cycle computer question

I’ve just bought a (cheap) cycle computer for my 29er. However, the biggest size on the wheel size chart is 700x40c whereas my tyre says 700x52. Will this make any difference?
Thanks.
Cathy

The chart might be just a quick guide to common wheel sizes; there may also be a way for you to enter the circumference or diameter directly. I measure the circumference by riding across a thin line of water and measuring the distance between it and the damp dot tracked by the tire after one revolution. I usually add just a couple of mm to at least partially account for the slight S-curve in the track. It’s been a while since I actually had my computer mounted on my uni though.

read the manual on it, it will probably say somthing about how you can type in your wheel size if its not on the chart. Mine did.

Re: cycle computer question

On Sun, 5 Mar, cathwood <> wrote:
>
> I’ve just bought a (cheap) cycle computer for my 29er. However, the
> biggest size on the wheel size chart is 700x40c whereas my tyre says
> 700x52. Will this make any difference?

Not much (theoretically, about 36mm per revolution), and you
shouldn’t trust the chart anyway.

The calibration is normally wheel circumference in mm (though might be
cm, or indeed any other unit). The instructions should mention this
somewhere, in which case the easiest thing to do is dab a blob of
paint on the tyre, ride in a straight line (or walk the uni in a
straight line pressing down on teh seat), and measure the distance
between the blobs on the ground.

If the instructions don’t tell you what the calibration value is, if
you tell what it says for 700x40, I can probably guess…

regards, Ian SMith

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Thanks.
The piece of paper (rather than manual) states that the settings value for 700x40c is 2224. It doesn’t say whether this is a measurement though.

Cathy

Re: cycle computer question

On Sun, 5 Mar, cathwood <> wrote:
>
> Thanks.
> The piece of paper (rather than manual) states that the settings value
> for 700x40c is 2224. It doesn’t say whether this is a measurement
> though.

In that case it’s circumference, measured in millimeters, but that
value is a little bit high.

So, blob of paint on the tyre, lean on the saddle as hard as you can
and roll it in a straight line. Measure the distance from the middle
of one mark on the road to middle of the next in millimetres and enter
that as the value.

The answer you’re heading for is something like 2250.

Does the tyre have markings like ‘47-622’? If so, the THEORETICAL
answer is (622 + (2 x 47)) * pi = 716 x 3.14159 = 2249. All 700c are
622, the 47 depends on the width or the depth of the tyre (depending
upon whether it’s adopting metric or american numbering). You can do
the calculation and substitute for 47 depending upon what’s on your
tyre. However, that doesn’t allow for the fact that the tyre squashes
a bit. Paint and measuring is better.

regards, Ian SMith

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Thanks very much Ian Smith.
Paint it shall be then.

Cathy