23mm v 25mm tires cateye readings

Frank pole
Frank pole Posts: 112
edited July 2012 in Road beginners
my cateye is setup for 23mm tires but im going to fit 25mm - would my actual speed be greater or less than if i was running 23mm

Comments

  • crankycrank
    crankycrank Posts: 1,830
    Actual speed will read less with 25mm.
  • rpd_steve
    rpd_steve Posts: 361
    with 23mm tyres the diamiter of the tyre overall is 622mm + 23mm +23mm = 668mm
    with 25 then 622 + 25 + 25 = 672mm

    Hence for each rev you travel further, so you go faster on 25mm compared to 23mm for the same wheel RPM.

    So you will be doing more speed than your comp says.
  • mcp73
    mcp73 Posts: 94
    Most Cateyes give you the ability to alter tyre size in the set up menu. Certainly, mine did at least.
  • route666
    route666 Posts: 7
    RPD Steve wrote:
    with 23mm tyres the diamiter of the tyre overall is 622mm + 23mm +23mm = 668mm
    with 25 then 622 + 25 + 25 = 672mm

    As 23mm and 25mm in this case generally refer to the width and not to the height, does this mean that bike tyres have a 100% profile - i.e. height = width ?

    If you look on the Schwalbe Tyre website (link here), you will see Approximate Tyre circumference values that you can use for your computer:

    Tyre Size Circumference
    23-622 2125 mm
    25-622 2135 mm

    These work ok in practice, but the perfect way to measure your circumference is to:
    o Pump up the tyres to the normal pressure
    o Rotate the wheel the sensor is on so the valve is at 6 o' clock
    o Put a chalk mark on the ground at this point
    o Get on the bike, move it forward in a straight line at least one, but preferably a few rotations, stopping when the valve is again at 6 o' clock.
    o Clalk the ground again where the valve is
    o Measure the distance between the 2 points, dividing this by the number of wheel rotations. This gives you the correct circumference for your bike with you on it.

    Of course, this won't give perfect results either :shock: The Cateye itself is probably not 100% accurate and if the sensor is on the front wheel (as most non-cadence ones are), then everytime you wobble the wheel from side to side as you struggle up a steep hill, the computer will overread.
  • thescouselander
    thescouselander Posts: 549
    Don't forget the radius will be less when you put weight on the wheel as the tyre deforms a little. My garmin has calibrated my 25 size gatorskins against gps as being 2106 mm radius.
  • route666
    route666 Posts: 7
    thescouselander wrote:
    Don't forget the radius will be less when you put weight on the wheel as the tyre deforms a little. My garmin has calibrated my 25 size gatorskins against gps as being 2106 mm radius.

    +1

    As in my 4th point above - "Get on the bike" ...
  • thescouselander
    thescouselander Posts: 549
    route666 wrote:
    thescouselander wrote:
    Don't forget the radius will be less when you put weight on the wheel as the tyre deforms a little. My garmin has calibrated my 25 size gatorskins against gps as being 2106 mm radius.

    +1

    As in my 4th point above - "Get on the bike" ...


    Oops, yes, I missed that you said that.

    The other way you can do it is to find 2 points that are known to be a mile (of 1 kmif you prefere) apart and see how far off your computer is when to go between the two markers and then adjust the circumference as necessary.
  • Bobbinogs
    Bobbinogs Posts: 4,841
    The Strada Wireless has the ability for the user to set 2 wheel sizes so, once you have calibrated it right for both sizes, you can just swap between them as approp. Might be worth checking depending on your model.

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