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PostPosted: Sun Feb 10, 2013 9:11 pm 
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I've heard of lots of people running 20-22s on Hed C2 rims(I think Martin has used Conti Attacks on his Hed's in time trials) without issue and Zipp specifically allows for use of their 21mm Tangente on their FC clincher wheels. To the extent that you think the tire width has to match external rim width, you are wrong. Allowable tire width does not depend on outside rim width, it depends on interior rim width between the tire beads.

So where Zipp uses ~16.5mm interior width in their wheels and a 21mm tire is safe according to Zipp, it seems entirely within reason that the 18mm width Pacenti should safely accommodate 23mm tires...

Further, as I posted in another thread, often times a tire narrower than the rim brake tracks is optimal for aerodynamics. Zipp's data clearly shows that FC rims are fastest with 21-23mm tires, not 24-25mm.


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Posted: Sun Feb 10, 2013 9:11 pm 


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PostPosted: Sun Feb 10, 2013 9:50 pm 
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bricky21 wrote:
Where does the idea that a 24mm rim means a 24mm tire is optimal come from? Optimal for what?



Well, the general rule of thumb is that to optimize aerodynamics, performance, and comfort... You want a tire width to match the outside width of the rim. Hence, a HED C2 rim outside width of 23mm is best used with a 23mm tire. This is what HED claims.

So, it stands to reason that if what i have written above is true... then a tire width that is substantially more narrow than the outer rim width is not only not optimized aerodynamically and performance-wise, but it can be downright dangerous. You could potentially roll that clincher tire when cornering hard.

So.... Lets say you have a rim with an external rim width of 23mm, you would get the best performance from a 23mm tire. A tire a lot bigger, ex: 25mm, would give that tire shape a bulbous look and performance would suffer. Likewise, a tire a lot smaller, ex: 20mm, would actually be dangerous for fear of the rider rolling the tire in a corner. So look... Bottom line is this. A 23mm outer rim width is best used for a tire that is 23mm wide. Of course, a plus/minus difference of 1mm is probably ok. So in reality any tire between 22mm and 24mm is ok. However, I wouldn't drop down to a 20mm tire if I hade a wide 23mm outer rim width.


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PostPosted: Mon Feb 11, 2013 12:24 am 
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Hi,

I don't know the exact data of the Pacenti rim but as far as tubular tyres are concerned a tolerance of + 1mm tyre width with respect to rim bed width should still be fine.
In general, for tubulars the idea is rim bed width equal to or + 1mm. Certainly not anything less as this would expose the rim bed to not only the effects of the winds but it would also be exposed to damage.

Quote:
Well, the general rule of thumb is that to optimize aerodynamics, performance, and comfort... You want a tire width to match the outside width of the rim. Hence, a HED C2 rim outside width of 23mm is best used with a 23mm tire. This is what HED claims.


For a clincher tyre you'd want a bit of a wider combo and ideally, a rim that has more of a flare beyond the rim bed width to catch (recapture) the wake and guide the turbulence caused by the crochet's edge and the void caused by the wider tyre.
Essentially, what you want to do is hide the crochet's edge by the use of a wider tyre.

For clarity's sake: to me rim width means rim bed width. Quite often I see this being mistaken for the widest point of the rim which obviously is not the same.

Ciao, ;)

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PostPosted: Mon Feb 11, 2013 12:56 am 
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I've posted it before, but I'll keep posting it:

The limited data I've seen shows that a narrower tire is faster. For example:
http://velonews.competitor.com/files/20 ... s-Zipp.png


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PostPosted: Mon Feb 11, 2013 1:11 am 
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For clarity.

Image

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PostPosted: Mon Feb 11, 2013 1:43 am 
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Hi,

So, how is a narrower than 23mm (I'm being generous) tyre going to fit any NACA profile using this rim width?
Obviously narower tyres will expose the rim's edges which inevitably will cause more turbulence.

Generally speaking, yes, narrower tyres are intrinsically more aero. You'll also have to fit them to the corresponding rim and they'd better have a devilishly low crr or all beneifit is gone.

Not something we didn't know already for the past 50 years or so?

Ciao, ;)

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PostPosted: Mon Feb 11, 2013 1:54 am 
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fdegrove, first off, I shouldn't speak in absolutes, some narrow tires are less aero than wider tires. Second, I agree that it is counter intuitive to have the rim "exposed" but it just seems to be that the narrower tire makes up fore the exposed ridges based on what I've seen. Jordan Rapp, who is a sponsored Zipp athlete and gear-head, has talked about this phenomenon because Zipp's tubulars are super wide and yet, even though a 21mm tire looks akward on them, its faster. It could be a lot of things though, because remember that a tire is a leading and a trailing edge, that one of the "rim edges" will be hidden if the wind is at yaw; and who knows what else.

Now of course, rolling resistance makes the decision that much more interesting but it makes this chart from Bontrager all that much more appreciated:
http://media.bontrager.com/images/spotl ... _force.png

It also demonstrates that a round tire, with a distinct tread above the casing can have bad drag even if its a narrow tire.


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PostPosted: Mon Feb 11, 2013 2:10 am 
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I'd buy a tubular set of these rims if they were drawn, no question.

Sent from my Galaxy Nexus using Tapatalk 2


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PostPosted: Mon Feb 11, 2013 2:22 am 
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Hi,

I think you are confusing a few things which you seem to take for facts. They're not.

What is however a fact is that an exposed rim edge is going to catch wind resistance.

Quote:
some narrow tires are less aero than wider tires


Not really. They easilly could when mated to the wrong combination though.

Quote:
Jordan Rapp, who is a sponsored Zipp athlete and gear-head, has talked about this phenomenon because Zipp's tubulars are super wide and yet, even though a 21mm tire looks akward on them, its faster. It could be a lot of things though, because remember that a tire is a leading and a trailing edge, that one of the "rim edges" will be hidden if the wind is at yaw; and who knows what else.


I read: "Sponsored". I also read "tubulars". And a lot of things in between.

Bottomline is that we do not know much about it yet.
Just do not announce the impossible as factual because some company with a commercial axe to grind said so....

Pace, ;)

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PostPosted: Mon Feb 11, 2013 2:26 am 
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Very interesting rims indeed, and a decent weight. I´m having problems with kimlin xr-200 rims, these look like a more sturdy option, and I like the idea of wider rim and/or 25mm tires... I wonder if the rolling resistant is decent. Comfort and performace in cornering must be much better for sure.

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PostPosted: Mon Feb 11, 2013 2:36 am 
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winky wrote:
Well, the general rule of thumb is that to optimize aerodynamics, performance, and comfort... You want a tire width to match the outside width of the rim. Hence, a HED C2 rim outside width of 23mm is best used with a 23mm tire. This is what HED claims.


A lot of people have silly notions that a 23mm clincher tire is 23mm wide regardless of what rim it is on. This is not true. For instance I recently mounted a 23mm tire on a 28mm wide Stan's MTB rim. Guess how wide the tire was? If you guessed 29mm you guessed right.

If a 23mm tire is 23mm wide on a 20mm rim, it will probably be >24mm wide on a SL23. It won't be a problem at all.

BTW, HED makes a lot of claims that you shouldn't necessarily believe.

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PostPosted: Mon Feb 11, 2013 2:54 am 
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Hi,

Quote:
If a 23mm tire is 23mm wide on a 20mm rim


Et voila. :thumbup:

Ciao, ;)

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PostPosted: Mon Feb 11, 2013 3:37 am 
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Looks pretty smooth to me. Tunnel testing will by the only way to be sure. The Firecrest and Smart line are significantly wider than the tires recommended, but they are also a lot deeper than these rims so I don't think it's a fair comparison. My opinion is at this depth a tire that is the same or very close to the width of the rim will work best. I just want confirmation. Anyone have wind tunnel time to donate? I have a set of wheels to send out if you do.

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PostPosted: Tue Feb 12, 2013 2:28 am 
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The Zipps (and Bontragers and other aero wheels) have a rim width wider than the actual tire width so the area behind the tyre is in a positive pressure gradient.

By doing so, if the boundary layer separates off the edge of the tire, it will re-attach as its flowing over the rim.

I personally wouldn't put too much weight into the 'aerodynamic' advantages of shallow allow rims.


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Posted: Tue Feb 12, 2013 2:28 am 


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PostPosted: Tue Feb 12, 2013 3:18 am 
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Looking at some independent tests, there have been some shallower wheels that did perform quite well. I know that the Smart wheels will perform better, but comparing a set of 3.4 clinchers I bet they hold thier own, they weigh only 50g more for a comparable build and are a bucket-load less expensive.

Horses for courses.

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