New (Wider) Rim + Same Tire = Same Clearance?
04-07-14, 06:21 AM
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New (Wider) Rim + Same Tire = Same Clearance?
My road bike fits with very tight clearance my Conti Touring Plus 700x28c tires. Other 700x28c tires that I've tried rub the frame. It's tight. The current rim is a 700x19. I'm looking at replacing the current wheels with CR-18s or something similar. Would the wider rim (or simply it being a different rim) present a clearance issue if I'm using the same tire?
04-07-14, 07:09 AM
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In theory, if you divide the change in rim width by pi (~3.1416), you'll get the change in mounted tire width. In other words, a 3 mm increase in rim width will give you about a 1 mm increase in mounted tire width. The real math is considerably more complicated, but this is a decent approximation in most cases. Since it sounds like you're already close to zero clearance, I'd be really cautious about changing rims.
04-07-14, 07:14 AM
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In general the same tire will run slightly flatter (lower) on a wider rim. The beads have to reach wider to hook up with the rim. That flattens the tire a little. The side walls shouldn't be much wider except maybe very close to the beads since in your case the tire is still wider than the rim unlike running a 23 mm tire on a 23 mm rim.
Last edited by rpenmanparker; 04-07-14 at 07:31 AM.
04-07-14, 07:15 AM
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Partly it depends on where the tires rub. If they rub the seatstays, chainstays or fork blades wider rims will make the situation worse. If they interfere at the seattube, brake bridge or fork crown a wider rim will have a very minor beneficial effect.
Last edited by HillRider; 04-07-14 at 07:29 AM.
04-07-14, 04:26 PM
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In general the same tire will run slightly flatter (lower) on a wider rim. The beads have to reach wider to hook up with the rim. That flattens the tire a little. The side walls shouldn't be much wider except maybe very close to the beads since in your case the tire is still wider than the rim unlike running a 23 mm tire on a 23 mm rim.
Tires are pinched in forming sort of an Omega shaped profile. Air pressure tends to make the internal chamber round, wxcept where constrained by the rim. Allowing the beads to move apart increases the working circumference by the change in rim width (circumference = flat width of the unrestricted wall of the tire + rim width). A bigger circumference means a bigger diameter, both vertical and horizontal.
This would hold true until the rim was so wide that the tire ws stretched into a low arch, in which your logic would hold. But tires aren't on rims that wide, so it doesn't.
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04-07-14, 05:05 PM
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Sorry, but you have this backward for the normal tire and rim configuration.
Tires are pinched in forming sort of an Omega shaped profile. Air pressure tends to make the internal chamber round, wxcept where constrained by the rim. Allowing the beads to move apart increases the working circumference by the change in rim width (circumference = flat width of the unrestricted wall of the tire + rim width). A bigger circumference means a bigger diameter, both vertical and horizontal.
This would hold true until the rim was so wide that the tire ws stretched into a low arch, in which your logic would hold. But tires aren't on rims that wide, so it doesn't.
Tires are pinched in forming sort of an Omega shaped profile. Air pressure tends to make the internal chamber round, wxcept where constrained by the rim. Allowing the beads to move apart increases the working circumference by the change in rim width (circumference = flat width of the unrestricted wall of the tire + rim width). A bigger circumference means a bigger diameter, both vertical and horizontal.
This would hold true until the rim was so wide that the tire ws stretched into a low arch, in which your logic would hold. But tires aren't on rims that wide, so it doesn't.
04-07-14, 05:14 PM
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No problem, but Hillrider was saying much the same thing as I did. I was sure I was relating the accepted belief with regard to benefits of wider rims with same tires. That the resulting lower tire profile gave better stability in cornering and allowed running lower pressures. Perhaps I had misunderstood all that.
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Last edited by FBinNY; 04-07-14 at 05:22 PM.
04-07-14, 05:53 PM
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No problem, but Hillrider was saying much the same thing as I did. I was sure I was relating the accepted belief with regard to benefits of wider rims with same tires. That the resulting lower tire profile gave better stability in cornering and allowed running lower pressures. Perhaps I had misunderstood all that.
04-07-14, 06:08 PM
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Yeah, I did some back-of-the envelope sketches and calculations. Here is where I think my mistake arose. The same tire on a wider rim is, as you say, wider and taller. BUT more of the circle roughly defined by the tire is below the top of the rim (imaginary). Not enough to shorten the tire relative to mounting it on a narrower rim, but enough to change the aspect ratio. So it resembles a taller, wider tire that is buried a bit more down in the rim. So it is taller but RELATIVELY shorter compared to width. I guess that explains the source of the widely promoted benefits. Does this make any sense?
