Gear

Technical FAQ with Lennard Zinn: The benefits of aerodynamic wheels in mud?

A reader wonders why deep-section carbon rims are so popular in the most non-aerodynamic discipline in cycling.

Q.Dear Lennard,
What is the purpose of using deep section carbon wheels for cyclocross? It would seem that the aerodynamic advantage that they offer would not be that useful in cyclocross.
Adam

2010 Koksijde World Cup, men's race, Tom Meeusen
Not a lot of wind resistance at these speeds, so what's the deal with all the aero' wheels? | Photo: Dan Seaton

A.Dear Adam,

The main reasons for using deep section wheels in `cross have very little to do with aerodynamics.

I certainly understand your point about the dubious aerodynamic advantages they would offer at those speeds, although I know it’s different for me riding alone compared to Tim Johnson riding alone. He’s going a heck of a lot faster, and the higher the speed, the more important aerodynamics become. So at Johnson’s speeds, especially on flat, dry courses, the aerodynamics of at least the front wheel might actually be significant. Although most of the time I’ve seen him winning, the gap over second place is way bigger than I would ever imagine the aerodynamics of an aero wheel with a very un-aerodynamic knobby tire on it could explain. And of course, the guys chasing him all tend to be on deep-section wheels as well.

That said, most riders spend a lot of time in ‘cross races breaking the wind themselves, and aerodynamics of an aero wheel are increasing their speed, although the difference might be incredibly small.

From my own experience, deep-section carbon wheels track much better through deep sand. On a recent race at the Boulder Reservoir, I raced one category in the morning on deep-section wheels and made it through the long sand pit pedaling every single lap. In the late afternoon, when I raced another category and used 25mm-deep aluminum tubeless clincher rims, I couldn’t make it through the pit without running a single time. In both cases, I’d line up on an existing deep track, but with the deep carbon tubulars, I could hold the bars loosely and just follow the track. With the aluminum wheels, however, I could not stay in the track and would get mired in the untracked sand.

Mud, particularly sticky mud mixed with grass, tends to collect on shallow wheels — both on the flat surface and around the spoke nipples, whereas it doesn’t get wrapped around the spokes and is shed off of the rims on deep carbon wheels.

Deep-section carbon tubular wheels also tend to be really strong. I regularly nail a fair number of obstacles un-gracefully – curbs, low railroad ties, steps and the like, and I find that aluminum clincher wheels get dented, often irreparably. But I have yet to dent or crack a deep carbon tubular wheel, even though I’m generally running my tires at 30psi or less, weigh 175 pounds, and am not the most adept at timing the lifting of my rear wheel on small bunny hops.

I also asked a few experts about it, and here are some of their answers.

From Shimano’s resident `cross expert Jesse Gascon, the reasons for using deep-section carbon wheels in cyclocross are:

“Lateral rigidity for ramming through the sharp turns for both front/rear wheels and overall strength and durability.”

From ageless many-time masters cyclocross national champion Steve Tilford:

“I think the Euro’ guys are using the deep section wheels because of the advantages they have in mud and sand. In Europe, mud and/or sand is virtually guaranteed. Plus the added strength/durability. You don’t want “vertical compliance” in your wheels in `cross. If I understand the use of the term, you use tire pressure for that. I’ve never understood the notion of compliance in a wheel. In virtually all aspects of the sport, especially `cross, I want a stiff, rigid wheel. Tires and tire pressure are the means to fine tune the ride. All that being said, the weight is important too. Mainly for the accelerations. Under perfect conditions, you’d have a full quiver of wheels with different tires on them for each condition. Probably not feasible for most riders.”

From Stu Thorne, manager of the Cannondale-Cyclocrossworld.com team:

For `cross racing, the 303s can be laced with 24 front spokes. Standard road stock is built with 18. | Michael Robson, michaelrobson.com
The Zipp 303: Aero' and popular in cyclocross

“We use primarily the Zipp 303 `cross wheels. We find that they work well in a variety of conditions. We like them for their ability to shed mud and while you mention that the weight of a slightly shallower dish is lighter, the weight savings is so negligible that it’s not really worth it. We have tried the Zipp 202 and the riders never really noticed the weight difference. The added bonus to the Zipp 303 is the rim width is quite wide (27.5mm), which aids in flat protection. The wider rim also gives the tire a larger surface area for gluing.

“In regards to the aero advantage of the deep dish rims, we have found that due to the wider tires used in `cross, the air is pushed around the rim, thus the rim profile doesn’t play into aerodynamics. All of our wheels are mounted up with Dugast tubulars. So with the combination of some of the most supple tires out there and Zipp’s toroidal rim profile, the vertical compliance is quite good.”

Ritchey media manager and avid `cross rider Sean Coffey says:

“First the deeper profile allows the wheel to cut through mud and sand better … a shallow rim profile sinks and gets buried, so you’re essentially pulling the wheel out with every pedal stroke. Deeper section wheels prevent mud/sand from coming over the top of the rim so it tends to roll faster through this sort of terrain.

“Second, a deep-section rim uses shorter spokes at a sharper bracing angle, which, combined with stiff carbon rim designs, gives you excellent lateral stiffness and precision.”

And finally, Zack Vestal, who recently went from being VeloNews Tech Editor and assigning me articles to a marketing position at Mavic, notes:

“The reason that Mavic athletes choose our Cosmic Carbone Ultimate (deep-section carbon tubular) wheels is because they really are the “ultimate” in our line. At 1185 grams, the CCU is our lightest wheel. It’s also built for fantastic lateral stiffness and extremely low inertia. As you know, cyclocross courses feature frequent changes of speed and direction. Our CCU wheels are very responsive for sprinting up hills or out of corners, thanks to their stiffness, light weight, and low inertia. They also happen to be super durable and the hubs are easy to service.

“Another incidental benefit of the Cosmic Carbone Ultimates is that they’re fully sealed at the rim, even through the valve hole. There are no openings at the spoke insertions for water or mud to enter. However, this feature is really just a byproduct of the rim construction, which is optimized for extreme stiffness, strength, and light weight.”

So, there you have it; a lot of rather good non-aero reasons to use them in cyclocross.
Lennard

Q.Dear Lennard,
Is there a reason for the complete lack of deep-dish aluminum wheels? The deepest you see are 31mm, the Reynolds Solitude and Velocity Deep-V being the two that come to mind quickest, and that doesn’t even get close to anything you see with carbon. It seems to me that having a deep dish aluminum wheel would be a good deal cheaper than any carbon set-up, and yet you don’t see any anywhere. I know you usually hear that they’d be too heavy, but it seems to me you could create one with around a 40-45mm dish that wouldn’t be that egregious in terms of weight. For example, the Solitude’s weigh in at around 1500 grams, and if they extended the rim down 10-15mm I doubt it would add more than 200-250 grams (and $25-50, which would still keep it under $600 retail), which would put them in line in terms of weight with Mavic’s Cosmic Carbone SLs.

Is there a particular design feature that causes a problem or something, cause I thought I’d seen one around there somewhere, but I cannot find any.
Sam

A.Dear Sam,
Actually, I think that’s a question with a simple answer — settle for either wrinkles or heavy weight. Aluminum rims start out as aluminum extruded through a die as a straight piece with the correct hollow cross section. They are then rolled into a hoop shape. A shallow-section extrusion is easy enough to roll into a nice, smooth circle without rippling. But a deep-section extrusion is extremely stiff along the tall profile, so rolling it into a hoop first of all takes a lot more force. More importantly, though, if the rim walls are thin, which they need to be in order to be lightweight, they will ripple when rolled. This was the problem with early deep aluminum rims, and the market rejected them. Nobody seemed to want to buy an expensive rim that had wavy sidewalls.

And of course, if the rims were beefed up enough to not ripple, they’d be too heavy. A case in point would be inexpensive colored wheels for fixies — they can be boat anchors, but of course they do look cool.

Or if softer aluminum were used that would bend easily without rippling, they wouldn’t hold up.

So don’t hold your breath for lightweight deep-section aluminum wheels.
Lennard

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Technical writer Lennard Zinn is a frame builder (www.zinncycles.com), a former U.S. national team rider and author of numerous books on bikes and bike maintenance including the pair of successful maintenance guides “Zinn and the Art of Mountain Bike Maintenance” – now available also on DVD, and “Zinn and the Art of Road Bike Maintenance,” as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Zinn’s column appears here each Tuesday.