The story behind the development of the aluminium 6-pot fixed caliper

Taking on the challenge of developing new technology to achieve quieter, smoother operation.

A world first, the path to center pin torque receiving mechanism.

Unprecedented structure achieves high levels of both braking force and quiet, smooth operation.

Converting kinetic energy into thermal energy when slowing down a vehicle: this is the role of brakes. In a disc brake, a disc rotor (referred to below as a “rotor”) rotating in conjunction with the wheels is pinched by a brake pad (referred to below as a “pad”), converting kinetic energy into frictional heat to generate braking force.

A caliper is one of the components of the disc brake, and performs the task of pressing the pad against the rotor. The relatively inexpensive type called the “floating type” or “cantilever type”, in which one or two pistons are positioned only on one side of the caliper gripping the rotor, is widely adopted and in general use.
On the other hand, large and high-performance vehicles utilize the “opposed type” or “fixed type” caliper in which a total of 4 to 6 pistons per wheel are positioned on both sides of the rotor and press the pad. Because it has a fixed frame structure, unlike the floating type, it has the advantage of weight reduction and high rigidity that achieves stable braking force at limit performance.

 

Differences in the structure of the two calipers

To absorb a higher thermal energy load for braking, it is necessary to enlarge the area of the rotor and the pad, and the caliper also inevitably gets larger. The opposed type caliper is particularly suitable for large brakes for high-performance and large-sized vehicles because it can press the pads more efficiently and uniformly with multiple pistons.
ADVICS, which within the Aisin Group is responsible for the development and production of brake systems, has been delivering 4-pot caliper (four pistons per wheel) for large luxury vehicles such as Lexus and Mercedes-Benz, as well as floating calipers for general passenger vehicles. However, the field of premium sports cars with top-class power performance exceeding 500ps output was new to ADVICS in the first half of 2010. The development of 6-pot fixed calipers, which unites the company’s various technologies, had begun.

 

The world’s first “center pin torque receiving mechanism”

It is recognized as technically difficult, with a conventional structure, to produce 6-pot fixed calipers with smooth, quiet performance and braking power suitable for a premium sports car. However, our development team members had a secret strategy: the world’s first “center pin torque receiving structure.”

With conventional opposed type calipers, the pad is suspended from the caliper body with a pin. During braking, the pad moves slightly in the tangential direction of the rotor to abut the caliper body and receive the braking force. More specifically, the pad has a movement allowance of 100μm during braking, and because the pad instantaneously abuts the caliper at the rotational speed of the wheel, the larger in area and heavier the pad becomes, the greater the energy that produces a percussive sound. The developers concluded that “for 6-pot fixed calipers, there are limits in a conventional structure to eliminate this noise.”

In order to overcome these issues, the development team held repeated discussions and worked through trial and error to devise a center pin torque receiving mechanism. The center pin torque receiving mechanism has the world’s first structure in which the pad is held by the inner peripheral pin (of the two peripheral pins, inner and outer, arranged in the center of the caliper) and the pad rotates around the inner peripheral pin. As a result, the momentum of the pad is mechanically reduced by about 25% compared to the conventional structure for more silent performance.

 

Model drawing of the conventional caliper and the caliper of the new structure

Movement of the pad during braking was reduced by 25% by changing from side movement to rotational movement. (Model drawing may differ in size and shape from actual product.)

 

Pin design posed the greatest challenge

However, even if this method was effective in theory, it was not easy to turn it into a marketable product. Above all the structure was completely different from that of conventional products, even other opposed type calipers. For example, since the force that was received by the entire caliper thus far is basically received only by the pin in this design, it was not clear at first how much strength the pin needed and what problems might occur.

A development team member who has been engaged solely in development of opposed type calipers since joining the company recalled that “the biggest challenge was pin design.”

“Because this was the world’s first mechanism of its type, at first we were completely frustrated with the pin design. If we made the pin thicker to increase its strength, it would be much more difficult to secure sufficient pad area. By making prototypes and carrying out many verifications with actual machines, making full use of strength calculations and CAE (computer aided engineering) simulations, we finally achieved optimal balance of pad size, pin thickness and shape, holding method and so forth.”

The member spoke about the first time driving a test vehicle fitted with a prototype.

“I already knew the product would work from the CAE analyses, bench tests and so forth. Still, the moment I actually drove the vehicle myself, applied the brakes, and felt the response, I thought ‘Yes, it works!!’ That was thanks to our hard work to achieve smoother, quieter performance.”

However, to put the product on the market, it was vital to prevent aging and corrosion of the pins even in harsh environments. Brake calipers are exposed to heat, brake dust, mud, and water generated during braking, and there is also the possibility of wheel cleaner and brake fluid adhering to them. Of all car parts, the brakes are exposed to some of the harshest environments, and as parts crucial for safety, absolute reliability is required.

The developers compared various surface treatments and conducted component analyses for each process, while engaging with material and surface treatment manufacturers. Even as the mass-production stage approached, we were still reviewing the compatibility of the metal components and materials, and the processes, which were finally able to achieve an optimal surface treatment.

The completed 6-pot fixed calipers body is impressive. However, the highlight of the new product is the pin that penetrates the center and literally forms its “axis.”

As the project neared its completion and the product was nearing the mass production stage, one of the development team members who had worked so hard to make it commercially viable said:

“Adapting existing technologies involves a degree of trial and error, but this time we truly lacked past expertise and experience and were really doing something new. There were no clues how to proceed and we had to figure things out for ourselves. That was quite a challenge.”

 

Caliper design was also an area of focus

For this new product, we were strongly committed to excellence in design and beauty, of the caliper. In a premium vehicle, the brake caliper is also part of the car design, and is one of the few functional parts that can be seen on the exterior.

At last, the world’s first vehicle to incorporate this aluminum 6-pot fixed caliper with a new mechanism was announced. It was the Lexus LC flagship coupe, released in March 2017.

For the Lexus LC 6-pot fixed caliper, we adopted a form that pursues functional beauty so as not to detract from the LC’s elegant styling. Furthermore, by collaborating with the paint manufacturer from the paint selection stage, we were able to achieve a beautiful black paint finish that conveys a distinctly Japanese sense of beauty. One might say that the Lexus LC’s styling is complemented by the Japanese-made brake caliper, which can be seen in glimpses between the wheel spokes.

Another feature of this 6-pot fixed caliper is weight reduction realized through a monoblock structure, where the entire caliper is a single component, and a center bridge mechanism. As a result, weight was reduced by 10% compared to competing products.

 

Looking ahead to new future challenges

The 6-pot fixed caliper created in this way has not only strong braking force, but also the smooth quietness and beauty suitable for a premium vehicle. A 6-pot fixed caliper of the same type was incorporated into the F SPORT, a sporty version of the Lexus LS flagship sedan, launched in autumn 2017.

What was this project like for the development team members?

“This was a tough project because we not only incorporated the latest technologies, but also worked to come up with entirely new ones. I am personally very pleased that we were able to see design and development through to the end and see the product adopted for mass-market flagship vehicles.”

“In the world of brakes, there is now demand for completely new technologies in the face of major changes such as automated driving and vehicle electrification. Being able to create a new mechanism unlike any in existence gave me a great deal of confidence going forward.”

For ADVICS, the adoption of this 6-pot fixed caliper for vehicles in the premium sports field is a big step forward. Moreover, for engineers involved in its development, the experience and confidence gained through this challenge will provide great impetus for the next new project.

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