Daily Archives: April 5, 2020

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Are Expensive Bicycle Wheels Worth the Money? Let’s Check the Physics

In this video, you see a cyclist testing new aerodynamic wheels from Zipp. Swapping your wheels may seem like a small change, but can make a big difference. From his tests, the rider discovers:

  • With conventional wheels, he can ride 20 minutes at an average speed of 41.12 kph with an average power of 379 watts.
  • With the Zipp 808 NSW aero wheels he rides 51 minutes at an average speed of 41.13 kph and average power of 344 watts.

Before looking at power and energy, I should go over two small details.

First, how do you measure power? Cyclists can measure power by installing a small computer, called a power meter, that measures the input torque at the pedals or crankshaft and records the rotation angle at timed intervals. If you know the torque and angle, you can calculate the input energy. Dividing this energy by time gives you power.

La te xi t 1

Second, this isn’t a perfect test of aerodynamics. If you really want to examine the effect of the new wheels, you probably would have to put a bike with a dummy in a wind tunnel. When the reviewer takes his second ride, many things could have changed—wind, body position, amount of sweat on the body—and impacted performance. Let’s assume the only thing that changed was the wheels.

Air Drag and Power

What happens when you ride a bike? If you are moving at a constant speed, then the net force on the bike-human system must be zero. In a slightly simplified view, I can draw the following force diagram:

Spring 2016 Sketches key

The vertical forces (gravity pulling down and the ground pushing up) don’t really matter here. Just forget about them and pay attention to the horizontal forces. First, let’s look at the air drag. Air acts in complicated ways when an object passes through it. But who cares when we can make a simple model of air drag force? Here’s an expression for the magnitude of this force:

La te xi t 1

In this model, the air force is proportional to the square of the bike’s speed (v). For the other terms, we have:

  • ρ is the density of air (around 1.0 kg/m3).
  • A is the cross sectional area of the bike plus the rider (how much of the object interacts with the air).
  • Finally, C is the drag coefficient. This parameter depends upon the shape of the object. If you change the wheels, it is the value of C that should change.

The second horizontal force is the frictional force. An interaction between the road and the tires propels the bike. I know what you’re thinking: Doesn’t the human propel the bike? In a sense, yes. But the reality is sort of complicated. The rider’s power goes through the pedals and chain to the wheel, which turns. But the force comes from the tire pushing against the road. So for our energy perspective on this problem let’s just say the human provides the friction force.

Clearly the faster the biker


Woman shot and died Friday in Drake neighborhood


A woman was shot while driving a vehicle in Des Moines’ Drake neighborhood on Friday afternoon and later died from her injuries, according to Des Moines Police.

Police received a call reporting the shooting shortly before 4 p.m. Friday in the 3300 block of Kingman Boulevard. When emergency personnel arrived, life-saving procedures were performed on the victim.

Police are treating the shooting as a homicide investigation, according to a news release from the Des Moines Police Department. Police identified the victim Saturday morning as Catherine Bobbitt.

No arrests have been made at this time.

Des Moines police spokesman Sgt. Paul Parizek praised medical personnel for quickly arriving at the scene and tending to the 33-year-old victim. Parizek said that by the time medics transported the woman to a hospital, she was speaking; however, the victim died at the hospital around 6 p.m.

A dispatcher reported that a caller said her mother was shot in the abdomen while driving her vehicle. Police said the caller was the victim’s 17-year-old daughter, who was sitting in the passenger seat and was uninjured.

Investigators have obtained surveillance videos from neighbors, Parizek said. The victim’s vehicle was traveling westbound on Kingman Boulevard when the shooting occurred.

Jerome Hilscher, 48, works on Kingman Boulevard and said he heard multiple gunshots followed by sirens. Other neighbors in the area said they saw a car drive off from the scene after hearing gunshots.

This story is developing. Check back for updates. 

Read or Share this story: https://www.desmoinesregister.com/story/news/crime-and-courts/2020/04/03/des-moines-crime-woman-injured-drake-neighborhood-shooting/2944387001/

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Alternator (automotive) – Wikipedia

Alternator (silver) mounted on a V8 engine

Alternators are used in modern automobiles to charge the battery and to power the electrical system when its engine is running.

Until the 1960s, automobiles used DC dynamo generators with commutators. With the availability of affordable silicon diode rectifiers, alternators were used instead. This was encouraged by the increasing electrical power required for cars in this period, with increasing loads from larger headlamps, electric wipers, heated rear windows and other accessories.


The modern type of vehicle alternators were first used by the military from WWII, to power radio equipment on specialist vehicles.[i] Post-war, other vehicles with high electrical demands, such as ambulances and radio taxis, could also be fitted with optional alternators.[1]

Alternators were first introduced as standard equipment on a production car by the Chrysler Corporation on the Valiant in 1960, several years ahead of Ford and General Motors.[1][2]

Magnetos in early automobiles[edit]

Some early automobiles, like the Ford Model T, used a different sort of charging system: an engine-driven magneto which generated low-voltage alternating current that was supplied to trembler coils, which provided the high voltage needed to generate ignition sparks. (This was different from a true ignition magneto, which generates high voltage directly.) Since such a magneto system only depended on the engine’s motion to generate current, it could even be used when starting a manually cranked engine, provided the crank was pulled sharply, so that the magneto would produce enough current for the coils to make good sparks.

The Model T incorporated its magneto into the engine flywheel. The first Model Ts used the magneto solely for the trembler coil ignition. Beginning with the 1915 model year, Ford added electric headlights, also powered by the magneto.[3][4] The magneto circuit was strictly AC, with no battery included. (There was a switch on the ignition coils to use a battery instead, which could be helpful when starting in cold weather, but Ford neither provided a battery nor did it encourage the use of one before it introduced an electric starter in 1919. The owner would have to install the battery himself and charge it externally.)

Starting in the 1919 model year, Ford upgraded the Model T to include an electric starter, which was standard for some models and optional for others. This starter installation also included a battery, charged by a conventional dynamo, and the lights were now powered by the battery. However, the flywheel magneto still powered the ignition, and since models without the starter had no battery, they continued to use magneto-powered lights.[5][6]

Advantages over dynamos[edit]

Alternators have several advantages over direct-current generators (dynamos). They are lighter, cheaper, more rugged, and can provide useful charge at idle speed. They use slip rings having greatly extended brush life over a commutator. The brushes in an alternator carry only DC excitation current, a small fraction of the current carried by the brushes of


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