Teaching with Bike Shop

About these notes

Bike Shop is a repair shop with a job board of twenty tickets and four stands: Gears, Shifting, Truing and Flats. A ticket is a bike a customer brought in. It closes when the bike passes its test. The test reads the bike, so a student cannot close a ticket by clicking around.

The Gears stand is the math. It fits a ratio and proportional relationships unit in grade 6 or 7 and can be used alone. The other three stands are troubleshooting: change one thing, test, read what happened. They fit a technology, engineering or STEM elective, a bike club, or a homeschool week.

Everything a student reads is plain text on the page, so a browser can translate it. The pictures carry labels only where a label is needed.

Three periods

Period 1 is the Gears stand. It needs nothing but the app.

1. One turn (10 minutes) On the projector, count the teeth out loud: 32 on the chainring, 20 on the cog. Ask how many times the back wheel will turn for one turn of the pedals. Press Pedal one turn and watch the paint dot on the back tire. The line under the picture reads 1.6 times and 3.06 meters.
2. Two chalk marks (10 minutes) Students pick the 14 tooth cog and pedal one turn: 4.37 meters. Then the 28 tooth cog: 2.18 meters, exactly half. Ask why half the teeth rolls twice as far. This is the moment for the words ratio and unit rate.
3. The hill (15 minutes) Students take the ticket Stuck on the gentle hill. The bike arrives in its hardest gear and the ride is a slow grind at 42 turns of the pedals a minute. They try bigger cogs until the ride is comfortable. The 22 tooth cog gives 66 turns a minute. Point at the speed: it is 10.9 kilometers an hour in every gear. The gear changed the legs, not the hill.
4. The same gear twice (10 minutes) The ticket Are there really 24 speeds? uses the city bike. Open Gear chart. Students find two rows that roll the same distance, 48 teeth to 24 and 28 teeth to 14, and pedal one turn in each. Both roll 4.34 meters. Ask for another pair on the chart.

Period 2 is the Shifting stand. Start with the picture: this is the back of the bike seen from behind, drawn wider than life so the 5 millimeters between cogs can be seen.

1. Name the parts (5 minutes) Open How it works and read the Words paragraph together: cog, derailleur, guide pulley, tension. On the free stand, click Bigger cog a few times and watch the orange pulley lead the chain across.
2. The slack cable (15 minutes) Students take the ticket Clicks but does not climb. The first click leaves the chain on the 14 tooth cog. They press Test ride and read what it found. Let them turn a limit screw first: nothing changes. Then the tip: click to a middle cog and turn the barrel adjuster out a quarter turn at a time. The line under the picture goes from Wrong cog to Ticking to Quiet. Eight quarter turns closes the ticket.
3. The stops (15 minutes) The ticket Chain went into the spokes arrives with the chain off. Students press Put the chain back on with nothing fixed, and it goes straight back in. Ask which screw is there to stop that. The L screw goes in, the barrel adjuster goes in, and the test ride passes.
4. Talk (10 minutes) Why did the limit screws do nothing for the first bike? Why did the test ride push on the stops by hand even when every shift was quiet?

Period 3 is Truing and Flats, half a period each.

1. One loose spoke (15 minutes) The ticket Rubs once every turn. Students spin the wheel, read where it rubs, click that spoke on the wheel and tighten it a quarter turn. All three strips move. Three quarter turns and the wheel is true. Then the ticket Every spoke cranked tight: the wheel is nearly straight and every bar is red.
2. The flat that came back (20 minutes) The ticket Flat again in two days. Most students put in a new tube and pump it up. The test ride goes flat before the corner. They take the tube out again, wipe the inside of the tire, and find the thorn. The ticket closes only when they name the cause.
3. Shop report (10 minutes) Students open Shop report, answer the questions and press Hand in.

What students get wrong, and where the app answers it

The first row is the common one: adding where multiplying is needed. The chalk marks are there for that. A student who expects 34 to 16 to match 32 to 14 can pedal both and look.

The twenty tickets

Each ticket has one idea in it. The right column is what closes it.

Most tickets have more than one way to close. A wheel can be trued by tightening one side or loosening the other. Any setting that leaves the bike right passes.

What the app simplifies, on purpose

• One made-up rider does every ride: 45 kilograms, 75 watts, comfortable between 60 and 95 turns of the pedals a minute, sitting down. The 75 watts is in line with published figures for teenagers. The comfortable range is a judgment based on adult figures.
• The rider makes the same effort in every gear. A real rider makes less at a very slow grind and can stand up on the pedals, so a real rider might get a slightly higher gear up the steep hill.
• The derailleur is one common 7 speed kind. Other bikes pull a different amount of cable on each click.
• How far one turn of a limit screw moves its stop, and how far off the chain can be before it ticks or jumps, are made up to behave like a real one.
• The view from behind is about four times wider than life across the cogs.
• The wheel is a front wheel with both sides alike. On a real rear wheel the two sides pull differently. How much a quarter turn moves the rim is a judgment; the directions are right.
• On a real stand a mechanic finds the wobble with the calipers as the wheel turns. The three strips show it all the time, so a quarter turn can be seen doing three things at once.
• A rim bent by a pothole or a curb usually cannot be trued straight. The app has no bent rims.
• Any pressure under the low number on the sidewall pinches on the curb. On a real bike that depends on the rider's weight and speed.
• There are no brakes, no front derailleur and no bent derailleur hanger.

Say the first one out loud. It is a good question to ask a class: why does the shop use the same rider for every ride?

Middle school and high school

The level is in the app's header. Middle school is the default: plain words, teeth, turns, meters, kilometers and miles an hour. High school adds watts and the push on the pedal in newtons on the Gears stand, how many millimeters the pulley is from its cog on the Shifting stand, and each spoke's pull in kilograms-force on the Truing stand. The tickets and the tests are the same at both levels.

For grade 5, use the Gears stand by counting: how many times does the paint dot go around for one turn of the pedals? Leave the decimals for later.

The Shop report, Setup and the answer key

Shop report is in the app's header. It lists the tickets the student has closed and asks nine questions, three on gears and two each on shifting, truing and flats. Hand in needs an answer to every question and says which ones are blank. Print gives the report on paper.

Setup is for the teacher. Untick the questions you do not want, add up to ten of your own, one on each line, and open the answer key. The setup is saved in the project, so set it up once, save, and hand the project out as the starter. Start over keeps the setup.

• Gears: a 36 tooth chainring and a 12 tooth cog. How many wheel turns for one pedal turn? (3)
• Gears: write a gear you pedaled and how far it rolled. How far on 10 turns? (ten times as far)
• Gears: is a hill less work in a low gear? (no: less push on each turn, more turns)
• Shifting: the chain is slow to climb. Which part, which way, and why not a limit screw? (barrel adjuster, out; the screws are only stops)
• Shifting: what does the L screw protect against? (the chain going into the spokes)
• Truing: the rim rubs on the left in one spot. Two ways to move it back? (tighten a right side spoke there, or loosen a left side one)
• Truing: what is wrong with tightening every spoke as hard as you can? (past the rim's range, rims crack at the spoke holes)
• Flats: two small slits side by side. What made them? (a soft tire pinched against the rim)
• Flats: a new tube is flat the next day. Two things to check? (the inside of the tire, and the rim strip)

With a real bike in the room

Turn a bike upside down or put it in a stand. Count the teeth on one chainring and one cog. Put a piece of tape on the back tire, turn the pedals once by hand and count the wheel's turns. Then roll the bike beside a tape measure for one pedal turn and compare with the app's number for the same teeth. Expect a few centimeters of difference: tires differ.

Keep fingers away from the chain, the cogs and the spokes while anything is turning. Do not adjust a bike a student will ride home unless someone who knows bikes checks it after, and check that both brakes work before anyone rides it.

Standards

• CCSS 6.RP.A.1: ratio language for the two tooth counts.
• CCSS 6.RP.A.2: unit rate, as wheel turns for each pedal turn and meters for each pedal turn.
• CCSS 6.RP.A.3, parts b and d: constant speed from pedal turns a minute, and converting meters a minute to kilometers or miles an hour.
• CCSS 7.RP.A.2, parts b and c: the constant of proportionality in distance equals meters per turn times turns, and how it changes with the gear.
• CCSS 7.G.B.4: the distance around the tire, as far as circumference.
• ITEEA STEL-8I (grades 6 to 8): use tools, materials and machines to safely diagnose, adjust and repair systems.
• NAEP Technology and Engineering Literacy D.8.17: diagnose a problem in a technological device using a logical process of troubleshooting.
• NGSS 3-5-ETS1-3 for grade 5, in part: fair tests in which variables are controlled and failure points are considered.

NGSS has no middle school performance expectation about troubleshooting or repair. These notes do not claim MS-ETS1 for the repair stands.

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