren't I ?

when I use other test data in my formula i.e. miles 10.5, Dist 10.3 10.5
miles in 10 mins 30 seconds for example the result is 60mph as expected.

Even if I skip the corrective adjustment I mention the result is still only
19.92 yet the Sigma BC1200 is reporting the average speed this morning as
20.12 Mph

TJ

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

Quoted message said:

Maybe I am missing something or is the computer really getting it wrong.

I have a SigmaSport BC1200 comp which is seemingly recording my average
speeds incorrectly.

example 5.03 Miles in 19m47s returns an Average Speed of 15.39 Mph

My own calcs make this 15.26 Mph

If the comp is right can someone explain why ?

Thanks

TJ

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