Wheel circumference - tables vs measurement

[Gixxerman]

I have recently bought a cycle computer (£15 from Aldi - a bargain!).
But I need to input the wheel circumference.
I have a Trek 7.3 FX with 700x32c tyres.
According to Sheldons tables, the value is 2155mm.
But I used a tape measure to measure the wheel around the fattest part of the tyre and the value I got was 2193mm.
Why is there such a big discrepency? Do the tables take into account the rolling circumference based on tyre deformation due to rider weight?
Do I need to do a rollout test to get and accurate value?
I suppose I could ride for one mile and compare it with my GPS and make adjustments accordingly.
But which to believe? Is the computer telling the truth or is GPS telling the truth?
I have 2 GPS systems so I suppose I could use them both and do a 2 from 3 vote.

 

[bauldbairn]

According to my "Nite Rider" cyclecomputer manual it's 2174 for a 700 X 32c wheel/tyre.

Slap bang in the middle of both of your figures.

 

[bauldbairn]

Oh I'd say the roll test is probably the best / most accurate method. I'd imagine different tyre manufacturers can have different profiles therefore giving different results.

 

[woohoo]

I always measure the circumference. The last time I changed tyres (old and new were 700x23), the circumference of the new tyre was 1 cm greater than the old because it had, in cross-section, a higher profile.

 

[Fiona N]

I have Michelin Pro Race 3 (700 x 23) which have a circumference of 2082mm according to a roll test (with me on the saddle)

 

[HJ]

The best way to do it is: Put a bit of tape on the tyre > find a flat floor where you can make marks (or tape a bit of masking tape to the floor) > put a mark on the floor in-line with the leading edge of the tape on the tyre > roll bike forward bike forward until the tape on the tyre reaches the floor again > mark the position of the leading edge of the tape on the tyre on the floor > measure distance between the two marks in mm > use this figure in you cycle computer.

If you are worried about deflection of the tyre by the riders weight, then ride it forward, although this means that you will need two people.

 

[Davidc]

If you look at the errors as proportions they aren't huge. 2% and 1%. That sounds reasonable for generic figures for the tyre size. Try the manufacturers web site to see if they publish a figure for the specific tyre you're using.

I measurd the circumference by putting a drop of paint on the tyre and 'riding' it forward on tarmac to give 3 marks then measuring the distance between the trailing edges. The result was within 2 mm, or about 0.1% of the figure on the Schwalbe website table for a marathon.

The mileage measured by the CYO computer when set up with a circumference measured that way corresponds very well with a Garmin Foretrex satellite unit.

 

[MJN]

Ignore the tables in the manual - they are provided merely as a 'get you going' approximation. The roll test is quick, easy and as accurate as required.

Mathew

 

[woohoo]

I have Michelin Pro Race 3 (700 x 23) which have a circumference of 2082mm according to a roll test (with me on the saddle)

I have the same tyres and the roll test (without me on the bike) came out at 2093mm (tyres at 100 psi). I've calibrated the bike against a car milometer and the car reading was just over 1% greater which agrees with a road test that I read about it. I wonder if the rims make any difference? (my PR3s were a bugger to fit although they've eased, as I found out during a recent "deflation event").

In any case, I find my bike computer can vary by about 1% over identical runs, so who knows (but it's good enough for me).

- edit for typo

 

[sgw]

The result was within 2 mm, or about 0.1% of the figure on the Schwalbe website table for a marathon.

Thanks for the pointer. Unfortunately I can't find a definitive stated rolling circumference for standard Marathons on the Shwalbe site.

A very interesting all round 36pg pdf on tyres HERE but it only gives specific dia for Marathon Supreme and XR.

I will stick to the roll measurement.

 

[Davidc]

I can't find it again either, but I'd recommend doing the measurement rolling while sitting on the bike.

Any of the ways suggested will work. An oversize tape measure would work too.

Good luck with it.

 

[chigman]

I can't find it again either, but I'd recommend doing the measurement rolling while sitting on the bike.

Any of the ways suggested will work. An oversize tape measure would work too.

Good luck with it.

Hi

I did mine whilst sitting on the bike and doing one full revolution using the valve as a marker. I did one mile on the bike and then the same mile in the car and there was only a couple of yards in it. So over say a sixty miler, I'm only 120 yards out at the end of it, and who's to say that the bike comp is out a gnats or indeed the car ? Accurate enough for me.

 

[Fiona N]

If you are worried about deflection of the tyre by the riders weight, then ride it forward, although this means that you will need two people.

Not is you use a marker on the tyre which leaves a mark on the ground. I use a blob of oil, but chalk will work if you've got a nice clean surface where you can see it - then roll forwards far enough that the tyre goes over the marker twice and you have a pair of marks on the floor which you can measure. I use the back edge of the mark (which, counterintuitively, is the first placed) on the basis that excess oil may be pushed upwards on the tyre surface as the tyre rolls so extending the mark forwards on the floor but the two marks are usually pretty much the same size and shape so I don't think it matters much.

 

[bigtrike]

Mesuring the circumference of the tyre will overstate speed and distance because the effective tyre size decreases under load ( think of a flat tyre, how the wheel hub sinks lower to the ground, i.e. the tyre effectivly becomes smaller.) As you have seen its not much, but if you can be accurate why not? A car spedo is unlikley to be accurate as by law it only has to (see below)
The amended Road Vehicles (Construction and Use) Regulations 1986 permits the use of speedometers that meet either the requirements of EC Council Directive 75/443 (as amended by Directive 97/39) or UNECE Regulation 39.

Stanadard speedometer reader is km/h however some countries (some part in USA) used mph but still km/h will be follow. mph will show as travelled distance only.

The Motor Vehicles (Approval) Regulations 2001 permits single vehicles to be approved. As with the UNECE regulation and the EC Directives, the speedometer must never show an indicated speed less than the actual speed. However it differs slightly from them in specifying that for all actual speeds between 25 mph and 70 mph (or the vehicles' maximum speed if it is lower that this), the indicated speed must not exceed 110% of the actual speed, plus 6.25 mph.

For example, if the vehicle is actually travelling at 50 mph, the speedometer must not show more than 61.25 mph or less than 50 mph.

 

[HJ]

Mesuring the circumference of the tyre will overstate speed and distance because the effective tyre size decreases under load ( think of a flat tyre, how the wheel hub sinks lower to the ground, i.e. the tyre effectivly becomes smaller.) As you have seen its not much, but if you can be accurate why not?

But then again think about how hard a road bike tyre is when full pumped up to 7.6 bar (110 psi), the contact area is very small and the deflection cause by the rider is probable within the margin of error of the measuring tape...