Setting tire circumference for computers

Discussion in 'Cycling Equipment' started by skydive69, Sep 19, 2004.

  1. skydive69

    skydive69 New Member

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    I have three different computers and they each list a different circumference setting for a 700x25 tire - what am I missing?

    I just purchased a new Cat-Eye 300 computer and want to set it up properly for my Specialized Roubaix Pro.
     
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  2. gruppo

    gruppo New Member

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    Measure the circumference of the tire and enter it manually.
     
  3. skydive69

    skydive69 New Member

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    Interestingly, that is not even treated universally. One company says to simply measure the circumference, and another says after measuring the circumference, subtract 4 mm to make up for the difference when the rider's weight is on the bicycle. I guess the engineer in me cries out for exact, universal information which apparently ain't going to happen!
     
  4. drewjc

    drewjc New Member

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    Apparently the best method is to roll out the distance whilst sitting on the bike. I do this by marking a spot on the tyre and rolling along a tape measure close to a wall to keep balance. Seems to work.
     
  5. skydive69

    skydive69 New Member

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    I concur. I finally decided to do the tire measurement manually, and using the method described in the Polar manual (measure the circumference and then subtract 4 mm), my measurement came out to exactly their recommended setting for a 700x25 tire - 2105.
     
  6. Tesla*

    Tesla* New Member

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    my measurement came out to exactly their recommended setting for a 700x25 tire - 2105.[/QUOTE]


    I usually use the manufacturer's default setting for tire size. Doing rollout involves probably more human error than the minute differences in tire size between manufacturers. I've checked my mileage on rides against another rider with the same size tires but weighing 30 lbs more than me and the difference was .2 mile over about 25 mfiles. One of us could have zigged or zagged that much, so I wouldn't get too worked up over the alleged precision of manually entered rollouts.
     
  7. skydive69

    skydive69 New Member

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    I usually use the manufacturer's default setting for tire size. Doing rollout involves probably more human error than the minute differences in tire size between manufacturers. I've checked my mileage on rides against another rider with the same size tires but weighing 30 lbs more than me and the difference was .2 mile over about 25 mfiles. One of us could have zigged or zagged that much, so I wouldn't get too worked up over the alleged precision of manually entered rollouts.[/QUOTE]
    So true - it's just that the engineer/pilot mentality that I am stuck with cries out for accuracy and precision.
     
  8. artmichalek

    artmichalek New Member

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    Having the engineer mentality myself, I'd do the roll test a bunch of times and average the results.:)
     
  9. e_guevara

    e_guevara New Member

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    What's with engineers and accuracy? I don't know, but I guess that makes me one with the mentality too.

    I once bought a cheapo computer and got results way off from my CatEyes. What I did was do some controlled runs on a measured straight course (wanted to use a theodolite but only steel tape was available :D) then adjusted the wheel size accordingly until I got a negligible enough error on the trip meter.

    Tedious? Yes. Fun? Hehehe
     
  10. Gonzo Bob

    Gonzo Bob New Member

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    Being really lazy myself, I simply measure the actual with of the tire and estimate the circumference with

    circ ~= (bead dia. + 2*width)*3.1416

    Subtract 2mm for better accuracy

    circ for 700x25 ~= (622 + 2*25)*3.1416 - 2 = 2109mm
     
  11. 531Aussie

    531Aussie Well-Known Member

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    I do mine at a velodrome that has the distance clearly marked
     
  12. rule62

    rule62 New Member

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    Jeez and I thought accountants were bad.... :p
     
  13. cheapie

    cheapie New Member

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    help! i put my polar 720 on my mountain bike and the book lists mtb tires up to 26X1.95. i have 26X2.1. is there a formula i can use to figure the metric circumference?
     
  14. meb

    meb New Member

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    If you were on the bike during the rollout, you ought not need adjust for the added weight since the weight was there during the rollout so it doesn't need be added.
     
  15. 531Aussie

    531Aussie Well-Known Member

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    I still like my idea :D

    Get to a velodrome and do a manual roll-out whilst riding right on the line -- on the bike, obviously

    Velodromes are usually 250m, but if not, they're usually clearly marked.
     
  16. meb

    meb New Member

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    Nominal formula for diameter is bead diameter + 2 X tire width.

    26 X 1.95 would be 559mm + 2 X (1.95 X 25.4)= 658.1 mm
    26 X 2.1 would be 559 + 2 X (2.1 X 25.4) = 665.7 mm.

    So take your calibration for 26 X 1.95 and multiply it by (665.7 /658.1)=
    1.0115.

    Should be able to make the adjustment from that.
     
  17. mitosis

    mitosis New Member

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    Pump your tires up to your normal riding pressure.
    Put a mark on a hard floor and roll the bike through 3 or 4 revolutions with you on it.

    Measure and divide by the number of turns you did and there is your circumference.
     
  18. mitosis

    mitosis New Member

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    So true - it's just that the engineer/pilot mentality that I am stuck with cries out for accuracy and precision.[/QUOTE]

    Your experience is far different to mine. The manufactureres numbers are a guide - and usually result in fairly flattering speeds and distances.

    Even different tyres make a difference. Unless you are lucky, you will be several kmph out setting your computer to the manufacturers circumference.

    Its no good comparing your speed and distance to your riding mates because they are likely to be wrong too. Do it the way I mention above and you are not cheating yourself.
     
  19. mitosis

    mitosis New Member

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    Problem with this method is you don't take into account the smaller diameter caused by your weight on the tyre. Sit on the bike and measure. There is no error to allow for with this method.
     
  20. meb

    meb New Member

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    Good point.

    Anyone got a model calculation that adjusts for weight on tire with sensor, and tire width and diameter?
     
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