Non-Pneumatic Bike Tires

Comments/Suggestion Section:

Pete after final presentation:

You say that glue is not a good binder for plastic bags… this is true of Elmer’s Glue. This may not be true of all glues. Surely, if a glue adhered to the plastic bag, then you might have yourself a rigid interlocking system of flexible material… sounds pretty compelling. Does the foam stick to the plastic bag material?

You have a considerable amount of typos or grammar mistakes.

One thing future groups might consider: One advantage of a pneumatic tire is that it gets hard after you put it on the wheel. I think that there may be considerable difficulty in getting a tire on the wheel if you get one that works well. You might start by sawing off one side of a rim on a wheel and try to build a nonpneumatic tire on it from one side.

Pete: Very nice. It’s nice to see so much feedback from other students too. How was your experience? How was the communication? what did you learn?
Your experimental exploration and the documentation of it is very inspiring… One glorious failure after another… yielding increased knowledge and curiosity!
New decision matrix? Outlook and summary? Where are people going to go with this project next time I teach this class?

• What does a “2” mean in your decision matrix? Maybe add a key.Also consider having your totals be .44(for first row) or mark that your total calculations are times 100
  • Done!
• Would love to see some lab photos

Not sure if you’re still interested in working with prickly pear juice, but if so let us know and we can leave you some next week. -Anna

Great job at assessing “Things To Try” for next time because you are more prepared for the next round of trials. Could cork crumbles work for rebound and structure or is the available materials and feasibility too low? Also, what is the “traditional braid” in your “Things to Try” area compared to the braid you did in your tire stuffing? -Audrey

You have some pretty creative ideas here. Do you think the size of the tire may affect the different materials you are trying? I’m wondering if the plastic bags me work better in a road tire as compared with a mountain bike tire. -Aaron

This is such an interesting project because I didn’t even imagine bike tires could be made out of something besides air! really creative! -Sheila (interesting tire designs:
http://ars.els-cdn.com/content/image/1-s2.0-S0263822311004909-gr9.jpg)

The structure of your matrix is interesting, I like the way its all weighted based on its comparison to a normal bike tire. – Shauna

Not sure if anything like this has been done before, but if it has or why it is a necessary technology (basically, some background info) might be a nice intro for your web page. -Pearce

A background/significance section would be nice, and a explanation (or a link) of the physics of a bike tire (how it deforms under different riding conditions, especially when turning, and what areas actually support weight, etc would be helpful. I would look into using a natural fiber like burlap or tightly wound yarn. Winding an elastic material like latex or rubber (discarded inner tubes?) might also work. I’m sort of thinking of the inside of an old-fashioned golf ball. – Sean

It is very interesting to see all the methods you are using to make these tires. I am hoping one of them will prove successful! Have you guys tried riding around on any of them yet? – Oren

There’s a product titled SLIME (http://www.slime.com/), which you may have heard of, that works in decreasing the number of flat tires. I personally haven’t used their product much, but have heard stories of people simply filling their tires with slime instead of tubes with air in them for a hefty bike ride. It gets to be decently heavy, and really messy if you do flat, but apparently works. I have a few bottles I can bring to class if you’d like to try some. – Nichol-B

I remember you mentioning something during your presentation about stuffing the spring with something – what about plastic bags or a rubber tube? -Daniel

I really like how your decision matrix is different from most and you compare the effectiveness to that of a normal bike, I think this is a great way to determine that your designs are not only feasible but better than what is available! – Jen

PROJECT BACKGROUND:

Bicycles are widely used across the globe for a variety of purposes. In many countries bicycles are critical modes of transportation used to transport basic necessities like water, taking goods to market to earn a living, or even carrying critical medicine.

With bicycles as a critical mode of transport for many things, a flat tire can have a much more significant impact on someone in a developing nation than it would on many of us. Bicycles have been around long enough that in most urban areas in developing countries repairing a flat tire may not be as troublesome. However in remote rural areas where roads are in poor condition or even non-existent, flat tires can be incredibly common as well as more of a burden to repair.

Though pneumatic tires have been the norm for a long time, some companies have been exploring alternatives:

Energy Return Wheel

The energy return wheel is a product still in development by a company called Britek. It incorporates a structure much more complex than that of a normal tire, and is promising for performance, but not for cost.

Michelin Tweel

The tweel is a product in development by Michelin. It uses flexible polyurethane ribs to create a strong yet flexible structure, while not adding much weight.

Nu-Tek Airless

A solid rubber/foam. They’ve been used, but are too heavy to outperform the pneumatic tire.

PROBLEM STATEMENT

To design an airless bicycle tire that is intended for and accessible by people in developing countries so that their livelihood is not at stake because of faulty transportation.

PROJECT GOALS

• Compatibility with standard bicycle wheel
• Similar performance to pneumatic tire
• Significantly outlast pneumatic tires
• Accessible material
• Simple installation process
• Low cost

PERSONAL GOALS:

• To gain hands-on design and fabrication experience.
• To creatively explore the process and context aspects of the design process instead of focusing solely on the product.
• To understand the realities of practical, sustainable, and appropriate product implementation in developing countries.

DECISION MATRIX:

The first matrix was our original. For our second iteration, we changed a few things, such as: breaking up the tire stuffing category into plastic bags and spray foam, deleting the solid tire category, and including the plastic insert category (a new idea we came up with after the initial decision matrix).

Rating Key:

• 2 – Much better than pneumatic tires
• 1 – better than pneumatic tires
• 0 – Same as pneumatic tires
• -1 – Worse than pneumatic tires
• -2 – Much worse than pneumatic tires

PROCESSES:

1. Tire Stuffing

   • Plastic Bags – We braided plastic grocery bags and stuffed them into an empty bicycle tire. With sufficient compression, we began to notice some rigidity, but not enough to feel like an inflated tire. We tried a few different methods of stuffing a tire with plastic bags:
     • Traditional Braid (Like braiding hair)
       • In order to compress the plastic bags, we tried braiding them together. The idea was inspired by the plastic bag bridge that was in the video about the Tinkering School (show link) that we watched at the beginning of the quarter. This gave decent rigidity when we stuffed multiple braids into a portion of the tire. The braided bags didn’t fill the tire shape evenly, however, so it didn’t quite work.
     • Binder (Elmer’s Glue)
       • We tried mixing plastic and glue by diluting the glue and compressing the plastic because we thought the glue would act as a binder. This attempt turned out to be unsuccessful. Glue is not a good binder for plastic
     • Maybe the simplest approach with plastic, we just shoved as many plastic bags into the tire as we could. With the tire half on, we started working in bags around the circumference until we couldn’t force any more into the tire. We rode on this tire too, and, surprise – it felt like a flat tire. We simply couldn’t get enough pressure inside the tire by shoving plastic inside.

• Shelf Liner – We tightly rolled shelf liner by hand and stuffed the roll into a tire. The shelf liner was much to soft to be useful in the bike tire.

• Spray Foam – We filled the tire full of spray foam; it took about 8 hours to expand and set. After setting, the tire was somewhat rigid, but it gave no rebound. More foam would have helped some, but overall, the foam isn’t elastic enough to be durable in the long run. We rode on this tire, and it felt pretty terrible… like a flat tire.

This is the tire we filled with spray foam

Bottom Line: With the materials above, simply stuffing them into a tire doesn’t produce the pressure necessary for rigidity. It would take an already stiff, but lightweight, material.

2. Wire Coil – We bought music wire and wound our own springs with the idea of stuffing a spring around the circumference of the tire. With the right coil thickness and working method, we thought we might be able to get the right mix of rigidity and elasticity. The different forming methods we’ve tried are:

• Heat and wind simultaneously – Terrible idea. Produced a very irregular spring with kinked coils. Didn’t fit in the tire.
• Wind first, then heat – Process worked well and yielded a consistent spring. Spring offered very good rigidity and maintained shape well enough to properly fit tire. We were able to get the tire bead to set with the spring inserted into a portion of the tire.
  • Spring didn’t have sufficient elasticity and deformed too easily with large loads on the spring. One reason for this is that we are loading the spring in the transverse direction, where it isn’t designed to be elastic. Also, our bending and heat treating process actually hardens the steel spring. In order to fix this, we could form the spring and then anneal it, which would “reset” its material properties, yielding a more elastic metal (as opposed to the work-hardened spring we tested.)
• Overwind to tighter radius coil without heat so that the coil will rebound to desired radius. (did you try this?)
  • We did try this method, with some success. The wire definitely rebounded to a larger diameter. However, getting the wire coil to rebound to the correct diameter proved more difficult. The behavior of the coil changed dramatically with wire diameter, and the smaller wire we tried was too small. It would take more experimentation to find the right wire diameter.
• Things to Try
  • More coil sizes and wire diameters. The few we tried were not nearly enough to find the ideal coil and wire size combo.
  • (maybe a metallic strip instead of a coil? Like a coiled up band saw blade like the prosthesis group was using?)
    • We investigated a method similar to this, but we came at it from a different angle and used plastic strips as the material instead of metal. The process and results are explained in the plastic insert section below.
  • Wind in same fashion, but heat to a higher temperature with acetylene torch or oven to actually anneal metal
  • Form composite with the wire coil as the backbone and another, more elastic, material as the matrix. (Did you try this?)
    • We began to experiment with this method during the last lab. We used a plastic sheet as the matrix and formed it around the spring. The process and results are explained in the plastic insert section below.

Different springs we have made

3. Plastic Inserts – We began exploring an approach that we believed would accomplish similar results to that of our plastic extrusion idea, and one that replicates the concept of the Energy Return Wheel but on a more basic level. We bought sheets of two different kinds of plastics (HDPE and PETG) with the goal of cutting them into strips that could then be bent to form fit the inside of the bicycle tire but add sufficient rigidity. We began by cutting small pieces to bend into wedges that would simply fill the distance between spokes on the wheel, rather than trying to tackle the whole circumference of the wheel at once.

• HDPE: We bought two thicknesses of HDPE, 1/32” and 1/16”. The 1/32” proved to be way too thin, though it served as a good medium for establishing what we thought was the necessary size for our test pieces. 1/16’ was also too thin though helped us to realize that where the two ends of the plastic met in the rim, a more rigid strip was necessary to keep the ends from shifting around as we loaded the tire rather than allowing the plastic to take the load. Our quick fix solution was a seam of hot-glue to hold the wedge in the desired shape. Though the 1/16” HDPE did not offer enough rigidity, the hot-glue stip made a noticeable difference in the apparent rigidity of the tire.
• PETG: We also bought two thicknesses of PETG, 1/16” and 3/32”. We would have preferred to buy consistent sizes with HDPE but they were not available, though this worked to our advantage. We began this time with the thicker 3/32” PETG and prepped a piece to make a test wedge. This thickness however was incredibly rigid, such that we had to bend it in a vice where we had normally been bending by hand and this ended up kinking the plastic. Once we wrestled this wedge into the tire it was instantly apparent that the 3/32” was way too thick and offered no give when the tire was loaded. The 1/16” PETG was next and it offered a lot of promise. Learning from the thicker plastic and with some hand forming by Pete, we bent a piece of 1/16” around a pipe to give us an insert with a radius similar to the tire, making it easier to install in the tire. Once installed, we concluded that the 1/16″ insert was a little to thin, as it kinked slightly after applying a load to the tire a few times. The feeling was much better than anything we tried, however. We feel this has promise, as we found the insert to be slightly too soft with the 1/16″ thick strip, and slightly too stiff with the 3/32″ strip. Maybe a 5/64″ thickness would be just right!
  • PETG was definitely the plastic of choice compared to HDPE, but maybe there is an even better plastic out there.

Tyler bending plastic

• For the final lab, we were not able to get 5/64″ thick PETG in time, so we focused on improving the forming method instead.

Our first forming attempt with a torch. WAY too much heat!

• We found that forming was easy when we used a torch to apply evenly distributed heat to the PETG. Both thickness (1/16″ and 3/32″) responded favorably to this method. We balanced the plastic strip on an aluminum pipe about the same diameter as a tire and applied heat as evenly as we could. The plastic slowly wrapped around the pipe. Once the weight of the plastic stip had caused the to form itself around the pipe into a “u” shape, we removed heat and pinched the ends together. This yielded a nearly perfect shape that, once cooled, still kept it’s elastic properties. We were on to something!
• Next, we formed a strip of plastic around a spring to investigate the composite idea we had back when we were experimenting with winding springs. First we wrapped a coil of wire around the aluminum pipe and clamped the wire coil in place. Then we took the strip of and formed it using the same torch method as before. We noticed that the spring would not hold its shape inside the formed plastic and would rebound as soon as we unclamped the ends, so we tried heating and pressing the spring into the plastic itself. This produced moderately good results, although we heated too much the first time, which caused the plastic to begin to melt (note the bubbles).

Formed plastic – note the spring indentations and the effects of excessive heat

We need a better way to make sure the ends of the spring stay in place

• For our final attempt, we used a larger, notched section of plastic and formed it around a spring. It turned out well, except the spring still popped out of place easily. Also, we concluded that the notches, as they had been cut, didn’t help the insert bend into the of the wheel circumference. In order for that to work, we would need to cut the notches higher and use heat to bend the insert into shape.

Placing the plastic in position

Heating and forming the plastic strip

Tyler hard at work. Ethan… not so much.

Forming the ends by pinching them together after heat was removed

Final result. Plastic insert came out great and kept it’s preheated properties, but the spring still rebounded out of the insert

CONCLUSION:

Throughout the quarter we tried a variety of methods and materials to get a taste for various approaches to this endeavor. Though some were simply explorations that confirmed an undesirable route, we finished with one approach that would be our choice to continue on, had we more time, as well as one that we still believe has unexplored potential.

If we were to continue this project, our path of choice would be the hybrid spring/plastic insert method. We believe this method to be effective for a variety of reasons. First, both materials utilized in this process are fairly inexpensive and simple enough to source. Also, the process of manufacturing this component, though it could easily utilize more advanced manufacturing techniques just as any other product, can be crafted in a fairly rudimentary fashion as demonstrated by us in the shop. We also believe this method can best blend our goals of replicating the performance of the pneumatic tire while maintaining a comparable weight, though in all likelihood still slightly heavier. With the proper sizing and setup of both materials, which would be our future exploration, we believe this method can successfully utilize the potential rigidity of the wound metal coil, while using the plastic to distribute the load more evenly to prevent kinking, provide a more even ride surface than we would achieve with the spring alone, as well as aid in lateral stability of the tire; something that was a serious concern with the spring by itself. The validity of this method in our minds is also reinforced by its similarity in structural concept to the Energy Return Wheel in development by Britek which seems to hold potential for market success.

We also believe that the concept of a solid foam type tire also holds some potential. Though our use of spray foam yielded sub-par results, during our post test evaluation of the process, we noticed that the foam in certain spots yielded resistance and rebound similar to a pneumatic tire. This observation, coupled with the limited market success Nu-Tek foam tires indicates to us that the idea has merit. We do believe, however, that achieving the goal of similar weight and cost will be incredibly difficult.

GROUP MEMBERS:

Corynne Umeda: I am a second year EE student on exchange from Hawaii.

Tyler Bierce: I am a fifth year architecture major. I’m sorta from California, but kinda from Oregon too.

Ethan Flory: Who am I? I don’t know, really. Most likely, a fifth-year mechanical engineering student. I’m from California.

GROUP DYNAMICS:

Group Consensus: Good. Enough said. But seriously, it was good; we worked well together and all contributed.

WHAT WE LEARNED:

We learned a lot of things while working on the project. Here are a few:

• Analyze existing technologies more closely with the intention of extracting successful techniques while avoiding unsuccessful ones.
• The context needs much, much, much more focus than we gave it. In an effort to get to work quickly, we did nowhere near as much background investigation as we would have liked.
• It’s better to physically test the concept quickly and assess if it’s worth dedicating more time to it. Sometimes an idea looks great on paper, but the moment you get your hands dirty, its flaws come out.
• Hawaiians get cold easy.
• You work much better when well fed.
• Ten weeks goes by quickly.
• Take more pictures.