Help, Bicycle Repair Man!

[skean]

Bike gurus, I has query. I have a Polar S-Series Speed Sensor for my Polar S625x HRM. Works great with my road bike.

Trying to fit it to my MTB for winter usage, I am coming unstuck. I have to set a wheel size to enable it to do its funky stuff. Now, my MTB wheel says "ETRTO 17-559". I have a chart with the manual that starts at something like 30-

Comments

[ravenbait]

559 means 26" so that's easy.

http://sheldonbrown.com/tire-sizing.html#iso

[ravenbait]

And look at the tyre wall, not the wheel. I know that might seem a silly thing to say, but it's a basic but plausible error.

[skean]

OK, now THAT I haven't done :-? I'll check when I get home...and then perhaps quietly delete this post ;-)

[skean]

Got that far, but Sheldon didn't help me on the "17" bit. Sadly the 26" is not enough to set it up

[andygates]

RB's right about looking at the tyre rather than the wheel - a fat tyre has a larger radius. 30x559 sounds credible for a fairly average MTB tyre.

I'm guessing that this computer doesn't allow you to do a rollout test?

[gedhrel]

Yeah. Presumably what's going on is that the rim circumference is the lower bound (obviously) of distance travelled per rev (assuming you cycle in a straight line, but if you're estimating your work done it doesn't matter much if you wobble, you'll get a lower bound again).

Tyre depth will increase the circumference. It's slightly complicated due to tyre deformation, but if you put up a sample of tyre types and the computer setting you use for that it might well be possible to fit something approximate.

[andygates]

Tyre deformation is why a rollout test is always best. Stick your bum on the bike and have a helpful minion measure one wheel-rotation along the floor (use the valve at the bottom for a reference).

[gedhrel]

Heh, you realise that exchange has neatly summarised us? I'm looking to fit a curve, you suggest riding the bike and measuring the distance :-)

Your way is by far the better.