Editor | On 10, Oct 1999

Chung-Ping Chiang and Ching-Huan Tseng
Department of Mechanical Engineering, National Chiao Tung University
Hsinchu 30056, Taiwan, R. O. C. E-mail: chtseng@cc.nctu.edu.tw
TEL: 886-3-5726111 EXT. 55155 FAX: 886-3-5717243

* Graduate Student
** Professor

Introduction

There are many different kinds of devices used to supply braking force for stopping or slowing a bicycle. For convenience and economical consideration, a caliper or cantilever brake assembly is the most common one used for gripping of the bicycle rim in response to operator manipulation of the brake lever. Such a brake assembly generally includes two moveable brake shoes which are mounted on the bicycle frame in the opposite sides of the bicycle rim. The brake shoe has a brake pad to supply friction force when the brake pad is engaging the rim for braking the bicycle (as shown in Figure 1).

Figure 1 – Brake assembly of bicycle

It is obvious that wetting and/or polishing surface will have a lower coefficient of friction and braking force. The caliper or cantilever brake of the bicycle is mounted unprotected on the frame structure. It means that the braking performance is easily affected by the weather condition. On the other hand, water films and grits adhered to the rim surface causes much lower coefficient of friction. Realistically, a bicycle is usually pre-designed to ride preferably on either dry or wet road. Therefore, it is necessary to develop a brake device which is suitable for all weather conditions, in the mean time, provides safe riding, and keeps the convenience of use.

In the past, engineers made more efforts to make a lot experiments in order to select the suitable material for improving braking performance, such as the reliability and stability, in all weather conditions. Al-alloy rims with a given brake-pad material gives the lower coefficient of friction in the dry and the higher coefficient in the wet [1] . This is the most common approach in recently years. Unfortunately, the softer Al tends to be machined out of the rim surface if a piece of grit gets under a brake pad. Pieces of Al become embedded in the brake pad and oxidize toAl₂O₃, and then the rim is worn more rapidly and the dry coefficient of friction can fall to a dangerously low level. The described problem is not a concern for a steel rim. However, the conventional method used to improve the braking performance in all weather conditions causes inconvenience of use and some harmful side effects. In another words, it is difficult to overcome the technical contractions by the traditional compromising approach. Therefore, TRIZ is needed to overcome the system contradictions.

Formulating From System Conflicts

In order to make a right formulation, it is necessary to re-analyze the system conflicts and re-define the design requirements. The requirement of discussed system is briefly defined as:

The braking shoe is suitable in all weather conditions, and the extra wearing of rim surface should be avoided. In additional, it is necessary to meet the requirements described without major changes in the system morphology.”

The core problem for developing suitable bicycle brake shoes in all weather conditions can be discussed from two different viewpoints. The first one is a brake pad formed of a material suitable for use only in one of the weather conditions, i.e. dry or wet, and then customer is advised to replace and adjust the brake pad in different weather condition. Secondly, when the rim surface is covered with a water film and dirt and grit from the machined rim, it will have lower braking performance under either wet or dry conditions. It is not possible to solve the problem before the core problem is found.

However, the conventional method for solving problems having conflicts are briefly stated as follows:

1. By the method of replacing or adjusting the brake pad, the reliability of braking can be improved in either wet or dry condition. But, it will lose the convenience of use..

2. Comprised among technical parameters, i.e. material properties of the pad and the rim, to improve the stability of braking under all of weather condition, and some harmful effects, i.e. grit, internally produced.

By using the Altshuller’s TRIZ Contradiction Matrix analysis and 40 principles, one does not need to make a compromise between system contradictions. The statement involved the contradictions can be re-stated as follows in the terms of the Altshuller’s Contradiction Matrix:

1. Attribute “Reliability” causes deterioration of the attribute “Convenience of use”.

2. Attribute “Stability of object” causes deterioration of the attribute “Harmful side effects”.

From the result of Contradiction Matrix in the terms of “Reliability/Convenience of use” technical contradiction, there are three principles suggested to solve system conflict. The recommended principles are listed as follows:

1. An inexpensive short-lived object instead of an expensive durable one (Principle 27)

2. Moving to a new dimension (Principle 17)

3. Composite materials. (Principle 40)

Using the same process as above, there are four principles recommended in the Contradiction Matrix in the terms of “Stability of object/Harmful side effects” technical contradiction. The recommended principles to solve conflict are listed as follows:

1. Transformation of physical and chemical states of an object (Principle 35)

2. Mediator (Principle 24)

3. Flexible film or thin membranes (Principle 30)

4. Mechanical vibration (Principle 18)

Conceptual Design Development

Considering the system requirements, it is reasonable to try a solution from principles “Composite Materials”, “Mediators” and both.

The principle “Composite Materials” suggests:

“Replace a homogeneous material with a composite one”.

The principle “Mediators” suggests:

“Use an intermediary object to transfer or carry out an action”

“Temporarily connect an object to another one that is easy to remove”.

A partitioned brake pad is one of the concepts that generated from the principle “Composite Materials”, and it is a useful way to overcome the “Reliability/Convenience of use” technical contraction. The U.S. patent for a partitioned brake pad was applied by Richard C. Everett and granted on Apr. 1999. The device was formed of different kinds of elastomers used separately to improve wet and dry braking with one pad. The section view of major invention shown in Figure 2.

Figure 2 – Patented Brake Shoe of bicycle

By combining the principles “Composite Materials” and “Mediators”, an ideal design can be generated and a patent application developed successfully. Referring to Figure 3, a movable member with soft rubber is mounted on the nearly front end of brake shoes in the manner shown, with a spring bias. The projecting surface of the movable member is pre-settled at an offset from the engaging surface of the brake pad. For braking, the movable member first touches the rim surface, and scrapes off water, grits, and dirt. When the brake pad contacts the rim, an undiminished braking effect under various weather conditions will be achieved. The system conflicts have been solved successfully by the TRIZ method.

Figure 3 – New concept for the brake shoe (U.S. patent pending)

Conclusions

In this study, the system requirements for the brake shoes of the bicycle have been provided. The right formulations for solving the system contractions are well established. A granted U.S. patent, shown in Figure 3, is provided for a study case. It is proven that the results of contractions matrix provides an excellent approach to innovate. In combination of the solving principles that get from Altshuller’s Contraction Matrix with the different system conflicts, a new conceptual design of the brake shoe pad that is suitable for all weather conditions can be developed successfully.

References

1. Frank Rowland Whitt and David Gordon Wilson, Bicycling Science, 2nd edition, The MIT Press, 1990.

2. Altshuller, Genrich, 40 Principles: TRIZ Keys to Technical Innovation, 1st edition, Technical Innovation Ceter, Inc., Jan. 1998.

3. John Terninko, Alla Zusman and Boris Zlotin, STEP-by-STEP TRIZ: Creating Innovative Solution Concepts, 3rd edition, Responsible Management Inc., 1996.

4. Victor R. Fey and Eugene I. Rivin, The Science of Innovation, TRIZ Group, 1997.

5. United States Patent, Patent Number: U.S. 5,896,955.