Measuring a tyre circumference for the computer.

[Racing roadkill]

The problem I always had with pre-GPS computers was that the tyre circumference settings go up in increments of 2mm or 5mm, or even 10mm in some cases, so regardless of the accuracy of your measurement, you couldn't enter the precise figure into the damn thing anyway.

Very true.

 

[Ajax Bay]

The best approach to this is to use a chart (for your wheel and tyre) and plug that in. Record that figure (213 say, my Cateye only does cm increments). Do a decent ride (without a stop ideally) with the GPS set up as well, starting/zeroing both computers at the start. If you take the GPS distance figure as gospel (echoing @ianrauk ) then work out whether the figure you've plugged in needs to be altered, and if so, reset the computer and enter the new figure. Record it. Ride.
Some may not consider accurate distance display important but, (as partly mentioned upthread) those who do audaxes and use the routesheet, value accurate distances. And a wheel sensor based computer is a good back up if the GPS dies or otherwise misbehaves. You will be very lucky to get it spot on, but if your cyclocomputer takes 4 figures (ie to the nearest mm) you should be able to get the readings agreeing within 0.2%, or even 0.1% with several iterations.
After 100km, my GPS only displays whole kilometres, so I appreciate the additional significant figures the Cateye gives me. Quite often it's useful to have an idea of tenths of a km. Mostly I use the readings as differentials, ie the difference between two readings when approaching a turning from a previous 'known point'.

 

[spen666]

... So how many of us, rather than rolling the tyre between two points using the valve as the 'marker' measure the overall diameter of said tyre and simply multiply by 3.142 - ?

That is just so inaccurate.. I prefer to use a far more precise value for pi. I never use less than pi to 1000 digits for my calculations

3.1415926535 8979323846 2643383279 5028841971 6939937510 5820974944 5923078164 0628620899 8628034825 3421170679 8214808651 3282306647 0938446095 5058223172 5359408128 4811174502 8410270193 8521105559 6446229489 5493038196 4428810975 6659334461 2847564823 3786783165 2712019091 4564856692 3460348610 4543266482 1339360726 0249141273 7245870066 0631558817 4881520920 9628292540 9171536436 7892590360 0113305305 4882046652 1384146951 9415116094 3305727036 5759591953 0921861173 8193261179 3105118548 0744623799 6274956735 1885752724 8912279381 8301194912 9833673362 4406566430 8602139494 6395224737 1907021798 6094370277 0539217176 2931767523 8467481846 7669405132 0005681271 4526356082 7785771342 7577896091 7363717872 1468440901 2249534301 4654958537 1050792279 6892589235 4201995611 2129021960 8640344181 5981362977 4771309960 5187072113 4999999837 2978049951 0597317328 1609631859 5024459455 3469083026 4252230825 3344685035 2619311881 7101000313 7838752886 5875332083 8142061717 7669147303 5982534904 2875546873 1159562863 8823537875 9375195778 1857780532 1712268066 1300192787 6611195909 2164201989

 

[snorri]

Neither, I checked mine using the kilometre posts at the side of the road.

Some of us would have to cycle more than 1,000km to find kilometre posts!
It was an exciting distraction whilst cycling the 30+km straight of the Afsluitdijk to watch the Cateye recording the passing of every distance marker .
https://www.holland.com/global/tourism/destinations/provinces/north-holland/the-afsluitdijk.htm

 

[robjh]

The circumference charts on Cateye instructions are accurate. You can confirm this by putting a blob of paint, Tippex or something like mustard on a tyre and rolling it then measuring.

My cycling buddy and I use the chart and on rides we are usually within 0.05 miles in our measurement, which is also usually within 0.05 miles of the measurement on Bikehike.

I've found the Cateye standardised circumferences to be slightly too low on my road bikes, by around 0.4% to 0.5%*, compared to rolling the tyre along a straight line on the floor and measuring by where the valve passes the floor. It's not surprising that some tyres should be slightly different shapes to others, so a standard size is unlikely to fit all cases.

* or out by 1 mile in every 200. Some might consider that accurate enough.

 

[Tim Hall]

I've found the Cateye standardised circumferences to be slightly too low on my road bikes, by around 0.4% to 0.5%*, compared to rolling the tyre along a straight line on the floor and measuring by where the valve passes the floor. It's not surprising that some tyres should be slightly different shapes to others, so a standard size is unlikely to fit all cases.

* or out by 1 mile in every 200. Some might consider that accurate enough.

With a tyre circumference of around 200cm and a resolution on the Cateye (the ones I've had in the past at least) of 1cm, that's as good as you're going to get anyway.

 

[smutchin]

Anyone worried about not racking up enough miles on their ride log can always "round up" the tyre circumference setting on their computer.

 

[presta]

Some of us would have to cycle more than 1,000km to find kilometre posts!

It's handy that the A-road at the top of the estate where I live has them. I don't know what qualifies an A-road to have them, at one time they were only on motorways.

I've found the Cateye standardised circumferences to be slightly too low on my road bikes, by around 0.4% to 0.5%*, compared to rolling the tyre along a straight line on the floor and measuring by where the valve passes the floor.

Your weight flattens the tyre when you sit on the bike, so it's rolling along the floor that gives a measurement that's too high, unless you sit on the bike to do it. (Which is why I used the posts at the side of the road.)

 

[snorri]

Anyone worried about not racking up enough miles on their ride log can always "round up" the tyre circumference setting on their computer.

Are you related to that Armstrong chappie?

 

[ianrauk]

If you take the GPS distance figure as gospel (echoing @ianrauk )

That's an unfair misquote Alex. I did not say that using a gps is gospel rather then saying isn't that what most people use these days.

 

[smutchin]

Are you related to that Armstrong chappie?

No idea what you're talking about.

 

[jefmcg]

The problem I always had with pre-GPS computers was that the tyre circumference settings go up in increments of 2mm or 5mm, or even 10mm in some cases, so regardless of the accuracy of your measurement, you couldn't enter the precise figure into the damn thing anyway.

Your tyres are roughly 2 metres circumference, so 2.5mm out is 0.125% error - that's about 200m over 100 miles.

And (thank you google, NYT and wikipedia) is about the accuracy of Jones counter which is used to measure olympic marathon courses where records are set. I think it's good enough for you.

 

[tyred]

Do the people who measure their wheel circumference take into account that it will change slightly when you sit on the bike depending on your weight, tyre construction, pressure and other variables?

I just use the values that you find on the chart that comes with the cycle computer. It can't be 100% accurate as there are variations between tyres of the same size from different manufacturers but I'm sure it's not hugely out and does it really matter if I think I've cycled 10 miles when I have actually done 9 3/4 miles or 10 1/2 miles? The road signs put up by the council are never that accurate either, nor is the speedometer in my car. A rough idea of what speed I'm doing or how far I've gone is perfectly good enough for my needs (and I suspect the needs of most people).

 

[Ajax Bay]

That's an unfair misquote Alex. I did not say that using a gps is gospel rather then saying isn't that what most people use these days.

I agree - most riders do, but this thread is about trying to set cyclocomputers.
I said "If you take the GPS distance figure as gospel (echoing @ianrauk )"
Ian - was meant to acknowledge your early post - I didn't 'quote' you; I 'echoed' you - apologies for generating a feeling of 'unfairness'. I was trying to say that a GPS distance reading can be taken as accurate (steady road, no fiddling around, no stopping) and that can be used to calibrate a cyclocomputer (in the manner I described).

 

[Ajax Bay]

does it really matter if I think I've cycled 10 miles when I have actually done 9 3/4 miles or 10 1/2 miles? The road signs put up by the council are never that accurate either, nor is the speedometer in my car.

The error of 1/4 mile in 10 is significant (2.5%). I tried to explain (above) why sometimes measuring distances accurately does matter. If you're trying to ride 100 miles, and you get home, plot your route and find that you only did 98, some people (me included) would be p**sed off. And besides, most people would like little screens to be reasonably accurate, otherwise why have the display there at all? YMMV.
General road signs (whether erected by the 'council' or the Highways Authority) giving mileage are not "never accurate" they're just occasionally inaccurate. If you wish to know how accurate your car speedometer is, the best way is to time how long it takes to pass 10 0.1km posts on a motorway (the distance between each post is (+ or - 1% but that error averages out to zero) 100m), keeping your speed at exactly 60mph by the display, and then do the math. NB Every tenth on has a '0' below the longer number. If it takes 37 seconds, your speedo is close to bang on. Less and it's under-reading (not recommended); longer and it's over-reading (useful as an explanation when young but increasingly bright children observe that you're 'speeding'.