Team 2

Dartmouth students riding bikes struggle to transport large and unwieldy packages from Hinman back to their dorms. 

• Lacking access to their hands, packages that don’t fit in backpacks are often placed in improvised, compromising positions, or simply can’t be transported.

• Bike attachments (bags, racks, baskets, etc.) are rigid, inconvenient...

Students need a: hands-free, adjustable, and removable backpack attachment that increases students load-bearing capacities on bikes, or when they otherwise can't access their hands. 

User-College students (with bikes, scooters, etc.)

• Specifically, Dartmouth students who use their Hinman box 

• Focusing on this specific user allows us to work within their existing systems i.e. using a backpack  

Purchaser 

• Dartmouth College Students 

  • Selling directly to students would be our preferred option and could be done online, in school stores, or in big box chains

• Hinman Services 

  • Providing package a solution to Hinman to then dispense to students as needed also works especially given this a product that would lend itself to quick mass adoption. 

These prototypes signal our experimentation with a strap bungee hybrid.  While we found them to have the greatest degree of adjustability prototype 2a struggled to hold its webbed shaped without getting tangled. In 2b we addressed this by using two bungees looped around and held together by certain anchor joining points. In 2b we also changed the center ring from a metal-rubber one to a bungee one 

This model stayed true to the basic principles we found to be working in 2b but iterated on them significantly. Users found the web joining structure to be too complicated so we used cut brass tubing which in addition to estatics didi a better job of holding the shape together. We fabricated our own parachute clips that slide directly onto the bungee rather then use zip ties making the design sleeker and stronger. We fabricated a new center ring out of metal which matched the aesthetics as well as hold more of the package load. We also changed the strap design that went around the bottom into a shape which did a better job maintaining package stability. Finally the joining points of the bungee were improved. In 2b we used adhesive covered with shrink wrap. While for our final prototype we experimented we prefabricated joining connectors like barrel connectors we found a system we designed of adhesive with brass tubing crushed and covered with shrink wrap was just as strong as premanufactured methods, a better aesthetic, and offered more flexibility in how we made the web.

Strength Analysis

Material and Connection Analysis

We calculated the strength of the system through a free body diagram analysis. First, we sketched the fully stretched model in 3D. Then, we found the areas of contact with the bottom of the package, where the force of gravity will be in effect. After identifying the different forces that act on these points, we simplified the forces into a single free body diagram. There are 4 tension forces, of which 3 denoted in purple are made of bungee cord, and the remaining 1 is made out of polyester. Two tension forces are diagonal with 45 degrees from the surface and two are vertical. Setting the mass as our goal of 35 lbs, we calculated the force each cord should be 49.8N. From the material analysis, we know that the bungee cord can hole up to 540N and the polyester strap can hold up to 1800N, which means both materials are strong enough. On the other hand, from the connection analysis, we found that the shrink cord connector can hold up to 48N while the brass connector can hold up to 186N. Since only the brass connector can hold our desired strength, we decided to proceed with the brass connections.

Capacity Analysis

To determine whether or not we fulfill another one of our crucial specs, capacity, we first mathematically calculated the minimum and maximum volume. When the system is not stretched at all, the system holds approximately the length and height of the backpack, with the width nearing zero. This means the user can comfortably store any package that might be too bothersome to put inside the backpack but does not have a fixed shape, like a vinyl-wrapped clothing package. To calculate the maximum, we based our calculations on the maximum elongation rate of 100%. Taking the web structure into account, we arrived at approximately 20" x 20" x 15" dimensions or 6000 cubic inches.

To check whether or not or hypothetical calculation holds, we tested the volumes of different packages the system can hold. We found that the system safely and securely hold the box shown in the picture. The box's dimensions were 16.5" x 11.5" x 24.5", and was one of the biggest boxes that can be found in Hinman. We demonstrated that the system allows the user to carry 4649 cubic inches of volume in addition to the volume that can fit inside the backpack (2772 cubic inches.)

Compactness Analysis

Our goal was to make our system compact enough that any backpack user can easily fit the package carrier in their front pocket. To make sure we meet the compactness spec, we measured the volume of the different prototypes in a vacuum sealed plastic bag. Prototype 2a was significantly large with 69 cubic inches of volume, where as prototype 2b and the final prototype took up less space because they were made with thinner bungee cord. Although our final prototype was slightly more bulky than prototype 2b, we decided to go for the final prototype because it had much better strength and durability. The final prototype measured 58.9 cubic inches and 150g, which any user can easily put in small pouch and store in their front pocket without feeling its bulkiness or weight.

Bungee cords are at the center of our product design, but they cannot be recycled. Currently, there are not many sustainable replacements in this sector, but we hope to explore other alternatives such as rope.

We are also hoping to incorporate recycled scrap metals into our designs and using a unified material for the center ring and connectors in order to decrease the number of materials in the overall product. However, we would also like to pursue further research in connections analysis with alternative materials to see if there is a stronger and more sustainable option we could use.