Sizes and cycle computors

viper

New Member
Hi

I have just bought a MTB and fitted a Topeak cycle computor to it ,

Setting the cycle computor was realitivley easy but the trouble came with the wheel size which is needed to programe it,

These are the sizes on the tyre ,

57-559 .......26 X 2.10 which i cant find any feference to on the net for circumference.

The data on the sheet supplied reads 57-559....26 X 2.215 circ 2133mm

I roughly measured the Dia at 660mm X 3.14 = 2072 circ

Confused i am , can anyone help please . It may not make much difference one way or another , but i am always thinking it is lying to me .

I have it set at the 2072 circ

Dave.

Grasen

New Member
My way of doing it

mark the tire
roll the bike so the tire does one rotation
measure the distance in mm
put that number in the computer

TheDoctor

Europe Endless
Moderator
That's the best way of doing it. Or, you can add the twice the width of the tyre to the tyre bead diameter and multiply that by pi.
(57+57+559) * 3.14159 = 2114.
This assumes that the tyre is as wide as it is high, which isn't necessarily so, but it's not far off.

alasdairgf

New Member
I wanted to get this a bit more accurate (no reason really, just for something to do! also I know what you mean about not trusting the numbers you're shown...) and take into account the change in the perimeter of the tyre when being ridden.

I programmed in the recommended mm circ for my wheel size and rode for a measured five miles (this is where I actually messed it up as I couldn't find a measured mile anywhere, so took an average distance from three online mapping tools). Then I just used a bit of rusty high-school algebra to figure out the ratio, as (recommended circumference / distance measured by computer) should equal (actual circumference / actual distance measured on map or by measured mile). After my 5-mile, I just calculated: actual circumference = distance on map x (recommended circ / distance measured).

Was surprised to see that my end result was only about 10mm out from the recommended circumference, but at least I now trust the figures (a bit).

MJN

New Member
Hi Viper,

If you want more than a rough approximation then definitely ignore the sidewall markings as not only are some of the figures notional but none of them cover the outer diameter of the tyre. The instruction manual will include a reference table for them as an it'll do start.

Grasens method is spot-on and dead simple. For added acuracy put your weight on the rolling bike (as it will reduce the effective circumference) and roll the wheel several times in one length and divide by the number of revolutions (this will reduce the relative error of measurement).

I'm not at all convinced by Alasdair's method (the online map measurement is an approximation at best in this context) but I've no objection to tackling such issues with the appliance of science so I'm not knocking it!

Mathew

OP

viper

New Member
Hi all

Used the " mark the tyre and roll it along " every time ending at the same mark , measured at 2061 mm, so pretty confident i will be

getting a true and accurate reading now .

Once again Thanks guys.

Dave.