Techies - Summer Biking

Now that spring is in full swing, I decided to break out my bicycle to start burning away some of that winter fat.  However, when I finally hit the trails, I saw that both my tires were visibly under pressured (I later found it to be at 20psi).  Remembering that only a month ago, I did a Tire Pressure Lab as part of my Mech 305 course at UBC, two thoughts came to mind.  For motor vehicles, the reduction in tire pressure causes the following two major concerns:

1. loss of stability;
2. reduction in fuel efficiency.

Shot of Vancouver from Canada Line Bridge

The US Department of Energy claims that for every 1psi drop below the recommended tire pressure rating, gas mileage can be reduced by 0.3%¹.  My focus today is on fuel efficiency and how the reduction of tire pressure reduces the efficiency of my bicycle. Of course, when we’re looking at efficiency, we’re looking at energy input vs energy output.  What better way to demonstrate the “feeling” of efficiency than physically being the energy input.  As I powered my bicycle, I realized that I reached fatigue much sooner than I used to.  At first I thought, “I must be so out of shape.”  However, once I made it to a gas station and pumped up my bicycle to the proper tire pressure (between 40-60psi) I soon felt a major difference.  Back on the road with optimal tire pressure and a brief break, I soon found powering my bicycle was a lot easier and that I made it home without reaching fatigue at all.

When I arrived home, as an engineering student, I eventually thought, “why does this even happen?”  So I set out to find some answers.  In an ideal case, we make several assumptions as the wheel rolls:

1. the wheel is perfectly circular;
2. there is a single point of contact;
3. the wheel is rolling without slipping.

However in real life, only rolling without slipping is true (unless you're trying to do a burnout).  In real life, the wheel actually deforms at the point of contact into a surface of contact.  Here I will offer two definitions:


Contact Patch:  Flat area of the tire that is in contact with the ground.²
Hysteresis:  During the deformation of a material, the energy of deformation is great than the energy recovered when the material returns to its original state.³
  
Because of losses to heat, sound and other forms of waste energy, hysteresis occurs.  Therefore, a loss of energy occurs.  Now you might ask, "why don't we pump the tire to it's maximum so that the deformation is minimum?"  There are two reasons for this⁴:a

1. Increased stress concentrations result in increase wear at the centerline of the tire;
2. high tire pressure stiffens the interface between road surface and car.

To explain the first reason, we see that if a tire is inflated to the point where it can almost be approximated by a circle, then the entire weight of the car is essentially sitting on zero contact patch.  The pressure in the tires approaches infinity (P = F/A where area is approaching a very small number)!  This high pressure results in high wear and the tires fail faster.


The second reason why we don't run tires at maximum pressure is more of a luxury.  Thinking back to Mech 364 - Mechanical Vibrations, the wheels of the car can be thought of as a bunch of springs: the higher the pressure, the stiffer the spring, the lower the pressure, the flimsier the spring.  At high stiffness, you can assume that there is no spring at all (a rigid connection).  As a result, you will feel every bump and pebble that your tires roll over, which serves as a very uncomfortable ride!



References:

1. US Department of Energy: http://www.fueleconomy.gov/feg/maintain.shtml - Retrieved April 27th, 2011.
2. How Stuff Works: http://auto.howstuffworks.com/tire4.htm - Retrieved April 27th, 2011
3. Wikipedia: http://en.wikipedia.org/wiki/Rolling_resistance - Retrieved April 27th, 2011
4. How Stuff Works: http://auto.howstuffworks.com/tire5.htm - Retrieved May 10th, 2011

3D Solid Model Samples

This edition of Tinker and Techies is going to be special. It will be updated regularly and is going to be a catalog dedicated to display 3D CAD designs I have done in the past. The purpose of this catalog is to demonstrate my ability to anyone who is interested in hiring my services as a freelance 3D Modeler or Designer.  It contains a brief background, description, and the time it took to design and model.  When reviewing the Design time and Model time, please keep in mind that I am a student with other commitments as well.

Wine Rack:  It was intended to be made out of wood and for my father's birthday.  The design of this includes a static failure analysis.  It's a simple design, but incomplete; I want to redesign the side panels to be more aesthetically pleasing.  I modeled the wine bottles myself as well by rotating a cross section.  Otherwise, a lot of straight lines; nothing special.  Design time: 18 hours | Modeling time: 6 hours

Card Catcher:  Designed during UBC's Mech 328 project course, the purpose of this was to catch business cards at the end of a process.  Four springs (not shown here) would deform as cards accumulated inside the box.  The plate inside is indeed slanted to utilize seating forces of the box walls.  This was not the final design, but was served as an important prototype.  The mating of the assembly was done to determine and simulate reasonable ranges of motion of the plate.  Design time: 60 hours | Model time: 4 hours

Multi-Element Airfoil Test Rig:  Designed during my first year with Formula UBC.  The test rig consists of rotating platforms attached to sliding links.  This allows the airfoils to be moved relative to the large, center airfoil.  The purpose was to be able to test various airfoil positions and angles of attack within a wind tunnel and find optimal lift vs drag ratio.  This was a very repetitive mating job and was the first time I learned to import points into a sketch.  Design time:  200 hours | Model Time: 4 hours

Gate Valve:  This piece was used as an on/off valve on a tiny water-air mixture propelled boat.  The model itself is not functional (ie the valve cannot be rotated) but could easily be simulated by using two components and mating them in an assembly.  This was a simple model to make and required me to take measurements of the actual part that we used.  Design time:  N/A | Model time: 1 hour

Flow Splitter:  This piece was to be a part of the same assembly as the Gate Valve.  The purpose of the piece was to take air and water feeding from two sources and expel the mixture out of the other end (the reverse of "splitting the flow").  This model, also requiring measuring of an actual part, was actually quite simple by using a mirroring tool.  Design time: N/A | Model time: 2 hours

Custom Propeller:  This piece was designed as the propeller on a miniature hovercraft.   The challenge behind this model was connecting the two blades to the central hub.  Normally this is quite easy if it was a flat surface, but it clearly was a cylindrical surface.  Also, the blades themselves were swept through changing airfoil shapes and sizes along the entire span.  Modeling this part taught me a lot about model techniques.  Design time: 20 hours | Model time:10 hours

Trap Door:  This assembly was to demonstrate how linear motion from another system could activate a trap door.  The mating within the assembly simulates realistic interactions between each component.  Design time: 1 hour | Model time: 2 hours

Orbiter:  Designed during UBC's Mech 223 project course in a team of 6 students, the objective was to launch this vehicle along a flat surface.  After a time, the vehicle would release a ball that was meant to reach a target.  After launch, the process would be fully automated.  In an attempt to incorporate as much detail as possible, I modeled the nuts and bolts that were used to hold the parts together.  Because the design process is iterative, parts of the model were changed frequently.  Design time = Model time:  160 hours

Star Wars - Tie Fighter:  This model was done as a personal challenge to incorporate as much detail as possible.  7 parts make up this assembly and is still an on-going project.  The accuracy of the model depends on "eye-ball" accuracy derived from pictures and scale models.  In a way, this is more of an artistic challenge.

Thank you for your interest.  Feel free to comment on any of the samples with questions, tips or even request a sample!  You may also e-mail me if you are interested in employing my skills.