Tire sizes for my CAT EYE

[tintintin]

Hi,

I have a slight problem that I am sure the more technically minded can help with.

I have two sets of tires for my Sports Tour: 26'' x 1.35'' Kojaks & 26'' x 1.6'' Supremes

As there seems to be no size that fits these on the tire chart when setting up my computer I have tried the following methods

Marking the wheel and rolling it on the floor to measure the circumference
Measuring the radius and using 2 x pi x R to calculate.

I think I must be a Numpty because what ever I do I end up with a different figure and the figure I get seems to be outside where you would expect to see the tire size sit between or correlate on the tire chart.

Any help appreciated - I'd hate to think I was traveling faster than I was - or Slower!

Cheers

[JimK]

On my Cateye meter one can set the wheel circumference right down to the millimeter, so one is not limited to any table of standard tire sizes.

The fun thing is, the effective wheel radius will change with tire pressure and the load on the bike! The more the tire gets squished down, the smaller the effective radius!

I took my bike out on the road and rolled forward to bring the valve to the bottom. Then I marked the spot with a stone. I then coasted slowly and counted three rotations of the wheel, until the valve again reached the bottom of the wheel rotation. I marked this second spot with a stone. Then I measured the distance between the stones and divided by three.

Really there is a limit to the precision possible with such things! But it's fun to push toward the limit!

 

[tintintin]

Thanks Jim,

As with yours, it can be set by the mm, but I can't seem to arrive at the same number or even near it... I'm going to try your method again and take a mean value I think.

Cheers

Will.

[JimK]

I have Schwalbe Marathon Supremes on my Nomad, 26x2 or 50-559. The Cateye table said that size would be 2050 circumference. The Schwalbe table said 2075. I measured 2008.

I inflate to around 50 psi on the front tire, where I have the Cateye sensor, plus I ride with a handlebar bag and a spare tire around my mid-section (that I just can't seem to lose!). So maybe my tire gets squished more than the standard tables anticipate!?

[Eric]

I’ve got 26 x 1.6 Supremes on my new RST. I measured the circumference using my usual Tipp-Ex method as 1999.
Upon riding in parallel with two good GPS systems (Memory Map 3500 and Satmap Active 10) it was till reading slightly over, so I reduced the diameter to 1970 and they agree!
I inflate to 45-50 psi; use a handlebar bag on dropped bars and am a ‘big bloke’.
Hope this helps.
 

[tintintin]

Thanks Eric,


I'll try that 

[Danneaux]

Hi Will,

Schwalbe have a useful chart providing *approximate* tire circumferences for their models at:
http://www.schwalbetires.com/tech_info/tire_dimensions

They make note that *actual* circumference varies with load and pressure, just as JimK did.  Still, it might be enough to get you in the neighborhood of correct computer settings provided you're using Schwalbes.

Best,

Dan.

[tintintin]

Thanks Dan, that's a great help.

Cheers Will