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Bike computer and wheel circumference question
in Public transport & cycling
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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.
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 should chill out mate! Don't let my quest for accuracy launch your blood pressure!
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!)
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.
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.
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.
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.
Obvious spammer building post count but anyway - no, you can't ride a bike without inflated tyres (unless they're solid)