# Computer calibration



## Panter (26 Sep 2007)

Afternoon all 

Took the new bike for a quick spin, but I think the computer calibration is out.

Anyone know what the diameter of a 700 X 26C is? I can't find that measurement on any table, and I'm finding it really hard to measure it what with the angle of the spokes, lack of 15 pairs of hands etc.

I tried the string on the floor trick but keep getting varying answers.

any ideas appreciated


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## Oldlegs (26 Sep 2007)

Are you sure it works on diameter ? All the computers I have use circumferance (roll out length).

These are 2,105 for 700x25 and 2,096 for 700x23 so presumably a 26 is about 2,116 giving a diameter of 673.


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## Panter (26 Sep 2007)

Thanks Oldlegs, yes you're quite right. I was going to multiply the diameter by 3.14 but was typing in a rush!

My chart gives:

700 X 23C 2133

700 X 25C 2146

700 X 28C 2147

So I took a guess of 2147.

I've since measured it by rolling the tyre along a tape measure which gives ~ 2120 so I'll go with your 2116.

I don't know why my chart is so far out though, still the speedo was definately being optomistic so I'll try it with your value


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## killiekosmos (26 Sep 2007)

I would go by what you measured on one revolution. It is worth noting that the difference between your measure and the table is 26mm or around 1% of the circumference. So this would not make a big difference to your speed measurement. For example if your speedo reads 15mph it could be out by 0.15mph - no big deal. (Cars get away with 10% tolerance)


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## gbb (26 Sep 2007)

Place you wheel so the valve is at the bottom (exactly)
Mark the floor.
Roll the bike forward till the valve has done exactly one revolution and the valves exactly at the bottom again
Make a new mark and measure between the two.

Thats your setting


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## Panter (26 Sep 2007)

> Place you wheel so the valve is at the bottom (exactly)
> Mark the floor.
> Roll the bike forward till the valve has done exactly one revolution and the valves exactly at the bottom again
> Make a new mark and measure between the two.
> ...


 That's exactly what I did  I went with Oldlegs guestimate as I would imagine its pretty close and was only 4mm difference to what I measured. Once my carcasse is on the bike I should think the diameter will squidge doen by more than 4mm anyway.




> I would go by what you measured on one revolution. It is worth noting that the difference between your measure and the table is 26mm or around 1% of the circumference. So this would not make a big difference to your speed measurement. For example if your speedo reads 15mph it could be out by 0.15mph - no big deal. (Cars get away with 10% tolerance)



Hmmm, thanks for that, I hadn't looked at the numbers really. That means there could be something a bit more sinister going on then, or a faulty computer at worst really, hardly sinister I 'spose but a bloody nuisance if it is.
I'm just a bit sceptical of its claim I hit 20mph while having a very leisurely 1st cruise on it, and the distance I went on seemed very optomistic.
I'll have a good spin on it at the weekend and get a better idea then.


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## Amanda P (27 Sep 2007)

Check whether it's reading in kilometres or miles. Maybe you only hit 20 kmh, not 20 mph.
Sounds silly, but...


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## Panter (27 Sep 2007)

Hmmm, I'm damn sure it was miles but it would give about the error I was experiencing if it was in kilometres. Maybe that was it.

Thanks for the thought, I'm riding it to work tomorrow so I'll get a good idea then. Its definately miles at the moment but I did a full reset after Yesterdays run to enter the measurement.


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## frog (27 Sep 2007)

Never had any accuracy with marking the floor. I use a tape measure around the tyre. It's very fiddley but accurate to a couple of mm's


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## Frazer (27 Sep 2007)

Sorry for aksing a stupid question...but doesn't a 700x23 have the same diameter as say a 700x28??? So the calibration should be the same....or am I just being thick?


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## Panter (27 Sep 2007)

I thought that, but I guess its down to the wider tyre also protruding further from the rim.

As I say though, thats my guess.

I wait to be corrected


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## Edge (27 Sep 2007)

http://sheldonbrown.com/cyclecomputer-calibration.html#ccc


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## Panter (28 Sep 2007)

Ok, did the commute this morning.

Mileage according to the car : 17.0 miles

" " " bike : 17.2 miles 

So thanks for the help peeps.

Nice link Edge, I didn't find that by Googling, typically enough.


Uncle Phil, I fear you may have been right. It must have jumped to Kmh as I tried to save the settings.


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## Amanda P (28 Sep 2007)

Sorry to be so annoyingly correct...

17 mile commute? Each way? Respect.


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## Panter (28 Sep 2007)

Uncle Phil said:


> Sorry to be so annoyingly correct...
> 
> quote]
> 
> ...


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