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We Be Prowlin’ started with a team of five engineering students from the University of Nevada, Reno with the purpose of creating a more adaptable version of the prowler sled.
The prowler sled is an exercise device that uses resistance for a high intensity, cardiovascular workout. Many problems arise with the current sled design and limit the overall functionality of the product. The most notable problems include operational noise, excess weight, and lack of surface-versatility.
A design implementing a wheeled base and braking system was used to overcome these problems. The wheeled base allows for significant noise reduction and the ability for the prowler to be used on a wide variety of surfaces. The new braking system in conjunction with the wheeled base eliminates the need for large amounts of weight to be added to the sled.
Introducing Plan C:
The prowler sled was originally developed for conditioning football lineman as it requires the user to maintain a low posture when exerting force on the sled. The current prowler, and it’s variations, functions as a simple yet effective device for the purpose of strength training as both lower and upper body regions are worked; however, a small number of flaws limit use of these sleds in certain circumstances. Below are examples of these flaws:
Immense Amount of Added Weight Required (315 lbs) =Reduced Portability
Noisy on Rough Surfaces = Limits use in Residential Neighborhoods
While a prowler sled is able to operate on concrete and asphalt, noise will occur due to the grating of metal skis on the ground. This may prevent use in a residential neighborhood if no empty lots or fields are present. The frictional contact may also cause damage to the ground or skis when high weight is added to the sled.
Altering sled resistance can be a challenge as weight plates are used as increments. Because of changes in friction, results across multiple surfaces may be vastly different for the same weight applied to the frame. This aspect also requires these extra plates to be carried with the sled, thus taking more storage space and making transportation of the device more difficult.
Prior to finding solutions to the aforementioned design problems, We Be Prowlin’ established a set of objectives to clarify our goals for this project
1. An alternative braking system must be devolved to provide a comparable resistance to current sleds.
2. The free weights required to operate the sled at maximum resistance must be drastically reduced to increase portability.
3. The frame must allow for easy, compact transport while reducing operating noise.
|1||Maximum Resistive Force||Maximum resistance should reach at least 150 lbf. This resistance is equivalent to 300 lbs of added weight onto a traditional prowler sled.|
|2||Varying Resistance||Should range between approximately 0 and 150 lbf|
|3||Maximum Weight of Sled||Should weigh no more than 75 lbs without external weight|
|4||Maximum External Weight – no more than 90 lbs weight should be added to the sled to achieve peak resistance||No more than 90 lbs weight should be added to the sled to achieve peak resistance|
|5||Minimum Size (inoperable)||While in storage form, the frame should condense to a space of 40” X 35”|
|6||Maximum Size (operable)||While in use, the frame should occupy a space of 40”X 45”|
|7||Maximum Noise Level||Noise produced from the sled should never exceed 60 dB above ambient conditions|
|8||Final Cost||Final cost of product should not exceed $300.|
After organizing our project goals, We Be Prowlin’ began compiling ideas to solve the braking problem. We wanted a system that would require little maintenance and allow easy operation by the user. The frame also needed to be easy to carry while preventing slip when at higher resistances.
These concepts led us to researching electric motors with magnets and an adjustable resistor circuit. This would allow for both concise and convenient adjustment of sled resistance while requiring infrequent maintenance. However, a large motor would be required to reach our intended maximum resistance, and the expense would nearly double our total cost for the project.
With a similar approach, we then decided on using magnets to create a magnetic field which would be disrupted by an aluminum flywheel attached to the front wheel. This would nearly remove the need for maintenance while allowing for easy resistance adjustment with a slight change of distance between the magnets and flywheel.
Plan A was to use a motor braking system, or rehostatic braking, which is widely used on electric cars and trains. Almost all hybrid cars on the market consist of a complicated rheostatic braking system. The idea is to stop a moving object by consuming the energy generated by the motor. After building the first prototype, it was determined that the maximum resistive force was not nearly as large as desired. In order to obtain the necessary force, a complicated circuit such as those used on hybrid cars would have been required.
For the second plan, we built our prototype with a magnetic braking system. As the front wheel spins, an aluminum flywheel disrupts the magnetic field created by an adjacent set of permanent magnets. This interference causes resistance to the motion of the aluminum disk and is proportional to the speed at which the disk rotates. Our second design failed simply because the flywheel could not spin fast enough to generate larger resistive forces. Using gears would allow a higher angular velocity to be reached, however the implementation would have increased the size and weight of the sled past the values of our specifications.
The third plan consists of a traditional band brake as a source of a resistive force. The concept of magnetic braking was given up, and a band brake was added to the front wheel. Upon completion of the band brake system and wheeled frame, all of our objectives and specifications were met.
In conclusion, we tested three different possible solutions that were considered upon beginning this project, and Plan C is the design that meets the design specifications. By implementing a band brake, we were able to build a wheeled prowler that is much lighter than a traditional sled. It’s easier to use, creates less noise and can be transported easily due to a contracting profile.
Testing for Plan C involved unconventional methods so that design specifications were met with certainty. With a simple tape measure, dimensions were easily verified as within our desired limits. The primary purpose of the dimensional constraints was to allow for portability as well as conservation of space. Due to its retractable frame, Plan C easily fits inside the trunk space of any mid-sized car.
The most challenging tests concerned the foremost design requirement: the resistive force. The critical aspect of Plan C was the implementation of a braking mechanism that allows for resistance levels comparable to those provided by conventional sleds.
The goal range for resistivity is from 0-150 pounds horizontally. This is the equivalent of 0-300 pounds loaded onto a conventional prowler. Extensive testing conducted on Plan C showed a horizontal force of at least 150 pounds was met, and even exceeded.
The sled designed by the engineering team was put up against the Butcher sled by Rogue Fitness (video below). An equivalent weight, 225lbs, was placed to match the resistance of the wheeled sled. After testing Plan C compared successfully to the traditional Prowler sled.
Given the design constraints, a special testing method was used to measure the actual sled’s resistance. Through the use of a standard spring bathroom scale, the horizontal resistive force was measured.
The figure below is a plot displaying force applied to the scale over time. From the plot, the maximum force recorded is approximately 170 pounds. This force well exceeds the engineering specification of 150 pounds as the maximum resistance.
Meet the Team
Matt Hanson: Team Leader, helped oversee all aspects of the project to completion. Main responsibility was communication between team members and professor.
Corey Veloz: Maintained up to date biweekly reports as well as SolidWorks drawings. Lead designer for ME capstone webpage.
Kevin Langley: Lead researcher for project concepts. Designed the MExpo poster and responsible for all forms of media.
Jesse Johnson: Editor of all written technical reports. Oversaw all manufacturing processes to the completion of the prototype.