Computer calibration for different size wheels?

[Sittingduck]

Hi All

Just got myself a Specialized Globe 08 hybrid with 700c wheels
Took the computer off my old mountain bike (obviously 26") but now it's telling me that the trip to work is 4.97 miles or something when before it was 5.13. Same on the trip home... 5.96 vs the old 6.2 or whatever it was.

This got me thinking that it must be to do with the distance from the hub of the front wheel that I have the sensor placed? If this is the case (presumably it is slightly further away on the smilghtly larger wheels), how do I know that the previous placement and readings were accurate on my old mountain bike?

Effectively, if I move the sensors further down the spoke & fork towards the hub will it register I am travelling faster and further?

Thanks

 

[Sittingduck]

Hmm

Seems I jumped the gun in posting. Have since found out about calibration tables etc from the excellent Sheldon Brown site http://sheldonbrown.com/cyclecomputer-calibration.html#ccc
and reset the calibration to reflect the new wheel size!

 

[domtyler]

Get a gps based computer and forget all this rubbish.

 

[Rob S]

Hi All

Effectively, if I move the sensors further down the spoke & fork towards the hub will it register I am travelling faster and further?

Thanks

You input the circumference of the tyre in to the computer and the sensor counts the revolutions...the distance of away from the hub doesn't come in to it.

 

[Steve Austin]

 

[HJ]

Get a gps based computer and forget all this rubbish.

And Dom will pay the 80% difference in price, he is generous like that...

 

[ufkacbln]

The simplest trick is to place the bike with the valve at the bottom of the wheel, and mark with chalk.

Now roll the bike forward to the point where the valve is back at the bottom. Mark again

The distance is the circumference.

If you want greater accuracy, roll a couple of revolutions and diveide.

 

[jay clock]

The method of rolling the wheel round is pretty good and certainly much better than using a standard table. However I have just bought a Garmin Forerunner for triathloning and that shows that one bike over-reads about 300m in 12km, and another under-reads a tiny bit. I just need to get my head round how much to adjust in the settings by!

 

[Globalti]

That's about 3.3% error - I doubt if the next increment up or down would cover such a small difference.

 

[Mr Pig]

The simplest trick is to place the bike with the valve at the bottom of the wheel, and mark with chalk.

I've found that to be quite close with maybe just a slight adjustment necessary.

 

[Vitesse]

Remember that the rolling diameter of the wheel depends on the weight of the rider and the tyre pressure!

 

[Mr Pig]

Remember that the rolling diameter of the wheel depends on the weight of the rider and the tyre pressure!

Yip. What I do is sit on the bike and roll it forward a couple of times. Still seldom bang on though, usually needs tweaking.