Nothing pulls more weight than semi...

Dear Car Talk

Dear Car Talk | Jul 01, 1991

Dear Tom and Ray:

I read your response to the letter about which type of transmission would require the fewest repairs for towing a trailer, an automatic or manual. I was surprised to see that you recommended an automatic since you said the additional weight being towed would put excess wear on the clutch. But if that were true, why do semi trucks use manual transmissions with clutches? Surely, nothing pulls more weight than a semi.

TOM: You're absolutely right, Corrine. Trucks do pull a lot of weight, and most of them do have manual transmissions. The difference is that they have a lot more gears than your average car, and those extra gears are what save the clutch.

RAY: As we explained in our previous column, the clutch "slips" when you let out the pedal. A certain amount of slippage is necessary so you can get the car off to a smooth start. But you want to keep slippage to a minimum, since that is eventually what burns out a clutch.

TOM: We also explained that when you add weight to your car--like by towing around a three-bedroom ranch house--you need even more slippage when starting just to keep the car's engine from stalling.

RAY: Trucks reduce slippage by adding more low gears. If you want to see how this works, try an experiment. Point your stick shift car up a hill, and try to start the car in third gear (on second thought, try this on someone else's car). See how much gas you have to use and how much you have to let the clutch slip just to keep the car from stalling? Now try it in second gear. You'll see that it's easier--that you need less slippage. Now try it in first gear. The clutch slips even less, right?

TOM: Now imagine that you had a gear even lower than first--like "sub-first"--based on your experiment, you can predict that you'd need even less clutch slippage to get the car started. And that's how trucks do it. Truck transmissions have as many as 18 gears--many of them very low gears used to get the vehicle started. If you watch a truck take off from a dead stop, you can see the driver shift five or six times in the same space where you would shift just once. They save those higher gears for pulling right up behind you at 65 mph and sit??ting on your bumper!

RAY: Trucks also come with something your car doesn't have: a truck driver. A good driver who knows how to pull a heavy load can add many miles to the life of a clutch. Eventually, though, even with a professional driver and all those gears, even a heavy duty truck clutch will wear out.

TOM: And that's why the best thing you could have for pulling a heavy load would be an automatic.

RAY: They do make automatic transmissions for trucks. You just don't see them very often. That's partially because they're so expensive, and partially to prevent boredom. Think of it from a truck driver's point of view. When you have 600 miles to drive in a day, don't you want something to do other than just put it in "D?"

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