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Fooled by tire diameter

Old 09-22-20, 06:29 PM
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zeeway
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Fooled by tire diameter

I had the bright idea of changing my worn 700c 25mm Conti Grand Prix 4000 tires to new 32mm Conti Grand Prix 5000 tires on my road bike. I checked width clearances at the chain stays and the fork, etc., and it looked like with a little adjustment of chain stay clearance, it would work.

But the idea failed because the new tires were significantly bigger in diameter, even though both were Conti and both were 700c tires. Big disappointment...not to mention surprise that diameters on 700c tires from same manufacturer were different. Oh well.
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Old 09-22-20, 06:33 PM
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Originally Posted by zeeway View Post
I had the bright idea of changing my worn 700c 25mm Conti Grand Prix 4000 tires to new 32mm Conti Grand Prix 5000 tires on my road bike. I checked width clearances at the chain stays and the fork, etc., and it looked like with a little adjustment of chain stay clearance, it would work.

But the idea failed because the new tires were significantly bigger in diameter, even though both were Conti and both were 700c tires. Big disappointment...not to mention surprise that diameters on 700c tires from same manufacturer were different. Oh well.
huh?
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Old 09-22-20, 06:42 PM
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Outside diameter?
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Old 09-22-20, 06:44 PM
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Originally Posted by zeeway View Post
I had the bright idea of changing my worn 700c 25mm Conti Grand Prix 4000 tires to new 32mm Conti Grand Prix 5000 tires on my road bike. I checked width clearances at the chain stays and the fork, etc., and it looked like with a little adjustment of chain stay clearance, it would work.

But the idea failed because the new tires were significantly bigger in diameter, even though both were Conti and both were 700c tires. Big disappointment...not to mention surprise that diameters on 700c tires from same manufacturer were different. Oh well.
Seem obvious to me.

This is why some cyclocross riders go to a 650b wheels when running very "wide" tires.
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Old 09-22-20, 06:47 PM
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Bike tires are essentially a circle when mounted and inflated so if they are wider they must also be taller. Has nothing to do with Continental.
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Old 09-22-20, 07:01 PM
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Originally Posted by zeeway View Post
I had the bright idea of changing my worn 700c 25mm Conti Grand Prix 4000 tires to new 32mm Conti Grand Prix 5000 tires on my road bike. I checked width clearances at the chain stays and the fork, etc., and it looked like with a little adjustment of chain stay clearance, it would work.

But the idea failed because the new tires were significantly bigger in diameter, even though both were Conti and both were 700c tires. Big disappointment...not to mention surprise that diameters on 700c tires from same manufacturer were different. Oh well.
Other things being equal, you would expect the 32s to be 7mm wider than the 25s. How much clearance did you have from the chain stays with the 25s?

Other things are not exactly equal as the ETRTO (European tire standards association) recently updated its specs for the rim size that tire makers should use to specify tire widths, and shifted to wider rims to reflect riders shifting to wider rims.

This happened between the introduction of 4000s and 5000s and should actually mean the 5000 series 32s should run slightly narrower than older Conti 32s (on the same rim). Still it sounds like more than what your frame can handle.

Also, I’m curious, what do you mean by adjustment of chain stay clearance? That’s pretty much fixed for a given frame and tire diameter.

Otto

Last edited by ofajen; 09-22-20 at 07:04 PM.
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Old 09-22-20, 07:16 PM
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"You've fooled me this time, 32mm tires! But next time, I shall have the last laugh! Hahahahaha!"
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Old 09-22-20, 07:20 PM
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Originally Posted by SurferRosa View Post
"You've fooled me this time, 32mm tires! But next time, I shall have the last laugh! Hahahahaha!"
Could I get some evil 32s?

Otto
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Old 09-22-20, 07:48 PM
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Yes, outside diameter. Guess this is part of my education. In the auto world, wider does not necessarily mean a larger outer diameter. Not so in bikes, I now know.
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Old 09-22-20, 07:52 PM
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Originally Posted by zeeway View Post
In the auto world, wider does not necessarily mean a larger outer diameter.
A 195/60-15 is 24.213" tall
A 205/60-15 is 24.685" tall
A 225/60-15 is 25.630" tall

The aspect ratio is 60% of the width.
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Old 09-23-20, 07:31 AM
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Originally Posted by GlennR View Post
A 195/60-15 is 24.213" tall
A 205/60-15 is 24.685" tall
A 225/60-15 is 25.630" tall

The aspect ratio is 60% of the width.
Semantics...perhaps. I said not necessarily...you can also buy 40 series tires with less sidewall height.
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Old 09-23-20, 07:47 AM
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Originally Posted by zeeway View Post
Semantics...perhaps. I said not necessarily...you can also buy 40 series tires with less sidewall height.
Yes, but you said "wider", not different aspect ratio.

Also a bicycle tire does not have the same profile as a car. If you compared it to a motorcycle tire you would of been better off.
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Old 09-23-20, 08:23 AM
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Originally Posted by zeeway View Post
Yes, outside diameter. Guess this is part of my education. In the auto world, wider does not necessarily mean a larger outer diameter. Not so in bikes, I now know.
So, the issue was lack of outer clearance with brake arms, brake stay and/or fork crown?

Unloaded bike tire profile tends to be roughly circular, so you can estimate resultant outer diameter by taking bead seat diameter (BSD) plus two times tire width.

In this case, BSD for 700C is actually 622mm, so outer diameter is roughly = 622 + 2 * 32 = 686 mm, or a radius of 343mm.

Otto

Last edited by ofajen; 09-23-20 at 08:27 AM.
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Old 09-23-20, 09:45 AM
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Originally Posted by zeeway View Post
Yes, outside diameter. Guess this is part of my education. In the auto world, wider does not necessarily mean a larger outer diameter. Not so in bikes, I now know.
Rectangular cross section for one (two numbers) versus round for the other (one number).

Narrower bicycle rims make the diameter larger.
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Old 09-25-20, 06:47 PM
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What everyone else. said. It's generally a Bad Idea to change tire sizes (widths) by a lot, at least on a road bike. There, a 23mm might fit fine but a 25mm won't. Mountain bikes tend to be a little more forgiving...
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Old 09-26-20, 09:42 AM
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Originally Posted by zeeway View Post
I had the bright idea of changing my worn 700c 25mm Conti Grand Prix 4000 tires to new 32mm Conti Grand Prix 5000 tires on my road bike. I checked width clearances at the chain stays and the fork, etc., and it looked like with a little adjustment of chain stay clearance, it would work.

But the idea failed because the new tires were significantly bigger in diameter, even though both were Conti and both were 700c tires. Big disappointment...not to mention surprise that diameters on 700c tires from same manufacturer were different. Oh well.
How do you "adjust" the chain stay clearance ?
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Old 09-26-20, 10:22 AM
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i needed this, thank you
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Old 09-26-20, 10:33 AM
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Originally Posted by zeeway View Post
Semantics...perhaps. I said not necessarily...you can also buy 40 series tires with less sidewall height.
Exactly. Automotive tire widths aren't locked into one aspect ratio.

But, as you now know, bike tires aren't automotive tires.
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Old 09-26-20, 01:46 PM
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Just like you can look up the diameter/circumference of a 195/60-15 and a 225/60-15 and find out they are different, so too can you look up the diameter/circumference of a 25-622 and a 32-622 and find out they are different.

(To a first approximation, a 32-622 is a 32/100-622, metric/aspect-metric instead of the auto metric/aspect-U.S.)

-mr. bill

Last edited by mr_bill; 09-26-20 at 01:49 PM.
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Old 09-27-20, 09:44 AM
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Originally Posted by branko_76 View Post
How do you "adjust" the chain stay clearance ?
I'm interested to see the answer to this question too!
I don't know what they are called but some bikes have inserts placed in the dropouts that prevent the axle from seating completely into the dropout. Removing this insert would allow a bit more rearward movement (with horizontal dropouts) of the wheel and potentially allowing additional clearance at the front end of the chainstays.
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Old 09-27-20, 12:18 PM
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You ‘adjust’ chain stay clearance by ‘dimpling’ the chain stay. Works well on steel frames...but risky on aluminum...and not possible on carbon frames.
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Old 09-27-20, 12:26 PM
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Originally Posted by zeeway View Post
You ‘adjust’ chain stay clearance by ‘dimpling’ the chain stay. Works well on steel frames...but risky on aluminum...and not possible on carbon frames.
ouch
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Old 09-27-20, 12:58 PM
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Originally Posted by zeeway View Post
You ‘adjust’ chain stay clearance by ‘dimpling’ the chain stay. Works well on steel frames...but risky on aluminum...and not possible on carbon frames.
Originally Posted by branko_76 View Post
ouch
Beats using a Dremel.
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Old 09-27-20, 01:33 PM
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Originally Posted by zeeway View Post
Yes, outside diameter. Guess this is part of my education. In the auto world, wider does not necessarily mean a larger outer diameter. Not so in bikes, I now know.
That is why car tires are specified by profile as well as width. 245x60 tires do have a larger diameter than 225x60 series tires. To keep the same height you need a lower profile.

Here is a good reference for the circumference (hence diameter) of bike tires by size.

https://www.cateye.com/data/resource...ENG_151106.pdf

A 700x25c tire has a diameter of 2105/3.14 mm while a 700x32c is 2155/3.14 mm
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Old 09-27-20, 02:12 PM
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Originally Posted by Pop N Wood View Post
That is why car tires are specified by profile as well as width. 245x60 tires do have a larger diameter than 225x60 series tires. To keep the same height you need a lower profile.

Here is a good reference for the circumference (hence diameter) of bike tires by size.

https://www.cateye.com/data/resource...ENG_151106.pdf

A 700x25c tire has a diameter of 2105/3.14 mm while a 700x32c is 2155/3.14 mm
Just for funsies, a 245/60 and a 225/60 can have the same diameter. You left out the third variable in car tire measurements which is the wheel size. The 245 or 225 mentioned are the tire width. The 60 is the sidewall height expressed as a percentage of the tire width. a 245/60R16 is the same diameter as a 225/60R17.
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