Based on performance data of unidirectional PBO fiber reinforced composites, this paper explores the application potential and engineering design solutions of PBO/carbon fiber hybrid spokes for high-performance bicycle wheel sets.

Tensile strength: 5.8 GPa

Tensile modulus: 270 GPa

Elongation at break: 2.5%

Impact absorption energy: 4 times that of carbon fiber

Specific gravity: 1.56 g/cm³

High-performance racing bicycle wheel spokes need to combine ultra-light weight, high rigidity, excellent impact resistance and superior vibration absorption capacity. Traditional carbon fiber spokes feature outstanding rigidity, yet they suffer from brittle failure and limited impact energy absorption, which have long been bottlenecks in engineering applications. As a super organic fiber with top-tier comprehensive performance, PBO fiber brings a new design approach for next-generation spoke materials when combined with carbon fiber via hybrid composite technology.

PBO fiber is the abbreviation for Poly-p-phenylene Benzobisoxazole fiber. Originally developed by the United States as a reinforcing composite material for aerospace applications, it belongs to the polyamide family containing heterocyclic aromatic structures and is hailed as a "21st-century super fiber".

PBO fiber is characterized by light weight, ultra-high strength, flame retardancy, high temperature resistance and good wave permeability, ranking first among all organic fibers in overall performance. It is regarded as a promising material for aerospace, military equipment, fire protection and other fields following aramid fiber. Due to its extensive military applications, PBO fiber has been listed as a restricted item under the Wassenaar Arrangement by the United States, Japan and other countries, which impose policies of military product ban and civil product sales restriction on China.

2. Overseas Development History

In the 1970s, the U.S. Air Force Materials Laboratory launched research on polymers with higher modulus and better temperature resistance than aramid fibers. The research was undertaken by the Wright Aeronautical Laboratory and SRI International, and PBO polymer was first publicly released in 1979. Dow Chemical subsequently obtained relevant patents and spent seven years improving polymerization efficiency and the yield of DAR monomers, but failed to realize fiber production. 

In 1990, Toyobo Co., Ltd. of Japan acquired PBO-related patents from Dow Chemical and overcame key spinning technologies. It built a 20-ton annual pilot production line in 1995 and a 180-ton annual industrial production line in 1998.

Independent Research by Domestic Scientific Institutions

Since the late 1990s, East China University of Science and Technology and Zhejiang University of Technology have carried out research on DAR monomers, the key raw material of PBO. Donghua University, Shanghai Jiao Tong University, Harbin Institute of Technology, Xi'an Jiaotong University, Tongji University, the 43rd Research Institute of China Aerospace Science and Technology, Harbin FRP Research Institute and other institutions have conducted in-depth studies on PBO synthesis processes, fiber preparation, material properties, as well as the performance and application of PBO-reinforced composites.

PBO fiber boasts the highest tensile strength (5.8 GPa) and tensile modulus (270 GPa) among organic fibers, along with excellent heat resistance and flame retardancy (Limiting Oxygen Index: approximately 68). When compounded with carbon fiber, it retains high elastic modulus while greatly improving impact toughness and vibration damping performance.

Bicycle spokes are mainly subjected to axial tensile load during riding. Tensile modulus determines the radial rigidity of wheel sets, while a low Poisson's ratio reduces the transverse shrinkage of spokes under tension and maintains the geometric accuracy of wheel sets.

The tensile modulus of PBO HM unidirectional FRP is approximately 1.1 times that of carbon fiber unidirectional FRP. Under the same cross-sectional area, PBO/carbon fiber hybrid spokes produce smaller elastic deformation, delivering more direct wheel response and higher power transmission efficiency during sprinting. Meanwhile, the similar Poisson's ratio between PBO and carbon fiber ensures predictable transverse deformation of tensioned spokes, facilitating the precise design of spoke tension layout for wheel sets.

Spokes bear instantaneous bending stress when subjected to lateral impacts (e.g., road gravel, speed bumps). Bending modulus and maximum bending stress jointly determine the resistance of spokes to non-axial loads.

Within a strain of 0.2%, the bending stress-strain performance of PBO HM unidirectional FRP is nearly identical to that of carbon fiber FRP, reflecting the high initial elastic modulus of PBO fiber. When the strain exceeds 0.2%, compressive buckling gradually occurs, leading to a slower increase in stress.

For spoke design, a sandwich structure (carbon fiber - PBO - carbon fiber) with hybrid lamination is recommended, where the tensile side is borne by PBO fiber to maximize the respective advantages of the two materials.

The interlaminar shear strength (ILSS) of PBO HM unidirectional fiber-reinforced plastic (UD-FRP) is approximately 40 MPa, roughly equivalent to that of aramid UD-FRP and lower than the 70 MPa of carbon fiber UD-FRP. In the design of hybrid spokes, special attention shall be paid to the interface between PBO fiber layers and carbon fiber layers. Surface modification is recommended to enhance interlayer bonding and avoid delamination failure.

The damping rate of PBO HM unidirectional FRP is about 0.015. While retaining elastic modulus comparable to carbon fiber, PBO fiber delivers remarkably better damping performance, which is one of its core competitive strengths. For bicycle wheel sets, this means road vibration can be absorbed more rapidly, reducing high-frequency vibration transmitted to the hub and frame, improving riding comfort and extending the fatigue life of other components.

The drop-weight impact penetration test (0°/90° multi-layer laminated FRP) directly reflects the ultimate load-bearing capacity and energy absorption of materials under sudden impact loads, serving as a core indicator to evaluate the anti-fracture performance of spokes.

The maximum impact load of multi-layer laminated PBO HM unidirectional FRP is about 3 times that of carbon fiber FRP and 2 times that of HM aramid fiber FRP. Its total absorbed impact energy reaches approximately 4 times that of carbon fiber FRP and 3 times that of HM aramid fiber FRP.

This performance is critical for spoke applications: even if local damage occurs under severe impact, PBO material can absorb massive impact energy to avoid instantaneous catastrophic fracture of spokes, greatly enhancing riding safety.

Radar Chart of Unidirectional FRP Comprehensive Performance

All indicators are normalized based on measured data from Toyobo.

The radar chart clearly shows the advantages and disadvantages of each material: Carbon fiber leads in bending strength and interlaminar shear strength, but suffers from poor impact energy absorption and vibration damping. PBO fiber outperforms others in tensile modulus, maximum impact load, impact energy absorption and vibration damping.

Performance Positioning of PBO/Carbon Fiber Hybrid Solution

The PBO/carbon fiber hybrid design aims to integrate the strengths of both materials: it maintains the bending strength and interlaminar shear strength of carbon fiber, while lifting impact resistance and damping performance close to the level of pure PBO fiber. This composite solution creates an all-round high-performance spoke material.

Calculations show that the hybrid design can boost the impact absorption energy of traditional carbon fiber spokes by 2 to 3 times, increase the damping rate to 3–4 times that of pure carbon fiber, with a tensile modulus loss of less than 10%.

PBO/Carbon Fiber Hybrid Spokes Open a New Era for High-Performance Wheel Sets

Combined with full test data of PBO fiber unidirectionally reinforced composites, the PBO/carbon fiber hybrid spoke design presents prominent competitiveness both theoretically and in engineering practice, and will become a core material solution for new-generation racing and high-end consumer-grade wheel sets.

The tensile modulus of PBO HM unidirectional FRP (fiber volume fraction: 59%) reaches about 150 GPa, exceeding carbon fiber and guaranteeing excellent radial rigidity and responsive pedaling performance of wheel sets.

Its total impact absorption energy is 4 times that of carbon fiber unidirectional FRP, which effectively prevents instantaneous fracture of spokes under sudden impacts and improves riding safety.

While maintaining high elastic modulus, its damping rate is about 5 times that of carbon fiber. It efficiently absorbs road high-frequency vibration and alleviates fatigue during long-distance riding.

The specific gravity of PBO fiber is only 1.56 g/cm³, lower than 1.76 g/cm³ of carbon fiber. Hybrid spokes can achieve further weight reduction under the same strength requirements.