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by Matt Wikstrom
February 27, 2017
Photography by Matt Wikstrom
TECH NEWS BROUGHT TO YOU BY BIKEEXCHANGE
For some riders, especially racers, the allure of carbon wheels is almost irresistible. Professional riders race on them all the time and results from wind tunnel testing tend to promise significant time and power savings. But does that mean a carbon wheelset is good value for money?
Australian tech editor Matt Wikstrom had a look at this question four years ago, and now he takes a fresh look at it in light of the introduction of disc brakes and the falling cost of carbon rims.
There is no other part of a bike that can rival the appeal of wheels when owners start to think about an upgrade. Indeed, I get more questions about wheelsets as an upgrade than any other part of the bike, and it’s surprising how well informed consumers are.
The popularity of factory-built wheelsets has no doubt contributed to consumer enthusiasm. Where once a wheel had to be assembled before the buyer could see the final product, now there are dozens of ready-to-ride products at a range of pricepoints with detailed specifications that can be easily compared. In short, upgrading a wheelset has become easier and more convenient than ever.
Sitting at the top of this market, like an exotic gemstone, is the carbon wheelset, the ultimate upgrade that promises to boost a rider’s power and performance. Such benefits come at a significant cost, yet the carbon wheel market has grown considerably in recent years, perhaps outpacing lower priced wheelsets. At the very least, the marketing effort has been immense, to the point where consumers have gained a new appreciation for the aerodynamic performance of wheels.
There has been a subtle shift in the carbon wheel market in recent years though. In the past, manufacturers have justified the opulent pricing of carbon wheelsets on the basis of the expense of the starting material and the amount of research and development that was required to create a cutting-edge rim profile. However, the rise of Asian manufacturing in the last decade and the inevitable trickle-down of technology has lead to a proliferation in small wheel brands and lower priced products.
After many years of exorbitant prices, carbon wheelsets are becoming more affordable.
Now there is room to wonder if the industry might 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 20 years ago when the material was too expensive, unproven and even unreliable?
The benefits of carbon composites are well known, where the low weight and high strength of the material accounts for the majority of its appeal. Indeed, the introduction of composite rims during the 1990s provided a major leap forward for wheel design and construction.
Consider Campagnolo Shamals from the mid-90s: the alloy rims were around 40mm tall and the wheelset weighed almost 2kg (1,980g), yet they were considered a breakthrough in aerodynamic design. By contrast, the composite version of that wheelets benefitted immediately from a weight saving of 300g.
Twenty years on, carbon wheelsets now feature taller rims and can weigh less than 1,500g (especially those with tubular rims) while the lightest wheelsets are well below 1,000g.
The impressive stiffness-to-weight ratio for composites means that even at a fraction of the weight, carbon wheelsets easily match (and often exceed) the performance of aluminium equivalents in terms of lateral and radial stiffness. At the same time, composites provide enormous freedom for sculpting the final product and carbon wheelmakers have been able to refine the final product in a way that was never possible for aluminium.
The original V-shaped NACA airfoil shape employed by the Shamal was aerodynamically sound, but one reason for its early popularity was that it was relatively easy to construct from aluminium. The pioneering work done by Hed and Zipp throughout the ‘90s soon proved that a wider and rounder toroidal profile was aerodynamically superior but construction of the rim ultimately depended upon the introduction of composite materials.
Such manufacturing challenges have been conquered and toroidal rim profiles have become ubiquitous now. Aerodynamic performance has become a commodity in the marketplace and the results from wind tunnel testing (and to a lesser extent computational fluid dynamics) are now commonplace, be it in published white papers, through independent testing or in a manufacturer’s claims. Consumers looking to maximise their marginal gains can weigh up data on the drag and stall of different rim or wheel designs along with other specifications (such as weight) when deciding on a new carbon wheelset.
More and more though, the price of carbon wheelsets has been falling, not because key brands like Zipp and Enve have been lowering prices, but because of the proliferation of small wheel brands. Rather than establishing their own production facilities, these brands typically partner with low-cost Asian manufacturers for the supply of rims and/or wheelsets. While these products are somewhat generic, they often match the weight and performance of established brands while trumping them on the basis of price, providing consumers with more choice and extra competition in the marketplace.
Thus, it’s fair to say that the value of a carbon wheelset has increased in recent years simply because of falling prices, however there are still a number of drawbacks to composite wheels that every prospective buyer should consider.
The data from wind tunnel testing is clear: high-profile carbon wheels provide an aerodynamic advantage over low-profile wheels (carbon or alloy), but the magnitude of those savings is small when viewed in the context of the bike and rider. Indeed, in the absence of highly controlled conditions (i.e. out in the real world), those time and energy savings are often difficult to reproduce.
It’s also worth pointing out that the significant time savings promised by the marketing material for any given wheelset are consistently calculated for high speeds over relatively long distances. In addition, the reduction in drag promised by a particular wheelset does not apply in every direction, such that minor changes in a rider’s direction relative to the prevailing wind can produce a huge change in drag. The bike can have a significant impact too, further diminishing the magnitude of the promised gains in the real world.
Thus, the time and energy savings that any manufacturer promises for a wheelset must be viewed as a theoretical maximum that may not be fully or consistently realised in the real world. For those buyers pondering an upgrade from an aerodynamically poor wheelset (such as low-profile alloy wheelset with 32 spokes), the increase in speed and performance will be tangible, while the minor differences identified for one brand over another are unlikely to be felt.
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.
With all of this in mind, it’s hard to argue for the value of the marginal gains offered by a carbon wheelset unless the rider is prepared to invest in complementary strategies that maximise his or her aerodynamic performance. This includes adopting an aerodynamically sound position on the bike, paying attention to tyre width and pressure, investing in an aero frameset, and opting for a wind-cheating helmet.
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. Nevertheless, manufacturers have worked hard to improve braking performance of carbon rims in recent years, so that it approaches that of alloy rims. However, overheating of clinchers still threatens to cause the rim to delaminate and/or a tire blowout after extended periods of heavy braking (such as long, steep descents).
There has been a rise in the number of proprietary brake pads designed to reduce heat generation, with the result that many buyers are locked into using a specific brake pad for their chosen carbon wheelset. Nevertheless, these new pad compounds seem to be reducing the risk of overheating but they do not eliminate heat generation during braking.
The heat-resistance of carbon rims has also been helped by careful selection of heat-resistant resins for the brake track. There is only so much that can be achieved with this strategy though, since raising the glass transition temperature of the rim renders the composite more brittle and susceptible to impact damage.
It’s getting harder for traditionalists to dismiss the relevance of disc brakes, especially for carbon wheelsets.
The risk of heat damage is therefore an unfortunate and unavoidable drawback for carbon rims. Indeed, the warranties offered by some brands specifically exclude heat damage. That isn’t to say that composite rims are dangerous, but owners must be prepared to exercise a certain amount of caution and mindfulness that isn’t required when riding alloy rims.
Such concerns disappear completely when carbon rims are paired with disc brakes. In fact, road disc brakes and bikes have arguably done more to increase the appeal of carbon wheelsets than all of the work done on new brake pad compounds and heat-resistant resins. Whether the gains in braking and safety are enough to compensate for the extra weight of disc brakes ultimately depends on the needs of the rider, but it’s very easy to see carbon rims becoming the standard choice for any road disc bike.
After more than two decades of manufacture, it’s fair to say that there are fewer concerns about the strength of carbon composites for framesets. In the same way, confidence in carbon wheels has been growing. Where once a carbon wheelset was considered race-day only, riders are now confident enough to use them everyday.
Nevertheless, carbon composites remain susceptible to impact damage, so owners should expect a costly repair in the event of a crash or when a run-in with a pothole damages the rim. This counts as perhaps the greatest disadvantage (and turn-off) for carbon composites — not only is the buy-in cost high, but so too is the cost of repair and replacement.
It’s hard to find marketing material from any carbon wheel manufacturer that proves the strength of the rims. And if there is any data on the durability of the rims (especially the brake tracks), then the manufacturers are reluctant to share it. There is anecdotal evidence to suggest that carbon rims are hard-wearing but it’s not enough to prove that they outlive alloy rims.
With this in mind, it’s surprising that some MTBers, including downhill riders, have started using carbon rims, which is enough to suggest that carbon wheels can appeal to riders other than aerodynamically-obsessed road racers.
While there is the promise of great strength and durability, every composite product is at risk of manufacturing defects related to wrinkles and voids that develop during the manufacturing process. Such defects are invisible to the eye yet they can have a profound effect on the lifespan of the product.
Similarly, an impact can produce sub-structural de-lamination invisible to all but ultrasound and x-ray inspection. Thus, a seemingly innocuous crash or collision can ruin a rim without any indication of the damage until it fails at a later date.
While carbon wheelsets are celebrated for their strengths, that does not mean they will satisfy the needs of every rider. For example, carbon composites are completely irrelevant to riders on a tight budget. Alloy wheels dominate this category and my advice for those shopping in this realm is to opt for the lightest wheelset with the widest rims that they can afford (and will fit their frame and brake callipers).
For those that prefer the versatility of a low-profile rim, alloy also tends to trump carbon, since wheelsets remain quite competitive on the basis of weight, offer superior braking for rim callipers, and can be used without fear of overheating (low-profile carbon rims are especially susceptible to overheating from rim brakes).
Carbon/alloy hybrid rims are still very relevant to riders using rim brakes.
The appeal of composites grows with the height of the rim profile, however alloy/carbon hybrid rims such as Shimano’s new C40 and C60, Hed’s Jet series, and Swiss Side’s Hadron range are able to match the weight and performance of a full-carbon clincher without compromising the quality of rim braking or suffering the risk of overheating.
Internet shoppers will no doubt be aware of the low-cost carbon rims and wheelsets that can be obtained from Chinese manufacturers. At face value, these products match higher-priced options from established brands, and with sometimes glowing anecdotal reports, appear to be a great bargain.
What may be missing though is after-sales service and a meaningful warranty. In the worst-case scenario, the rim may be riddled with voids that will lead to de-lamination with use. Such defects are invisible to the eye, so there is no way to distinguish a defective product from a sound one without more advanced inspection.
That isn’t to say that all Chinese-made rims are defective; in fact, the risk of voids applies to all brands of composite rims. The difference is, established manufacturers typically rely on robust manufacturing processes and quality control procedures to mitigate this risk.
Does that mean that all low-cost Chinese rims are manufactured in the absence of such precautions? Not at all but consumers have no way of knowing what they are getting, regardless of whether they pay top-dollar or not.
The carbon wheel market has followed the same trend as carbon bikes in that as time has passed, lower-priced alternatives from new brands have slowly appeared rather than the market leaders dropping their prices. As discussed above, an increase in Asian production has given consumers cheaper carbon wheels, but most were created to rival more expensive brands.
Giant wasn’t the first to start supplying carbon wheelsets as a stock option for its complete bikes, but it has managed to bring them to lower pricepoints.
Now low-cost carbon wheelsets have started appearing amongst the inventory of some well known online retailers, such as Wiggle and Chain Reaction Cycles. Wiggle’s Cosine range of all-carbon clinchers retail for less than AUD$1,000 while Chain Reaction Cycles offers its Prime brand of carbon wheelsets for less than AUD$1,500. Neither brand is positioned to compete with the market-leaders but they may be staking out new territory for carbon wheels with what might be best described as entry-level products.
Fifteen years ago, the notion of an entry-level carbon wheelset would have seemed ludicrous. It will be interesting to see if more of these products appear in the next year or two, but my guess is that, just like carbon framesets, these products will work to increase the general appeal of carbon wheels.
In the first version of this article more than four years ago, I concluded that while there were some significant shortcomings, carbon wheels had a lot to offer the consumer, especially performance-oriented riders. That view hasn’t changed, and indeed, with the introduction of road disc bikes and a growing number of lower-priced products, I can only see that appeal increasing.
Now it’s feasible for riders to buy a pair of mid-profile carbon hoops purely for aesthetic reasons. That a major manufacturer in Giant is specifying its own carbon wheelsets for mid-level bikes (both road and off-road) is another sure sign of where the industry is headed. In short, what was once exclusive exotica is becoming commonplace.
That doesn’t mean all riders will be forced onto carbon rims in the years to come. At the very least, alloy wheels will continue to serve low-priced entry-level bikes, and as long as rim brakes persist, there will always be a strong argument for aluminium rims.