Bicycles - How Things Work - Slides | PHYS 140, Study notes of Classical Physics

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4.1 Bicycles

4.1 Bicycles

Clicker question

Clicker question

Which vehicle is most stable?

(A) tricycle

(B) Bicycle

(C) This chopper bike

(D) An SUV

The tricycle and SUV The tricycle and SUV

are very good at are very good at

standing still. standing still.

But both the SUV and tricycle have But both the SUV and tricycle have

major problems turning! major problems turning!

This is not funny – 1000s of people die in SUVrollovers every year…and that rate is increasing!

Statically stableDynamically unstable

Statically unstableDynamically stable

Statically stableDynamically unstable

Statically unstable?Dynamically unstable?

A tricycle is statically stable

A tricycle is statically stable

It’s base of support is atriangle defined by thepoints of contact (tiresand ground).

c-o-m

Tips over to a newstable equilibrium

If we go too far…

Gravitationalpotential energyincreases

Energybecomessmaller in thisdirection

mgh

mg

h

Stable equilibrium means that Stable equilibrium means that

gravitational potential energy rises gravitational potential energy rises

when the object is tipped when the object is tipped

Tricycle:staticallystable

Bicycle:staticallyunstable

You can also understand this using torque You can also understand this using torque

Why does the SUV roll? Why does the SUV roll?

F

friction

r

Too much torquearound c-o-m!

SUVs and tricycles are dynamically unstable because the total forcedoes not point to thecenter-of-mass.

Supportforce

Total force

There’s a torquearound c-o-m fromthe centripetal force!

Clicker question Clicker question

Why does Lance ride a Why does Lance ride a

bike and not a bike and not a

tricycle? tricycle? (What makes a bike (What makes a bike

dynamically stable?) dynamically stable?) (A) A cyclist can lean on a bike and change the torque around his center of

mass.

(B) The pedals on a bike are connected to the rear wheel (but the front wheel

on a tricycle), making it possible for the cyclist’s feet to work independentlyof the motion.

(C) A bicycle has brakes, so that the cyclist can change speed quickly.

Leaning during Leaning during

turns allows turns allows

total force total force

to point towards to point towards

center of gravity, center of gravity,

so no net torques! so no net torques!

Vehicle

Vehicle

s Dynamic Stability

s Dynamic Stability

Dynamic effects can aid a vehicle’s stability

• by shifting base of support to under the center

of gravity (meter stick)

• by dynamically stabilizing an equilibrium, i.e.,

reduce net torques (bicycle)

And, can make a And, can make a

statically unstable vehicle statically unstable vehicle

dynamically stable dynamically stable

A bicycle steers automatically!

A bicycle steers automatically!

• Gyroscopic precession• “Trail” – the shape of the fork

Trail Trail

is the dominant effect at low speed is the dominant effect at low speed

A bicycle has A bicycle has

some special some special

features which features which

make it incredibly make it incredibly

dynamically stable dynamically stable