Why Carbon Races Wheels? A Geeks Guide To Your Number One Aero Advantage.
Carbon Composite Race Wheels
Whether it be speed or distance, human bicycle racing performance has always been enhanced by the quality of the equipment used by the athletes. The thrill of competition has driven bicycle manufacturers to continually push the performance limits of the equipment they design. From advances in frame design to the addition of better quality and lighter components, cycling equipment has constantly evolved to be lighter, more aerodynamic, and of superior quality. It is clear that carbon fiber composite has made a distinctive impact on cycling design and has dynamically progressed cycling equipment into what it is today. Almost every aspect of today’s bicycle and its components has been revolutionized by the advent of composite design. From the frame to the wheels, almost every part can be and is made from carbon fiber composite. However it is the wheels that have the most impact on overall speed and efficiency, to which much effort has been made to improve.
By the late 1980’s bicycle racing wheels were taking lessons from the disc wheel, constructed from laminated thin foam layers, which had already proven itself as a record holding aerodynamic design used in track racing. In terms of aerodynamic drag, the disc wheel is theoretically ideal when considering the wind from a 0°direction; however it is highly sensitive to cross-winds which are a constant factor in almost every outdoor and real world riding condition. Conversely, a wheel with a narrow and thin side profile is exponentially less affected by cross-winds. Keeping this in mind, designers and engineers pushed for a wheel that was both aerodynamic and versatile in all wind conditions, and would enhance an athlete’s racing ability.
Many of the early designs formulated by such manufacturers as Nimble, Specialized, HED, and Spinergy in the early 1990’s, utilized carbon fiber composite spoked wheels containing 3-5 spokes of airfoil cross section, which produced less drag than the metal round spokes and box section rims of traditional wheels. These wheels were somewhat revolutionary in both thinking and design, and quickly gained acceptance by both manufacturers and athletes. However, because the design utilized a low count of very large cross-section spokes, the wheels were still subject to some of the cross-wind issues of the disc. The overall design concept was sound, but the stiff and rigid one piece composite spoked wheels exhibited mild sensitivity to aerodynamic induced vibrations and an overall abusive ride when coupled with a stiff and efficient frame design. Although the aerodynamic advantages of the new wheel design proved to be beneficial, most athletes found that the tradeoffs in handling were simply not worth the additional speed.
The next logical step in the progression of race wheel design was to remove the composite spokes and build a hybrid wheel with traditional metal spokes coupled to a deep-section composite rim utilizing a “V” or Toroidal shape rim. This step was very progressive in that it changed the dynamic of the wheel from that of a harsh and stiff riding wheel to one that had similar ride properties to traditional spoked wheel. The initial testing of the new wheel design showed that about 90% of the aerodynamic advantage was retained and that the depth of the rim and the number of spokes were the primary contributing factors to drag. By the mid 1990’s the standard for composite race wheel design utilized a deep section rim built with either bladed metal spokes or 3 to 5 aero-section composite spokes.
The lessons learned in the early days of race wheel design continue to contribute ideas and principals to the current designs which push aerodynamics and wheel technology to new limits. Manufacturing processes and material costs have evolved to dramatically reduce the cost and increase production to a point where a high quality carbon wheel set is cost competitive with an aluminum wheel set. Current composite race wheels are both incredibly light and are highly tuned to be very strong and efficient, and still maintain a comfortable ride.
Why Carbon Fiber Composite?
For decades, bicycle wheels have been made from aluminum, since it is one of the lightest extrudable metals, and yet durable enough for daily riding. It is historically easy to manufacture through a relatively simple process. Once it is extruded into a desired profile, cut to length, and rolled to a loop, it is simply joined at the ends to create the rim. Because it is an isotropic material, aluminum has the same value (roughly 68Gpa) for both its tensile and compressive modulus, which is an important design factor. By comparison, a carbon composite structure must be designed anisotropically, and demonstrates a much higher tensile modulus (typically 140 GPa to 240 GPa). However, carbon composites compressive modulus is much lower (typically 11 GPa to 15 GPa) and must be accounted for when considering the rims design. Carbon composites require a more complex stress and strain calculation process than traditional metal materials and are only recently accumulating enough history to solidify its stature as a superior engineering material.
Because of its greater tensile strength to weight ratios, it possible to significantly improve bike performance with carbon rimmed wheels. Carbon composite’s higher tensile modulus increases rim stiffness, which reduces power loss as the rider’s energy is transmitted to the rear wheel. Achieving the same stiffness in an aluminum wheel requires the use of more material, which adds weight and produces an abusive ride. In contrast, a lighter and less stiff aluminum rim requires a greater number of spokes to maintain overall wheel stiffness, which again adds weight and reduces ride quality. The stiffer carbon rim requires fewer spokes to maintain the same rigidity as a higher spoke count aluminum wheel. A lighter rim reduces the rotational weight of the wheel, which means the rider expends less energy to maintain forward motion and during acceleration. Contrastingly, carbon’s very low compressive modulus does a better job than aluminum of damping vibration on the road or rough terrain which is a key factor in rider comfort.
Both a significant reduction in wheel rotational weight and an increase in wheel stiffness are the best way to improve riding performance. These two factors are the easy to perceive and are often the difference between winning and simply performing your best. Typically a rider will use a basic aluminum wheel-set for daily riding and training and exchange for the faster carbon race wheels for ultimate race day performance. Training on a heavier and less aerodynamic aluminum wheel-set enhances the perceived performance from the lighter and more aerodynamic race wheels.
Rim Design and Construction
The design and manufacturing process of most composite rims is highly specific to the end use of the wheel. Like many composite products, the choice and type of material can be adapted to tailor the properties of the product to the conditions which it is used. The prepreg fabrics, number of plies and their orientations is specific for each wheel and is dependent on the type of rim and its intended use. Once the final shape and general architecture is established, the layup schedule is mapped and is typically a mixture of plies that are angled to stiffen the wheel as needed, considering the rim type and shape as well as ride compliance.
Depending on the manufacturer, the layup is typically optimized by adding reinforcement in areas where it is needed, again specific to rim type. For example, extra material is used to reinforce the areas where the spokes interface with the rim to prevent pull-through due to the highly concentrated point load the spoke tension creates. Many manufacturers reinforce the braking surface with a Kevlar fiber weave to improve the durability against the harsh brake pad wear and pressure. Although Kevlar fibers have 1/3 the stiffness of carbon, they are about 3 times more resistant to impact, abrasive wear, and shear forces. The added Kevlar helps distribute the compaction forces generated during braking further down the rim sidewalls, thus dissipating braking heat and shear stresses away from the braking surface. In some cases the braking surface and tire mounting surface are encapsulated with a woven silica/ceramic fabric to further dissipate the compressive forces due to braking, and add durability and toughness at the tire seating and braking area.
Typically carbon rim designs incorporate uni-directional fibers to handle all of the tensile and compressive loading within the rim, giving the final product a mix of strength and stiffness without sacrificing the compliant ride quality. Woven fibers are used exclusively in spoke-hole areas, braking surfaces, and damage prone outer skin areas to ensure the maximum possible strength in these high stress areas. Overall, the combination of fiber types and orientation results in a spoke pull through strength of more than 50% greater than the ultimate tensile strength of a 14 gauge spoke which insures a solid connection to the hub. The mix of fibers also produces an increased durability, a higher stiffness to weight, and strength to weight ratio than a traditional aluminum rim.
Because of the delicate nature of raw carbon prepreg fabrics, a complex procedure is required to prepare a carbon rim in a mold. The work of preparing a rim can typically only be done by human hand to insure that all the small parts of prepreg fabric are correctly put together. The process is arduous and typically takes up to an hour to prepare. Once the molds are checked and pressurized internally with a bladder, the filled form moves into an autoclave where a detailed and precise “baking process” completes the wheel blank. The resulting wheel blank comes out of the form in a raw state and requires a finishing process to refine the rim. Finishing specialists clean the blank, mill the residual epoxy at precisely defined regions and prepare the rim for final assembly by the manufacturer.
The final assembly of a carbon composite race wheel is also normally done by human hand. The spoke installation and lacing is typically done through a mechanized system by the larger manufacturers, but the final tensioning and truing is normally done by hand. The choice of spoke is again specific to the end use, and is normally made by another manufacturer who specializes in spoke design and fabrication. For most aerodynamic wheels, a bladed stainless steel spoke is used and is specifically heat treated to improve its tensile strength and durability. Although the hubs are also typically produced by another manufacturer who specializes in their design and manufacturing, they are generally specific the wheel producer who also contributes key design criteria to the final product.
Fatigue and impact testing as well as field trials are integral to the design process, and continually enhance the performance for each rim type. Like many other progressive products that utilize a material like composite, the durability testing and failure modes testing are critical for defining the overall strength and longevity of the wheel. Carbon fiber composite wheels are still in their infancy when compared to traditional aluminum wheels, but are proving to be a better material for most racing applications.
Top Manufacturers and Current Designs
Today’s top race wheel manufacturers have roots grounded in the early years of carbon wheel development, and have success built on the failures and achievements of early designs. Current wheel designs have been tested to standards which are normally reserved for aircraft and are wind tunnel proven to be aerodynamically efficient. Incorporating hundreds of hours of wind tunnel testing allows manufacturers to hone their design and prove the aerodynamic advantage when paired with various type s of bicycles and riders. Integrating such progressive technologies as ceramic wheel bearings, surface dimpling, tire to wheel matching, and low friction surface coatings are becoming common practice and push wheel performance to new limits. Manufacturers such as Zipp, HED, and Bontrager, are recognized as progressive designers and have introduced many of the technologies that are commonplace in the bicycle racing arena.
As the industry leader in carbon race wheel design, and a rich heritage in Formula One race car composites, Zipp is known for advancing wheel design technologies to an uncompromising level. No other manufacturer of race wheels devotes as much time and effort to real world and wind tunnel testing of their product. The engineers at Zipp have pioneered countless numbers of patents, including the hybrid-Toroidal rim shape, multi-material co-molding, and rim dimpling. Zipp sets the standard of design to which most other manufacturers aspire to. As the largest supplier to professional level race teams and triathletes, Zipp has gained an enormous following of athletes. Zipp produces a wide array of wheel sets ranging from a shallow 30mm rim section to an ultra deep 1080mm rim section, there are wheels for every riding condition. Even the classic disc wheel has been given a face lift to a new design which incorporates a Toroidal rim and generates lift with the same principal as an airplane wing.
Opening its doors in the mid 1980’s, HED cycling began humbly and quickly grew to become known as a leader in aerodynamic design. HED cycling products designs and manufactures 16 different models of carbon composite wheels and are used in such prestigious events from the Tour de France to the Hawaii Ironman World Championships. Every single wheel is still handmade, though production has become somewhat automated to increase production. HED wheels are affordable for most athletes, and are built upon sound aerodynamic and durable designs.
Currently as a division of Trek Bicycles, Bontrager Components was born during the Mountain Biking revolution of the 1980’s. Founded by Keith Bontrager, an engineer and designer from California, Bontrager Components produces a wide range of cycling components for Trek Bicycles and aftermarket consumers. Bontrager wheels currently utilize such unique designs as low count spoke pairing, Swiss made hubs, and Trek’s patented OCLV (Optimum Compaction Low Void) carbon composite molding process. All Trek bicycles come standard with Bontrager wheels and some models are available with the Aeolus Carbon Aero wheels produced in a partnership with HED Cycling.