How to calculate metres travelled per crank revolution?

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deliquium
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How to calculate metres travelled per crank revolution?

Postby deliquium » 26 Mar 2019, 4:28pm

Whilst trying to locate, photograph and log local remaining Ordnance Survey Benchmarks, I need a way of measuring distances up to ~250 metres, whilst out on the bike.

If I could find a gear to pedal in that equated to say 5m per crank/pedal revolution, this would be very helpful.

GPS positioning isn't near enough.

Determining (like today's failed find) 149.7m North of Junction Wall on West Side of Road, would be easier counting right hand pedal revolutions

I bet lots of you know the answer and may suggest alternative methods of semi precise locating?

:)
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TrevA
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Re: How to calculate metres travelled per crank revolution?

Postby TrevA » 26 Mar 2019, 4:32pm

https://www.bikecalc.com/gear_meters_of_development

This website should help you calculate it. It might be difficult to get exactly 5 metres per pedal Rev ( that is what the development is).

48 chainring X 20 tooth rear cog is 5.04 metres on a 700c wheel.

Someone will be along soon to give you the formula for working out the gear yourself :D .

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NUKe
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Re: How to calculate metres travelled per crank revolution?

Postby NUKe » 26 Mar 2019, 4:45pm

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cycleruk
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Re: How to calculate metres travelled per crank revolution?

Postby cycleruk » 26 Mar 2019, 4:45pm

Front ring teeth divided by rear sprocket teeth x tyre circumference.
Say front ring = 48T
Sprocket = 12T
Tyre circumference = 2 x Pi x R = 2 x 3.142 x radius of back tyre (13" ?). = 81.69 "

48/12 x 81.69 ~ 326 inches =
1 rev = 27.23 ft.

Insert your own figures to suit.
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deliquium
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Re: How to calculate metres travelled per crank revolution?

Postby deliquium » 26 Mar 2019, 5:05pm

Brilliant - thank you all :D
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Brucey
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Re: How to calculate metres travelled per crank revolution?

Postby Brucey » 26 Mar 2019, 5:50pm

counting wheel revolutions, esp on on a larger wheeled (26-27") wheeled bike can be useful too.

For example a 25-622 tyre will roll ~15m for every seven turns.

BTW you will find the rollout of wheels with pneumatic tyres will vary slightly depending on the load on the wheel. Whether this is significant enough to worry about for your purposes I couldn't say, it is a relatively small error.

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531colin
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Re: How to calculate metres travelled per crank revolution?

Postby 531colin » 26 Mar 2019, 9:07pm

deliquium wrote:….....GPS positioning isn't near enough...........

I'm surprised by this. With the 2 1/2" map on my phone, I reckon its accurate down to a metre or two.....as in which side of the hedge the bridlepath is.

Grandad
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Re: How to calculate metres travelled per crank revolution?

Postby Grandad » 26 Mar 2019, 11:01pm

This takes me back to my time as an RTTC (now Cycling Time Trials) course measurer.

Distances were calculated using the number of revolutions of the front wheel. The rev counter was a version of the star wheel cyclometer -the height of technology then. Every session started with 2 runs over a very accurately measured mile to establish the number of revolutions for the wheel, tyre and tyre pressure on that day. The session concluded with another ride along the measured mile to ensure that nothing had happened to change the constant. Needless to say a puncture usually wrote off the day's work. At speeds above 12mph there was a risk of the striker jumping so downhill sections had to be ridden with the brakes on. To cater for possible future changes to a course measurements were taken at most junctions and several at roundabouts

Lengthy mathematical calculations both on the road and later at home were needed to convert the revs to miles. I really appreciated the arrival of the hand held calculator towards the end of my time doing this :)

I suspect that provided a measured mile is available this is still the most accurate way to measure distances

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deliquium
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Re: How to calculate metres travelled per crank revolution?

Postby deliquium » 27 Mar 2019, 8:24am

531colin wrote:
deliquium wrote:….....GPS positioning isn't near enough...........

I'm surprised by this. With the 2 1/2" map on my phone, I reckon its accurate down to a metre or two.....as in which side of the hedge the bridlepath is.


If mine was that accurate I'd be very pleased. My iPhone SE GPS tends to jump around by +10s of metres. Even after letting it "settle down" it can still be off, as it is right now, by some 8 metres. Perhaps it's a heavily tree covered Snowdonia mountain thing?

And ten meters in both directions of a moss grown stone wall hiding an OS benchmark, or worse an OS rivet, can be tedious and even dangerous if one is hands on knees on a busy road :shock:
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Mick F
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Re: How to calculate metres travelled per crank revolution?

Postby Mick F » 27 Mar 2019, 8:42am

531colin wrote:
deliquium wrote:….....GPS positioning isn't near enough...........

I'm surprised by this. With the 2 1/2" map on my phone, I reckon its accurate down to a metre or two.....as in which side of the hedge the bridlepath is.

Me too.
My Garmin Montana will get down to 7ft accuracy.
(Well, it did do before I sent it back to Garmin UK.)
Mick F. Cornwall

Brucey
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Re: How to calculate metres travelled per crank revolution?

Postby Brucey » 27 Mar 2019, 10:09am

GPS units vary in accuracy depending on how many satellites they 'see' and how clever the software is.

I'm not au fait with the capabilities of current units but there was talk of new generations of chips being able to pick up GPS, Galileo, and Glonass signals, therefore providing civilian GPS functionality with much higher accuracy than any (civilian version) of each system could provide by itself.

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Mick F
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Re: How to calculate metres travelled per crank revolution?

Postby Mick F » 27 Mar 2019, 10:24am

Brucey wrote:GPS units vary in accuracy depending on how many satellites they 'see' and how clever the software is.
Yep.

Montana will do GPS and Glonass. Let it settle, and it'll get down to 7ft.
Near high buildings is an issue of course. Also, whizzing along on a bike under tress isn't so good as it doesn't have time to settle.
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Mick F
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Re: How to calculate metres travelled per crank revolution?

Postby Mick F » 27 Mar 2019, 10:30am

Compare these two. Same ride, one under the trees, and the other in the open.
Screen Shot 2019-03-27 at 10.26.27.png
Screen Shot 2019-03-27 at 10.28.00.png
Mick F. Cornwall

Brucey
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Re: How to calculate metres travelled per crank revolution?

Postby Brucey » 27 Mar 2019, 10:49am

unless you had a liquid lunch.....?.... :lol: :wink:

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geomannie
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Re: How to calculate metres travelled per crank revolution?

Postby geomannie » 27 Mar 2019, 11:39am

Why not work it out for your specific bike?

    Measure the circumference of your rear tyre (sit on bike and roll for one complete revolution, measure distance travelled. Repeat for more accuracy).

    Work out your gearing ratio(s) i.e chain ring teeth dived by chosen cassette ring teeth

    Multipy to get distance per crank revolution.
geomannie