Wheel/Tyre circumference and CATEYE Cycle computers.

[TheCyclingRooster]

Hi to you all out there. For more years than I can remember I have used a simple but very easy to fit,use,calibrate a Raleigh Cycle Computer albeit the table for wheel sizes does not have the circumference dimension for a 700 x 23c wheel/tyre combination - the nearest is a 700 x 25c at 2124mm.

I have recently picked up a CATEYE CC-MC100W Micro Wireless cycle computer and I am quite seriously perplexed by the table of sizes that comes as a part of the overall retail package.
The circumference for a 700 x 23c is listed as being 2096mm,that is a full 28mm less in circumference than the listing on the Raleigh table of sizes.

I have just fitted a brand new Vittoria Rubino 700 x 23c to my Benotto and physically measure the out surface of the tyre with a Dean Tape (soft tape) and I am getting a measurement of 2020mm - that is a difference of 24mm to what the CATEYE table of sizes quotes of 2096mm.

How do you check your tyre sizes and set your computer tyre/wheel circumference ?
Do you fit the sender unit to the front or rear of your bikes ?

 

[snorri]

I have never concerned myself with the tyre size when setting up the computer as only the circumference needs to be determined. The circumference could vary slightly depending on tyre pressure, tyre wear and weight of rider.
Put a generous chalk mark on the tyre then sit on the bike and propel it forward so that the front wheel is rotated one and a bit times leaving two chalk marks on the floor, measure the distance between these marks to obtain the wheel circumference.
Refer to the table in the Cateye instructions to set up your computer.
I fit the sender on the front wheel, things tend to be cleaner around the front wheel, also keeping sender and receiver in as close proximity as possible can only be good for accurate data transmission.

 

[slowmotion]

Put the sender on the front forks. The wireless transmitter to the bar display is very limited in range and may well not work on the back wheel.

For best accuracy, do a "roll out" test. Pump the tyres up to working pressure, get in the saddle with the bike on a hard surface, and scoot along for one complete wheel revolution while keeping your weight on the saddle. One way is to put a chalk mark on the ground with the valve stem above it, and mark another point when the valve stem is again above it one revolution down the road. If you want to get obsessive, do multiple wheel revolutions and take the average measurement.

EDIT: @snorri got there first!

 

[raleighnut]

Alternately tell it you've got 32mm tyres on and watch your average speed rocket.

 

[Ajax Bay]

Just set it at 2100 (or whatever the measurement (as per advice above or Sheldon) is), go out for a long ride (preferably avoiding wiggly roads and tree cover) with both the Cateye and your GPS zeroed/started at the same time, take the readings at the end, do the maths and if necessary reset the Cateye with a revised calibration. If yours can be set to the nearest mm then you should be able to get it spot on.

I am getting a measurement of 2020mm - that is a difference of 24mm to what the CATEYE table of sizes quotes of 2096mm.

 

[Racing roadkill]

Get a tape measure, stick it to the ground. Position your front wheel with the valve in line with the forks. Place on '0' on the tape measure. Roll the bike forward until the valve is back where it started, having completed one rotation. The number that it stops at on the tape measure, is the number you enter on the cycle computer. It's the way I've always done it. Fit the sender unit to whichever wheel you used to take the measurement.

 

[GuyBoden]

 

[Racing roadkill]

Is that (r) all the way to the outside of the tyre, from the centre point? At which pressure? Is the wheel / tyre perfectly round? (r) assumes it is.

 

[GuyBoden]

Is that (r) all the way to the outside of the tyre, from the centre point? At which pressure? Is the wheel / tyre perfectly round? (r) assumes it is.

Measuring the radius from the axle to the ground just seemed easier to me, that's how I found my wheel's circumference.......

 

[I like Skol]

I have that very same computer on all my bikes and my road bike has 700x23c Vittoria Zaffiros and the computer is set to 2103mm. This figure is arrived at by a number of methods.

• Initially I physically measure the circumference by rolling the bike in as straight a line as possible for 4 or 5 complete revolutions then divide the measured distance by the number of revolutions to give the rolling circumference. The reason for doing multiple revolutions is to minimise any measurement errors.
• I then do a ride of around 20 miles and compare the computer measured distance against the distance from an online route planner such as MAPOMETER and also against the same route plotted on my Tomtom satnav.
• I then tweak the cycle computer to match the average of the other two distances (if there is a difference) to get an accurate mileometer on the bike.

The default benefit of all this is that even if you don't particularly care about logging accurate distances it does mean that the speed reading is about as accurate as you can get because the speed is purely a function of time and distance. The time part is precise so if the distance is correct the speed has to be right.

The reason that you might have to tweak the initial rolling circumference figure is because a loaded tyre behaves a bit differently to an unloaded tyre.

 

[I like Skol]

Measuring the radius from the axle to the ground just seemed easier to me, that's how I found my wheel's circumference.......

No need to complicate things with geometry (not everybody's strongest point) just do the roll-out measurement which will be far more accurate than trying to measure r and then multiplying any error by 2π.

Also, don't bother driving the same route in your car to check the distance as car speedometers and milometers are not very accurate at all.

 

[Globalti]

Just put a blob of paint on the tyre, get on the bike with the tyres at the normal pressure and ride it then measure the distance between the blobs it leaves on the ground. You can check the measurement against something like Bikehike, you'll find it remarkably accurate.

My cycling buddy and I have the same setting on the same Cateye Velo computer on the same tyres at the same pressure and we are usually within 0.1% of each other at the end of a ride.

 

[TheCyclingRooster]

Hi to you all again. Firstly let me say that I am aware the different tyre sizes :- 23,25,28 etc,etc will produce different measurements of circumference - that goes without saying - not Rocket Science - as they say.

The method that I personally use is a soft flexible tape (Dean Tape) and rotate the tyre and mark it from the valve position and the at 1500mm then from there to the valve position to complete the cycle.

In real terms the tyre is rock-hard and therefore the only way an appreciable difference would register is if the tyre was soft to the feel or deflated. As it is it would probably only reduce the rolling circumference by 3/4mm.

Chalk marks are by no means accurate enough.

 

[biking_fox]

Measuring the radius from the axle to the ground just seemed easier to me, that's how I found my wheel's circumference.......

Only accurate with you sitting on the bike. Can easily be a mm or so out. Which multiplied up makes quite a difference.

Chalk mark, sit on and roll. Repeat a few times. done.

 

[si_c]

My GPS device has a speed sensor, which it autocalibrates from the GPS track to estimate the tyre circumference allowing for pretty accurate values. It's currently showing as 82.68" (2100.072mm) for my 700x23c Rubinos. I run them at around 115-120psi.