I've read all the threads here about open model carbon rims, and while the width does seem to make a difference for clincher performance, it's harder to get a read on potential real-world differences when riding tubulars. It does appear the rim bed radius is important in that a shallow bed works better for wider CX tires and that some road width tires could have gluing issues on those same, flatter rims.

I'm wrestling with current choices of a 38x20.5mm carbon rim (older style, and less expensive) vs the 38x23mm newer versions. In 50mm profiles the choices are better (or worse, depending on your point of view): 20.5mm , 23mm, or now 24mm U shaped profile.

How much of a difference does it make in the tubular world, and what criteria would one use to decide?

Are you asking about handling difference in windy situations, or drag?

Which 23x38 and 24x38 tubulars are you considering? I'm after something along those lines as well.

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Bikes: Raceur, Beateur

@wassertreter -- Asking for overall opinions. Not overly worried about wind pushing me around as I'm a big boy. These would be for fast club rides, weekend group events, etc; not a licensed racer. I'm looking at the usual suspects on the lower-cost end of things: FarSports and Yoeleo are the leading candidates.

I haven't found a 38x24 u-shaped rim that's on the market yet; both have 28x23 (I think the same mold as the schematics I've received from them look the same, as does their 50x23. FarSports does have their 50x24 out now, and I'm still considering some of the 38x20.5 rims since pricing are starting to drop on them as they shift over to 38x23s or wider.

The width of both a tubular and clincher rim will have just as big of an effect on ride quality as depth. It is a factor that is overlooked and underrated all the time by the everyday cyclist.

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I know in Cross a wider rim helps keep the tire from rolling (supposedly), maybe applicable to road tires and wide rims?

@Zen, no question it makes a difference on clinchers. Love my Open Pave CGs on my Hed C2s, and happy with the A23s I'm running on my CX bike.

Most commentary I've read from wheelbuilders is that rim width for tubulars is mostly irrelevant unless you're a top level rider looking for the last aero watt or two, and that it's shape more than width (and as you've posted in this forum, the shape of a rim like the Enve does make a difference in handling in windy conditions).

The tire profile doesn't change when glued up to a narrow rim or a wider rim, right? So how does width play into the ride quality picture?

@Colin, I think the CX situations is that fatter tubies (32+mm) have a shallower radius due to their cross section, so a wider rim provides more gluing surface. In the case of a rim like the Major Tom, the rim bed is slightly shallower to match that wider tire profile. I've read of a few folks who have run road tires on Major Tom rims, but there are some questions about about road tires potentially lifting at their edges....

Shape and width go hand in hand relative to performance. tire profile matters as well, and its not a "last watt" thing, it's lots of watts (though watts are more or less important to some folks and I'm not being critical one way or the other.


Wider clincher rims matter more than making a tubular rim wider because the casing deflection and shape is determined a lot by the width of the tire bead. Tubular tires are not pinched between rim beads so they'll more easily flatten out (at similar pressure) versus clinchers.

that contact patch / casing shape that is primarily a ride quality improvement. It means that a 23 section tire can have the character of a slightly larger tire profile because the casing shape is a little different... The advantage for performance is that you're running a smaller section tire that will have comfort similar to either a larger section tire and behave more like a tubular but it will also match better with the rim profile for better aerodynamics.


With tubulars, the wider rim benefits are primarily in aero performance.

The new Zipp firecrest U shape design is a pretty careful combination of shape, width and depth... The people copying it seem to only be concerned about getting something that looks like a U and are ignoring the shape of the U to some degree and the width to a larger degree. Most very simply don't have the capability to even reverse engineer the firecrest shape to the degree necessary to understand it...

But for the most part, the tire sections that will work best for the u shaped wheels is a 22-23 section. And because tubulars don't bunch the tire between beads, you're getting all of the comfort/casing shape and deflection that benefit wide rim clinchers.

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charles@pezcyclingnews.com

As usual, very detailed and helpful info, Charles.

I'm thinking strictly about tubulars here, so considering I'm likely going w/24/25mm Veloflex Roubaixs or something similar on these wheels (I have my Nemesis set if I need to go fatter:)) the question seems to be one of aerodynamics.

If I were a rich man (cue "Fiddler" music here) no questions the Zipp/Enve/Bontrager wheels would be primo, as it appears the newer November wheels would be as well. I'm looking to build up a set of good, but inexpensive wheels so I'm in the open-mold world -- from the more reputable resellers like Farsports or Yoeleo.

With those parameters, how significant could the differences be between the older profile, wider but still old-profile 23 mm rims, and the now-hitting the market 24 U-shaped rims? All questions of opinion, of course, since none of us are putting all the variations in a wind tunnel or doing the repeatable road tests of all the options:)

Width in and of it's self doesnt really dictate any aero benefit. youde have to give a rim model or shape being copied to try and take a guess at it...

And virtually all wheel makers suggest that a 23 section tire is the largest you can put on the wider rims before you damage aero performance.

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charles@pezcyclingnews.com

Hi,



That being only because of these newly shaped rims are made wider as we now believe wider than 20mm tyres have lower rolling resistance at realistic bilateral loads or have the laws of aerodymica changed since we last studied them?

Bottomline is: a tyre and its shape need to match its rim. Narrower is better aerodynamically but since no one seems to know how to make a fine tyre anymore, we settle for the next best compromise.

IOW, a better tyre rim match can be manufactured that will combine both ideals. It is however not an economically viable solution so we sell you the wider is better idea instead. (Totally misread by the public but who cares)
So, IOOW, when mere speed is the goal, keep the frontal area narrow, i.e. narrow tyres, and then we really are talking about Watts saving.

A more hollistic aproach may well yield in far better results than current projections though.

Ciao,

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The U shapes test better at 22-3-4 rim width. It's not a sales pitch as much as the best resulting width / shape combo. Much wider isnt really doable with current bike / brake standards. The tire side shape (out-side) needs to match reasonably well with the inside rim shape and the tire needs to not stick out / be wider than the wide point of the rim. The result is that tires need to be 23 or less to work the best...

And because the OP is sticking to tubular, there's not much advantage in ride quality going slightly larger anyhoo as theres no clincher beads to deal with and the tire is a little more free to deform.

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charles@pezcyclingnews.com

I guess that handling in the crosswinds is what I meant by enhanced ride quality, on the tubys at least.

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Thanks, Zen -- got it on the width/shape and handling. Sounds like the big boys (Zipp/Enve/Bontrager) really have it dialed in on that aspect of rim shape.