Tyre sizes

[Paul A]

I'm looking at getting some new tyres for my bike, I usually buy Specialized Roubaix Pro.
I don't have access to my bike at the moment but I'm almost certain that the last pair were 700c x 25.

But, when looking online at both the Specialized site and Tredz they're showing the size to be 700c x 25/28 ?
Are they 25 or 28 ? Surely they're one size or the other?
The reason I ask is I'm not sure if I have clearance for 28s . The bike is a Colnago Ace.

Thanks

[gregoryoftours]

From the product reviews/q&a section on the tredz site, it looks as if the second (bigger) number is the width of the tyre. So the 23/25 is a 25mm width etc.

[Paul A]

Yes, perhaps the 28 relates to the tyre width and the 25 relates to the height/depth of the tyre from bead to the outer edge of the tread?

[Cugel]

The actual width of a tyre in use is determined to a significant degree by the inner width of the rim the tyre's mounted on. For example, I had some older 700C wheels with 15mm inner rim widths on which "28mm" tyres shrank to 26.5mm wide when mounted; the ladywife has some 700C wheels with 20mm inner rim width on which the same "28mm" tyres become 31.5mm wide when mounted.

As I recall (although I may be wrong) a "28mm" tyre will be at that actual width on rims with an inner width of 17mm. I may have read that on either the Schwalbe website of the Hunt website.

Some manufacturers make their "28mm" tyres a slightly different size than the "28mm" tyres from other manufacturers. This tends to confuse things even more!

Cugel

[Jdsk]

...
But, when looking online at both the Specialized site and Tredz they're showing the size to be 700c x 25/28 ?
Are they 25 or 28 ? Surely they're one size or the other?
...

I'm having a chat with Specialized:
https://www.specialized.com/gb/en/roubaix-pro/p/156469

Have you got a URL for a tyre where Tredz show two numbers, please?

Jonathan

[tatanab]

Have you got a URL for a tyre where Tredz show two numbers, please?

Jonathan

Specialized's own website does that https://www.specialized.com/gb/en/rouba ... 998-156469

[Jdsk]

Have you got a URL for a tyre where Tredz show two numbers, please?

Specialized's own website does that https://www.specialized.com/gb/en/rouba ... 998-156469

Yes. that's the URL that I posted. Is there something similar from Tredz?

Thanks

Jonathan

[Jdsk]

..
But, when looking online at both the Specialized site and Tredz they're showing the size to be 700c x 25/28 ?
Are they 25 or 28 ? Surely they're one size or the other?
...

From today's online chat with Specialized:

This measurement is a bit confusing. Essentially, depending on the profile of your rim. i.e, the internal width of the rim, the tyre will 23mm or 25mm wider depending (if you went with the 700 x23/25c.

... depending on the rim the tyre may come up a bit narrower or wider.


As above:

The actual width of a tyre in use is determined to a significant degree by the inner width of the rim the tyre's mounted on.
...

Jonathan

[axel_knutt]

Schwalbe, and quite probably other manufacturers, have a +/-3mm tolerance on tyre width:

And then you can add variation with tyre pressure on top of that:

And it depends where you measure:

[Paul A]

This is the link to Tredz ...

https://www.tredz.co.uk/.Specialized-Ro ... _45919.htm

Thanks for the replies.

Hmm.. so if clearance is tight you won't really know if they will fit until they're mounted on the wheels.

[tim-b]

It's laid down by the European Tyre and Rim Technical Organisation
ETRTO specifies the rim width for tyre measurement and the pressure that the tyre is inflated to. Measurements must be taken 24 hours after mounting the tyre at the specified pressure, which in your case is 6 bar

A nominal 23mm tyre should be measured on a 15mm internal width rim
A nominal 28mm tyre should be measured on a 17mm internal width rim

You only measure the casing width, extra rubber in the form of side/shoulder knobs and reinforcement should be ignored. The tyre you linked to appears to have extra rubber, maybe that's what they mean. I know from experience that side knobs on a nominal width tyre do cause a problem with some mudguards

An extra 1mm of rim width (for example) shouldn't add 1mm to the measured tyre casing width and there is a manufacturing tolerance as well

[Jdsk]

This is the link to Tredz ...

https://www.tredz.co.uk/.Specialized-Ro ... _45919.htm
...

Thanks.

Today's chat with Tredz.

Jonathan

Me:
What does 23/25 mean, please?

Tredz:
Hi! Welcome to Tredz Live Chat! You’re through to xxx, the width of the wheel that would be

Me:
Thanks. I'm used to tyre sizing, but why does it have two numbers?

Tredz:
Casing: 25c 120 TPI casing
Tread: 23c dual compound

Me:
Thanks. I was wondering if the tyre took different widths depending on the rim size and profile?

Tredz:
No problem at all is there anything else I can help with?