IH01-MarshallAnderson
From IEOR 170 Spring 2007
Shimano's Rapid-fire mountain bike shifters
The first product I decided to review was Shimano's “Rapid-fire” type bicycle shifters. These shifters allow the rider to “shift” the chain between different sized sprockets. This changes the ratio of angular velocity between the pedals and the rear wheel. Rapid-fire shifters have been around for more than ten years and are common on many mid-range to high-end mountain bikes. Shown below are three pictures that illustrate the general layout of the rapid-fire shifters.
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As one can see from the pictures above, the shifter is located next to the handle bar grips and has two levers. The thumb lever extrudes slightly from the rear of the handle bars while the finger lever is positioned towards the front of the handle bars. The position of the levers and the shifter as a whole makes it an ergonomically superior design because the rider is not required to change the position of their hands to switch gears.
Also from the picture, it can also be observed that the thumb lever has a large pad while the finger lever features a smaller pad. In addition, the rider will immediately notice the unsupported sides of the levers. These slight differences in geometry along with the positions of each lever are affordances that indicate to the user to use their thumb and index finger to use the shifters. Overall the Rapid-fire shifters make the act of shifting gears an automatic task. The rider is allowed to focus on riding the bike without thinking about how to shift gears.
Another important note is that the thumb lever pivots forward while the finger lever pivots backwards. This is not only a natural movement for a hand that is grasping a handle bar, but also the motion could be considered to be mapped to going faster or slower. That is, the thumb lever “up-shifts” the chain and allows the rider to go faster while the index finger lever “down-shifts.” The motion of the levers maps well to the act of traveling faster or slower.
Bridgeport milling machine
The second product I reviewed was a Bridgeport milling machine. Milling machines are used to cut and shape a range of materials and are a vital piece of equipment for most manufacturing operations. As with any cutting tool, it is important to have a logical and consistent method to changing the cutting speed. However, Bridgeport mills suffer from a poor interface design that causes confusion when users try to change the speed. The sources of the problem are illustrated in the pictures below.
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As one can see from the pictures there is a lever on the right side of the mill and a knob on the left side. The lever on the right switches between high and low gears while the knob can be toggled to high-range, low-range or to a neutral position between the two. An operator turns on the mill by turning the knob to either of the two non-neutral positions.
To cut at high speed the user must ensure the high gear is engaged and the knob is turned to high range. Similarly, to cut hard materials like steel the user must ensure that the low gear is engaged and the knob is turned to low-range. Simple enough? Below is a chart that gives the various outcomes.
The interface is flawed for several reasons. First, the gear lever is not visible from the normal operating position. The knob, on the other hand, is visible and may lead the user into believing that they can operate the mill at a low speed by simply turning the knob to low-range. Secondly, the interface includes multiple modes and users can quickly form habits in one of the modes that cause serious errors when the mode is changed. For example, if a user repeatedly cuts soft materials and then switches to a hard material, even if they remember the gear lever, they are likely to turn the knob to high range out of habit.
Both of these common mistakes can cause irreversible damage to the cutting tool and the work piece. A simple fix for the problem is not easy. For instance, relabeling the knob for clockwise and counter-clockwise will not work because the labeling would be backwards as soon as the gears are changed. I also thought about moving the knob to the right side of the mill next to the gear lever, however, that would make it impossible to complete some tasks that require the user to use both hands. The only improvement that can be made without changing the design significantly is to post a highly visible c.
To correct the problem the interface should use a gear sensor and a knob labeled for clockwise and counter-clockwise. The sensor would sense what gear was engaged and the user would use the knob to turn on the mill to the correct rotation. A small processor would get the signals from the knob and the gear sensor then output the correct command to the motor. For a machine that costs approximately $10,000 adding a sensor and processor should not increase costs significantly.
