Asics Corporation spikeless CFRTP sprinting shoe
The global sports shoes market was valued at close to USD 19.0 billion in 2018 and is anticipated to witness significant growth in future. Many sports shoe companies are focusing on the development of innovative products and trying to use various raw materials such as carbon fibre to produce advanced shoes and to meet various consumer requirements. Constructing a mass production technology is one of the important tasks for Asics to capture the sports shoes market.
Sprinting shoes play an important role for athletes to provide traction and efficiently transmit their power to the ground because they are the only interface between the athlete and the ground. Therefore, they need to be lightweight while keeping an ideal bending stiffness distribution and sufficient traction. In general, a sprinting shoe has several metal spike pins on the bottom of the sole. However, based on the available data and the insights from various athletes, Asics Corporation hypothesized that if the penetration and pull-out behaviour of spike pins were minimized, sprinting performance could be improved because 1) when sprinting in spike shoes, it may take too long to generate sufficient traction by penetrating the spike pins into the track surface, 2) the traction generated by the penetration of spike pins can be excessive, and 3) increasing traction without spike pins can be possible by increasing the contact area of the shoe sole with the ground.
When developing the new sprinting shoe, the challenge was twofold:
- The first challenge was to develop a CFRTP spikeless sole structure. In the prototyping process, all the metal spike pins were replaced with a set of hexagonal-shaped “gripper” elements made of carbon fibre-reinforced thermoplastics. It was impossible to manufacture this complex structure with traditional sheet moulding compounds (SMC) due to their lower flexibility. Thus, a CFRTP made from randomly-oriented ultra-thin UD tapes was used to reinforce the “grippers” from base to top effectively.
- The second challenge was to maintain a good balance of light weight and bending stiffness distribution for the new sprinting shoe. Utilizing parametric design, computer simulations, and 3D printing technology, the CFRTP was employed over the whole area of the sole and the prototypes were engineered for the best balance of bending stiffness distribution and lightweight properties that current sprinting shoes ever achieved.
How it’s made: The prototype stage of design thinking
The innovation was developed through an industry-academia collaboration. Product, composite material, and matrix resin manufacturers as well as the university were challenged to create an out-of-the-box sprinting shoe. At first, Asics Corporation proposed a unique concept using carbon fibre-reinforced thermoplastics (CFRTP) mainly based on conversations with various athletes and scientists. Then, they designed the sprinting shoe sole utilizing parametric design and finite element analysis.
Preformed randomly-oriented ultra-thin CFRTP tapes make it possible
to achieve the finest detail moulding with CFRTP
The shoe’s performance in terms of the athletes’ biomechanical values and mechanical properties was evaluated after prototyping. Wear trials were conducted with world-class athletes under various track surfaces and weather conditions.
During the prototyping stage, the composite manufacturer developed the CFRTP material based on randomly-oriented ultra-thin tapes for sprinting shoes. They adjusted the tape length for excellent mouldability and durability through basic research with the university. The matrix resin manufacturer redesigned the matrix’s glass transition temperature to prevent the sprinting shoes from melting due to the frictional heat between the shoes and the ground.
The manufacture with innovative technologies
Asics Corporation opted for randomly-oriented ultra-thin CFRTP tapes for this project as random-fibre composites have the potential to achieve fine-detailed mouldability for shoe sole structures. However, manufacturing high-quality thin-walled shoe soles using the current random-fibre composites is quite difficult.
To realize this project the team developed five key innovative technologies:
- Precise spreading to fabricate ultra-thin tows
- High-speed fabrication of high-quality ultra-thin unidirectional (UD) tapes
- Uniformly scattering of chopped UD tapes
- Unique preforming technology for all shoe sole shapes
- Appropriate control of fibre length and resin properties
Manufacturing process of spikeless CFRTP sprinting shoe sole
with randomly-oriented ultra-thin CFRTP tapes
The precise spreading technology makes carbon fibre tows thinner at a thickness of 0.03mm. An in-situ polymerizing thermoplastic epoxy resin is employed as the matrix. As the viscosity of thermoplastic epoxy resin before polymerizing is very low, resin impregnation into spread tows is quite effective.
Furthermore, resin impregnation becomes more effective when the company’s original impregnation technology is employed, so that void-free ultra-thin UD tapes can be fabricated at high speed. The UD tapes are then uniformly scattered and laminated. The preform for the sprinting shoe sole is automatically manufactured by a unique preforming technology where the thickness of a shoe sole can be specifically engineered for 20 separate areas with a precision of 0.2g.
Comparizon of cross section after manufacturing
between typical sprint spike and spikeless CFRTP sprinting shoe
Additionally, material waste reduction was improved and the fibre length and resin properties were adjusted for the manufacture of the sprinting shoe sole structure.
Tested by world-class athletes
The spikeless sprinting shoe prototype was first introduced at the event celebrating the 100th anniversary of ASICS founder Kihachiro Onitsuka on 15 May 2018. The current development stage is the product testing phase by world-class athletes. Many athletes have been wearing the prototypes of the spikeless CFRTP sprinting shoe at their trainings for several months, and some of them already used the prototypes at their races, including the World Athletics Championships 2019 at Doha. The company is now collecting wear-out, fatigue strength and traction property data from the wear tests, especially the traction properties under various weather and surface conditions as well as, most importantly, feedback from the athletes regarding the perception of the spikeless sprinting shoes.
Typical photograh of world-class athletes wearing the prototypes in competition
Thanks to the unique preforming technology for ultra-thin carbon-fibre chopped tapes, the mouldability of this CFRTP random sheet involves much less restrictions than typical SMC composites. This made it possible to flexibly design the geometry and direction of the “grippers” and to fabricate the complex three-dimensional structure. Additionally, the bending stiffness distribution of the spikeless sprinting shoe sole can be modified flexibly by controlling the amount of CFRTP. As the next step, combining the mechanical/performance data and the athletes’ feedback will be considered to make the sprinting shoes even better and further optimized for each athlete.
The company already started mass production tests of the spikeless sprinting shoe sole to meet the expectations of not only top athletes but also general consumers such as college athletes. The resulting mass production process can be expanded to other categories of sports shoes such as football shoes.
Social and environmental significance
Health consciousness is growing all over the world and the social significance of sports is also growing more than ever in modern society. Sports are an important element in living longer and more healthful lives, and in feeling more fulfilled. Asics Corporation’s mission is to provide goods and services that promote healthy and fulfilling lifestyles throughout the world.
The CFRTP matrix used in this development is a thermoplastic epoxy resin that is expected
to have high forming and recycling efficiency when used in composite materials.
The spikeless CFRTP sprinting shoe was originally developed for track and field athletes to maximize their sprinting performance. Many athletes have commonly trained with prototypes, and several athletes proved that this product significantly contributed to improving their performance by clocking the personal bests. It was shown that this innovative product contributed to fulfilling the athlete’s life. The company would like to provide the same advantages to general consumers if the day comes when the mass production process is fully developed.
The significance in terms of environmental impact is that the CFRTP matrix used in this development is a thermoplastic epoxy resin that is expected to have not only sufficient interfacial bonding properties but also high forming and recycling efficiency when used in composite materials. Thus, it offers a huge potential to reuse the spikeless CFRTP sprinting shoe sole by re-press moulding after long-term usage. Furthermore, the unique CFRTP preforming method contributes to the reduction of waste materials. This should also have a positive impact on global environmental issues.
The spikeless CFRTP sprinting shoe
Industries: Sports, Leisure & Recreation