QUAL O REAL VALOR DE UMA RODA DE CARBONO?

fonte: http://www.cyclingtips.com.au/2012/08/where-is-the-value-in-a-carbon-wheelset/

 

August 1, 2012

By Matt Wikstrom, CTech Editor

 

Professional riders use them all the time while many manufacturers claim that you will benefit from significant time and power savings, but does that mean a carbon wheelset is good value for money? When coffee shop conversation turns to upgrades, a lot of riders think of wheels. Indeed, I get more clients asking about wheelsets as an upgrade than any other part of the bike, and it's surprising how well informed they are. Forgive the generalisation, but for an enthusiast it seems wheels are to road bikes what engines are to cars.

Judging from the way the factory wheelset market has grown over the last ten years, I’m guessing plenty of riders are acting on the urge to upgrade their wheels. Sitting at the top of this market, like an exotic gemstone, is the carbon wheelset, the ultimate upgrade that promises to deliver the same kind of boost to a rider’s power and performance as nitrous can deliver to a car. Such benefits come at a significant cost, yet this end of the market has grown too. Now lower-priced options are starting to appear and I wonder if the industry is poised to abandon aluminium for wheelsets in the same way aluminium was abandoned for frames. As unlikely as that may seem, how many riders would have predicted the coming dominance of carbon framesets 15 years ago when the material was too expensive, unproven and even unreliable?

The wheels that were at the leading edge of the rise of carbon wheelsets, the Lightweight Obermayer.

The value, according to the hype

According to the marketing hype, the value of a carbon wheelset lies with its weight and superior aerodynamics. The weight saving afforded by carbon is well known and readily demonstrated; the material also ushered in the current era of aerodynamic wheel design. Consider Campagnolo Shamals from the mid-90s, the alloy rims were around 40mm tall and the wheelset weighed 1980g, yet they were considered a breakthrough in aerodynamic design. Now carbon wheelsets feature rims that are as tall (or even taller) and weights have dropped below 1500g.

Carbon may come in only one colour, but there is plenty of variation in the number finishes that are available.

 

Another advantage of carbon is that it can be molded and the companies leading aerodynamic wheel design have been able to design wind-cutting profiles with the help of wind tunnel testing and fluidics. Current marketing efforts have provided riders with a basic understanding of aerodynamics along with hard data (of sorts) so they can compare the drag and stall of different rims or wheel designs before making a decision to buy.

A typical example of the drag data that is generated from wind tunnel testing. Note the characteristic dip in drag for a lot of wheels between 10 and 15 degrees yaw demonstrating that the lowest drag for any wheelset can only be achieved at a discrete angle rather than over a broad range of wind angles. Data taken from Bontrager's 2011 white paper on the development of their Aeolus D3 wheels (http://media.bontrager.com/images/features/201108_aeolus/bontrager_aeolus_d3_wheels.pdf).

Carbon rims and wheels also benefit from an impressive stiffness to weight ratio. We all know from the performance of carbon frames that the material can be very stiff, and carbon rims are typically engineered to be very stiff so as to allow low spoke counts. At the same time, carbon wheelsets can offer a smooth and compliant ride, so gram for gram, they easily outperform aluminium in this arena.

Low weight, superior aerodynamics and stiffness to weight, these traits are of enormous value when performance is the primary consideration, but at what point does a set of carbon wheels seem overpriced?

The shortcomings of carbon wheels

Consider the drawbacks of carbon wheels for a moment: tall rims may provide some aerodynamic advantage in favourable conditions, but they will rob a rider of their confidence (or worse, control of their bike) in crosswinds. Clearly, such wheels are far from versatile. In addition, the reduction in drag promised by a particular wheelset does not apply in every wind direction, such that minor changes in a rider’s direction relative to the prevailing wind can produce a huge change in drag that is associated with the familiar and consistent dip in drag at 10-15° yaw (see this article for more detail on this phenomenon). I don’t doubt that the data from wind tunnel tests is reliable and reproducible, but I have a question about how well it translates to the real world. The scientists devoted to measuring aerodynamic drag insist on a highly controlled environment to measure drag because variation in wind speed and direction can ruin their ability demonstrate a significant change. Put another way, differences in aerodynamic drag are typically so small that they can only be measured in isolation of all other factors. Hence, a lot of the available data has been measured for the performance of the wheel only, rather than after it is fitted to a bike. Bontrager explore the difference a bike can make to aerodynamic drag in their recent white paper, albeit with a highly aerodynamic TT frameset, and found that the savings in drag were maintained or even enhanced, depending on the angle of the wind. However, it remains unclear if this is the case for all bikes, so at best, the time or power savings promised for a wheelset have to be interpreted as a theoretical maximum that may not be fully or consistently realised in the real world.

It is now a lot more common for wheelsets to have warning stickers regardless of whether they are carbon or aluminium.

I find there is an amusing irony in the poor braking performance of carbon rims that have been engineered to be faster than standard alloy rims. Manufacturers have worked hard to improve braking performance of carbon rims so that it is starting to approach that of alloy rims, however, overheating of clinchers still threatens to cause a tire blowout after long periods of heavy braking (ie long, steep descents). Some companies (eg Reynolds, Enve) have developed specific brake pad compounds to reduce heating and strictly insist on their use. The problem still persists, and now some organisers of mass participation rides in the US are preventing riders from using carbon wheels. Red Kite Prayer recently highlighted this issue and found that overheating and rim failure may be more common for carbon clinchers. Unfortunately, the industry remains coy about the issue, but it seems more effort is required to solve the braking problems of carbon with some predicting that the move to disk brakes will address the issue. For now, it is a definite shortcoming that weakens the value of a carbon wheelset.

Swissstop Yellow King brake pads work well on a lot of carbon rims yet the company claims their new Black Prince pads are more effective and generate less heat.

So what of the strength and durability of a carbon wheelset? After more than a decade of manufacture, it’s fair to say that there are fewer concerns about the strength of carbon for framesets (and perhaps wheelsets), however the material is still susceptible to impact. Sifting through the marketing material from various wheel manufacturers, few emphasise the strength of their rims, so if there is any data on the durability of the rims (especially the brake tracks), then the manufacturers are reluctant to share it. Zipp highlights their use of Kevlar reinforcement for the edge of their tubular rims (“Carbon Bridge“) while they employ woven carbon fibre (instead of the uni-directional fibres that are used throughout the rest of the rim) around their spoke holes for added strength. In contrast, Enve mold the spoke holes into their rims to improve the strength of their rims. I can’t say whether these strategies deliver on their promises, but they are the sort of details lacking from lower priced and nameless options from China or elsewhere.

A lot of riders that own carbon wheels keep them in the cupboard for race days or special events. Personally, I don’t think there is a lot of value in a wheelset that spends more time off the bike than on, but I can understand the extra caution where thousands of dollars are concerned. Replacing a carbon rim is a costly exercise even when taking advantage of the crash replacement programs offered by many companies. Are carbon wheels tough enough for every day use? Aside from proposed weight limits, the manufacturers can’t dictate how their wheels should be used, but some (eg. Zipp) seem to favour the race day only approach. In contrast, other companies (eg. Enve) promote their rims and wheels for everyday use. As far as I can tell, the only concerns associated with everyday use is a greater risk of mishap and an increased rate of wear. There is no hard data on how long it takes for a carbon rim to wear out, though some anecdotal reports suggest it is longer than aluminium. On balance, I don’t think there is much evidence that supports the limited use of a carbon wheelsets, as long as you accept the associated risks. Jason Woznick fromFairwheel Bikes shared a great video in support of this notion–the boys from Arizona ride their carbon wheels as hard and as often as alloy wheels:

Considering the alternatives

It is possible to buy or assemble a set of alloy wheels that dip below 1200g for less than $1500, but there isn’t much choice. Stan’s offers their Alpha 340 rim, which is tubeless ready (385g) that can be paired with a lightweight set of hubs (eg. Dash), while American Classic has two wheelsets, a new tubeless wheelset that weighs in at 1179g, and their magnesium wheelset (1108g). Just recently, I’ve heard about Spada Oxygena wheels that also weigh in under 1200g. However, don’t expect these wheels to be as robust as a standard alloy wheelset. Just like carbon, they have specific requirements or shortcomings when it comes to spoke tension, brake pads, and/or tire mounting (eg. see the recent review of the American Classic tubeless wheelset at Pezcyclingnews). I also doubt any of them will satisfy a sprinter’s needs.

Final thoughts

When I set out to write this article, I didn’t have a preformed conclusion though I suspected that the question of value depended on a rider’s resources. The major brands may not have solved all of the shortcomings (yet) but as a package, carbon wheels are hard to beat. Alloy wheels cannot offer the same combination of weight, stiffness and aerodynamics but they are undeniably more cost-effective. My time as a bike mechanic and wheel builder has proven to me that the axiom, you get what you pay for, applies without fail to bikes and bike parts. The value of a wheelset (or any other product, for that matter) ultimately depends on how well it serves the needs of the buyer/rider. If a buyer wants a high-priced set of carbon wheels because they enhance the appearance of their bike, then they may get their money’s worth. In contrast, a struggling C-grader with limited time to train may feel a little ripped off when the race wins don’t come any easier after dropping $3K on the same set of wheels. My advice, the only sound advice, is first decide on your needs, and then set about finding the product that completely satisfies them.