The trend to fat, bulbous rims for aerodynamics – misguided?

[Samuel D]

Over the last few years, ‘aero’ rims for road bikes have become wider and blunter with a corresponding increase in tyre width, allegedly to improve drag at high yaw angles (i.e. conditions with a large crosswind component).

The marketing supports this:

• Easton video
• Zipp 404 Firecrest Carbon Clincher promotional drag chart
• Flo Cycling (buzzy start-up that promotes a “Net Drag Reduction Value” weighted toward drag at 10–20° yaw angles, which they call the sweet spot).

Promotional material should be taken with a large pinch of salt, but it’s safe to say bulbous wheels are designed to reduce drag at high yaw angles. In still air or otherwise low-yaw conditions their advantage over a narrow box-section rim with loads of spokes is not actually very great, even according to the purveyors of aero wheelsets.

What’s more, at low yaw angles the bulbous wheels aren’t significantly faster than old-fashioned ‘V-notch’ aero wheels. Sometimes they’re even slightly worse in wind-tunnel tests.


•••


Slowly it dawned on me that, although the wheel companies talk a lot about high yaw angles, I very rarely draft other cyclists at such high offsets. Usually the best position to draft is almost directly behind. In other words, high yaw angles are rare in my own cycling.

So I tried to find some information to back up my feeling that high yaw angles are rare. I came up with this recent PDF paper by Catalyst Cycling, another start-up.

I didn’t follow all the maths, but the bits I did follow certainly supported the devastating conclusion:

“The main conclusion that can be drawn then is that wheels should be designed for much lower yaw angles than those that are currently being used. This means using narrower tires and narrower rims, as well as less bulbous rim shapes.”

Doesn’t this suggest the current trend toward fat, bulbous rims and wide tyres on road bikes is, charitably speaking, misguided?


•••


On a related note, this Tour de France I noticed that Richie Porte and Chris Froome, riding for marginal-gains Team Sky, often use shallower rims than the competition. The exception appears to be on stages with a risk of crosswinds (high yaw angles) and echelons, when Sky breaks out the deep-section rims. (Of course the review websites would tell Froome he needs shallow-section rims in crosswinds, supporting my extreme suspicion they aren’t worth reading!)

All very interesting, isn’t it?

[mig]

something 'new' to sell to people maybe?

[Brucey]

I agree there is a lot of guff wafting about the place concerning this subject. The wind tunnel tests are mostly misleading because they are not usually done using a wheel that is either rotating or seeing disturbed air. Both these things can alter the results dramatically. I agree that real-world yaw angles are not commonly in that league except in windy time-trials or when you are on the front in an echelon.

It all goes to show that in racing, every last fraction of advantage is both fought and (more importantly for the manufacturers) paid for.

cheers

[CREPELLO]

Apart from the aerodynamics of the rim shape, a wider width seems to be all part of the deal. I read this blurb that accompanies the H Plus Archetype rim...

The well accepted 23mm wide profile mimics a tubular tire, where the advantage can be felt immediately while taking corners as the tire is no longer shaped as a light bulb, flopping over with high load

This runs counter to much of the received wisdom on this forum at least. I've also read that rolling resistance on these wider rims is lower than a narrower rim. And that they are less prone to pinch flats.

My own impression when I ran 28mm Conti Gators on 24mm wide Mavic A719 rims (1mm wider than the Archetype above, which is suggested would run well with a 23mm wide tyre) was that they were a harsh ride. I have to admit though that this was run on a tourer which had a stiff frame and forks.

So I'm not sure what to think. More empirical evidence needed, me thinks. But I'm not rushing out and buy these stuff - I've only just finished building a new wheelset, with nice and narrow Ambrosio Excellence rims (19mm wide) with lurvely Ultegra 6600 hubs. They seem comfortable enough to me with 25mm Schwalbe Stelvios.

[MartinC]

Crepello, my experience matches yours. A while ago I was running 28mm Contis on Mavic T224 rims (24mm external/19mm internal width) and they were certainly harsher than on narrower rims. I've no real feel for the impact on rolling resistance though.

[Keezx]

Wider rims have advantages , but the aerodynamic story is IMO the least interesting part for the average cyclist.
A couple of years ago I got interested in wider rims while active on German forums.
Decided to try it and built myself a set with Kinlin XC279's (17 mm inside).
Used them first with 24mm Conti GP's because I had some stock of those.
With the usual >7 bar pressure they felt harsher than on older 13 and 15C rims.
After expirimenting with tyre pressure I found 6,0/5,5 bar rolling much smoother and still keeping good stability in fast corners, while rolling resistance is only a tiny bit higher, (not notable but tests are obvious)
After many thousends of km's over a large variation of road surfaces I'm completely convinced of this concept.
A we speak I have 3 wheelsets with 17C/18C rims and use them with 25 mm tyres, sold all the other stuff.
Aerodynamic? Maybe, but smooth riding, good stability and grip in corners are the keypoints for me.
If you pump the same as in the old days there are no advantages, tyre pressure is crucial.

[Samuel D]

Apart from the aerodynamics of the rim shape, a wider width seems to be all part of the deal. I read this blurb that accompanies the H Plus Archetype rim...

The well accepted 23mm wide profile mimics a tubular tire, where the advantage can be felt immediately while taking corners as the tire is no longer shaped as a light bulb, flopping over with high load

This runs counter to much of the received wisdom on this forum at least. I've also read that rolling resistance on these wider rims is lower than a narrower rim. And that they are less prone to pinch flats.

Yes, these claims about the effect of wide rims on tyre performance are also contentious. And many of them seem to conflate wider rims with effectively wider tyres. There’s a difference in benefits arising from a wider contact patch (which could equally be achieved by fitting a wider tyre to a narrow rim) and any benefits arising specifically from the greater rim-width-to-tyre-width ratio.

After many thousends of km's over a large variation of road surfaces I'm completely convinced of this concept.
A we speak I have 3 wheelsets with 17C/18C rims and use them with 25 mm tyres, sold all the other stuff.
Aerodynamic? Maybe, but smooth riding, good stability and grip in corners are the keypoints for me.

Not having tried this myself, I remain doubtful! It seems to go against the principles of tyre performance as I understand them.

However, I agree that the trend to wider rims and tyres, though perhaps driven by misguided concerns about drag at high yaw angles, has benefitted many cyclists by putting them on wider, more comfortable, and maybe even faster-rolling tyres.

[Samuel D]

OLD THREAD ALERT!

It is interesting to note that FLO Cycling, a company that designed wheels for yaw angles I thought were clearly far too high (see my original post), has just this month admitted it got it all wrong:

“We have updated our 2012 NDRV formula. In 2012 we thought a rider spent 80 percent of their time between 10 and 20 degrees of yaw. Now we know that a rider actually spends 80 percent of their time between zero and 10 degrees of yaw. This discovery forced us to focus on designing wheels that were more aerodynamic at shallower yaw angles instead of focusing on designing wheels that were fast between 10 and 20 degrees of yaw. Our old estimates for how much time a 2012 FLO wheel would save you aren’t as accurate as they could have been, and we have recalculated them below. We have compared those to the new 2016 models.”

I find it astonishing that all the earlier 63-seconds-saved-over-40-km type of claims have been brushed under the carpet just like that… Poof!

Now the new and improved yaw model is being used to make claims for time saved that are just as precise as before but entirely different.

My tip? If you don’t like the numbers, just wait for a new yaw model to come along!