Thorn Cycles Forum

Technical => General Technical => Topic started by: Dirk on July 09, 2007, 11:39:19 AM

Title: Cateye MC100W set up query.
Post by: Dirk on July 09, 2007, 11:39:19 AM
I just fitted 26 x 2,25 tyres and want to adjust the computer, but in the manual there is only the circumference setting stated for 26 x 2.125  and 26 x 2.35
Does anybody know the circumference of a 26 x 2.25 tyre in mm's
Thanks.
Title: Re: Cateye MC100W set up query.
Post by: pdamm on July 10, 2007, 03:43:08 AM
Dirk

Not sure of the circumferrence but you can use the rollout test described on this page.

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

Peter Damm
Title: Re: Cateye MC100W set up query.
Post by: ahconway on July 13, 2007, 01:40:17 PM
This is serendipity - I was just looking for advice on a similar matter!

I just installed a Cateye Mity 8 (I've finally lost patience with the wireless bit of nonsense that I bought with my Raven AT). But on my first venture out, it seems that either I'm a candidate for next year's TdF, or the setting is wrong. I'm running 26 x 1.5 tyres, and the Cateye manual says I should use setting 201 - clearly wrong.

Sheldon's page, though, says I should be on 199 - I'll try that next.

I actually ran across the man himself on the Minuteman Bike Path (in Lexington Massachusetts) the other day - he's as fascinating in person as you would expect him to be!

And incidentally, I've just discovered that the Cateye website has an English-only version of the manual for my cyclometer, much friendlier than the 10-language poster that comes in the box. http://www.cateye.com/sites/cateye/upload/manuals/en/MT400v1_E-1.pdf

Andrew
www.velofellow.com
Title: Re: Cateye MC100W set up query.
Post by: Fred A-M on July 13, 2007, 02:16:21 PM
Whilst there's a thread running on this, maybe someone can help solve a mental block that I have with the logic of these settings?   Why should a 26 X 1.5 setting be any different to say 26 X 1.75, 2, 2.25 etc as surely it the wheel circumference that determines how far you travel and not the width?  

I've set my devices up in the past by measuring the distance covered by the 1 revolution of the wheel, using the valve position as start and end points with pretty accurate results (as far as I can tell) and so just can't get my head around settings would vary (as stated in manuals, and as experienced above, inaccurately in some instances)according to tyre width?  

Enlightenment appreciated!
Title: Re: Cateye MC100W set up query.
Post by: stutho on July 13, 2007, 03:11:37 PM
Hi Fred, yes you are right it is the circumference that matters BUT it is the circumference of the tyre that we are looking for (as the width of the tyre increases so does the distance between the tread and the rim)
so the answer is

Total circumference = [the diameter of the rim + [tread to rim distance * 2(ie both top and bottom)]] * pi
Title: Re: Cateye MC100W set up query.
Post by: Fred A-M on July 13, 2007, 09:46:38 PM
Cheers Stuart, that's an end to a lifetime spent failing to appreciate that tyre width also means tyre depth!!!! :-0
Title: Re: Cateye MC100W set up query.
Post by: ahconway on July 16, 2007, 12:30:56 PM
Just to update my previous posts above...

I did a roll-out test and found that the circumference of my wheel is actually 198cm!
Title: Re: Cateye MC100W set up query.
Post by: stutho on July 16, 2007, 01:16:23 PM
Hi Ahconway,

Was there any weight on the bike when you did the rollout.  If not then you may find that the circumference (for set-up) may need to be lowered a bit.

(A deflection in the tyre of 1.6mm due to weight would reduce the circumference by 10mm)
Title: Re: Cateye MC100W set up query.
Post by: ahconway on July 18, 2007, 03:02:57 AM
quote:
Originally posted by stutho

Hi Ahconway,

Was there any weight on the bike when you did the rollout.  If not then you may find that the circumference (for set-up) may need to be lowered a bit.

(A deflection in the tyre of 1.6mm due to weight would reduce the circumference by 10mm)



Who knows? I did 110 miles on Sunday and my cyclometer was spot on, so I'm not going to worry about it any more!