Tech Report: The Real Story Behind Carbon Clinchers

During Zipp’s carbon clincher development, they did thorough testing on how much heat was generated during heavy braking.

Wheels have always been one of the most talked-about bicycle components, as they should be. After all, few things can have as big of an impact on performance as what wheels you’re rolling on. Unsurprisingly, as with so many other aspects of cycling, wheels too have become part of the fashion equation. Currently, the ‘aero look’ is in, and the industry is responding enthusiastically-just as it did with the ascendance of carbon fiber.

While the last decade of the carbon wheel market has been dominated by the tubular wheel, the industry has always known that real growth in the market would only be found outside the small number of tubular diehards that don’t mind lugging a spare tire with them on every ride. The really big potential lies with the design, manufacture and sales of a carbon clincher, although some of the biggest names in high-tech wheels have yet to jump on the carbon clincher bandwagon-as we all know, the ‘market’ has never been a rational entity. Like time itself, the consumer market waits for no one; it goes where it wants, when it wants.
But now, in the summer of 2011, we can announce that the era of the carbon clincher has officially arrived. Right this moment the market is being flooded by new and old brands offering rim depths, weights and prices that were never before thought attainable with carbon clinchers. We’re told it’s the ultimate all-in-one training/race wheel. We wanted to find out if this was true.

French professional team AG2R uses Michelin tires on Reynolds full carbon clinchers for the majority of their races.

In 1998 Lew Composites introduced the first full-carbon clincher rim. Company owner Paul Lew, an avid triathlete, wanted the same weight savings and aerodynamics that a carbon tubular could offer, but with the ease and versatility of a clincher. ‘I knew that if I could build a wheel with the best of both worlds, people could have one set of wheels that would be practical for training while still being light and aerodynamic for racing,’ explained Lew.
At the time, the carbon being used in tubular rims cost between $15-$18 per pound, but the high-modulus carbon that Lew Composites used in order to deal with the heat issues for the new clinchers cost between $90-$110 per pound. While Paul was able to create his vision of the ideal wheel, they were far from practical-at least economically. Due to the extreme expense of the materials and the process involved, other rim manufacturers were hesitant to jump into the ring.
Within a few years after Lew Composites was purchased by Reynolds Composites in 2001, a couple of other players emerged on the market. Lew attributed this to ‘prices coming down on material, while epoxy resin had improved. Technology was higher quality and had become more affordable.’ In other words, it was only a matter of time.
In theory, carbon clinchers have the potential of being the perfect wheel: light, laterally stiff and aerodynamic-all with the convenience of a hook-bead clincher. Unlike carbon tubulars and alloy rims, carbon clinchers have always had one major hurdle to overcome: heat. As with all carbon wheels, the heat generated while braking during long and steep descents has always been a challenge for the resin and carbon engineers designing the wheels.
However, while a tubular rim receives the same braking heat as a clincher, it’s the rim’s profile that makes the difference. A tubular rim has an open profile where the tire sits, spreading pressure across the entire surface of the rim. On the other hand, a clincher has all the pressure of the tire and tube pushing outward on the rim’s sidewall. It’s the combination of the heat buildup and the outward pressure that have made the development of the carbon clincher so vexing.
Two words: glass transition (Tg). Tg is the temperature at which the epoxy begins to deteriorate. In the case of clinchers, when the epoxy reaches Tg the outward pressure of the tire and tube can cause the rim’s sidewall to deform, potentially resulting in a catastrophic failure.
On a long descent, braking temperatures can reach 300 degrees Fahrenheit; resins not designed for high heat will reach their Tg point well below that, resulting in rim failure. Raising the Tg point of the resin is the obvious answer, and one that carbon clincher makers have been trying to perfect for years.
When talking about Zipp’s resin woes in developing their carbon clincher rims, Josh Poertner states, ‘The thing we had the hardest time with was finding the right resin to use. High-temp resins are usually not tough enough, while highly toughened resins make great handlebars but can’t necessarily handle high temperatures. What you dope the resin with to get one feature comes out at the expense of the other feature. We sat down with nearly every resin vendor around the world and said we need both A and B. We finally found a small vendor that specializes in Formula One racing who was able to provide the specs we needed.’
While the resin development has evolved to allow for a higher Tg temperature, brake pad technology has evolved too. ‘We developed a polymer brake pad in conjunction with a polymer brake track. Together they lower the braking temperature by 100 degrees Fahrenheit,’ said Lew of Reynolds.
Reynolds isn’t the only company to have designed a specific brake pad to be used with their rims. Keith Williams of Williams Wheels stated, ‘We’ve been involved in a variety of brake pad tests, trying to develop a pad that keeps the braking heat down but doesn’t wear out too quickly either. We went with a pad that has ceramic dust in it, which lowered the temperature up to 90 degrees Fahrenheit.’
Just about every carbon clincher on the market comes with a set of brake pads that are specifically designed for use with their own rim. Ensuring that the right pad is used with the right rim is essential to keeping braking temperatures- and ultimately braking performance-in a safe range.

Outward pressure on the clincher’s sidewall from the tire can be too much if braking heat reaches Tg, which can result in a rim failure.

By contrast, due to their open profile, tubular rims are not affected by the same sidewall failures caused by heat and tire pressure that clinchers are.

During Zipp’s development of their carbon clincher, they built a test machine that replicates descents up to 50 mph so they could test braking temperatures. What they found was that their rim could withstand more pressure and heat than the best Kevlar-beaded tire out there. The tire would blow off well below the resin’s Tg temperature was met. Zipp’s Andy Paskins, ‘We can confidently say that our carbon clincher is as safe as an aluminum rim.’
But can everyone say that? The influx of new carbon clincher brands is mostly due to the fact that Asian manufacturers are producing open-mold (non-proprietary) carbon clinchers at prices far less than what was available on the market just three years ago. The majority of the new brands are purchasing their rims from these manufacturers and without investing the resources that the established wheel brands have. It’s safe to assume that R&D and testing is sometimes minimal to nonexistent for many of the new players. This isn’t to say that the rim manufacturers themselves are not doing their own extensive testing to ensure the rims they’re selling are safe, but it’s not to say they all are either.
In fact, one wheel expert joked about the overnight arrival of so many new wheel brands: ‘Look closely and you’ll find that the only proprietary design among many of those wheels are the graphics!’ And in truth, as we’ve surveyed the wheel market, we have discovered far more identical or shared parts within the carbon clincher family than we’ve ever seen among competing wheel categories.
Ironically, the most reassuring message we heard on the reliability of all the carbon clincher upstarts came from the man who first came up with the idea. Paul Lew predicts, ‘A few years from now, just about all the companies’ carbon clinchers will be safe. There is some good stuff coming out of Asia right now, but it’s still hit-or-miss. The raw materials are there and the engineers are there; it’s just a matter of time. Years back there was a small group of engineers who knew the process, but over the years some of those engineers have gone in different directions and helped other companies develop their own rims.’

A popular alternative to full carbon clinchers are co-molded, deep-section rims. While not suffering from braking heat issues, they d come with a slight weight penalty.

Curiously, as active as the carbon clincher market has become, it’s odd to find some of the most distinguished names in wheel design absent. French wheelmaker Mavic, the biggest and oldest name in bicycle rims manufacturing, is certainly the most prominent name among the missing. The company that introduced the first alloy rim in 1926 isn’t fully buying into the carbon clincher attributes according to product manager Maxime Brunand. ‘We have tried many lay-ups, and nothing is up to our expectations for rim track pressure and braking. We are taking a conservative approach to the carbon clinchers, since we feel we already offer wheels that are comparable in weight and aerodynamics with better braking and no threat of delamination from brake heat.’
Mavic, along with other notable names such as HED, Rolf, American Classic and Bontrager, produces wheels with carbon nose cones comolded to alloy rims (Bontrager also makes a full carbon clincher). These hybrids offer deep aerodynamic rim profiles with brake tracks that are not affected by brake-heat issues, but the alloy rim generally adds some weight compared to full carbon clinchers.
The carbon clincher market has come a long way since Lew’s original design 13 years ago. Some of the early safety hurdles have been minimized through a combination of extensive testing, innovation and better materials. While only a few companies are responsible for being the catalyst behind the R&D of carbon clinchers, over time, many other manufacturers have been able to capitalize on the lessons learned. 
There are definitely some safety concerns to be aware of if you decide to take the plunge into the carbon clincher market. Riders over 200 pounds, or those who ride in especially mountainous regions where long descents could cause high braking temperatures, would be well served to research the testing that went into a wheel before purchasing one. Another option to consider would be the carbon/alloy co-molded wheel options that are available.
An additional point to take into account is that most of the rims and brake pads have been engineered as a proprietary system designed around reducing brake heat. Using the correct brake pads is essential to ensuring your own safety. Consumer demand for a lightweight, aerodynamic, stiff, versatile and fashionable wheel is driving the industry, which makes one thing certain: carbon clinchers are here to stay.

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