How can the width of a tyre affect it's circumference?

Bustacapp

How can the width of a tyre affect it's circumference? I noted on my cycle computer quick guide chart that same diameter but different width tyres have differing circumferences. How can that be?

rolf_f

Imagine that the cross section of the tyre is circular. Then the width of the tyre is the same as the diameter of the cross section. Increase the width and the tyre becomes taller as well as wider.

Look at MTBs - small volume tyres on those have very noticeably smaller overall rolling diameters than fat big volume tyres.

As for your computer - the air pressure in your tyre will affect the effective diameter as well; lower pressure will result in a reduced effective diameter. It's best if possible to measure how far the bike actually travels and enter the info manually rather than relying on the table (assuming you are that fussed).

motdoc

I don't think it can independently of diameter. To me width = diameter.

Bustacapp

Rolf F wrote:

Imagine that the cross section of the tyre is circular. Then the width of the tyre is the same as the diameter of the cross section. Increase the width and the tyre becomes taller as well as wider.

Look at MTBs - small volume tyres on those have very noticeably smaller overall rolling diameters than fat big volume tyres.

As for your computer - the air pressure in your tyre will affect the effective diameter as well; lower pressure will result in a reduced effective diameter. It's best if possible to measure how far the bike actually travels and enter the info manually rather than relying on the table (assuming you are that fussed).

I thought about that but what confuses me is that the diameter of these tyres is 700. Is that diameter to the outer part of the tyre or the inner bead? If the former then these tyres cannot all be described as 700??

lostboysaint

That's an internal diameter. The diameter to the rolling surface will be much greater on a 25mm than a 23, a 28 than 25 etc. for those reasons previously given - basically it needs to remain an air supported "circular" structure so needs to be deeper as it gets wider if it's not to square off completely or need ridiculously high pressures.

Monty Dog

Just to confuse things the International Standard for 700c tyres is that the bead seat diameter is 622mm i.e. a 700x23mm tyre actually has an external diameter nearer 670mm- you'd have to fit something like a 40mm width tyre to get nearer 700mm

Bustacapp

I was about to say the inside diameter being 700 would make sense but now I'm more confused than ever.

Bustacapp

If I measure from outer tyre to outer tyre and multiply it by 3.142 will it give me the exact circumference?

freezing77

Most accurate method, mark a point on the tyre and place at the bottom. Mark the ground, roll the wheel until the same mark is at the bottom, mark the ground measure between the two ground marks.
For greater accuracy use more than one revolution.

tiredofwhiners

If all 700mm wheels were coloured blue, and the instructions said for blue wheels, us X, you wouldn't worry about how the size of the wheels varied - just that they are blue.

A big fat tyre on a 700mm rim will be bigger than a thin, tiny type on a 700mm rim. Think of adding a football on to a vertical drain pipe versus adding a tennis ball onto a vertical drainpipe - which is taller ?

Don't overthink things, just use the number on the table

cougie

Best to do the measurement with you on the bike - or you may be measuring it too high.

Froomes Edgar

Bustacapp wrote:

If I measure from outer tyre to outer tyre and multiply it by 3.142 will it give me the exact circumference?

No, use 3.141592653589793238462643383279502884197169399375105820974944592307816406286 208998628034825342117067982148086513282306647093844609550582231725359408128481 117450284102701938521105559644622948954930381964428810975665933446128475648233 786783165271201909145648566923460348610454326648213393607260249141273724587006 606315588174881520920962829254091715364367892590360011330530548820466521384146 951941511609433057270365759591953092186117381932611793105118548074462379962749 567351885752724891227938183011949129833673362440656643086021394946395224737190 702179860943702770539217176293176752384674818467669405132000568127145263560827 785771342757789609173637178721468440901224953430146549585371050792279689258923 542019956112129021960864034418159813629774771309960518707211349999998372978049 951059731732816096318595024459455346908302642522308253344685035261931188171010 003137838752886587533208381420617177669147303598253490428755468731159562863882 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MattC59

It depends on make of tyre, size of tyre and rim width.

lotus49

Bustacapp wrote:

How can the width of a tyre affect it's circumference? I noted on my cycle computer quick guide chart that same diameter but different width tyres have differing circumferences. How can that be?

Strictly speaking, it can't. The circumference of a circle is pi x diameter and has nothing directly to do with the width of the tyre. However, tyres clearly aren't rectangular so a slight increase in width will usually also increase the height. It won't make much differenc tthough, 2mm extra tyre height will only increase the circumference of the tyre by roughly 1.26cm.

freezing77 had the right idea though. It would make most sense just to measure the circumference of your tyre exactly as suggested. Do sit on the bike rather than just pushing it though as your weight will reduce the height of the tyre.

rolf_f

lotus49 wrote:

Bustacapp wrote:

How can the width of a tyre affect it's circumference? I noted on my cycle computer quick guide chart that same diameter but different width tyres have differing circumferences. How can that be?

Strictly speaking, it can't. The circumference of a circle is pi x diameter and has nothing directly to do with the width of the tyre. However, tyres clearly aren't rectangular so a slight increase in width will usually also increase the height. It won't make much differenc tthough, 2mm extra tyre height will only increase the circumference of the tyre by roughly 1.26cm.

Errrrr, it can! The width is sort of equivalent to the diameter. Of course, inflated tyres are not perfectly round in cross section but generally, width is proportional to rolling diameter - it's not so easy to design a tyre that, when filled with air at 100psi, only expands in height and not width.

lotus49

Rolf F wrote:

Errrrr, it can! The width is sort of equivalent to the diameter. Of course, inflated tyres are not perfectly round in cross section but generally, width is proportional to rolling diameter - it's not so easy to design a tyre that, when filled with air at 100psi, only expands in height and not width.

I don't think you read my post very carefully. Firstly, the question was whether the width (not the diameter) could affect the circumference and I said "Strictly speaking it can't" and it can't. The formula for calculating circumference is related only to the diameter and the value of pi.

If you had read the rest of the post you would have seen that I went on to say that in practice, wider tyres will usually have a greater diameter and so in practice wider tyres will usually have a larger circumference. I even put a number on it.

MattC59

lotus49 wrote:

Rolf F wrote:

Errrrr, it can! The width is sort of equivalent to the diameter. Of course, inflated tyres are not perfectly round in cross section but generally, width is proportional to rolling diameter - it's not so easy to design a tyre that, when filled with air at 100psi, only expands in height and not width.

I don't think you read my post very carefully. Firstly, the question was whether the width (not the diameter) could affect the circumference and I said "Strictly speaking it can't" and it can't. The formula for calculating circumference is related only to the diameter and the value of pi.

If you had read the rest of the post you would have seen that I went on to say that in practice, wider tyres will usually have a greater diameter and so in practice wider tyres will usually have a larger circumference. I even put a number on it.

You're assuming that the tyre cross section (if complete) is round, and that the rim width allows te tyre to maintain this cross section.

lotus49

MattC59 wrote:

You're assuming that the tyre cross section (if complete) is round, and that the rim width allows te tyre to maintain this cross section.

You are quite right I did, but I was just using the numbers as an illustration. As I went on to say, the best way to find the circumference is to measure it not to try to calculate it.

rolf_f

lotus49 wrote:

I don't think you read my post very carefully. Firstly, the question was whether the width (not the diameter) could affect the circumference and I said "Strictly speaking it can't" and it can't. The formula for calculating circumference is related only to the diameter and the value of pi.

You confuse me. I think your post below is a bit muddled. I am assuming that all that stuff about the circumference of the circle must relate to the circumference of the wheel (or it just doesn't make sense). That is neither here nor there. The point I already made was your second point below. The circumference I am talking about is that of the tyre and the width of that does impact the rolling radius of the wheel/tyre combination. I don't think we are actually disagreeing but your first sentence is incorrect in terms of Bustacapps comment and your own third sentence. Apologies if I'm wrong re the above but I do agree entirely with your 3rd and fourth sentences below.

lotus49 wrote:

Strictly speaking, it can't. The circumference of a circle is pi x diameter and has nothing directly to do with the width of the tyre. However, tyres clearly aren't rectangular so a slight increase in width will usually also increase the height. It won't make much differenc tthough, 2mm extra tyre height will only increase the circumference of the tyre by roughly 1.26cm.

lotus49

I have obviously expressed myself very poorly. I'll have another go.

The circumference of a tyre is determined by its height not its width. However, on a bike the two will usually increase in proportion to each other so wider tyres will usually have a larger circumference.

rolf_f

lotus49 wrote:

The circumference of a tyre is determined by its height not its width. However, on a bike the two will usually increase in proportion to each other so wider tyres will usually have a larger circumference.

I did misunderstand what you are getting at though actually you are still incorrect (unless you are talking about car tyres which are a completely different thing). As the tyre bead position is fixed whatever tyre you use, if you increase the width of the tyre, you increase the rolling diameter of the tyre. Basically, the height of the tyre is (simplistically) the same as the width of the tyre. You don't increase one without the other. Of course, tyre shape does vary but probably not by so much and the table of values that Bustacapp was talking about reflects this proportionality.

We should have just listened to Tiredofwhiners who said it most simply and elegantly - illustrated below.

Bustacapp

lotus49 wrote:

I have obviously expressed myself very poorly. I'll have another go.

The circumference of a tyre is determined by its height not its width. However, on a bike the two will usually increase in proportion to each other so wider tyres will usually have a larger circumference.

If you look at this chart, there are lots of '700' tyres that have different circumferences (due to wide tyres etc). Therefore why are they described as 700 diameter when that is obviously not the case?

Pross

Bustacapp wrote:

lotus49 wrote:

I have obviously expressed myself very poorly. I'll have another go.

The circumference of a tyre is determined by its height not its width. However, on a bike the two will usually increase in proportion to each other so wider tyres will usually have a larger circumference.

If you look at this chart, there are lots of '700' tyres that have different circumferences (due to wide tyres etc). Therefore why are they described as 700 diameter when that is obviously not the case?

It's just a nominal size, basically a standard at which a certain size of tyre and a certain size of wheel are compatible.

lotus49

Pross wrote:

It's just a nominal size, basically a standard at which a certain size of tyre and a certain size of wheel are compatible.

Exactly as you say. It's a nominal size so really it's just a name that was based originally on a measurement.

I stick to what I have said several times now in this thread. The width of a tyre does not affect its circumference, only the height does. The two do usually go hand in hand but wider tyres only have a larger circumference because they are taller, not because they are wider.

rich164h

Those figures are ok as a rough guess but do go an measure it (use the mark on the tyre method - with you sitting on the bike) and get the number correct. you can see in that table that the difference between the 25 and the 28 tyre is ~1% by my calculation and whilst that doesn't sound like much I'd be pretty annoyed if I thought I'd completed my first century ride but I'd only completed 99 miles!! It only takes 2 mins to make the measurement.

It should also be noted that the different tyre manufacturers (and even different products by the same manufacturer) do have different width vs height characteristics so that 1% could easily be more than that.

tiredofwhiners

Rolf F wrote:

We should have just listened to Tiredofwhiners who said it most simply and elegantly - illustrated below.

Thank you