Tyre Circumference

[Road_Runner]

I personally just use the Pi button found on 95+% of calculators and calculator programs/apps. That way i get a 0% rounding error.

As for measuring the circumference of the wheel, i use the sit on the bike and roll it method. You can measure the diameter and calculate it but the tyre is under little load and won't be compressed giving a different diameter. For example, if you calculate the circumference of a 700x23C wheel, the circumference will calculate to be approximately 2125mm. Using the same wheel, if you use the sit on and roll method, you'll find the measurement is approximately 2096mm. The two measurements are out by approximately 1.4%, which is far more significant than a Pi rounding error.

Let's be honest, the readings your computer gives will never be 100% correct because of the various factors like, inflation, tyre wear, load, scrubbing during cornering. Not to mention the various manufacturing tolerances and how accurate your computer is.

So i personally would just use the table table shown by @vernon, because the tyre circumference that i ride with seems pretty accurate to the table within a couple of millimetres. A 2mm difference would give me an error of 0.095%.

 

[Kestevan]

Buy a gps computer.

 

[Nigelnaturist]

Buy a gps computer.

less accurate.

 

[Road_Runner]

less accurate.

I would agree with @Nigelnaturist because GPS units only poll every x ms/s. Within that time, you would have moved and if it's not in a straight line, it's got an error involved. Secondly, the accuracy of GPS units are weather dependent.

 

[compo]

I would agree with @Nigelnaturist because GPS units only poll every x ms/s. Within that time, you would have moved and if it's not in a straight line, it's got an error involved. Secondly, the accuracy of GPS units are weather dependent.

I also read that GPS doesn't take into account hills or climbs on a route but just measures as if everywhere was flat. I don't know how much difference this would make over a longish ride.

 

[cardiac case]

^^^^^

It's a well known fact that it's further up a hill than it is down it.

Paul G

 

[Road_Runner]

I also read that GPS doesn't take into account hills or climbs on a route but just measures as if everywhere was flat. I don't know how much difference this would make over a longish ride.

I would say that is probably dependent on the GPS unit in question. GPS units inherently place you on a 3 dimensional grid with how they operate. So i imagine that it's all down to the GPS unit and how it sums the vectors together to calculate the total distance.

 

[Cyclopathic]

mark the bottom of the tyre and a flat floor with chalk then wheel the bike forward until the chalk mark is at the bottom again and mark the floor at that point then just measure with a tape.

This is what I do when setting one up. The actual figure always differs from the recommendations so is more accurate imo.

 

[morrisman]

I personally just use the Pi button found on 95+% of calculators and calculator programs/apps. That way i get a 0% rounding error.

<pedantry>
The calculator will still be using an approximation to pi, so there will still be a rounding error, albeit probably quite small.
</pedantry>

 

[youngoldbloke]

Will this do for everyday?
3.1415926535897932384626433832795028841971693993751058209749445923078164062862089986280348253421170679
8214808651328230664709384460955058223172535940812848111745028410270193852110555964462294895493038196
4428810975665933446128475648233786783165271201909145648566923460348610454326648213393607260249141273
724587006606315588174881520920962829254091715364367892590360011330530548820466521384146951941511609..
Reckon you have problems measuring the distance between chalk marks anyway - or knowing when your mark on the tyre is actually on the ground. Is there a formula which gives you the loaded deflection of the front tyre, if measured unloaded. Does it matter?

 

[Road_Runner]

<pedantry>
The calculator will still be using an approximation to pi, so there will still be a rounding error, albeit probably quite small.
</pedantry>

True. At what point does someone round down to 0% error? . At what point does pedantic pi behaviour becomes far more insignificant than measurement errors? Excel - 14 decimal places. Windows Calc - 32 decimal places. Android inbuilt calc - 10 decimal places.

....
Reckon you have problems measuring the distance between chalk marks anyway - or knowing when your mark on the tyre is actually on the ground. Is there a formula which gives you the loaded deflection of the front tyre, if measured unloaded. Does it matter?

I agree. This is why i stated that my measurement was approximately correct to the table within a couple of millimetres. As i struggled to spot when the wheel had completed an exact rotation. I used one of the rubber sprues on the side wall against a tape measure - but this measurement technique has it's own issues but i think it's the most accurate out there for us mere mortals.

With the formula - i doubt it because it would depend on side wall thickness and the exact compound of the rubber (as well as other factors), which i doubt could be measured accurately enough and/or be given by a manufacturer. And tyres like everything else is manufactured within tolerances.

I don't think it matters that much. In my opinion, so long as it's within a few millimetres, it's good enough. During a ride there are various conditions that will alter the measurement anyway. Traction between wheel and floor, surface of the floor, gradient of floor, weight distribution, weight changes (sweat, rain, atmospheric changes, fluid consumption), tyre wear, cornering (Does the usable diameter of the wheel change whilst cornering? - I want to say it does).