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Textbook:
Bicycle racks are constantly under cyclical loading, from the constant loading and unloading of the bike onto and off the rack to the vibrations and shock from the road translated through the car to the rack. Therefore, we designed ours with this in mind.
Cyclical loading affects materials differently than pure static or dynamic loading since it is an uneven mixture of the two. This causes the materials to experience a phenomenon known as fatigue. When performing this analysis, the stress-life method will be used. Although the least accurate, especially for low-cycle applications, it is also the least costly to perform and has a wide range of applications with ample supporting data.
Since a bike rack does not need extreme tolerance measurements and it is built with a considerable safety factor, the stress-life method is best suited. As the number of stress cycles increases, the fatigue strength of the material decreases. This means that the material can withstand less force and pressure after many cycles compared to when it was new. The rack is also made with cost in mind. As the ability to withstand more cycles increases, so too will the cost for both the material and the tests. Thus, the bike rack is designed for finite life, defined as less than 107 cycles. Our materials are both strong enough and durable enough to withstand repeated loads.
Patent:
Thus “Bicycle carrying system” patent no. US8998050B1 consists of a rack that is attached to the exterior rear of the vehicle at three separate anchor points. There are two bumper-mounted platforms which support the bicycle wheels and are held via straps. There is an additional set of straps which secures the bicycle frame to the vehicle as well.
This rack is unique in the way that it is broken up into three non-connected sections. This allows for more compact and lightweight storage, but it also means there must be multiple intrusive anchor points on the car.
However, there are a few areas we have improved on when designing our rack. First off, the way that the bumper platforms are mounted (via threads) leaves them prone to rotation. This is not wanted because it could cause the bike to become unsecured while traveling, leading to a hazard. There is a knob to tighten the platform so there is no rotation, but it is awkward for the user. Another issue is there is more permanent disfiguration of the vehicle due to two platforms being inserted through the bumper.
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