3/15/15 9:04 PM

Is there a web site anywhere with a list of actual measured tyre sizes, as opposed to the manufacturers (usually) fictitious claims. I usually use 28mm tyres for commuting / training / light touring, but a couple that I've purchased recently have been considerably smaller than the claimed size: Vittoria Rubino Pro 700x28 measures 25mm and Conti GP 4 Seasons 700x28 measures 26mm. Both these measurements with the tyres mounted on Velocity Aerohead rims and a week after being inflated to pressure.

3/15/15 10:16 PM

Other than googling each one for actual size listed in forums by the users, I have not seen one.

I will say my experiences, in fact it is here already on the site in a few places/threads.

The 28 Conti All Seasons run small.

All the numbers I am about to mention are widths, not height. The GP4KS Chili 25s I have get to 28 on wide rims. The 23s are real 23s on 15-17C rimes but on 19C Rim I get 25 for the 23GP4s, and 25s Chili as I said, 28mm.

25mm Rubinos I get 25 on wide rims and 24 on narrow rims with the threaded ones. The slick Rubino 23mm on a narrow rim I get 23.3 after a few days.

I got some NOS 25mm Axial Pros last year for 12.00 each. Those sit out at 28.8mm on the wider rims. Nice for when you want that.

I just mounted some Pro4 Service Course on Pacenti SL23 rims and that is the widest of all these rims I have for road. VS the HED Belgiums and A23s I have here. The 23mm Pro4 SC after a few days and a few rides is 24.8.

3/16/15 5:35 AM

...is to ask here or on other forums where people can actually measure them and provide direct information. I'm not aware of any tire width databases, just tire weight databases.

3/16/15 7:17 AM

I have Vittoria Randonneur "700-28" tires mounted on Shimano WH-R500 rims and they also measure 26 mm wide as installed.

I also have Vittoria Zaffiro Pros 700-23 on Mavic CXP-33 rims and they measure 23.6 mm and Rubino Pro Slicks7 00-23 on Shimano WH-R560 rims and they measure 24.5 mm or actually larger than labeled.

3/25/15 12:34 AM

The effect of rim width on tyre size:

I just purchased a pair of DT Swiss R23 wheels (the rims have an 18mm internal width and 23mm external width). The wheels with Mavic MA2 rims that I was previously using had Schwalbe Ultremo ZX 700x28 mounted that measured 27.5mm wide by 27.5mm high on those rims. The MA2 rims have a 13.5mm internal width.

The exact same tyre, when moved to the R23 rim, which has an 18mm internal width, now measures 29.5mm wide by 28mm high...

3/25/15 7:11 PM

As previously reported, a Continental GP 4000s 25mm tire measures 1 mm wider on a Velocity A23 rim vs. a Velocity Aerohead. I don't know the internal width of those two rims off the top of my head but since the external width is 3 mm larger on the A23 I assume the internal width is also 3 mm wider.

3/28/15 7:54 PM

A wider rim effectively increases the circumference of the tire, which means that the diameter should increase by ~1/3 of the rim width change.

3/29/15 7:25 PM

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A wider rim effectively increases the circumference of the tire

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3/29/15 7:30 PM

Think Y axis

3/29/15 7:46 PM

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Think Y axis

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3/29/15 7:55 PM

Circumference of the roundness of the tire itself, not the wheel? As in a 25C tire being 25 or 27mm across the Y axis.

3/30/15 7:01 AM

Well, if you think about the total circumference of the enclosed space, the wider rim contributes to a greater total circumference (arc of the rim plus the arc of the tire). If you assume the space is round, the increased circumference must lead to an increased diameter.

3/30/15 9:41 AM

I'm with Kerry. This assertion seems counter intuitive to me.

In my thinking, spreading the bead surface of the tire over a wider rim dimension (with the same air pressure), would result in dropping the center of the tread closer to the rim surface and should lead to a decreased effective radius, and thus a smaller circumference.

Not sure I have visualized how the opposite will work???

3/30/15 10:51 AM

I think we're visualizing two different dimensions here. Some of us are referring to the circumference of the tire around the axis of the hub while others are talking about the circumference of the cross-section of the tire/wheelbed.

3/30/15 11:16 AM

I was referring to the tire cross-sectional diameter, not the overall diameter.

3/30/15 11:25 AM

It was more math than Zen [i think]. LOL.

3/30/15 3:03 PM

Take the limiting example of a rim so narrow that the two tyre beads touch each other. In this case the tyre forms the entire circumference of its cross-section, and both its width and height above the rim wall are at a minimum. As you gradually widen the rim, both the width of the tyre and its height above the rim wall will increase.

3/31/15 7:12 PM

There are two circumferences here. One is of the cross section of the tire (which includes the rim width) and the other is the circumference of the tire as mounted on the rim and which therefore determines the number of revolutions per unit distance that the wheel displays.

Widening the rim increases the tire's cross sectional circumference and decreases the circumference of the tire as measured for calibrating a bike computer.

3/31/15 8:34 PM

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Widening the rim increases the tire's cross sectional circumference and decreases the circumference of the tire as measured for calibrating a bike computer.

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4/1/15 7:22 PM

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Both measured by a roll-out test and by the height of the tyre centre above the rim wall, a wider rim makes a tyre taller and therefore a longer roll-out.

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