Measuring a tyre circumference for the computer.

[bpsmith]

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.)

My tyres only flatten at the bottom, so it's only a small part of the circumference that would be inaccurate...

 

[hobo]

My tyres only flatten at the bottom, so it's only a small part of the circumference that would be inaccurate...

The difference is the diameter of the tyre x pi minus the diameter loaded x pi.
Also true distance traveled doesn'ttake into account the road surface imperfections

 

[deptfordmarmoset]

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).

Also.....the front wheel will travel further than the rear wheel because of the meandering caused by making balancing corrections. (You can check this by cycling through puddles and tracing the tracks back to the wheels.) So, while the rear wheel ''cuts corners'' and the front wheel wobbles, which is the distance we're measuring? Somewhere in between?

 

[bpsmith]

The difference is the diameter of the tyre x pi minus the diameter loaded x pi.
Also true distance traveled doesn'ttake into account the road surface imperfections

Think about it..."It's only flat on the bottom".

I was joking.

 

[Ajax Bay]

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.

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?

My tyres only flatten at the bottom, so it's only a small part of the circumference that would be inaccurate...

(Note: he was jesting)

The difference is the diameter of the tyre x pi minus the diameter loaded x pi.

Simplistic - does the diameter of the tyre change under load?
NB - for fun.
100lbs load on tyre. Pressure=100psi. Approx one square inch contact when loaded. Width of ellipse (contact area) 20mm, so length of ellipse roughly 40mm. Difference between chord (the tyre is flattened) and arc (the tyre is circular) is 0.02mm. The overall circumference of the tyre is (say) 2120mm (making the assumption that this does not change). This is one thousandth of a per cent difference.
Conclusion: Differences to the effective circumference of a tyred wheel caused by the tyre deflection under load is negligible, for cyclocomputer (and probably all) purposes.

 

[snorri]

No idea what you're talking about.

Sorry, it was a weak joke.

 

[Tim Hall]

No idea what you're talking about.

Yeah right. I bet you put two magnets on the wheel "just to balance it". The 100% increase in indicated speed is just a happy by product.

 

[smutchin]

I'm admitting nothing.

 

[jefmcg]

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.

Why? Your speedo - unless you just input some random circumference - will be much more accurate in how far you actually travelled than dropping points on a map and letting an algorithm join them up.

 

[youngoldbloke]

To measure the diameter accurately I take the wheel off, support it vertically (against a table, using a spirit level) on a truly level floor. I then use the level placed on the highest point of the tyre, and transfer this distance to a piece of paper taped to the table leg. Measure from this mark to the floor.Then x 3.142 for circumference. I have a number of 700c wheelsets, carrying different tyres, and they are all different diameters. Then calibrate the computer/s (Sigma) as closely as possible.

 

[presta]

Simplistic - does the diameter of the tyre change under load?
NB - for fun.
100lbs load on tyre. Pressure=100psi. Approx one square inch contact when loaded. Width of ellipse (contact area) 20mm, so length of ellipse roughly 40mm. Difference between chord (the tyre is flattened) and arc (the tyre is circular) is 0.02mm. The overall circumference of the tyre is (say) 2120mm (making the assumption that this does not change). This is one thousandth of a per cent difference.
Conclusion: Differences to the effective circumference of a tyred wheel caused by the tyre deflection under load is negligible, for cyclocomputer (and probably all) purposes.

The circumference around the part of the tyre not in contact with the road isn't relevant because the tyre compresses as it passes across the road surface. What you need is the actual radius of the wheel at the point of contact with the road thus:

Taking your chord length of 40mm, and an uncompressed radius of 350mm, the compressed radius is sqrt(350^2-(40/2)^2) = 349.43

1 - 349.43/350 = 0.16%

 

[jefmcg]

If you want the most accurate measurements, measure out say 300m (A). Cycle it. See how far your computer says it was (C).

Then divide A by C, and that gives a multiplier M: M=A/C. From then on you just have to multiply the record distance by M to get the correct distance.

Easy and totally worth doing

 

[presta]

General road signs (whether erected by the 'council' or the Highways Authority) giving mileage are not "never accurate" they're just occasionally inaccurate.

I emailed the DoT to ask how accurate the kilometre posts are. They never replied.

 

[Ajax Bay]

By "plot" I guess you mean "upload a GPX track and see how far Strava/RWGPS/ANOther site/local software thinks you have gone."
I'm a member of a number of challenges, including the CC monthly ones. If I want my ride to qualify for one of these, I always take care to exceed the target distance by at least a couple of kms as insurance

Esteemed Dt. I use a GPS in tandem with a cyclocomputer but I do not "upload a GPX track" (how could that be 'plot'?) During planning before (maybe) and then edited afterwards I plot the route I've taken on RwGPS. It invariably matches (within tolerances) the distance recorded on my GPS.
As you know (Dt) we both pursue these little 'challenge' indulgences (but I have yet to step up to RRTY). I, too, always go the extra mile or two, for, as you put it, "insurance". But what is your 'insurance' if your GPS goes down? No GPX file. No accurate idea of how far you've cycled? Do an extra half hour in case, perhaps?
October's looking dodgy, for reasons well covered on the relevant 'chat' thread.

 

[Ajax Bay]

"This is one thousandth of a per cent difference." My maths on this has been niggling at me (more off than on) for a few hours - it seemed much too low.

0.16%

Thank you for getting it right. Negligible, then.