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Safe assumption re: tire dimension with wider wheels?

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Safe assumption re: tire dimension with wider wheels?

Old 12-12-15, 10:15 AM
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Sy Reene
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Safe assumption re: tire dimension with wider wheels?

Though eg. a 25mm tire will be wider when mounted on eg. a 23mm vs 17mm rim, it will not be as tall (ie. on wider rim, the tire will have more clearance above the tire to eg. the seatstay bridge or fork crown). Is this safe or not to assume?
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Old 12-12-15, 11:42 AM
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I believe that is correct.
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Old 12-12-15, 12:00 PM
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I wouldn't bet on the tire not being as tall.
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Old 12-12-15, 12:02 PM
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I think the difference would be very, very small, if any at all. Try it. Let us know if that's the case.
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Old 12-12-15, 12:05 PM
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The wider the rim, the less tall the tire will be. Sources: geometry, life.
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Old 12-12-15, 12:51 PM
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Originally Posted by pacificaslim View Post
The wider the rim, the less tall the tire will be. Sources: geometry, life.
Life will show that there are variables that geometry doesn't take into consideration.
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Old 12-12-15, 12:54 PM
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Wrong. They are not shorter. There is a graph showing the comparisons. Wish I knew where to find it for you. Someone else will.
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Old 12-12-15, 12:59 PM
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Originally Posted by rpenmanparker View Post
Wrong. They are not shorter. There is a graph showing the comparisons. Wish I knew where to find it for you. Someone else will.
Actually I'm just hoping that they would be no taller; not looking for shorter necessarily. I have acceptable clearance now on narrow rims, but maybe an issue if a slightly wider rim yields an actually taller tire. Geometry suggests it shouldn't be, but at the same time, geometry doesn't account for inflation and stretch capabilities of tire.
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Old 12-12-15, 01:18 PM
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Old 12-12-15, 01:18 PM
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If you think about of the wider rim defining a larger arc, the available arc supplied by the tire adds up to a larger circle.
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Old 12-12-15, 01:35 PM
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Originally Posted by rpenmanparker View Post
If you think about of the wider rim defining a larger arc, the available arc supplied by the tire adds up to a larger circle.
It's an interesting diagram.. if you imagine each colored arc though as a piece of string and laid them out.. I think the diagram is showing a longer piece of string being used for the same tire on a wider-rimmed wheel.
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Old 12-12-15, 01:39 PM
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Originally Posted by Robius View Post
See OP. What did I tell you? It's like game theory. You can always count on someone else doing the grunt work when you can't. Thanks, robius for coming up with the graph.
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Old 12-12-15, 01:40 PM
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Originally Posted by Sy Reene View Post
It's an interesting diagram.. if you imagine each colored arc though as a piece of string and laid them out.. I think the diagram is showing a longer piece of string being used for the same tire on a wider-rimmed wheel.
I don't think so. The lines for the wider rims don't reach as far to the middle.
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Old 12-12-15, 02:24 PM
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Originally Posted by pacificaslim View Post
The wider the rim, the less tall the tire will be. Sources: geometry, life.

I take this back. The tire will only get wider after the beads are separate to a certain point - and that point is probably much wider than available road bike rim widths. It's the point at which the beads are wider than where they'd be for the tire to form a perfect half circle. As the beads come closer together than that, the tire must bulge up higher. Wide than that point, the tire must give up some height to be used for width.
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Old 12-13-15, 09:43 AM
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Originally Posted by rpenmanparker View Post
I don't think so. The lines for the wider rims don't reach as far to the middle.
Maybe not.. looked to me that eg. you could slide the red 25mm diagram (on Ardenn+) due right to overlay the 25mm (white) rule on the 19mm rim so bottom of left rim side is in same place. Then wouldn't it be obvious the red line is extending much further by being taller and finishing way off to the right of the white line?
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Old 12-13-15, 10:22 AM
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Tires don't form a perfect circle. The tread of some tires has a defined shape, which in combination with the more flexible sidewalls influences the final shape of the tire. It seems that this is why the height and width don't change in a linear fashion as the inner rim width grows.
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Old 12-13-15, 11:34 AM
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I just did a quick test. I have the same 25mm Conti gp4000sII tires mounted on three different width rims. Here are the results

Open4CD ext 19.5mm, int 13.1mm Tire width 25.27mm Rollout 209.55cm
Velocity A23 ext 23 int 17.8 Tire width 26.8 Rollout 210.65
Farsports 28 ext 27.5 int 19.5 Tire width 27.46 Rollout 209.70

I can't claim complete accuracy but the rollout compares nicely with the gap between the tire and the brake arch. The A23 had the least gap and the Open4CD and the Farsports were almost identical. A lot probably depends on the shape and size of the bead.

Surprising that the widest and narrowest rims had nearly identical rollouts (total diameter). I tried measuring the distance from the rim to the edge of the tire but couldn't get consistent results so I did the rollout measurement.

Edit: All tires were at 90lbs and the Farsports had a brand new tire. The other two are in the 1k mile range.

Last edited by Bluechip; 12-13-15 at 12:04 PM.
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Old 12-13-15, 11:41 AM
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Consider this.

Air pressure within causes the tire to assume a nearly circular profile, with a circumference equal to the tire's flat bead to bead width + the rim width.

So--- Wider rim -----> bigger circumference -----> bigger diameter -----> taller height over rim.

The exact effect will vary with the suppleness of the tire, with the most supple tires being the most perfect circle.
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Old 12-13-15, 02:41 PM
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Originally Posted by Bluechip View Post
I just did a quick test. I have the same 25mm Conti gp4000sII tires mounted on three different width rims. Here are the results

Open4CD ext 19.5mm, int 13.1mm Tire width 25.27mm Rollout 209.55cm
Velocity A23 ext 23 int 17.8 Tire width 26.8 Rollout 210.65
Farsports 28 ext 27.5 int 19.5 Tire width 27.46 Rollout 209.70

I can't claim complete accuracy but the rollout compares nicely with the gap between the tire and the brake arch. The A23 had the least gap and the Open4CD and the Farsports were almost identical. A lot probably depends on the shape and size of the bead.

Surprising that the widest and narrowest rims had nearly identical rollouts (total diameter). I tried measuring the distance from the rim to the edge of the tire but couldn't get consistent results so I did the rollout measurement.

Edit: All tires were at 90lbs and the Farsports had a brand new tire. The other two are in the 1k mile range.
Awesome info which confirms what folks have been saying. Thanks. So the 2107mm circumference wheel (A23) means a 671mm diameter. The 2096mm wheel (Open4CD) yields a 667mm diameter.. so roughly 4mm diff = 2mm height added. The 19.5mm internal width rim starts to revert back to a flattened circle apparently.. BTW, that Farsports rim had as somewhat large internal vs external width differential.
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