Tyre Circumference

[memory]

Hi everyone,

New to cycle chat. Can anyone help? Trying to setup my bike computer and i need to input the tyre circumference. The markings on the tyre are 700 x 38C.
Can anyone tell me how to work the circumference out from this?

Many thanks, memory

 

[glasgowcyclist]

Hi everyone,

New to cycle chat. Can anyone help? Trying to setup my bike computer and i need to input the tyre circumference. The markings on the tyre are 700 x 38C.
Can anyone tell me how to work the circumference out from this?

Many thanks, memory

Memory, don't tell me you've forgotten!

C = 2πr



GC

 

[youngoldbloke]

Measure the diameter accurately (this will vary depending on your brand of wheel, tyre and pressure - I have 3 bikes with same size but different wheels and tyres and the circumerences are all different) and multiply by 3.142

 

[raleighnut]

mark the bottom of the tyre and a flat floor with chalk then wheel the bike forward until the chalk mark is at the bottom again and mark the floor at that point then just measure with a tape.

 

[Rickshaw Phil]

Hi and Memory.

Another vote for measuring as described by Raleighnut and Adrian. It's surprising how much difference there is between tyres even when they're marked up as the same size.

 

[themosquitoking]

Measure the diameter accurately (this will vary depending on your brand of wheel, tyre and pressure - I have 3 bikes with same size but different wheels and tyres and the circumerences are all different) and multiply by 3.142

Is pi to only three places accurate enough for millimeters?

 

[Archie_tect]

Ask @vernon ....

 

[youngoldbloke]

Don't know - I use 3.1416

 

[compo]

everytime I Measured my tyre i got a different figure. In the end i put a dab of paint on the tyre then rode the bike a few yards along the pavement leaving a little line of paint marks. Simply measured the dabs, leading edge to leading edge and had a good measurement.

 

[vernon]

Ask @vernon ....

Either use the values from the table below or use the method described in the text above the chart to determine the circumference of the tyre.

It all depends how anal you are about the accuracy of the value that your cycle computer displays.

Using 3.1416 instead of 3.142 introduces an error of 0.0127% and this assumes that you have accurately measured the diameter of the tyre.

I just take the value given by the table. I once measured the rolling circumference for a 700 x 28c tyre and found it to be about 5mm out from the table value, an error of 0.23% - nothing worth losing sleep over.

 

[raleighnut]

22/7 works as well.

 

[machew]

pi = 3 is good enough for the kids at school.

 

[youngoldbloke]

I remove the wheel involved and place it on a flat smooth surface against a table with a deep frame as near vertical as possible (use spirit level). I then hold the level against the top of the tyre and with a sharp pencil and using the level mark this point on a piece of paper taped to the table frame. Measure this distance (keep rule/tape vertical) and you have the diameter. I find it difficult to measure the roll out method accurately enough. Maybe I'm anal.

 

[Archie_tect]

Vern, I was more interested in pie rather than pi...

 

[sreten]

Hi,

FWIW your missing the ~ 15% tyre drop for optimum pressure, which
reduces the effective radius of the tyre, so you have to do an
effective roll out test, or approximate the reduction of the
measured diameter / radius, hence 22/7 is all the pi
you will ever need if measuring and calculating.

rgds, sreten.