Tri-Pedal Cutter Brake System
Background on Conventional Cutter Brakes
There are three conventional ways for a car to implement a cutter brake into its braking system. The first is using three pedals, one for the right rear brake, one for the left rear brake, and one for all four brakes. This technique requires a lot of foot box space and in the realm of Mini Baja the more foot space occupied by the driver the less suspension travel the car can have in the front end. The second style of cutter brake is to use a dual lever assembly, one lever activates the right rear and the other activates the left. The third approach is to use a double action single lever assembly, which consists of one lever that when pressed forward activates one rear brake and pulled back activates the other. The order in which the brakes are activated is left up to the installer.
Both of the lever cutter brake systems are inherently dangerous in that while using the cutter brake it requires the driver to remove one hand from the steering wheel doing this while turning only makes the procedure more dangerous.
Proposed Tri-Pedal Cutter Brake
The purpose of the proposed tri-pedal assembly is remove the risk associated with lever style cutter brakes with out jeopardizing the suspension travel in the case of the conventional three pedal designs. The purposed assembly does this by essentially shrinking down the three pedal design into the size of one traditional pedal thus permitting the driver to keep both hands on the wheel while still being able to use a cutter brake system with out sacrificing front end suspension travel.
Concept
The Tri-pedal assembly is essentially three independent pedals in one assembly (see figure 1 shown as a negative for clarity). Thinking of each pedal as a lever actuating a brake cylinder will make understanding this concept easier. The left most pedal or lever actuates the left rear master cylinder, the middle pedal actuates the front master cylinder which in turn activates both the front left and right calipers, and the right most lever/pedal actuates the right rear master cylinder. This setup is designed capitalize on the drivers instinct. If the driver wants to brake the right side of the vehicle all he or she needs to do is press the right side of the brake assembly, if they want to brake left they press on the left side of the brake assembly, and should the driver need to come to a complete stop they just need to stomp on the entire assembly.
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| Figure 1. Tri-Pedal Proto-type Display |
Drawing number 3007 shows a schematic of how the pedals and master cylinders interface to help clarify the pedal/master cylinder/caliper interaction.
Technical
The brake assembly consists of the three braking circuits one for the front brakes, one for the right rear and one for the left rear. Therefore each brake assembly is required to have its own master cylinder. Tuning the assembly can be very difficult and is achieved one of two ways. The first option is to size the master cylinders and brake calipers to apply the desired amount of braking force to each wheel. The second choice is once the brake system has been calibrated with common master cylinders the pedals can be re-fabricated to reflect the associated change in push rod lengths. Due to the tight tolerances of the design, balance bars must be fixed therefore any change in push rod length is directly reflected in the distance from the pedal surface to the drivers foot. In order to compensate for this, pedal shapes should be custom fabricated to aid in brake tuning.
The three pedals are fabricated to fit any wildwood reverse swing mount dual master cylinder mounting brackets. This set up allows the pedals to share a common pivot (see drawing number 3007).
Results
First attempts at making the purposed tri-pedal cutter brake assembly function failed due to fabrication. Given the initial time constraint team members attempted to melt steel into aluminum in an effort to develop a temporary assembly and proof the concept, but unfortunately as predicted the pedal assembly failed at the weld site (see figure 2).
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| Figure 2. Top View Pedal Assembly |
It was at this point that the team encountered other more important design challenges not related to the brake system and chose to terminate the progress on the new cutter brake system and use a traditional braking system in order to address the unrelated design changes.
Recommended Abatements
- Use similar metals (duh) – The use of steel will solve the initial fabrication issues, the weight of the steel is undesirable however speed holes can be machined into the pedals with out jeopardizing structural integrity.
- Change front master cylinder orientation - The front master cylinder linkage is cantilevered in a way that will produce extremely high torsion and shear stresses on both the pedal and push rod. Changing the layout of the master cylinders can completely relieve this problem.
- Use bearings at linkage and push rod union – The purposed pedal assembly used a thread bushing as a means to connect the pedal linkage to the push rod (see figure 3), in traditional braking systems this is not a problem because the linkage/balance bar is allowed to rotate with respect to the pedal, however by fixing the balance bar to the pedal (thus making it a linkage) removes that degree of freedom. This forced the linkage to have to rotate inside the threaded bushing at the push rod union; this rotation effects the push rod alignment with the master cylinder, which is undesirable. Replacing the threaded union with a bearing style will alleviate this problem.
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| Figure 3. Front Master Cylinder Profile |
Next Generation Abatements
- Steering Sense – Using a tri-pedal cutter brake design eliminated all issues associated with using a lever style cutter brake, however because the controls of the pedal are so close together the design becomes a human factors and controls concern. It can be difficult to activate only one pedal at a time while trying to navigate through rough terrain. The cutter brake system will only be used in extremely tight cornering maneuvers. In these tight maneuvers the steering wheel will be turned through its full rotation.
Based on this, another purposed abatement that was not tested is to interface potentiometers with the steering column. Use these sensors to determine steering wheel rotational position and then through microprocessor control interfaced with transducers capable of actuating a brake cylinder create a dynamic braking system. The brake system can then essentially sense the extremity of the corner and apply the cutter brakes to the desired wheel with out having any driver intervention. This purposed abatement will continually reduce the human factors risk associate with trying to actuate the correct pedal or lever while attempting to maneuver a tight corner in rough terrain.
Conclusion
The purposed tri-pedal cutter brake system failed due to fabrication. In the best interest of the team further progress was terminated to allow for more focus on certain unforeseen critical design aspects not related to the brake system. In conclusion the 2007–2008 Mini Baja cars will use the traditional single lever double action cutter brake system. The concept of the tri-pedal cutter brake assembly is worth revisiting because it allows the driver to maintain control of the vehicle while implementing a cutter brake maneuver. The tri-pedal cutter brake assembly remains a step in the right direction for progress. It is through the generation of innovative ideas that Utah engineers will be competitive on an international level and receive positive exposure crediting the University Of Utah College Of Engineering.