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PostPosted: Mon Aug 22, 2016 6:02 pm 
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Posts: 542
I keep seeing people posting about aero rims then they mention how they use it with a 25mm clincher tires.

Based on the latest post on the silca blog, there is some great data on the "105%" rule. To get the benefits of your aero rim, your external rim width has to be at least 5% wider than your tires measured width. Given that clincher tires tend to balloon up well over their marked size on wider modern rims, they simply cancel almost all the aero benefits you would get from your rim.

There are a few exception where you can use a 25mm tire with modest aero penalty, like in the case of Zipp 303/404 or the newest ENVE wheels. But in general, running clincher rims with anything wider than 23mm tire will ruin your aero benefits and probably you are better of using 22mm tires. On the other hand, if you go narrower, you will get a hit in the Crr, so I'd question the whole point of using an aero clincher rim that's narrower than 27mm.

Link to the post: https://silca.cc/blogs/journal/part-5-t ... rodynamics


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Posted: Mon Aug 22, 2016 6:02 pm 


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PostPosted: Mon Aug 22, 2016 9:13 pm 
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If that's your conclusion then why does the blogpost show lower drag for the 25c tire on a Zipp 404 than a 23c tire? (ignoring the fact that Conti's run wide on wide rims and the 25c is more than likely 27mm wide in the test scenario). I'd call that an aerogain, not a penalty ;)


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PostPosted: Mon Aug 22, 2016 10:25 pm 
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snaaijer wrote:
If that's your conclusion then why does the blogpost show lower drag for the 25c tire on a Zipp 404 than a 23c tire? (ignoring the fact that Conti's run wide on wide rims and the 25c is more than likely 27mm wide in the test scenario). I'd call that an aerogain, not a penalty ;)


In the graph you mention the fastest tire is a 23c gp4000. The other three line in the graph shows the aero penalty of higher tire pressure in the case of 25c. Also the 404 is wider than most of the other clincher rims at 26.53mm. Also the inner width of the rim is 16.25mm so the 25c tire won't balloon up that much.

Let's put the graph here:
Image


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PostPosted: Mon Aug 22, 2016 11:07 pm 
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Ahem. My bad. I mixed up the different plots...


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PostPosted: Mon Aug 22, 2016 11:14 pm 
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Location: Glermsford, Suffolk U.K
Well thats why i put 23mm tyres on my wide aero rims and wider tyres on my shallow wide rims. It is quite obvious.

The down side of course to using tyres narrowr than the rim is that it exposes the rim to damage. Of course the tyre wear thing is great for people who sell tyre.

What is not clear is how important this is in a road race. In a tt it will be seconds, in a road race will it make any difference.

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PostPosted: Mon Aug 22, 2016 11:25 pm 
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And what it doesn't take into account is the benefits of running a wider tyre in comfort, speed and handling, but is that enough to overcome the slight aero disadvantage, don't know, but I'd rather run the wider tyre...


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PostPosted: Mon Aug 22, 2016 11:50 pm 
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tb123 wrote:
And what it doesn't take into account is the benefits of running a wider tyre in comfort, speed and handling, but is that enough to overcome the slight aero disadvantage, don't know, but I'd rather run the wider tyre...


Sure, but that's not the point of the article. It points out that there is no reason to run a deep aero rims if your tire is wider than the rim.

It would be interesting to see more on this subject and factoring Crr into the equitation. Based on the latest tests by Tom Anhalt great Crr often can make up for sub-optimal aero properties in the case of tires:
http://bikeblather.blogspot.com/2016/03 ... party.html


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PostPosted: Mon Aug 22, 2016 11:51 pm 
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nemeseri wrote:
But in general, running clincher rims with anything wider than 23mm tire will ruin your aero benefits


This statement is not supported by your evidence. The relevant comparison would be to compare a wide tire on an aero rim to a wide tire on a non-aero rim. To "ruin" the aerodynamic benefits, for example, a 25mm tire on a Zipp 404 would result in the same drag as a 25mm tire on a Mavic Open Pro. Is that the case?

The evidence you've shown just supports the general notion that a narrower tire is more aerodynamic. That seems like a reasonable assertion, but I've already decided I want to run at least a 25mm tire, and now I'm looking for the best performing wheel on which to mount that tire.


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PostPosted: Mon Aug 22, 2016 11:57 pm 
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Another wheelset that was designed in accordance with this principle is the Roval CLX 64.

The new venge comes fitted with a 22 mm front tyre on a ludicrously wide 20mm (internal width) wheel.

I have 24mm tyres on my cl64's and note that the rim is the widest point.

More aero, yes, but it also opens the rim face up to damage as the tyre doesn't provide protection.


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PostPosted: Tue Aug 23, 2016 12:07 am 
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prototoast wrote:
This statement is not supported by your evidence. The relevant comparison would be to compare a wide tire on an aero rim to a wide tire on a non-aero rim. To "ruin" the aerodynamic benefits, for example, a 25mm tire on a Zipp 404 would result in the same drag as a 25mm tire on a Mavic Open Pro. Is that the case?


No. But 404 has a narrow tire bed and a quite wide rim and I highlight this in the original post too. In the case of a wider rim bed, or 23mm outer rim width the difference might be surprisingly small. Especially if you compare the performance to something like a modern lightweight rim with 16-20 spokes.

From the article above:
Quote:
The Rule of 105 states that the rim must be at least 105% the width of the tire if you have any chance of re-capturing airflow from the tire and controlling it or smoothing it.


This statement is supported by the fact that HED recommends 22mm tires for their clinchers for optimal aero benefits. And those rims have a 25mm outer width. Also I find it interesting that the new flo wheels dropped their inner rim width to 17c instead of the previous 19.4mm. May or may not because this way the tires stay narrower resulting in better aero results with a wider selection of tires.


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PostPosted: Tue Aug 23, 2016 3:19 am 
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Fastest would be a 28mm tire on a 17mm iw rim with a 30mm ow. But then the rim would be heavy. So you cannot win really.


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PostPosted: Tue Aug 23, 2016 4:28 am 
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F45 wrote:
Fastest would be a 28mm tire on a 17mm iw rim with a 30mm ow. But then the rim would be heavy. So you cannot win really.


Based on the article a 24mm tire with good crr (ie. Specialized turbo cottkn) on a zipp 303 would work great (ow: 28.5mm). So there are working combinations out there for sure.


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PostPosted: Tue Aug 23, 2016 5:19 am 
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The other question is, although a 23mm tyre on a 404 has better aero principles than a 25mm tyre, is a 25mm tyre on a 404 still more aero than a 25mm tyre on a box section rim - I imagine it is.


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PostPosted: Tue Aug 23, 2016 6:08 am 
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tb123 wrote:
The other question is, although a 23mm tyre on a 404 has better aero principles than a 25mm tyre, is a 25mm tyre on a 404 still more aero than a 25mm tyre on a box section rim - I imagine it is.


Based on the graph above I'm pretty sure it's more aero than a box section rim. But the amount depends on the tire pressure and probably tire usage in this case.

I'd rather see a comparison with Mavic/Fulcrum deep clinchers against modern shallow alloy rims, but I doubt anybody has the balls to do that.


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Posted: Tue Aug 23, 2016 6:08 am 


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PostPosted: Tue Aug 23, 2016 6:22 am 
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Posts: 560
So when some manufacturers are claiming you should be running wider tire with their now wider new aero rims to gain benefits in handling and comfort (which is true), they did not forsee this aero drawbacks or haven't done enough research? Brands like Campagnolo actually claims it's more aero to run wide tire with wide rim (no evidence, studies, or any figures to back up the claim).

It comes down to where your put your priorities. All out aero or better comfort and handling, which save time and power and not in a minimal amount either.


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