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Why Your Car Wheels Need to Disagree in a Turn

8th grade nonfiction passage about the car differential: 275 words, six questions, an answer key and vocabulary.

8th grade nonfiction the car differential 275 words

Why Your Car Wheels Need to Disagree in a Turn

Imagine driving around a tight corner. The car follows a curved path, so the wheels on the outside of the curve must travel along a bigger circle than the wheels on the inside. That means the outer wheel covers more ground than the inner wheel in the same amount of time. If both wheels were locked to a single solid axle, they would be forced to turn at exactly the same speed. One tire would have to skid or hop across the pavement, which would wear the tires and make steering harder.

The solution is an ingenious arrangement of gears called a differential. On most cars, the engine's power reaches the drive wheels through a shaft that turns a large ring gear, and bolted to that ring gear is a small cage holding a few little gears that can spin freely. Each drive wheel has its own half of the axle, and each half ends in a gear that meshes with the little gears inside the cage.

On a straight road, the little gears simply ride along with the cage, and both wheels turn together. In a corner, the road holds the inner wheel back slightly. The little gears begin to rotate, which lets the outer wheel turn faster while the inner wheel turns slower. The power is shared between the wheels, but their speeds are free to differ.

The design does have one weakness. If one tire rests on slippery ice, it can spin rapidly while the other tire, on dry ground, receives almost no push. To correct this, engineers created limited-slip differentials, which resist large differences in speed between the two wheels.

Questions

  1. Why does the outer wheel travel farther in a turn? in the text

  2. What would happen if both wheels were locked to one solid axle? in the text

  3. Where do the little gears of a differential sit? in the text

  4. What do the little gears do on a straight road? in the text

  5. Why can a plain differential be a problem on ice? think about it

  6. How does a limited-slip differential improve the design? think about it

Words to know

differential
a set of gears that lets two wheels turn at different speeds
axle
the rod that a pair of wheels turns on
mesh
to fit together, as the teeth of gears do
pavement
the hard surface of a road
resist
to work against or hold back
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One passage, three levels

The same topic and facts written three ways, so a mixed class can read and talk about one text. Each group’s sheet shows only a shape; the key says which is which.

● Easier · 248 words · Flesch-Kincaid grade 3.2

Think about driving around a sharp corner. The car goes along a curve. The wheels on the outside travel along a bigger circle than the wheels on the inside. So the outer wheel goes farther than the inner wheel in the same time. If both wheels were fixed to one solid axle, they would have to turn at the same speed. One tire would have to skid on the road. That would wear out the tires and make steering hard.

The fix is a set of gears called a differential. On most cars, the engine turns a shaft, and the shaft turns a big ring gear. A small cage is bolted to the ring gear. Inside the cage are a few little gears that spin freely. Each drive wheel has its own half of the axle. Each half ends in a gear that fits with the little gears.

On a straight road, the little gears just ride along, and both wheels turn together. In a corner, the road holds the inner wheel back a little. The little gears start to turn. The outer wheel can turn faster, and the inner wheel can turn slower. The wheels share the power, but they do not have to share the speed.

The design has one weak spot. If one tire is on ice, it can spin fast while the other tire gets almost no push. Engineers made limited-slip differentials to fix this. They stop big speed gaps between the wheels.

▲ This passage · 275 words · Flesch-Kincaid grade 7.4

Imagine driving around a tight corner. The car follows a curved path, so the wheels on the outside of the curve must travel along a bigger circle than the wheels on the inside. That means the outer wheel covers more ground than the inner wheel in the same amount of time. If both wheels were locked to a single solid axle, they would be forced to turn at exactly the same speed. One tire would have to skid or hop across the pavement, which would wear the tires and make steering harder.

The solution is an ingenious arrangement of gears called a differential. On most cars, the engine's power reaches the drive wheels through a shaft that turns a large ring gear, and bolted to that ring gear is a small cage holding a few little gears that can spin freely. Each drive wheel has its own half of the axle, and each half ends in a gear that meshes with the little gears inside the cage.

On a straight road, the little gears simply ride along with the cage, and both wheels turn together. In a corner, the road holds the inner wheel back slightly. The little gears begin to rotate, which lets the outer wheel turn faster while the inner wheel turns slower. The power is shared between the wheels, but their speeds are free to differ.

The design does have one weakness. If one tire rests on slippery ice, it can spin rapidly while the other tire, on dry ground, receives almost no push. To correct this, engineers created limited-slip differentials, which resist large differences in speed between the two wheels.

■ Harder · 291 words · Flesch-Kincaid grade 7.9

Imagine steering a car around a tight corner. The vehicle follows a curved path, so the wheels on the outside of the curve must travel along a considerably bigger circle than the wheels on the inside. That means the outer wheel has to cover more ground than the inner wheel in exactly the same amount of time. If both wheels were locked to a single solid axle, they would be forced to turn at an identical speed, and one of the tires would have to skid or hop across the pavement. The result would be accelerated tire wear and steering that felt stiff and unwilling.

The solution is an ingenious arrangement of gears called a differential. On most cars, the engine's power reaches the drive wheels through a shaft that turns a large ring gear. Bolted to that ring gear is a small cage holding a few little gears that are free to spin. Each drive wheel has its own half of the axle, and each half ends in a gear that meshes with the little gears inside the cage.

On a straight road, the little gears simply ride along with the cage, and both wheels turn together. Around a corner, however, the road holds the inner wheel back slightly. The little gears begin to rotate, which lets the outer wheel turn faster while the inner wheel turns slower. The power is shared between the wheels, but their speeds are free to differ.

The design does have one notable weakness. If one tire rests on slippery ice, it can spin rapidly while the other tire, on dry ground, receives almost no push. To correct this problem, engineers created limited-slip differentials, which resist large differences in speed between the two wheels.

Download as one PDF

How to use it

  1. Press Read it big (or B) to put the text on the projector, or Full screen (F) to fill the whole screen.
  2. Ask the questions one at a time and use Show answer when the class has decided. A shows or hides every answer.
  3. Print student copy for the class, or Print with answers for yourself.

Ideas for class

  • Before reading, put differential, axle, mesh, pavement and resist on the board with their meanings from the Words to know section and have students predict what the text will be about.
  • Read it aloud once while students follow along, then have them reread it alone or with a partner. At a calm 100 words a minute it takes about 3 minutes to read aloud.
  • Use the Read it big button (or B) to put the text on the projector, and Show answer on one question at a time so the class commits to an answer first.
  • Question 5 asks the reader to infer. Ask students to point to the words in the passage that led them to their answer.
  • Ask students to write one question the passage did not answer, then look for the answer in a book or on a classroom screen.
  • Print the student copy for independent work and keep the answer-key copy for yourself.

Related

Questions

What reading level is "Why Your Car Wheels Need to Disagree in a Turn"?

It was written for 8th grade. It has 275 words and works out to a Flesch-Kincaid grade level of about 7.3. That is a formula's estimate from sentence length and syllables, and it is not a Lexile or a guided reading level, so check it against your students.

What do the questions ask?

There are six. 4 ask for something the text says directly and 2 ask the reader to work something out from clues. Each has an answer in the key, and the answers are written as a teacher would accept them, not as the only possible wording.

How long does it take to read?

About 3 minutes read aloud at 100 words a minute. For a one-minute fluency check, paste it into the fluency timer and use the word count.

Can I print it?

Yes. Print student copy gives the passage, the questions with lines to write on, and the word meanings. Print with answers gives the same page with the answers filled in. Both are made in your browser.

Is this passage from a published book?

No. It was written for Peabody Teach, so there is no book to buy and no login to read it.

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