1. Engineering/Failure Analysis
Team Icarus
John Brooks
Ivan Olendan
Kyle Reno
Endalk Tegegne
David Wu

2. Last Week: Design Layout
Decided to go with a modified triangular design rather than T design
A lot of room for improvement in the caster wheel design
Maintenance/Failure is a major area for improvement over current designs

3. Topics for Today Qualitative Caster Angle/Trail effects. Trail effect
Positive vs. Negative caster angle
Failure Analysis
Static Analysis – Pin shear and bending with factor of safety. Load Rating of wheels.
Impact Analysis – Spring model of wheel assembly to estimate impact forces using strain-energy.
Materials Selection
Wheel selection
Frame
Bearings/Pins
New Design Idea

4. Qualitative Caster Analysis
Mechanical Trail (distance between steering arm and wheel contact)
Although the scientific understanding of bicycle steering remains incomplete,[Whitt] mechanical trail is certainly one of the most important variables in determining the handling characteristics of a bicycle. A higher mechanical trail is known to make a bicycle easier to ride "no hands" and thus more subjectively stable.
For our purposes, trail will increase how well the dolly tracks, but it will also increase bending stresses that exist in the caster pin (due to a larger moment arm).
Whitt, Frank R.; Jim Papadopoulos (1982). "Chapter 8", Bicycling Science, Third edition, Massachusetts Institute of Technology. 

5. Qualitative Caster Analysis
Caster Angle (source is based on cars)
Positive caster angle
Improves tracking
Makes wheels harder to turn (in cars and motorcycles power steering is often required for significant caster)
Reduces Flutter
Negative caster angle
“Vehicle might wander as a result of negative caster”
Too much negative caster can cause sensitive steering at high speeds.
Unequal caster angles cause the vehicle to steer toward the side with less caster. For us, if one pin bends and changes the caster angle and the other pins doesn’t bend as much this would be problematic.

6. Static Failure Analysis
This is for a ¾” pin with current fork length and fork angle. They currently use a 5/8” pin.

7. Modeling Impact (to determine max force that arises)
Spring model
For springs in series:
Max force exerted on tires=3620N=814 lbs
Max bending stress in pin = GPa
Bending factor of safety (failureyielding) for a ‘hardened’ steel~1.027
When 50 lb dolly is dropped 1 foot 

8. Wheel Spring Model Assume a rectangular shape carries load in tire
N/m

9. Fork Spring Model
Treated both sides of fork as cantilevers. Bending spring rate is much smaller than axial compression spring rate, so axial component is neglected.
K= N/m

10. Pin Stresses
From our model, the bending stresses exceed the yield strength for many metals. The pin may be permanently deforming without fatigue.
There’s also a shearing stress in the pin because the fork is in compression, but the static analysis showed this is 100 times smaller than the max bending stress

11. New Dolly Design Cradle Support Bar Wheel Rub
Interference Problem with Pilot
Keel Rest

12. Material Selections
Square tube steel for the front part of frame, aluminum used as much as possible elsewhere.
Decided on urethane Never-Flat Tires. They come up to a 10 inch diameter and have a load rating of 350 lbs.
Hardened steel for caster pin (need very high yield strength). Looking at aircraft grade chromoly steel steerer tubes for pin connection (used in bikes, easy to add bearings with this)
Galvanized steel for bolt connections.

13. Next Week
Part Drawings
Fully dimensioned drawings of dolly.