Pivoting Crank Arm for Cyclists with Limited Knee Flexion

Designers: Winston Lynk, Megan Toney, Stephanie Tupi
Client Corrdinator: Sue Cheng
Supervising Professor: Larry Bohs
Our client is an independent, athletic woman who enjoys cycling. However, she was unable to ride because of limited left knee flexion due to car accident. The Pivoting Crank Arm allows her to cycle again. The device consists of a modified crank arm with a pivot joint. By allowing the pedal to drop to a lower height at the peak of the pedal motion, the pivot decreases the degree of knee flexion required for pedaling. The device was optimized for the client’s capabilities, but it can be adjusted for individuals with different degrees of knee flexion.

Prior to her accident, our client used to bike 25 to 35 miles every weekend. After the accident, she started swimming, but she “very much missed the physical activity in the open air that [she] got with cycling on country roads.” After using the device, she commented, “Thank you all very much for enabling me to bike again. I have been waiting for this moment for 6 years! It’s so exhilarating!”

The Pivoting Crank Arm (Figure 1) is comprised of a shortened mountain bike crank arm, an aluminum bar used in an overlapping fashion as a pivot, and a shoulder screw with associated bearing.

The crank arm is cut at a location designed to maximize the client’s power generation with each pedal stroke while still adhering to her range of motion. The outer section of the crank arm, containing the pedal, is 78 mm long, while the inner section, attached to the axle, is 127 mm long. These dimensions were determined after testing an adjustable version of the device with the client to establish her maximum amount of flexion within a comfortable range of motion, while keeping the total length from axle to pedal at the standard 170mm.

The aluminum bar attaches to the outer crank arm portion with four ¼”-20 machine screws. The pivoting sections attach together with a shoulder screw, which rides in a sealed ball bearing in the aluminum bar to minimize rotational friction. After passing through the bearing, the shoulder screw threads into the inner portion of the crank arm. This threaded connection is secured with thread-lock compound as well as a set screw. A slight gap between the overlapping portions of the crank arm and aluminum bar accommodates a delrin washer to facilitate smooth rotation.

Figure 2 shows a picture of the client riding her modified bike. The replacement cost for the pivoting crank arm is about $90.

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