Please help mathematically challenged rider

Tumbleweed said:

.....thats me :-).

Mileage indicated by computer was 26.5
Wheel size entered in computer was 2124.
Actual wheel size as just measured with me sitting on it(two wheel revs
divided by two) = 2100

So I *think* I actually did less miles, by a factor of 2100/2124 = 98.87% ,
this = 26.2 actual miles.
Interestingly my GPS unit said I'd done 25.6 but it wasnt in a good position
(it was in my pocket) so it may have missed some signals. Next time for the
exact same route I'll have it better placed.

Anyway, how is my maths, does it make sense?

Your maths is fine. Your GPS is not good for measuring distances. It
approximates a curve by a series of discrete chords so measures a
shorter distance than actual.

Tony

Tumbleweed said:

Interestingly my GPS unit said I'd done 25.6

GPS always read lower than cycle computers, in my experience. Even
when everything is accurately calibrated and GPS positioned for best
possible reception and maximum resolution.

regards, Ian SMith

"Ian Smith" <[email hidden]> wrote in message
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Quoted message said:

Tumbleweed said:

Interestingly my GPS unit said I'd done 25.6

GPS always read lower than cycle computers, in my experience. Even
when everything is accurately calibrated and GPS positioned for

best

Quoted message said:

possible reception and maximum resolution.

2100mm for wheel circumference corresponds to a wheel diameter of
26.3".

Of course, if a 700C wheel really did have a diameter of 700mm, and
pi was equal to 3, like the bible says, 2100mm would be exactly
right.

Jeremy Parker

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

Quoted message said:

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

Quoted message said:

Tumbleweed said:

Interestingly my GPS unit said I'd done 25.6

GPS always read lower than cycle computers, in my experience. Even
when everything is accurately calibrated and GPS positioned for

best

Quoted message said:

possible reception and maximum resolution.

2100mm for wheel circumference corresponds to a wheel diameter of
26.3".

Of course, if a 700C wheel really did have a diameter of 700mm, and
pi was equal to 3, like the bible says, 2100mm would be exactly
right.

Jeremy Parker

According to the manual, a 700 with 32C tyres should have a figure of 2155.
But with me sitting on the bike*, (and I don't weight that much :-), and
reducing error by doing two revs and dividing by 2, the actual measurement
is 2100. I might do it again just to double check, but I think it will be
the same.

--
Tumbleweed

*which AFAIK is the correct procedure when doing this measurement

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tumbleweednews at hotmail dot com

"Tumbleweed" <[email hidden]> wrote in message
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Quoted message said:

"Jeremy Parker" <[email hidden]> wrote in message
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Quoted message said:

"Ian Smith" <[email hidden]> wrote in message
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Quoted message said:

On Sat, 9 Apr 2005, Tumbleweed <> wrote:

> Interestingly my GPS unit said I'd done 25.6

GPS always read lower than cycle computers, in my experience. Even
when everything is accurately calibrated and GPS positioned for

best

Quoted message said:

possible reception and maximum resolution.

2100mm for wheel circumference corresponds to a wheel diameter of
26.3".

Of course, if a 700C wheel really did have a diameter of 700mm, and
pi was equal to 3, like the bible says, 2100mm would be exactly
right.

Jeremy Parker

According to the manual, a 700 with 32C tyres should have a figure of
2155. But with me sitting on the bike*, (and I don't weight that much :-),
and reducing error by doing two revs and dividing by 2, the actual
measurement is 2100. I might do it again just to double check, but I think
it will be the same.

--
Tumbleweed

*which AFAIK is the correct procedure when doing this measurement

Come on Joe, this is all getting a bit too precise - we'll know when we've
arrived, regardless of far it is! (You do realise that you have to make
periodic adjustments the computer settings to allow for tyre wear!)

Rob

Ian Smith said:

On Sat, 9 Apr 2005, Tumbleweed <> wrote:

Quoted message said:

Quoted message said:

Interestingly my GPS unit said I'd done 25.6

Quoted message said:

GPS always read lower than cycle computers, in my experience. Even
when everything is accurately calibrated and GPS positioned for best
possible reception and maximum resolution.

I find my GPS (etrex Summit) agrees with my bike computer within 2%,
except when for some reason it's lost reception for a while and
straightened out some curves. I say a while because it often loses
reception for a few dozen yards, but that's usually when I'm
travelling in a fairly straight line and it makes very little
difference to the total.

I set up the bike computer carefully, taking into account usual tyre
pressure and my weight. The GPS is attached to the bike handlebars.

It matters how detailed a track your GPS takes. Some have optional
levels of detail. The Summit doesn't, but does make unusually detailed
tracks (as do some of its relatives, forget which).

--
Chris Malcolm [email hidden] +44 (0)131 651 3445 DoD #205
IPAB, Informatics, JCMB, King's Buildings, Edinburgh, EH9 3JZ, UK
[http://www.dai.ed.ac.uk/homes/cam/]

RG said:

Come on Joe, this is all getting a bit too precise - we'll know when we've
arrived, regardless of far it is! (You do realise that you have to make
periodic adjustments the computer settings to allow for tyre wear!)

I expect you'd also find that a front-wheel mounted magnet will
register a greater distance over the same course when going slowly, as
the small sideways steering motions to maintain balance are that much
greater.

So you should probably ensure you're moving at a reasonable speed when
measuring tyre circumference.

-dan

"Daniel Barlow" <[email hidden]> wrote in message
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Quoted message said:

I expect you'd also find that a front-wheel mounted magnet will
register a greater distance over the same course when going slowly, as
the small sideways steering motions to maintain balance are that much
greater.

So you should probably ensure you're moving at a reasonable speed when
measuring tyre circumference.

-dan

...for the pedantic only...was leaning slightly against a wall for the entire
distance so it was a dead straight line :-)

--
Tumbleweed

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