Going Down Fast!

Apr 21, 2008

RAY: This was sent in by Tom Ireland and I had to mess around with it a little.

He writes:

"Where I live there are quite a few hills, many of them very steep. Some of them are quite a challenge for any vehicle, even those that are high performance. In fact, a few of these hills are so steep that I often have to ride up the whole way in low gear… especially if I happen to catch a red light at the bottom, like I did the other day.

A few days ago I found myself going not up one of these hills, but down a long steep grade. I found myself gaining speed at an alarming rate. Shifting even into the lowest gear didn't help at all and I had to apply my brakes almost the entire way down to keep from going so fast that I'd lose control and crash.

The brakes got really, really hot, but I was able to stop safely at the red light at the bottom of the hill. Here's the interesting part: There's nothing wrong with my vehicle. It's in perfect working order."

So the question is, why did I have to use my brakes to maintain a safe speed going down that hill? And why, despite the fact that I put it in first gear, did it not slow me down in the slightest?
RAY: I thought I did an admirable job of obfuscating this. Here's the answer: When you shift your car into low gear, you are using the inertia - the mass of the engine to slow the car down. So when you're in first gear for example, you are asking gravity to pull the car down the hill, and you're asking that gravity to turn the pistons and the crankshaft and all of that stuff. And that's more likely to slow the car down, better than being in second, or third or fourth gear.

TOM: Yeah.

RAY: However, that will not work with all vehicles. Like bicycles. So if you are riding a bicycle, it makes no difference what gear you're in going down the hill because there are no pistons or crankshaft. You are the engine and you are coasting and the only thing that can slow you down going down the hill --

TOM: Is the brakes.

RAY: Or a tree! A tree works really well. So who's our winner?

TOM: The winner this week is Beverly Levine from Greensboro, North Carolina. And for having her answer selected at random from among the pile of correct answers that we got, Beverly's going to get a 26-dollar gift certificate to the Shameless Commerce Division at cartalk.com, with which Beverly can get a copy of our brand-new four-CD set called Car Talk's Field Guide to the North American Wacko.

RAY: Congrats, Beverly!

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