Monday, August 31, 2009

Equipment - Rim Or Fairing?

So you're like me, watching the Vuelta on Universal Sports, and you see all the pros packing the flat, narrow Dutch roads, flying along, pedaling umpteen thousand dollar bikes. If you're like me, you'll ooh and aah and gasp in the final few minutes of the race. But even if you're not me, you're bound to notice something - gobs and gobs of those "carbon fiber wheels".

Why do they ride them? What's the big deal with the wheels?

Ultimately, the wheels are light, but with pros' bikes regularly dipping below the UCI mandated minimum weight, they can afford to put some heavier parts on their bike, if the additional weight gets them some other function. You'll regularly find aluminum bars, stems, and cranks on pros' bikes, because of their increased reliability and the ability to ride a bent aluminum piece (versus a broken carbon fiber one). Many pros have powermeters on their bikes, whether cranks or hubs, and such devices inevitably add weight to the bike.

And finally, many pros use tall profile wheels, aero wheels if you will. They're a bit heavier than the lightest non-aero wheels but they offer some functional advantages to the featherweight wheels.

They reduce the air resistance of the fastest moving part on the bike, the wheels. Okay, the tires move the fastest (the top of your tire moves twice as fast as your ground speed), but there seems to be limited gains made with "aero" tires. Rims, though, make more of a difference, based on the aerodynamic drag they do or don't have.

Although not necessarily aerodynamically correct, I stand by my layman's way of explaining one way how tall profile rims help deal with wind.

Regardless of how the actual aerodynamics work, many studies have found that aerodynamics trumps weight in all but the hilliest of races. And, honestly, if I were that concerned about weight, I could stand to lose, oh, say, 4,540 - 9,080 grams off my own body, much more than the 500 or 800 grams separating my wheels from the lightest wheels available.


So what's the deal with this "carbon fiber" stuff?

Carbon fiber is a mix of epoxy resin (plastic if you will) reinforced with carbon fiber. Both are extremely brittle in their native state, but together they form a very strong material. The resin helps distribute the load across the carbon fiber threads, and the threads keep the resin from breaking too easily by spreading the load to more than one or two CF threads.

I should point out that carbon fiber is still brittle at point of failure. It doesn't fail by bending, it fails by shattering. Although there are ways to help prevent the pieces from scattering all over the place (using aramid, or its trademarked name Kevlar, doesn't make a difference since only DuPont makes the stuff), such things are safety features and don't allow the broken piece to do much of anything. You may have a aramid-lined carbon fiber handlebar, but when one side breaks off, it'll just dangle on some aramid threads. Not much use to me or you.

Is it a fairing or a rim?

There are two kinds of "aero" carbon fiber wheels, and this has nothing to do with tire type. There is the "faired" rim and the "structural" rim (my terms).

Faired Rim

The Jet 6, a fairing wheel from HED. Note that the spokes come out from the sides of the fairing, not the tip. Note too the elongated spoke holes. (If you click on the picture, you'll get a bigger version of it, and it's easier to see there.)
Image from HED Cycling

The fairing wheels are typically aluminum rimmed wheels with a carbon fiber (or just plain plastic) fairing on it. You can tell a fairing wheel because the holes for the spokes are big, huge actually, relative to the spokes. This is to allow the spokes to move laterally in the fairing - they do so because the spokes are not anchored in the fairing, they're anchored up at the rim, and the rim naturally flexes relative to the hub. It's a fairing, right, so it's not structural.

Big companies that make fairing wheels are HED, Bontrager, and the aforementioned Mavics (I think their Cosmic Carbone were the first fairing wheel with a structural fairing). HED's fairing wheels are labeled Jets, including the Jet Disc.

A fairing wheel have certain "default" characteristics. They are typically more comfortable because they use full length spokes, longer than structural tall profile rim spokes by at least 2-7 cm. They are typically weak for a given weight since the fairing is not really structural. Usually they have a higher spoke count or a heavier rim, to make up for the fact that the voluminous fairing has no structural function. They tend to be more flexible laterally for a given weight, again because of the full length spokes and the need to either increase rim mass or increase spoke count.

I haven't had the opportunity to cut up a faired wheel, but my understanding is some of the wheels are made with fairing specific rims, with a cross-section optimized for strength, not for appearance or aerodynamics. A rim could be manufactured with the fairing in mind, with specific adhesion areas to anchor the fairing. All this is a good thing and could actually justify the official reason for faired wheels.

Which leads me to how a fairing could be legal in the bike racing world, at least at the UCI level.
I never understood how fairing wheels could be legal because aerodynamic fairings are not allowed by the UCI. Well, I read how. Basically manufacturers claim the fairings are "rim stiffeners" to increase rim performance. This is official mumbo-jumbo to get around the actual benefit of the fairings, namely the aerodynamics.

Although I'd say that a true carbon fiber fairing may stiffen the wheel a bit, the Mavic initially used a vacuum-formed black plastic fairing in their first generation Cosmic Carbone, one so flimsy you could squeeze the halves together. Try doing that with a structural carbon rim - you can't squeeze them at all.

I was initially disappointed in those first generation Cosmic Carbones when I realized the fairings were just fairings, as compared to my various other structurally rimmed wheels. However, the rims were designed to be faired, and so that kind of eased my disappointment. Nowadays, with a true carbon fiber fairing, and over a decade of optimizing rigidity, I'm sure the wheels work better than the first generation of wheel.

A faired wheel has one significant disadvantage - in almost all circumstances (the exception below) you must remove the tire to true the wheel. This is because the fairing hides the spoke nipple. This also discounts such wheels from being fully tubeless clincher compatible, because ideally a tubless tire rim will have no spoke holes in it. A fairing wheel with no spoke holes under the tire would be impossible to true.

The only exception to the above would be a wheelset which had the spoke nipples were in the hub.

I'd consider a faired wheel for myself just as readily as a structural one, especially for a training clincher wheelset. In fact, given the opportunity, I'd use Jet 6 wheelset, a front/rear pair of the rear wheel pictured above. The typically heavier weight of a faired wheel is fine for training, and the clincher rim makes removing the tire easy for any necessary truing.

Finally, using tall profile wheels in training, especially front ones, is critical in developing a honed feel for the bike's handling characteristics with such wheels. Reserve a tall profile set of wheels just for race day and you'll be either asking for problems or find yourself a bit uncomfortable in gusty or unpredictable windy conditions. Both are not good when you're in the middle of a tightly packed field with a mile to go before the sprint.

Structural Rims

A structural rim, the Stinger 6, sister to the Jet 6 shown above. Note the spoke nipples poking out of the rim.
Image from HED Cycling

The Zipps, Reynolds, and HED Stinger wheels are made with structural carbon fiber rims. The spoke nipples are anchored in the "v" of the rim, whether hidden (Reynolds) or not (Zipp, HED). HED3s and other 3-4-5 spoke wheels are structural simply because if you remove the carbon spokes (or "wings", since they're typically 7 or 8 cm wide), there are no spokes left. Anyway, a structural carbon rim, a tall one (I'm skipping the climbing specific non-aero ones) will have a taller rim and correspondingly shorter spokes.

These wheels, if the rim is constructed properly, are stiffer laterally. The shorter spokes and hopefully laterally stiff rim contribute to this rigidity. To be fair the initial structural carbon fiber wheels were not very stiff laterally. Structural carbon fiber rims are usually lighter too, mainly because you can use carbon for the rim material since the rim/fairing is one integrated piece.

Having stated my willingness to train on Jet 6s (oh, if life were only so rough), one of my ideal set of race wheels would be the Stinger 6s. Or a Stinger 9 rear, since a taller rear wheel has almost no disadvantages.

Tire Type?

It's much easier to make a clincher wheel using the fairing method. Just get a normal clincher rim, glue a fairing onto it. The aluminum rim is plenty strong for the job, efficient, reasonably light, and very cost effective.

Making a carbon clincher, on the other hand, is tough. Carbon fiber works best in tension, when the threads are in line with forces pulling on both ends. Clincher bead walls don't exhibit that kind of stress so they do not allow a manufacturer to use carbon fiber in an ideal fashion.

This is all relevant because clinchers rely on the rim to hold in air pressure, a substantial force at 100-150 psi (typical range, although it can go higher). There may be 70 or 80 square inches on a rim, so a total of 700-1200 pounds of force trying to blow the two sides of the rim off.

It's relatively easy to make all-carbon tubular rims because the tires themselves hold all the air pressure. Initially that's all you saw, tubular carbon wheels, and Zipp still refuses to make carbon clincher rims.

Some manufacturers have figured out how to make strong carbon clinchers. Reynolds is the one that comes to mind, I think others include Lightweight and LEW, the latter being closely tied to Reynolds. They use heat and pressure, keys to controlling carbon fiber quality in the curing process. They also skip any cosmetic top coat, resulting in a sort of smudgy looking grey finish. Surprisingly, the familiar carbon fiber weave is not necessarily the best structural material for specific purposes. More and more manufacturers acknowledge this, with bikes and rims and everything else starting to lose the familiar woven pattern initially associated with carbon fiber.

Campy, or as someone pointed out, Corima for Campy, make carbon rims but due to poor design or construction, the rims are limited to 118 psi for 700x22 tires, lower for wider ones (according to page 5 in their manual). In contrast, Reynolds recommends a maximum pressure of 150 psi for their carbon clinchers on their FAQ page.

That may be fine and all, but the unofficial info I've learned is even better. I had the opportunity to talk with a (now-former) Reynolds engineer that gleefully recalled inflating tires to insane pressures until either the tires blew off or the rims exploded. They were repeatedly blowing tires off the Reynolds clincher rims before the rims blew apart, and at pressures well into the upper 200 psi range. No other carbon rims withstood that abuse, and yes, that meant that they blew apart a very expensive Campy wheel.

And yes, those experiments were done for work, not as a drunken Friday night "Hey, I wonder if..." kind of experiment.

Note: Such experiments were carried out under controlled circumstances, with proper safety gear. If you try the same experiment with your clinchers, carbon or not, I take no responsibility for whatever happens. At best you'll end up with something similar to what's pictured below.

Picture of an aluminum clincher rim after a 150 psi tire blew the rim apart.
From Bethel Cycle, sponsor of the Bethel Spring Series.


It's clear that the market has two types of tall profile wheels - the fairing wheels and the structural tall wheels. Each has their place, their strengths, their weaknesses. If and when you go looking for a set of aero wheels, keep the two types in mind.


ycantelar said...

nice write up! very informative.
here's a good interview with one of the engineers at Zipp. Maybe you'd enjoy the read as well

Aki said...

Really interesting article. It seems very frank, very open. The comments are interesting too, but the engineer comes across well. He's obviously very proud of what he does, and that's a good sign.

hobgoblin said...

I like what you had to say about the Stinger 6. Oddly enough, I was just about to e-mail you and ask your opinion of that wheelset!