How to calculate metres travelled per crank revolution?

[pyruse]

Surely by far the simplest way to calculate it is:
1. Find an empty car park
2. Put a chalk mark beside ground contact point of rear tyre
3. Pedal forward one revolution of the cranks.
4. Put another chalk mark where the bike is now
5. Measure the distance between the marks.
6. Repeat for different gears.

Much simpler than trying to calculate it, and probably more accurate.
I also find this is the best way to calibrate a cycle computer. Just make marks and roll the wheel one complete revolution forward.

[Brucey]

that is roughly the method used in 'medium gear' events. It is not at all accurate, primarily because it is (without a spirit level on the cranks or similar) almost impossible to accurately judge exactly one complete turn of the cranks. It also involves a lot of very tedious (and pointless) measurement if you propose to use this technique for every gear combination.

It is by contrast much easier to measure exactly one wheel revolution accurately, and to multiply by the gear ratio.

cheers

[BigG]

A truly accurate measurement is difficult by the system you suggest as your wheel diameter can only be approximated. Its true rolling diameter depends not only on the size of the tyre but also on its pressure, your weight (or the proportion of this that is carried on the back wheel) and the amount the tyre is compressed when you are riding. It is probably accurate enough to add twice the tyre width to the 622 mm rim diameter and then subtract about 3 mm for local flattening. This gives a total of about 2.1 m with a 25 mm tyre and any figure using an additional decimal place is unlikely to give a better result. Using a 28 mm tyre only increases the distance per wheel revolution by about 0.75%. The difference caused by riding in a slightly wobbly line compared with a ruler-straight one are nearly as great as this.

[Mick F]

Also, when climbing a hill, the rear tyre deforms and the effective circumference is reduced a tad more.

When I had my Garmin 705, you could display the effective rear wheel circumference using the Speed/Cadence device information. It measured the rear wheel revs per distance travelled and you could see the readout.

Checking it on the flat or descending, it was a higher figure than when climbing a hill.
I have/had a thread on this very subject some years ago.

[Brucey]

fwiw the tyre construction affects how much the tyre rolls out differently as well as the pressure. For example a radial tyre construction more nearly rolls out the same length regardless of tyre pressure; the tread belt behaves a bit like a tank track, whereas with a cross-ply construction the tread shortens when the tyre is loaded.

Also note that under traction, the tyre slips a bit more; this reduces its rollout length (effective diameter) too. It'd be interesting to instrument both rear wheels on a one-wheel drive trike and see how much difference there is; apart from traction the loads on each tyre ought to be the same.

cheers

[deliquium]

It's distances up to a max of 200 metres I'm trying to reasonably measure - cos parking the bike on sometimes dangerous fast (aren't they always to us no car cyclsists), bendy narrow main roads and then pacing out an approximation could be dangerous

Yesterday I consulted an OS 1913 6" to the mile map (largest I can find for here in Nantgwynant). It indicated a cut benchmark less than 400 yards from my home. It is not listed in the most recent OS list of benchmarks from the 1970s.

But with the help of a trowel and the excellent resource https://maps.nls.uk/geo/explore/#zoom=1 ... yers=6&b=1

It indicated the benchmark was approx 27metres along my track from the centre of an adjoining public footpath. So I paced 27 large 5'8" tall human steps and started scraping the moss off the very delapidated trackside low wall, stone by stone - looking especially for large and flat enough typical stones where BMs usually lurk.

Bingo! Uncovered after gawd knows how many years? The horizontal (almost) mark is where the 'bench' would have been 'leaned into', then levelled and compared with the previous BM. This one is listed as 224.2 feet (NB mix of Imperial and decimal) above the mean sea level based on Newlyn Harbour (that's in Cornwall for the English and foreigners who might be reading this

[RickH]

It's distances up to a max of 200 metres I'm trying to reasonably measure - cos parking the bike on sometimes dangerous fast (aren't they always to us no car cyclsists), bendy narrow main roads and then pacing out an approximation could be dangerous

Yesterday I consulted an OS 1913 6" to the mile map (largest I can find for here in Nantgwynant). It indicated a cut benchmark less than 400 yards from my home. It is not listed in the most recent OS list of benchmarks from the 1970s.

But with the help of a trowel and the excellent resource https://maps.nls.uk/geo/explore/#zoom=1 ... yers=6&b=1

It indicated the benchmark was approx 27metres along my track from the centre of an adjoining public footpath. So I paced 27 large 5'8" human steps and started scraping the moss off the very delapidated trackside low wall, stone by stone - looking especially for large and flat enough typical stones where BMs usually lurk.

Bingo! Uncovered after gawd knows how many years? The horizontal (almost) mark is where the 'bench' would have been 'leaned into', then levelled and compared with the previous BM. This one is listed as 224.2 feet (NB mix of Imperial and decimal) above the mean sea level based on Newlyn Harbour (that's in Cornwall for the English and foreigners who might be reading this

Well done!

Would the info listed under "What is the exact circumference of my tire?" (sic) on this page of Schwalbe's be close enough for you?

[mercalia]

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.

cheers

also the "gps" on a mobile phone may not be pure gps but wifi and cell mobile tower assisted that aint too accurate