notes by Alex K.
Today’s agenda: Lecture with Bill Becker on Wheels
Bill Becker biography:
- Retired architect, architecture undergraduate at Case Western and Masters from Penn
- Retired as a motorcycle designer and builder
- Favorite thing about motorcycles: amazing and functional pieces of design, where aesthetics and engineering work together flawlessly
- Likes working with vintage machine and upgrading them to a higher standard than when they were new
Lecture:
Why can’t bicycles or motorcycles stand by themselves? “Because they’re too (two) tired!”
Wheel from a French moped: has a hub and brake in the middle, essentially analogous to motorcycle wheel
History of wheels:
- The wheel is a form, as long as it is circular with a pivot it is consistent
- Spokes: an innovation to make wheels lighter and springier
- First motorcycle has two different wheels
- front wheel: spoked, moves freely through space, only pressure is force of gravity
- back wheel: solid wheel, applying pressure from drive chain onto the ground to move the bike
Three subjects: physics, engineering, and material science
Physics
- Mass: defines how a matter in one object relates to matter in another object (on earth, define by weight: relation of mass of earth to mass of object)
- Objects are subject to gravity and motion – inertia or acceleration
- “weight is not a friend of the wheel”
- on a railroad car, heavy wheels are OK
- but on a racecourse/motorcycle, you want wheels to be light so that less work is required to accelerate them ––> spokes; thin rim; aluminum hub
- Vectors: ways to describe the direction and magnitude of forces we are talking about
Material science
- Athletes look for certain things in different classes, e.g.
- Wrestlers want a high strength to weight ratio
- Gymnasts want flexibility, strength, and small mass
- Basketball players want strength, flexibility, and height
- Materials of a motorcycle also look for different qualities for different needs:
- Aluminum: strong, flexible, low weight, corrosion resistant, machinable, malleable, inexpensive; it can be cast into unusual shapes (like in engine case)
- Cast iron: strong, porous (can absorb oil), reduces friction, dissipates heat well but brittle
- Steel: can be hard, soft, corrosion resistant (hardened steel: ball bearings; woodrift keys) (softened steel: fender, needs to be bent into shape)
- Rubber: flexible, not very compressible but high tensile strength, corrosion-resistant, lightweight
- More: polyester, brass, chromium, asbestos, plastic, air
- Material scientists will select from these to do different jobs in the wheel
Engineering
- Terms:
- Deadload – the weight of the thing itself, i.e. motorcycle with fluids in it
- Liveload – weight of it with a rider on it
- Static loading: object is stationary, just applying gravitation forces
- Dynamic loading: object is moving through space, loading due to contours of the road, turning, impact, acceleration/deceleration
- Forces acting on a materials:
- Compression – some materials (e.g. brick) are very good in compression, others (e.g. rubber/sand) are very compressible
- Tension – another structural force that is just “pulling” on things – aluminum, steel are good; glass is bad
- Bending – just a combination of compression and tension
- Shear – forces are opposed off-axis to create a “tearing” effect
- Thomas Young: physician, material scientist
- relationship between stress and strain
- Force on an object in compression: expect straining (i.e. getting fatter/shorter)
- Force on an object in tension: expect stretching
- Young’s models:
- Increasing strain increases stress, up to plastic deformation
- Linear relationship btw. stress and strain during elastic deformation
- concave-down parabolic relationship between stress and strain during plastic deformation
- Fractures at the end of plastic deformation phase
Wheels:
- Patterns of four spokes at a time (36 spokes in total on most bikes; 40 on British)
- Spokes evenly spaced around the rim
- On a motorcycle wheel, spokes are not arranged to intersect the axis of the wheel because of a differential between forces on the hub and forces on the wheel
- Spokes are off-angle to make a rigid geometry by translating forces between hub and rim
- Triangulation both in the plane of the wheel and laterally (i.e. coming out of the plane of the wheel)
- Spokes are in pairs of four to resist opposing and proposing rotation; out of plane bending
Loading of the wheel and tension of the spokes:
- Spokes are great in tension and bad in compression because of their slender shape and size + how they are connected to the rim
- Static loading of gravity of the rider – top spokes are doing the work to hold the hub up
- Impact loading – all the spokes that are opposite to the point of impact go into tension
- Acceleration – half of the spokes (the ones that point in the direction of acceleration along the hub) go into tension
- Turning – rigidity out of the plane of the wheel go into tension
Nowadays, many wheels are cast instead of spoked because of the cost of labour; 88 parts in our spoked wheel but only 1 in a cast wheel. There is some debate as to which is more aerodynamic.
The size of a spoke is determined by its material strength, trying to be maximally aerodynamic while still supporting the weight of the bike. Bicycle spokes are very thin because they are lightweight and very little torque is transmitted by the rider.
This singular spoke brought to demo can hold about 800 lbs! All the spokes in tension at any moment can hold up to 8,000lbs.