Calibration, Cateye Strada

woodbloke
woodbloke Posts: 35
edited August 2011 in Road beginners
I'm gradually building up the miles on the bike and went out for a short run yesterday. At the end of the ride, the total distance shown was around 17 miles but on checking the distance this morning on the RAC route planner, the distance was calculated at 18.27 miles.
The computer was set up according to the manual (ie ref finding the linnear travel of one wheel rotation) and the corresponding figure was punched in. However, the magnet is set about 120mm or so from the rim, so that the actual linear distance travelled by one rotation of the magnet is going to be less than the original distance on the circumference.
So...the question is, do I need to recalibrate the Stada with the circumference of the wheel at the magnet positon? (ie the sum is going to be the hub - magnet distance x 2 x 3.142) - Rob

Comments

  • JimboM
    JimboM Posts: 380
    No need to recalibrate, the sensor should just count the revolutions and then multiplies this by the distance you programmed in for the wheel travel.

    Not sure why there is such a big difference to the RAC site but could be

    1. Sensor not lined up properly with magnet so misses a count sometimes
    2. Wheel travel distance hasn't been input correctly
    3. RAC site possibly uses a best guess for the distance traveled
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  • woodbloke
    woodbloke Posts: 35
    I was pretty careful with inputing the wheel trave distance and I guess I got it correct to the nearest cm, but I'll check the whole set up again tonight - Rob
  • petemadoc
    petemadoc Posts: 2,331
    I'd say your cat eye is more accurate than the RAC thingy. Just double check your wheel size figure is correct then you should have full confidence in it.
  • Nairnster
    Nairnster Posts: 602
    RAC will be done shortest driving route. Did you apex any bends in the road when you could see nothing coming, turn off onto another road a little sooner and clip over the incorrect side of the road. May all add up, but without knowing exactly how they measure the route distances, its hard to say.

    As PeteMadoc says though, as long as you set up your strada correctly, it probably is accurate.

    When you measured the wheel travelled distance, did you do it with someone sat on the bike? Or just freewheeled the bike by hand? The weight of a person deforming tyres slightly etc, will have a slight effect on circumference.

    Slightly OT, but how are you finding the Strada generally. Easy to use and read the display?
  • 2Phat4Rapha
    2Phat4Rapha Posts: 238
    A quick rule of thumb derived from your tyres can check your roll-out measurement.

    You need to read your ISO/ETRTO numbers, eg for a 700c wheel you might have an ISO/ETRTO of a "28-622" for a 28mm tyre. This is the Inner Rim Width (IRW) and the (Bead Seat Diameter)

    The circumference is approx the BSD plus twice the IRW times Pi (2.Pi.R, or Pi.D)

    Circ = (BSD + (2 x IRW)) x Pi

    Hence approx circumference = (622 + (2 x 28)) x Pi

    Which should be within 1% or 2%.

    However, it does depend on tyre pressure, tread and rider weight as mentioned earlier.

    If you've done the roll out test a few times and averaged the results, plus checked it against the rule of thumb above, then believe your own numbers!
    I may be a minority of one but that doesn't prevent me from being right.
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  • petemadoc
    petemadoc Posts: 2,331
    Nairnster wrote:

    Slightly OT, but how are you finding the Strada generally. Easy to use and read the display?

    I have a Cat eye and find it generally very good. Nice and simple, easy to use and sensor is pretty secure.

    Negatives - no backlight so you simply cannot read it at night time. I have the strada double wireless that reads cadence, the cadence sensor has to be pretty close and can be fiddle to set up but once secured it's ok. That's it really. Backlight is the big disappointment for me.
  • Nairnster
    Nairnster Posts: 602
    Thats cool, am awaiting delivery of a double wireless myself. And the bike to put it on lol
  • woodbloke
    woodbloke Posts: 35
    Nairnster wrote:
    When you measured the wheel travelled distance, did you do it with someone sat on the bike? Or just freewheeled the bike by hand? The weight of a person deforming tyres slightly etc, will have a slight effect on circumference.

    Slightly OT, but how are you finding the Strada generally. Easy to use and read the display?
    When I set it up, I sat on the bike and rolled it across a level floor (as instructed) and then measured the distance...it made a difference of about a cm. I'm finding the Strada quite easy get on with and the lack of a back light doesn't worry me unduly as I don't intend riding in the dark - Rob
  • MattMTB
    MattMTB Posts: 95
    With my Cateye Strada wireless the instruction booklet tells you what value to input in for the wheel size. IIRC it was something like 2093 for a 700x23 wheel / tyre. There's not really a need to do the rolly measurement thing.

    I ride with a mate who has a garmin GPS and over a 37 odd miles route our respective units differ in distance by about 1/100th of a mile so I'm guessing it's pretty accurate.

    As someone said above, it just works on numbers of times the magnet passes the sensor - it doesn't matter if you put the magnet and sensor right next to the rim or right next to the hub - so long as they line up.

    Cheers,
    Matt
  • woodbloke
    woodbloke Posts: 35
    Having checked it tonight, I realized that I'd input the wheel size slightly too big :oops: which was probably accounting for the discrepancy...thanks for all help received though - Rob
  • JimboM
    JimboM Posts: 380
    If the wheel size was too big you'll find that the discrepency would be even more now !
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  • anto164
    anto164 Posts: 3,500
    JimboM wrote:
    If the wheel size was too big you'll find that the discrepency would be even more now !

    No he won't.

    He's making the wheel smaller, which means less distance travelled per revolution of wheel. Therefore shorter distance.
  • JimboM
    JimboM Posts: 380
    Errr yes it will

    Originally the computer said he'd done 17 miles which was 1.27 less than the RACs 18.27

    If he changes the wheel size to something smaller the computer is going to think he's done even less than 17 miles which will be a bigger difference to the 18.27
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