Comments | skean: Help, Bicycle Repair Man!

skean

Help, Bicycle Repair Man!

Jan 28, 2008 10:53

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- ( Read more... )

polar hrm, bike

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andygates January 28 2008, 11:17:40 UTC

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 January 28 2008, 11:49:51 UTC

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 January 28 2008, 13:16:25 UTC

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 January 28 2008, 13:31:12 UTC

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.

andygates January 28 2008, 14:01:59 UTC

I would (of course) agree, but you're the kind of guy who would make and sell the computer. I'm the the mook who'd buy it ;)

andygates January 28 2008, 14:07:12 UTC

And I'm the kind who can plausibly muck up a ride out test (which the HRM will accomodate BTW) so want to do it with a number someone gives me :-)

andygates January 28 2008, 14:17:20 UTC

They're really really easy. Just run the wheel along a line on your floor. Big jessie!

skean January 28 2008, 14:56:09 UTC

I once tried to de-ice the fridge. I called the fire brigade because of the freon gas outburst. And then bought a new fridge.

andygates January 28 2008, 16:10:21 UTC

Do make sure that photos of you doing the "I'm confused!" face are put online, there's a good chap :)

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