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How much time do I lose with rear tire "ballooning" out wider than disc wheel?
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I have an old Renn 575 disc wheel, with 21mm outside rim width. I'm riding 23mm Conti GP4000S II tires, which bulge out beyond the width of the rim. I know that having the tire bulge out beyond rim width is slower and that'd I'd be faster with a wider wheel. I have decided that the increased rolling resistance of going to a narrower tire pumped to higher pressure is probably greater than what I'm losing in aerodynamic drag, but it's just a guess. I could also put a disc cover onto a lightweight rear wheel I recently built that has a 24mm outer width rim.

How much do I actually lose aerodynamically by having a wider tire than the disc? It's all in "dirty air" anyway, so maybe not that much? On the other hand, it creates the trailing edge of the whole bike/rider system, so it might make small eddies at the back that are significant?
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Re: How much time do I lose with rear tire "ballooning" out wider than disc wheel? [nilloc] [ In reply to ]
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I don't recall a test that compared tire width on a disc. But your reasoning is sound.

The performance of a wheel cover is highly dependent on how uniform it is. IME they are never perfect. But a very good fit would perform just like a disc.

There are tires that are considerably better than the GP4000 for Crr. The GP5000, GPTT, and Attack are all "safe" bets. The Corsa Speed and Supersonic are a bit less safe, but faster.
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Re: How much time do I lose with rear tire "ballooning" out wider than disc wheel? [nilloc] [ In reply to ]
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If you have a frame that shrouds the bulging tire, the difference is probably too little to be measured in a wind tunnel, on your bike.
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Re: How much time do I lose with rear tire "ballooning" out wider than disc wheel? [rruff] [ In reply to ]
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rruff wrote:
I don't recall a test that compared tire width on a disc. But your reasoning is sound.

The performance of a wheel cover is highly dependent on how uniform it is. IME they are never perfect. But a very good fit would perform just like a disc.

There are tires that are considerably better than the GP4000 for Crr. The GP5000, GPTT, and Attack are all "safe" bets. The Corsa Speed and Supersonic are a bit less safe, but faster.

Thanks. It's good to have somebody else corroborate my thoughts. I knew the GP4000 S II isn't the fastest, but for a long time seemed like a pretty fast tire that was still pretty safe. Whenever I replace the current tires I think I'll move to the GP5000. IIRC, they're at least as fast as the GPTT and Attack, right?
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Re: How much time do I lose with rear tire "ballooning" out wider than disc wheel? [nickwhite] [ In reply to ]
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Thanks, Nick. I'm on a 1st gen. Speed Concept. Looks like I'll just stick with my current disc.
nickwhite wrote:
If you have a frame that shrouds the bulging tire, the difference is probably too little to be measured in a wind tunnel, on your bike.
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Re: How much time do I lose with rear tire "ballooning" out wider than disc wheel? [nilloc] [ In reply to ]
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nilloc wrote:
IIRC, they're at least as fast as the GPTT and Attack, right?

No not for Crr, and if Conti upgrades the compound in the GPTT and Attack then the gap will be substantial. Currently they are pretty close. And the GP5000 is a bit smaller I think, which will help with the trailing edge aero.
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Re: How much time do I lose with rear tire "ballooning" out wider than disc wheel? [nilloc] [ In reply to ]
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nilloc wrote:
Whenever I replace the current tires I think I'll move to the GP5000. IIRC, they're at least as fast as the GPTT and Attack, right?

Full aero and RR comparisons: https://www.aero-coach.co.uk/gp-5000-data
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Re: How much time do I lose with rear tire "ballooning" out wider than disc wheel? [RichardL] [ In reply to ]
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Thanks for the link!
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Re: How much time do I lose with rear tire "ballooning" out wider than disc wheel? [nilloc] [ In reply to ]
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I wondered the same with my P4.
Testing was a bit of a wash but found that the 21 hid better but suffered on RR, the 25 was worse and the 23 hit the spot. The RR of the Conti TT outweighed the aero of the GP4000, but I reckon the best of all could be the GP5000.
The P4 has a narrowish rear wheel cut, so other frames might need a 25, however I think even then a 25 might well win overall when RR is taken into account.
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