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Disc v Rim Aero
Last Post 12/17/2014 05:55 AM by Dave Kirkpatrick. 10 Replies.
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dkri
Posts:95
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| 12/11/2014 06:37 PM |
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I don't know who is still hanging around, or who all reads where ever else, but the disc versus rim aerodynamics topic is getting pretty hot. The test that we did with our former hosts got published in their print edition on Monday. That is now available for download. Specialized also published a test from their tunnel on Tuesday. That was a heck of a coincidence.
I don't know if it's surprising or not, but our results differ substantially from theirs. I have no idea how to account for the differences. Our test could not have been any more straightforward, and I reviewed the raw files today, the error bars are nearly nonexistent.
Anyhow, just like anything else, some people will think that the differences are in Normas, and some people will think they are beyond insignificant. I'm just glad that it's finally out there, and the conjecture and eyeball aerodynamics can stop. |
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| formerly dkri |
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jmdirt
Posts:775
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| 12/11/2014 09:57 PM |
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I saw it in my mag but didn't read it yet...I will in just a minute now!  |
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Cosmic Kid
Posts:4209
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| 12/11/2014 10:53 PM |
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I've been following the ST thread on the Specialized test. Interesting stuff, especially the reaction of some who are violently opposed to discs. Funny how they laud Specialized for all their testing until they get results they don't like.....
dkri, you say your results are different than Specialized. I haven't seen the new mag..... can you give us a quick synopsis? |
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| Just say "NO!" to WCP!!!!
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dkri
Posts:95
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| 12/12/2014 07:19 AM |
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We found the drag penalty to be higher, and more asymmetric than Specialized did.
Wheels only, a Rail 52 disc with 24 spokes still has a notable advantage over an aerodynamically quite competent 28mm deep alloy front. People decry wheel only testing but the results from it are shown to translate at full strength into on-bike results.
There's also a bit of a "burn the heretics and their wind tunnels" sentiment - I notice that too, CK. Not just on ST. I think people are mostly opposed because they think the S test gives the industry to shove disc down their throats. I don't know. We showed worse results for disc and we honestly don't give a rat's patoot whether people prefer discs or not, we are happy and well positioned either way. Our new frames ship from Italy today, and there's a disc frame among them. |
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| formerly dkri |
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jmdirt
Posts:775
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| 12/12/2014 08:43 AM |
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"Our new frames ship from Italy today, and there's a disc frame among them."
I don't see pictures on your web site so I'm assuming those will follow your post soon. |
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dkri
Posts:95
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| 12/12/2014 10:47 AM |
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Yeah, we invited some of existing customers in to a private sale, sort of a pre-launch thing, and are getting those with our first stock of frames in shortly. So we've put them on our FB page and on our Instagram (http://instagram.com/novemberbicycles) and then we will put them on the site soon. We're also launching a new site - we're great at biting off more than we can chew and then just gutting through it. It'd be cooler if we didn't do that to ourselves but we always seem to.
We were really surprised that no one chose the disc option for the private sale. We figured that at least a couple would. Nope.
We kind of went nuts on the paint scheme. We've been sick of the "do you have anything that's blacker than black" look from day 1, so we kind of blew that look completely up. |
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| formerly dkri |
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Hoshie
Posts:134
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| 12/14/2014 10:48 PM |
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Surprising - I think those with an investment in high end wheels may wait on discs.
Can one get a wider volume tire in your new disc frame?
I personally see one advantage as the ability to fit 28s or 30s occasionally beyond all the other actual braking benefits.
Good that you are testing - it's an important thing to do.
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dkri
Posts:95
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| 12/15/2014 09:07 AM |
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Piece went live on Velonews.com this morning - http://velonews.competitor.com/2014/12/bikes-and-tech/discs-drag-wind-tunnel-testing-disc-brake-road-bikes_355621
A tire that measures 28 will fit in the disc frame. Phrasing of that is important - a Conti GP4000 "25" on an Open Pro is going to be narrower and shorter than a GP4000 "23" is on a Rail, because of the bead seat width. Conti GP4000 23 on a Rail is 25.8mm wide and 23.1mm tall, same tire is 24.0w x 22.8t on a 404. There are also at least two mold for Conti GP4000 23, and they have the better part of .5mm difference in their inflated size on a Rail. A GP4000 "25" measures 27.3 x 24.3 on a Rail.
A very rough guide is that for every 2mm of bead seat width you add in the rim, you will make the tire 1mm wider. Height depends too much on bead hook shape to give that clean a guideline.
Tire height is most often the limiting factor. With rim brakes, the tire can hit the brake arms. With rim or disc, the tire can hit the back of the seat stay. People usually think of tire width as limiting factor, but that's only sometimes the case. |
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| formerly dkri |
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ChinookPass
Posts:809
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| 12/16/2014 11:48 AM |
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Why is the delta larger from the drive side?
![]()
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Cosmic Kid
Posts:4209
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| 12/16/2014 06:56 PM |
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I'm spitballing here, CP, but my guess would be two reasons:
1) discs are acting like a block in the "dirty air" side of the bike when wind is from the drive side. It is catching the already disturbed airflow and making it worse.
2) Slight "sail" effect from a broadside presentation of the disc when on the prevailing wind side.
Again, complete guesses.... maybe dkri can provide more info. |
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| Just say "NO!" to WCP!!!!
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dkri
Posts:95
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| 12/17/2014 05:55 AM |
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| CK got it. |
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| formerly dkri |
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