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# Bike computer and wheel circumference question

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• Posts: 401 Forumite
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shape and therefore circumference will also vary with rim width, rims have become wider of late so that could account for some of the difference. The only where you're going to get better than taking the middle figure referred to is by making several measurements of as many rotations of the wheel as you can fit on a flat straight surface, or by trial and improvement over a known distance.
Of course of your ultimate precision you have to take into account the bumpyness of the surface as well and any deviations from a straight line. I imagine the back wheel will run slightly straighter than the front but of course carries more weight and is more likely to slip in either direction.  Then you start getting into fractal theory.
To be honest taking the mid-figure is about as good as you're going to get.
• Posts: 8,380 Forumite
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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
Thank you.

Assuming your assertion that any weighted tyre deflection will be so minimal as not to affect the rolling circumference of the wheel is correct, then it really isn't necessary to physically measure the circumference of a wheel roll (or multiple rolls) at all, is it?  All I need to do is to measure the diameter (or radius) of the tyre on the wheel and calculate the circumference from there.  (And if you are correct, then I don't see why you suggest on the other hand applying weights to it at all?)

Somewhat to my surprise (I expected it to be smaller) the diameter of the tyre on my front wheel is 69.9cm - or at least that is the closest I can get by eye with a steel tape and the average of three attempts.  That gives me a rolling circumference of 219.6cm.  On the face of it, that suggests that both the Cateye and ridewithgps websites are wrong in returning a figure of 216.8 cm, and thet the CyclingUK website is more accurate at 2.19m

Or - it suggests that Cateye and ridewithgps might be feeding in an approx tyre compression of about 0.5cm (which is what I guessed in my second post above but which you seem to think can't happen) to arrive at their figure of 216.8cm.

I don't know which is right - I was hoping somebody would come up with some useful suggestions as to what would explain the differences between the websites rather than getting bogged down in the practical details of actually measuring the circumference.

(Regarding your observation on the Subaru Forester:  I was once being taken by taxi along a narrow lane partially blocked by a stationary lorry.  The lorry driver opined to my taxi driver "You could drive a 'bus through there" to which my taxi driver responded "I know mate.  Problem is I'm driving a taxi not a 'bus..."

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, ... If you're determined to use an ancient computer and want to be crazily accurate, ...

You should chill out mate!  Don't let my quest for accuracy launch your blood pressure!

• Posts: 8,380 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.
Will there be little compression though?  My calculations above suggest that there might(?) be 0.5cm, but I don't really know. I'm happy to be shown to be wrong - or right.

And despite Farfetch's apparent apoplexy that I just need to do as I've been advised, I'm not convinced that measuring a single or multiple wheel rolls is as accurate or reliable as people have suggested - for the reasons I've already given, despite Farfetch clearly knowing better than me.

I'd love to check it on a longish ride of known distance...

Obviously I am not going to get a GPS bike computer.  (I'm surprised you even said it!)

• Posts: 8,380 Forumite
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rdr said:
shape and therefore circumference will also vary with rim width, rims have become wider of late so that could account for some of the difference. The only where you're going to get better than taking the middle figure referred to is by making several measurements of as many rotations of the wheel as you can fit on a flat straight surface, or by trial and improvement over a known distance.
Of course of your ultimate precision you have to take into account the bumpyness of the surface as well and any deviations from a straight line. I imagine the back wheel will run slightly straighter than the front but of course carries more weight and is more likely to slip in either direction.  Then you start getting into fractal theory.
To be honest taking the mid-figure is about as good as you're going to get.
Brilliant!  I can't tell you how happy I am to read that!  Somebody at last who begins to understand and appreciate my "problem"!

Yes - the back wheel will run slightly straighter and less far than the front.  And my concern is that over any long "known" distance, the distance travelled by the front wheel (the measuring wheel) won't be the same as the "known" distance anyway, because I wouldn't be able to keep to a consistently straight or "shortest" line.

Anyway, no-one can give me any insight as to why the CyclingUK web figure is so different from the Cateye and ridewithgps ones.  (Other than my own realisation that CyclingUK may have simply calculated an unweighted and unadjusted circumference).

I've now asked on the CyclingUK website if anyone knows where 2.19m comes from.

Thank you everybody.
• Posts: 0 Newbie
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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
Thank you.

Assuming your assertion that any weighted tyre deflection will be so minimal as not to affect the rolling circumference of the wheel is correct, then it really isn't necessary to physically measure the circumference of a wheel roll (or multiple rolls) at all, is it?  All I need to do is to measure the diameter (or radius) of the tyre on the wheel and calculate the circumference from there.  (And if you are correct, then I don't see why you suggest on the other hand applying weights to it at all?)

Somewhat to my surprise (I expected it to be smaller) the diameter of the tyre on my front wheel is 69.9cm - or at least that is the closest I can get by eye with a steel tape and the average of three attempts.  That gives me a rolling circumference of 219.6cm.  On the face of it, that suggests that both the Cateye and ridewithgps websites are wrong in returning a figure of 216.8 cm, and thet the CyclingUK website is more accurate at 2.19m

Or - it suggests that Cateye and ridewithgps might be feeding in an approx tyre compression of about 0.5cm (which is what I guessed in my second post above but which you seem to think can't happen) to arrive at their figure of 216.8cm.

I don't know which is right - I was hoping somebody would come up with some useful suggestions as to what would explain the differences between the websites rather than getting bogged down in the practical details of actually measuring the circumference.

(Regarding your observation on the Subaru Forester:  I was once being taken by taxi along a narrow lane partially blocked by a stationary lorry.  The lorry driver opined to my taxi driver "You could drive a 'bus through there" to which my taxi driver responded "I know mate.  Problem is I'm driving a taxi not a 'bus..."

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, ... If you're determined to use an ancient computer and want to be crazily accurate, ...

You should chill out mate!  Don't let my quest for accuracy launch your blood pressure!

I am chill, I have the sense to have a modern computer that routinely automatically calibrates wheel circumference based on number of rotations and distance travelled, plus I don't really care if, over a 50 or 100 mile ride, it could be a few metres plus/minus given the GPS records and known street data from mapping firms are far more accurate than using a tape measure.

The sole reason I mentioned weights was because you were saying you felt the circumference would be meaningfully affected by using the bike vs wheeling the bike which it won't but I still gave you a solution.

Different wheel rims have different widths, while the wheel may well be 700mm in diameter, some rims will allow the tyre to spread out more (increasing comfort) and calculations assume averages on say a 25c tyre being X mm wide due to pressure

I never said tyres don't deform, don't lie.

You have already been provided with explanations as to why wheel circumference isn't exact.

I'll leave you to your tape measure, I prefer accuracy
• Posts: 14,810 Forumite
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Bear in mind the circumference will also change due to tread pattern and tyre wear, so any calculator will only give you an estimation based on the ERTO size (622xsomoething).

The most accurate way to measure it is by measuring the radius at the centre of the trim, either with chalk on the pavement, or running some string round it. Taking the diameter and calculating should also work.
• Posts: 5,195 Forumite
edited 14 July 2021 at 5:11AM
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What do you mean by your wife's Garmin?

If it is an edge bike conputer, does she have a speed sensor?

Garmin came to recognise that the GPS was not accurate enough for some cyclists so added a speed sensor, which measures revolutions of your wheel as your Cateye will do. The computer uses both GPS and the number of wheel revolutions to get as close as it can. You have a choice of setting the circumference yourself, or the computer will calibrate it.

I think it's unlikely you will ever beat an Edge with a speed sensor for accuracy with a simple computer. If she's using one of the wrist type GPS then it might be possible to come close.

For getting a known distance, is there a local cycling club? If so they will have a measured 10 mile time trial course. Particularly if it is used for open events the measurement will have to follow known methodology. You could find out where it is and calibrate using that.

Finally, and I appreciate you weren't happy with the suggestion, but I'd do the single circumference and measure it. As you are concerned about not keeping the wheel straight, use a piece of string stretched out, paint a line alongside it and follow that with your wheel. That will ensure any variance as a result of not being straight is negligible.

Different brands of tyre vary markedly, as do different sizes and tyre pressures. My bike will drop 4-6 pounds of pressure in a week. My road bike is pumped up before every ride, but my commuter used to be only pumped up once a week.

Anyone producing a chart will have to use their own methodology for brand, width of rim, size of tyre, pressure used, weight of rider etc to produce their figures. I consider the variance you are referring to as quite possible.
• Posts: 3,173 Forumite
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With respect, you are *way* overthinking this. The 'measured roll of the wheel' method is as accurate as you are going to get in the real world. Like you, I love accuracy and precision. I use this method every time I install a bike computer (probably 10 in the last 5 years) and they all agree with each other - bikes ridden side by side - and with Map My Ride and a GPS speedo app on my phone. I would say this method gives you accuracy within 1-2%, which is streets better than is legally required in a car, for example.

There are so many factors that can affect the speed reading that are not in your control. You won't get perfection. What you don't do is use the 'standard' circumferences given on the scrap of paper they laughingly call a user manual for the computer. Those are averages for a wide variety of tyre widths and depths, and are not accurate at all.
If someone is nice to you but rude to the waiter, they are not a nice person.
• Posts: 8 Forumite
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if I`m right - inflated tires needs not for every situations
• Posts: 0 Newbie
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ElHar said:
if I`m right - inflated tires needs not for every situations

Obvious spammer building post count but anyway - no, you can't ride a bike without inflated tyres (unless they're solid)

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