Bike computer and tyre circumference

[Manxman]

Hi all

Hope this is the right board for this question, apologies if not.

Don't laugh but I have a 30 year old Cateye Vectra CC-7000 bike computer. I'm recalibrating it for a new tyre (Bontrager 700 x 35 according to the sidewall) and before attempting to measure the circumference or anything I thought I'd check what sort of figure I'd be looking at.

The old printed manual for the Cateye suggests 217cm and this seems to be confirmed by the Cateye website (2168mm or 217cm) https://cateye.com/data/resources/Tire_ ... rt_ENG.pdf and also by this website (2168mm) https://ridewithgps.com/help/wheel-circ ... ce-setting

However, the CyclingUK website gives a significantly larger circumference of 2.19m https://www.cyclinguk.org/cyclists-libr ... tyre-sizes

I'm curious to understand why the CyclingUK website gives such a different figure from the others. Does anybody know why, or is 219 (or 217) simply an error? I appreciate that there may be a number of other variables, but 2cm difference on only a little over 2m seems like quite a lot to me.

(Just to clarify - (1) I'm not at this point particularly interested in the practical ins and outs of actually physically measuring the circumference of the tyre and (2) I don't really want an in-depth seminar on the differences between UK/US/imperial/metric/ISO tyre measuring methods - unless those differences provide an immediate answer. I'm just wondering why the difference and which is likely to be the more reliable).

Thanks

[DaveReading]

Hi all

Hope this is the right board for this question, apologies if not.

Don't laugh but I have a 30 year old Cateye Vectra CC-7000 bike computer. I'm recalibrating it for a new tyre (Bontrager 700 x 35 according to the sidewall) and before attempting to measure the circumference or anything I thought I'd check what sort of figure I'd be looking at.

The old printed manual for the Cateye suggests 217cm and this seems to be confirmed by the Cateye website (2168mm or 217cm) https://cateye.com/data/resources/Tire_ ... rt_ENG.pdf and also by this website (2168mm) https://ridewithgps.com/help/wheel-circ ... ce-setting

However, the CyclingUK website gives a significantly larger circumference of 2.19m https://www.cyclinguk.org/cyclists-libr ... tyre-sizes

I'm curious to understand why the CyclingUK website gives such a different figure from the others. Does anybody know why, or is 219 (or 217) simply an error? I appreciate that there may be a number of other variables, but 2cm difference on only a little over 2m seems like quite a lot to me.

(Just to clarify - (1) I'm not at this point particularly interested in the practical ins and outs of actually physically measuring the circumference of the tyre and (2) I don't really want an in-depth seminar on the differences between UK/US/imperial/metric/ISO tyre measuring methods - unless those differences provide an immediate answer. I'm just wondering why the difference and which is likely to be the more reliable).

Thanks

2 cm over 2 m is of course 1%. I'd have thought that, all other things being equal, differences in tyre inflation pressure could produce most, if not all, of that variation.

[thirdcrank]

If it bothers you, find a local measured mile and check your result.

Not for the first time, I'll mention that long ago, I rode to and fro on the measured mile near what was until recently Ellis Briggs' bike shop. (Other measured miles are available.) At the end of each mile I adjust the Mity2 up or down and after a while doing this I checked and found the it was back at the default setting for 700C.

[Psamathe]

I've generally found the measurements on https://www.sheldonbrown.com/cyclecompu ... ation.html pretty good (it gives a table for different tyre sizes).

Earlier this year I went to great lengths (with help) measuring a new bike (1st time with that sized wheels), carefully did it sitting on bike, etc. but the result was terrible so I reverted to Sheldon Brown's table. To check the setting I do a straight ride with GPS and with phone GPS and compare distances and if the 2 GPSs match then I treat is as a 2 out of 3 gives a majority.

Ian

[Paulatic]

When I did that sort of thing I’d find a slight slope of nice black tarmac. Put a bit of Tippex on tyre sit on the bike and roll down the slope. Then measure dot to dot on the road.

[Mick F]

There's only one way of doing it, and that's to sit on the bike and measure. Get someone to push you along and keep you steady and count the wheel revs using chalk and the valve in the rim.

Whatever figure you come up with and input into the Cateye ............. you MUST believe it.
It's true and it's fact, and don't let anyone convince you otherwise.
We can use every device known to man, and they will all - without fail - give different distances.

If you use a front wheel sensor, the front wheel weaves along of course.
If you use a rear wheel sensor, that weaves less, but the tyre deforms up hills under power.

Believe me, the numbers vary.
All you need to do, is believe what the device tells you.
Been out with two GPS devices on the same bike on the same ride.
Neither agreed with each other.

Ancient Chinese proverb: Man with one clock knows the time. Man with two clocks is never sure.

[thirdcrank]

I thought that was the gears in inches discussion. I don't think anybody suggests that distance can be measured accurately by reference to wheel revolutions if you don't know the wheel's circumference

[Mick F]

Yep.
Measure the circumference.
Sit on the bike and count the wheel revs using chalk and the valve on the ground.

Input that figure into your Cateye.
Believe it. It's true. You measured it.
Compare it to someone else's bike and you will doubt it, but you know you are correct, coz you measured yours.

[Jdsk]

Just to clarify - (1) I'm not at this point particularly interested in the practical ins and outs of actually physically measuring the circumference of the tyre...

Good try.

; - )

Jonathan

[rareposter]

As @MickF said - there's no maths involved, no reading of tyre values (most values are not anything close to what the manufacturer suggests for many reasons - rim width, pressure, ideas of how tyres are measured...).

Run a steel tape measure out along a smooth flat level hard surface to about 2.5m. Inflate tyres to the pressure you intend to ride them at then roll the front wheel along the measure for 1 full revolution - you can measure using the valve or just put a dot of tippex/permanent marker/chalk on the tyre wall. Repeat a few times then take an average.

If you want to be super precise you should be riding the bike so the tyre can deform as your weight is on it but frankly the logistics of that and the actual difference it makes is not worth it.

Take that value in mm and input it into the computer. Job done, the computer does the rest of counting revolutions, adding up the distance travelled each time and converting to miles or km.

Once out in the real world you can compare the readings against known distances or against a GPS - they're unlikely to match precisely but they'll be to within a few fractions of a %.

[RickH]

Also don't forget that the rear wheel will travel a shorter distance on a ride than the front wheel! When you go round a corner the rear wheel follows a smaller radius curve. Plus, even when riding in a nominally straight line, you are always riding in a series of gentle curves.

Back in the 90s I had a computer (Cateye I think) with a rear wheel & cadence sensor. Some folk I rode with had the same computer but with a simple front wheel sensor. No matter how carefully we calibrated the computers (& I think the settings went down to single mm), over 100 miles I would consistently get a couple of miles less registered than they did.

[thirdcrank]

even when riding in a nominally straight line, you are always riding in a series of gentle curves.

Fun fact: I remember from one of the comics (not the comic) of my childhood dating from before I could ride a bike that it's always possible to tell the direction of travel of a cyclist riding in fallen snow because the sharp and of those deviations will point in the direction of travel. Not a tracking skill I ever remember putting into practice.

[Mike Sales]

even when riding in a nominally straight line, you are always riding in a series of gentle curves.

Fun fact: I remember from one of the comics (not the comic) of my childhood dating from before I could ride a bike that it's always possible to tell the direction of travel of a cyclist riding in fallen snow because the sharp and of those deviations will point in the direction of travel. Not a tracking skill I ever remember putting into practice.

Sherlock Holmes (or at least Conan Doyle) was onto this.

In Conan-Doyle’s short story The Adventure of the Priory School Sherlock Holmes solved a mystery by deducing the direction of travel of a bicycle. His logic has been minutely examined in many studies, and it seems that in this case his reasoning fell below its normal level of brilliance.

https://thatsmaths.com/2015/10/08/which ... icycle-go/

Or, more simply,

https://gdaymath.com/lessons/gmp/1-9-wh ... onderings/

[Jdsk]

Jonathan

[PH]

I'm just wondering why the difference and which is likely to be the more reliable).
Thanks

What it says on the side wall and what the size actually is has some variation, even the same tyre and a different rim will vary. All the charts you quote are guidance, an estimated average, it'd be wrong to see them as anything else. Use any for an approximate reading (Which for most people is close enough), but none of them are likely to be precise for your tyre, if one is, it'll be coincidence rather than reliability.
EDIT - And don't forget to always stop with the magnet in the right place, or you'll miss a whole wheel revolution each time you start