Bike computer and wheel circumference question

Hi guys

I use an ancient (30 years old!) Cateye Vectra CC-7000 and I'm wanting to re-calibrate it for a 700 x 35 tyre.  (That's the measurement given on the side wall - I haven't got a clue about the mysteries of the different metric, ISO, imperial and US systems of measuring tyres and life's too short to learn about them now).

According to the Cateye manual I need to set the computer for a 217 cm circumference, and that seems to be confirmed by the Cateye website ( Tire_size_chart_ENG.pdf ( ) and by this one  Wheel Circumference Setting | Ride With GPS Help

But the CyclingUK website gives a completely different circumference of 219 cm (2.19m)  A guide to cycle tyre sizes | Cycling UK.

So, assuming the tyres are "correctly" inflated (ie towards the upper end of the recommended range on the sidewall) what is the correct circumference?  Or does it depend on some further esoteric aspect of tyre sizing that I don't really want to be bothered about?

(I know it's only a couple of centimetres difference and therefore < 1%, but I'd like to to be able to use the correct figure so far as possible).




  • glennevis
    glennevis Posts: 642 Forumite
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    Tyres from different manufacturers will produce slightly different results due to design, tread depth, tyre wear etc.

    For a definitive answer, measure the actual circumference yourself. Start by making a mark on the ground. Set up the front wheel with the valve nearest the mark on the ground. Walk forward until the valve is once more nearest the ground. Make another mark on the ground. Measure the distance in cm.

  • Manxman_in_exile
    Manxman_in_exile Posts: 8,380 Forumite
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    edited 11 July 2021 at 1:20AM
    Yes.  Thanks.

    Absolutely take on board what you're saying, but I'm not certain if it gets me closer to an answer.

    The difficulty I have with what you suggest is that it's not clear to me whether that method is sufficiently precise and repeatable to give me a more reliable answer than the 217 - 219 cm I already have.

    First, if I'm walking the bike forward rather than riding it, I can't make any accurate allowance for tyre compression when the bike is weighted with me sitting on it riding it - which is what I need to replicate.  I suppose I could guess a compression of (I don't know) 0.5cm, but it's a pure guess.  I assume a compression of 0.5 cm with the bike weighted would reduce* the rolling circumference of the tyre by about the same as the 2cm I'm already querying.  (So does that mean I need to reduce any measurement I get by 2cm?  As 0.5cm is a stab in the dark, I simply don't know).  

    Second, just walking the bike forward I wouldn't be confident of keeping a sufficiently straight line over the required 2m plus to give me a repeatable answer better than the 217 - 219 I've already got.

    I see exactly the theoretical value of what you suggest, but taking the above two factors together, I'm just not sure how accurate and reliable the method would be in practice in helping me choose between 217 and 219 (or a completely different figure, or three completely different figures if I repeated it twice and did it three times!).

    While I definitely do want to know what the "correct" figure for the circumference is (whatever "correct" means here**... ) I suppose what I'm really getting at is why the discrepancy between the CyclingUK website and the two others?

    Thanks again

    * I've roughly worked that out in my head based on c = 2*3.147*r, but could be way off!  It's too late for this...

    **What I do mean by "correct" in this context is that if my bike computer says I've ridden 10 miles, I'd prefer it to be more likely that i've ridden ten miles rather 9.9 miles or 10.1 miles.  I just like numbers to be right...
  • Just to add I also appreciate the other variables you mention like tyre profile, tread depth, tread wear etc.  I'm happy to accept uncertainty and variability in those parameters as I've got no control over them and (in a sort of Donald Rumsfeld sense) I know that I don't know them.  The only variable I can put into the computer is tyre circumference and that's all I want.  I'm not, for instance, going to decrease it as the tyres wear or increase it after I've inflated the tyres.

  • glennevis
    glennevis Posts: 642 Forumite
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    edited 11 July 2021 at 11:06AM
    If you have a smartphone then it's easy to accurately calibrate your cycle computer to within 0.46% (the limit of accuracy if the cycle computer uses whole numbers). Just use a mapping app and record your track as you go for a ride. Adjust your calibration number after comparing the actual distance travelled in the mapping app with the cycle computer recorded distance.

    By "mapping app" I don't mean Google Maps but an app that uses offline maps and GPS satellites to accurately determine your position to within 2m in open air. I use and can recommend AlpineQuest for this task. Other mapping apps are available.

    (By the way pi is 3.1416)
  • molerat
    molerat Posts: 31,826 Forumite
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    edited 11 July 2021 at 11:43AM
    The most accurate way is measuring one roll of the wheel, maybe with someone helping by pushing the bike along with you sitting on it.  You are never going to get it 100% accurate, in the same way that a car speedo is not accurate, as there are too many variables in play but one roll of the wheel with you on the bike is the closest you are going to get. The figures quoted in the manual are purely a guide taking tyre size variations into account.
  • glennevis said:
    If you have a smartphone then it's easy to ...

    I'm using a 30 year old Cateye bike computer ( :| ) - I don't have a smartphone otherwise I'd already be trying it - not that I'd necessarily be persuaded it was any more accurate* than either 217 or 219.

    What I want is a way of being able to externally "verify" or "certify" the way I calibrate my bike computer that I'm happy with. (ie by reference to Cateye or CyclingUK website).  What I don't understand is how Cateye and CyclingUK can produce such different figures - it might even be more than 2 cm depending on what the figures were before rounding - I think Cateye is 216.8 cm and CyclingUK might be 219.4 for all I know.

    (I know what the value of pi is - I was simply trying to illustrate that your first suggestion at trying to physically measure the circumference of the tyre might not be as accurate in practice as you may have thought.  Now you've suggested a different measurement method).

    But thanks for taking the time to respond again.

    *I like accurate measurements - and I understand that 100% accuracy is unattainable for various reasons.  One of my current issues is determining whether when measuring running routes my bike computer is more accurate then my wife's Garmin GPS.  I don't think her GPS is as accurate as she thinks it is - and I'm not sure how you satisfy your self that any mapping app is as accurate as you - or anybody else - may think it is.

    And I don't necessarily believe that my bike computer is more accurate than my wife's GPS because I'm not sure what the appropriate circumference is for my bike computer.  I'd just like to know which is "more accurate and more reliable".  I fully appreciate that without a surveyor's wheel it will be difficult, if not impossible, to determine...  Even then it won't be "right"...

    Again, I'm very curious why the Cateye (and others) website differs by 2cm from the CyclingUK site

  • molerat said:
    The most accurate way is measuring one roll of the wheel, maybe with someone helping by pushing the bike along with you sitting on it.  You are never going to get it 100% accurate, in the same way that a car speedo is not accurate, as there are too many variables in play but one roll of the wheel with you on the bike is the closest you are going to get. The figures quoted in the manual are purely a guide taking tyre size variations into account.

    Yes - I fully understand that 100% accuracy is not achieveable and that is not what I'm looking for.  I also fully appreciate that the Cateye and CyclingUK websites will have to make several compromises and generalisations to account for differing variables.  However, I don't think those variables would account for as much as 2cm divergence on approx 215cm.

    I'm afraid I don't believe that measuring one roll of the wheel, whether walking, riding or being pushed by someone else would give a more accurate and reliable figure than the 217 or 219 I already have.  I'm not convinced that you could travel in a sufficiently straight line to get an accurate circumference. (Even if I did it 100 times and threw out the highest ten and the lowest ten, and then averaged the remaining 80, I wouldn't be assured it was consistently reliable).

    I suppose what I was hoping for was someone to reply something like:  "Yes - they will differ because Cateye will be using US/Imperial measurement conventions whereas CyclingUK will be using ISO/European conventions. [Or whatever].  The ones most applicable here should be blah... blah... " or simply say "Yes.  It's inexplicable that the two figures should differ so much".

    I'm just curious that the figures are so different.  (I consider 2cm on 217cm quite a lot).

    I guess I'll have to ask CyclinUK how they arrive at 219.
  • You've been given advice on how to measure it to a high degree of accuracy and yet you refuse to do it because you don't like the advice, what is the point in asking for help and then rejecting it?

    One rotation of the wheel and measuring it will give a perfectly usable degree of accuracy, wheels that are 80+ psi are not going to deform so much that you get a huge variance between riding and simply wheeling. If you're determined to use an ancient computer and want to be crazily accurate, hang some weights off the bar/saddle or get someone else to sit on it and wheel it along.

    I saw a site where someone tested the variance of a subaru forester SUV where 2 psi on that resulted in a chance of 3/16" to the circumference or around 1/2 a cm. With a bike tyre rather than the weight of a car, it's really not going to be a huge change on a 700x 25
  • Keep_pedalling
    Keep_pedalling Posts: 16,607 Forumite
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    You will have very little tyre compression on the front, so it is going to make very little difference. Do an initial set up as suggested, then go out on a longish ride of known distance to check for accuracy then adjust as necessary. 

    Alternatively treat yourself to a GPS bike computer. 
  • martindow
    martindow Posts: 10,215 Forumite
    Name Dropper First Post First Anniversary
    If you work on the middle figure of 218, the error is going to be +/- less than 0.5% which seems to be a very good tolerance.
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