get it in gear

Part VI in The World’s Best Mechanical Engineer Explains It All for You series, with Special Guest Star: The World's Best Mechanical Engineer.

Joe-Henry,

Q. How do gears work?

A. Gears are an entire science of their own. They have to be designed and manufactured very precisely in order to function properly. The little bumps on gears that push on each other are called teeth.

Gears are used to speed things up, slow them down, make them more powerful, less powerful or to change the direction of rotation. But the thing to remember about gears is that there are no free rides. If you use a big gear to drive a little gear, the little gear will spin faster than the big one. But it will have less torque. Torque is a way to measure how hard it would be to stop something from spinning, like if you squeezed the shaft between your fingers.

If you use a little gear to drive a big gear, the big gear would spin slower than the big gear, but the big gear would have more torque. So more speed equals less torque, and less speed equals more torque. There is always a trade. Welcome to the first law of thermodynamics.

Gears are also used to change the direction of your rotating power. Older rear wheel drive cars had the engine in front, a drive shaft running down the middle underneath the car, and a gearbox attached to the axle. That gearbox is called a differential.

This brings me to the experiment. Normally I wouldn't suggest an experiment of this magnitude, but in our current situation I don't think it will happen any slower than the other experiments.

Look on E-bay and find yourself a 1966 Ford Galaxy 500 convertible. Have your Mom buy it. She may balk a little. Here are your sales points:

• It won't go down in value, it's collectible.

• She'll look great with her hair flowing out behind her and the top down.

• It's so mechanically simple, you can fix it for her.

• By the time you're 16, gas will be so expensive you'll only be able to drive around the block once a week. That will help keep you out of trouble.

• It's the engineers’ choice.

You'll also need a pair of jack stands, a jack and of course, a case of Mountain Dew in cans. Take a spin around town with the top down, and a Mountain Dew in hand. Your Mom will feel SO GOOD she'll realize she should have listened to all of B1-67er's engineering advice. Tune the radio to an oldies station, preferably something with some soul. If you hear any song by the band "WAR" (the World is a Ghetto, Low Rider, Summer, Me and Baby Brother, etc, etc) you will reach Engineering Nirvana. Ask your Mom to spin by Radio Shack for some of that motor wire.

When you get home, have your Dad or Mom help you put the rear end of the car up on the jack stands. Set the case of Mountain Dew under the car too for added safety and to get the Mountain Dew into some shade. With the emergency brake released, spin one of the rear wheels of the car. Surprisingly, the other rear wheel will turn the opposite direction. That's because the differential (the gearbox at the back of the car) is a very special gearbox that allows the two back wheels of your car to travel at different speeds. This makes a car drive smoothly around corners. If you think about it when you turn a tight corner in your car, the two wheels on the inside of the corner don't travel very far, but the two on the outside of the corner travel a long ways. If it wasn't for the differential, your car would kind of have to bounce and skip around corners.

Pop open a few Mountain Dews and admire your Galaxy glimmering in the sun. And here's a little tip for when you're 16. If reverse isn't working too good in the galaxy, don't park with your girlfriend with the front wheels pointed downhill against a parking block. If you don't believe me, ask B1-66er: he tried it.

There is an alternative to the admittedly costly but rewarding Galaxy experiment. You can stop by Midas and ask them if they have any rear wheel drive cars up on their lifts. If so, ask them to spin a back wheel for you.

— The World's Best Mechanical Engineer

Also in this series:
• busta dew
• holy hydraulics, batman!
• solenoid spectacular
• springs & things
• the world's best mechanical engineer explains it all for you

[Photo credit: b1-67er, aka The World's Best Mechanical Engineer]