From HED website

The HED C2 rim not only improves cornering performance because of the extra width, but its air volume is effectively increased, meaning it can be run at a lower pressure for better vibration absorption and more speed.

A wider rim is more aerodynamic too. The HED C2 rim sets the tyre sidewall in line with the rim, eliminating the bulge of a 23mm tyre on a 19mm rim.

Just because it is written on the HED website it doesn't make it true.

For a standard 19 mm rim with a 23 mm tyre, the boundary layer will most likely separate at the point of maximum tyre width (i.e. before the flow reaches the bead).

For a 23 mm rim on a standard 23 mm tyre, the boundary layer may remain attached as it flows over the tyre bead and onto the rim. However, it will still separate once the flow reaches the rear section of the rim, because the rapid curvature of the C2 rim will create a strong adverse pressure gradient which will cause almost immediate boundary layer separation.

I've worked as an aerodynamicist for over 10 years and there is nothing in that profile of the HED C2 that convinces me that a boundary layer would be able to remain attached as the flow approaches the trailing edge of that rim.

Or to put it another way, have you ever seen an aerofoil that has the same curvature of an HED C2 rim?

If HED were to produce 3rd-party CFD/Wind Tunnel analysis which proves that the HED C2 rim is more aerodynamic than the Velocity A23, or even Mavic Open pros with a 23 mm tyre I'll change my mind. But I'm extremely skeptical of any aerodynamic claims made for that rim shape.

I'm sure they are great rims and I'm considering buying a pair myself to build up on White Industries/Alchemy rims with 28/32 spokes for a strong training/touring wheel.

But I'm not buying it for the aerodynamic benefits. If I wanted aero I'd buy the Zipp 101s.

Here is another perspective. My current wheels are Campagnolo Omega V-shaped rims on Ultegra hubs. I bought these wheels 18 years ago. The bike shop sold me the 'V' shape by saying that they were stronger (which makes sense to my mechanical engineering-trained brained). There was never ever mention of any aerodynamic benefits.

The laws of physics haven't changed in the past 18 years. The Omega has a rim width of 20 mm and depth of 22 mm, so if you put a 20 mm tyre on it they would be more aerodynamic than a HED C2 (which has a width of 23mm and a depth of 24 mm).

I can see your point... but it doesn't seem to be quite so simple. Shimano's older 21mm wide x 24mm deep rims always had good aero, and the new Zipp and Hed carbon rim shapes are not obvious. Check out this test of aero wheels... specifically the 22.5mm wide x 23.3mm deep (very rounded shape) Corima compared to the 101.
http://www.tour-qtr.com/epaper_4_2011" onclick="window.open(this.href);return false;

The benefit is probably more for the trailing part of the wheel than the leading part.



The rim needs to be rounded, so it makes a good leading edge. The Omega isn't. But I've always argued that the 20mm wide x 22mm deep Stan's Alpha 340 makes a pretty good aero profile with a 20mm tire.

I suspect that this new 23x28mm rim will have good aero properties because of it's rounded shape.

Not really. Fatter tires are always waaaaaaaaay more comfortable than skinny tires. Always.

In this case, I think the U-shape of a rim beats a V-shape. And the U-shape would favor a wider tire. If not, why did the winner of the World TT championships use real wide tires on his bike? If I remember correctly, I think he used a 25 or 28mm tire. And time trials are all about aerodynamics. His team must have known that wider tires are more aero when paired with a wide rims.

What? Rim shape and tire selection are two different things.

Anyway Tony Martin rode clinchers, 23mm Conti GP TT up front but a 22mm Attack on the rear.

Zipp's data shows that even though they've been designing around a 23mm tire since the wide 303 in 2009ish; their wheels are usually still faster with a 21mm tire.

A U beats a V no matter the width most likely. And your tires, if aero is your goal, should mate well to your brake track in width. Definitely not wider.

Oh. Oops. I thought he had, at least a 25mm. You're right. My bad. If you have a wide rim, matching it to the same profile tire makes it more streamlined and aero.

im impressed by how much of that you made up yourself.

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Mine are built and ready for pick up from my LBS ... wheels are all black to go with my NEO Hyper

Front: 654g
SuperLight Wide Front Hub - 71g
BHS C472w 20-hole - 479g
Sapim CX Ray
Alloy nipples

Rear: 857g
SuperLight Rear Hub - 211g
BHS C472w 24-hole (16:8) - 477g
Sapim CX Ray
Alloy nipples

I had read that review before, but I never took noticed of the Corima results. Very interesting.

I've read various journal articles and online reports which show the benefit of a rounded rim at yaw angles (both for the tyre-leading and tyre-trailing section of the rim). This makes perfect sense and likewise the fact that the Corima can outperform the Zipp 101 at high yaw angles isn't surprising.

The fact that it does at zero yaw angle is also very interesting. Having a look at the profile shows that is has fairly shallow curvature, until it reaches the rounded trailing edge. This would be more aerodynamic than a HED C2 which curves rapidly to a sharp trailing edge.

I might do some quick photoshopping of the various rim profiles to help explain better what I mean.

I ordered a pair of H Plus Son Archetype polished rims from China yesterday to replace the wooden Ghisallo rims I recked at Liege-Bastogne-Liege last weekend. I will probably go with Sapim Race of DT Comp to lace the rims onto my trusty DA 7700 hubset.

As soon as the wheels are built and I have had a chance to ride them, I will get back to you with first impressions.

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My Renovo R1 wooden bike project

Check it out: the most amazing vintage Colnago, Merckx and Pinarello collection

Ok so I've drawn some pretty lines around the rim profiles of the A23 (left), HED C2 (middle) and Bikehubstore C472 (right).



Based on a decade or so of professional experience, this is how I presume to the flow to behave over all three rims. The curvature of the trailing edge of the A23 is so sudden that the boundary layer will separate at point 'X'. This of course would create a large wake.

For the HED C2, the initial curvature is smaller, however it then quickly begins to taper so the bounday layer would most likely separate slightly further downstream. A smaller wake would be created.

The C472 is a longer rim and its trailing-edge curvature is even smaller. Hence the boundary layer would most likely remain attached almost until the trailing edge. This rim profile would be the most aerodynamic.

If you look at the profile of the Corima Winimum it resembles a shorter version of the C472, rather than the Hed C2. By that I mean the actually trailing edge is thicker, however the curvature that leads towards the trailing edge is more gentle than on a HED C2. The actual thickness of the trailing edge doesn't make much difference when dealing with such a short aerofoil section (treading rim depth as aerofoil chord). It's the curvature leading up to the trailing edge that is the crucial design parameter that will determine boundary-layer separation.

Of course, these diagrams assume the 'worst-case' position of the rim in the 'tyre-leading position' where the rim is at right-angles to the flow. As you move towards the top/bottom of the rim the effective chord length grows.

When dealing with 'rim-leading' position, there are other good reasons why a thicker and rounder trailing edge is better, primarily for slight angles of yaw.

One day I might throw these profiles into a CFD package (e.g. FLUENT, Open Foam) and see what sort of numbers pop out. You'd have to make some assumptions about tyre size/shape though, my tyres in the pretty picture above don't seem to have enough sidewall.

I think I might buy a pair of C472 for my next wheel. Same weight as the HED C2, slightly better aerodynamics and about half the cost. Who makes them? Kinlin?

Haha, I like you! Thanks for the great visual and detailed breakdown.

It seems the suspicion is that Kinlin makes the 472 but I have yet to see it confirmed.

99% sure.

Your tire profile looks pretty good for a 20mm tire, but a 23mm tire will be ~25mm wide on these rims. Also, they are a little heavier than the Hed rims, which have run ~455g recently. Both of my C472s were 480g.

So, you're saying there's a chance they might not be made by Kinlin.