Its lasting high Young’s modulus at higher temperatures is one remark­able advantage of rayon as reinforcement for high performance mechanical rubber goods. 
Copyrigth: all Cordenka

Its lasting high Young’s modulus at higher temperatures is one remark­able advantage of rayon as reinforcement for high performance mechanical rubber goods.
Copyrigth: all Cordenka

Cordenka PP brake fig 1

Fig. 1: Rayon’s tenacity / elongation behav.

Among the many challenging demands placed by consumers on today’s automobiles, safety definitely remains the core topic. When constructing tires and brake systems for performance vehicles, therefore no compromises should be made on quality.
The excellent volume constancy – even at higher temperatures – is one of the major benefits of brake hoses reinforced with Cordenka rayon. It constitutes an advantage of rayon over thermoplastic fibers in this application. Rayon reinforced brake hoses make sure that the braking force directly gets to where it is needed and that a touch of the brake pedal – in all driving conditions – gives the driver the sense of safety that car makers aim to provide. In 2012 Cordenka MA1, 1100 den was commercialized as advanced rayon brake hose reinforcement, representing the state-of-the-art rayon material for this application since then. Cordenka MA1, 1100 den provides excellent volumetric expansion uniformity at elevated temperatures on the one hand and optimized whip test durability on the other. Having proved its worth in premium sports car applications, Cordenka MA1 is expected to capture significant brake hose reinforcement market shares.
This paper compares Cordenka MA1 fiber properties with competing materials and introduces brake hose benchmark test results that demonstrate the state-of-the-art rayon brake hose grade’s potentials.

Fig. 2: PVA’s tenacity / elongation behav.

Fig. 2: PVA’s tenacity / elongation behav.

Rayon fiber performance

The tenacity/elongation behavior in the operational elongation range of brake hose reinforcements illustrates rayon’s constant dimensional stability which is widely independent of ambient temperatures (figure 1). The constantly high Young’s modulus at temperatures up to 120 °C is visualized here. The tenacity/elongation behavior of thermoplastic PVA fibers – a material applied in commodity-type brake hoses – shows some remarkable differences in comparison with rayon. Figure 2 shows a very good initial modulus for PVA fibers at room temperature, but a significantly lower performance at temperatures of 60 °C and higher.
Its Young’s modulus indicates the tensile stiffness of a reinforcement material. This parameter is the crucial factor for the dimensional uniformity of a reinforced hose under pressure load. The maintenance of high Young’s modulus values at elevated temperatures separates rayon (topmost curve in fig. 3) from thermoplastic reinforcements.

Rayon reinforced brake hoses feature a superior dimensional stability at every day’s driving conditions and contribute to the reduction of pressure / volume tolerances within the brake system.

Rayon reinforced brake hoses feature a superior dimensional stability at every day’s driving conditions and contribute to the reduction of pressure / volume tolerances within the brake system.

Brake hose performance

Brake hose volumetric expansion tests at different temperature levels confirm the superior dimensional stability of rayon reinforced brake hoses and correspond with the yarn measurement results discussed before. Volumetric expansion tests were carried out according to the FMVSS 106 method. This test measures the volume of brake fluid present in a brake hose at a predefined pressure. Figure 4 shows that rayon and PVA reinforced brake hoses exhibit similar levels of volume expansion under increasing pressure at room temperature.

Fig. 3: Reinforcement’s Young’s modulus as function of the testing temperature

Fig. 3: Reinforcement’s Young’s modulus as function of the testing temperature

The comparison of two brake hoses applied by two different European car manufacturers demonstrates the typical small advantage of rayon reinforced brake hoses at room temperature. The volume expansion of the rayon reinforced hose com-pared to that of the PVA reinforced hose is 10 – 20% lower.
Figures 5 and 6, however, reveal the superior dimensional stability of rayon rein-forced brake hoses at common temperature levels for brake hoses in actual use. At 60° C and 50 bar pressure, the rayon reinforced hose contains a volume of brake fluid 49% greater than that measured with the same hose at room temperature and 50 bar.
The volume increase of 161% exhibited by the PVA reinforced brake hose is more than three times higher. The stroke distance of the test cylinder inducing the pressure reflects this relation in linear fashion.

Fig. 4: Volumetric brake hose expansion with rayon and PVA reinforcement at 23 °C.

Fig. 4: Volumetric brake hose expansion with rayon and PVA reinforcement at 23 °C.

Application relevance

The necessary elasticity of the system’s single parts poses a challenge for brake-system designers. The deployment of braking force requires a steady volume of brake fluid under pressure. The volume consumed for pressure generation should therefore be as small as possible, in order to minimize pedal travel and facilitate short braking distances. That’s why a tight brake system provides a sense of safety at a touch of the brake pedal. Another driver for the minimization of pressure dissipation within the brake systems of modern cars are the requirements of short response Electronic Stability Control Programs (ESCs). Within fractions of a second, ESCs steer the braking of individual wheels to stabilize cars in critical situations, like skidding. Rayon reinforced brake hoses contribute to the maintenance of fastest reaction times, independent of temperature conditions.

Fig. 5: Volumetric brake hose expansion with rayon reinforcement at 23 and 60 °C.

Fig. 5: Volumetric brake hose expansion with rayon reinforcement at 23 and 60 °C.

Given

  • Rayon’s superiority with regard to dimensional stability at higher temperatures and
  • Common brake-hose service temperatures of greater than 40 °C
    it is comprehensible that rayon is the material of choice for the reinforcement of performance brake hoses in the premium segment of Europe’s automobile industry.

    Fig. 6: Rayon’s & PVA’s Young’s modulus as function of the testing temperature incl. 40 °C.

    Figure 6: Volumetric brake hose expansion with PVA reinforcement at 23 and 60 °C.

Yet the application of rayon-reinforced brake hoses is not limited to the premium segment alone. High performing, rayon reinforced brake hoses can also be found in compact cars made by demanding manufacturers around the world. The compliance of rayon-reinforced brake hoses with all relevant standards and specifications has been proved with their application in the cars of leading OEMs.

Fig.7: Rayon’s & PVA’s Young’s modulus as function of the testing temperature incl. 40 °C.

Fig.7: Rayon’s & PVA’s Young’s modulus as function of the testing temperature incl. 40 °C.

To further validate the application relevance of the foresaid, additional comparative Young’s modulus measurements have been carried out on rayon and PVA brake hose grade yarns. Figure 7 displays absolute Young’s modulus values of rayon and PVA brake hose yarns measured also at 40 °C.

Driving tests

Titelstory_Picture 1

Picture 1: Installation situation of the thermo­couples on the brake hose.

Temperature measurements also were carried out on brake hoses in car driving tests. In brake systems of motor vehicles, brake hoses are exposed to mechanical and thermal loads. Both, direct heat conduction from the caliper and heat transfer by convection occur.

 

Fig. 8: Driving test brake hose temperatures measured at sensor position a).

Fig. 8: Driving test brake hose temperatures measured at sensor position a).

The aim of the experiments was to show how high these thermal loads actually are in a normal car under typical operating conditions. The temperatures were measured directly at the brake hose as well as in its environment. For the driving test a Ford S-Max 2.0 Duratorq (diesel / 103 kW) was used. The in-stalled alloy wheels allow for good ventilation conditions at the wheel brake. The temperature was measured using Type K thermocouples on the left front axle wheel brake directly at the point of attachment of the brake hose to the caliper (Position a)
at a distance of 5 mm from the brake hose near the saddle (Position b)
The temperature measurements were carried out respectively at or after sporty driving on the following test tracks:

Fig. 9: Driving test brake hose temperatures measured at sensor position b)

Fig. 9: Driving test brake hose temperatures measured at sensor position b)

  • City:15 km, brisk drive / stop-and-go
  • Country: 31 km, country road in mountain location
  • Motorway: 48 km, adaptive braking with high delays
  • Mountain: 3 km, average gradient 9% / maximum gradient 16.5%

All driving test were carried out at ambient air temperatures of 16-20 °C and dry roads. Figures 8 and 9 illustrate the temperatures measured at the brake hoses during the test runs as well as those measured with the engine running at the same places five minutes after the end of the test runs.
In the driving tests the thermocouples were exposed to the headwind which obviously is not the case for the brake hose reinforcement that is covered by the brake hose‘s rubber layers. Therefore the temperature measurements carried out five minutes after the end of drive with running engine are considered to provide the relevant indication of the temperature level the reinforcement is exposed to during the use of a car.

Brake hose benchmark – MA 1 in comparison

Cordenka MA 1 rayon brake hose reinforcement is found in state-of-the-art brake hoses in two braids with linear densities of 1100 den (1220 dtex) each. Competing brake hoses typically are reinforced with two braids of Kuralon 1239 PVA with linear densities of 1200 den (1330 dtex) each. In benchmark tests of according brake hoses the data in tables 1 and 2 was determined. Table 2 data was determined at the same hose types used for table 1 analysis. It proves that 1100  den Cordenka MA 1 can be regarded as efficient brake hose reinforcement also.

Conclusions

Its lasting high Young’s modulus at higher temperatures is the remarkable advantage of rayon as reinforcement for high performance mechanical rubber goods. Rayon reinforced brake hoses feature a superior dimensional stability at every day’s driving conditions and contribute to the driver’s sense of safety when steering his or her car. Moreover it can be shown that 1100 den Cordenka MA 1 rayon is an efficient brake hose reinforcement, the use of which leads to advantageous and balanced brake hose properties. n

Primary publication:
Einsiedel, R. (2020) Advanced rayon brake hose
reinforcement. In: Pfeffer P. (eds) 10th International
Munich Chassis Symposium 2019. Proceedings.
Springer Vieweg, Wiesbaden

About the author

Rudolf Einsiedel

Head of Sales & Procurement Cordus
Sales Director MRG & New Business Cordenka
Cordenka, Obernburg, Germany