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?
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.
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....
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.
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:)
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.
But for the most part, the tire sections that will work best for the u shaped wheels is a 22-23 section.
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.
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.
teleguy57 wrote:So how does width play into the ride quality picture?
I guess that handling in the crosswinds is what I meant by enhanced ride quality, on the tubys at least.
Depth (mm)x BT Width (mm)
38mm x 20.5 / 23 / 25
50mm x 20.5 / 23 / 25
60mm x 20.5 / 23 / 25
88mm x 20.5 / 23 / 25
Issue 1: rim width variants.
As far as I know, most 20.5 widths are the older, more 'V' shape profile, while most of the 23mm or 25mm widths are now more 'U' shape. They don't appear to be as advanced as a Zipp Firecrest shape because they seem (from photos and videos) to be more narrow at the spoke hole-side and less rounded. The 23mm and 25mm versions do often bulge out from the brake track, however, giving us two width metrics when viewed in profile: brake track width and bulge width. For example, the 50mmx25mm Yoeleos have a bulge width of 26.27mm. This is the shape most of us want, the one that is closest to a Firecrest (which I'm placing as the industry's most advanced standard). Short (generalized) story: the wider more bulging U-shape, the more aero for most types of riding (as of early 2014).
Issue 2: tire width + rim width aerodynamics
As many on here point out, you should also consider your tire width when choosing rim width. Wide tires on narrow rims aren't as fast (the theory goes) than matching widths more closely. Lots of variables in there, but the numbers on our gold standard 404 Firecrests, again, show them being best with 23mm tires. Short (generalized) story: match your tire width to your rim's brake track width.
Issue 3: rim bed design
Lots of mis-information and guessing about this, but for the most part people (amateurs in online forums, like us) seem to think rim width doesn't matter to tubular glueing. I'm not convinced. I think more clarity is needed when discussing this factor: rim bed profile matters more than rim width. You can have a 25mm rim width that is better designed for 23mm tires (aka Zipps). I'm guessing some 25mm rims do not have such an optimal rim bed. The main concern I have is A) having a tubular tire be less secure because of less contact on a rim bed and B) rim bed design negatively deforms the shape of the tubular tire affecting rolling resistance or cornering ability.
I'd like to know if the base tape on a narrow tubular tire might not fully contact the rim bed on wide rim. Example: a 23mm tire on a 25mm rim. Differing opinions, but I'm hesitant to go that route, and will likely go 23mm tire on a 23mm U-shape rim. Even found a comment direct from Yoeleo about their rims and that combo:
"The 25mm wide wheels are U shape rims, please use the 25mm tire or 28mm tires, 23mm tire can not fit well.and we will able to offer the 23mm wide u shape carbon wheels in 60mm and 88mm depth this month [Sept. 2013]" — Source (in comments below video): http://www.youtube.com/watch?v=x6BbopKHnf0
What they mean by "can not fit well" isn't 100% clear. 23mm tires don't glue well to that rim? Or the tubular tire doesn't shape well? Or the aero properties don't match well? I'm guessing the first option because I don't think Yoeleo is that precise with their aerodynamics or care that much about tire shape, but they may have tested a few glued tires and felt it wasn't glueing strong enough.
As for mismatched rim beds deforming tubular tire shape, I'm less concerned but looking for a more expert opinion or examples.
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