Washboard Roads: Fast or Slow?

Dear Car Talk

Dear Car Talk | Oct 01, 1996

Dear Tom and Ray:

This might be a silly question for you asphalt cowboys, but for my wife and me it has caused a bit of contention. We live in the country, three miles down a sand-and-clay road. The road is usually in a condition we call "washboard." The wife and I differ on how we drive it. I say when the road is real bad, you go slow so as not to cause any undue damage to the car.

She says to drive over it as fast as safely possible. That way, the car rides over the top of the washboard bumps and does less damage than by going slowly over each hump. What do you say?

-- Ron

TOM: I think the answer depends on whose car you're driving, Ron. If it's your own car, go slow. If it's your wife's car, run the rapids!

RAY: Logic would dictate that you should go as slowly as you possibly can over bumps. The more you "bang" your car around, the sooner things loosen up, break and fall off. Makes sense, right?

TOM: But I suppose that under certain conditions, your wife's theory may be correct. I mean, if the distance between the bumps were just right, and the speed of the vehicle were just right, you could conceivably go from top-of- bump to top-of-bump, couldn't you?

RAY: Yeah. Evel Knievel tried that at the Snake River Canyon. Remember that?

TOM: Well, let's assume -- just for the sake of argument -- that the tops of the bumps are two feet apart, OK? In order to minimize damage, you have to be airborne from the top of one bump to the top of the next. If you lose altitude, and you crash into the rise of one of the bumps, that's like a hitting a curbstone, which we can all agree is bad for the car.

RAY: OK. So if you remember your high school physics, you know that y=1/2gt squared[DB1] _ where y = your loss in altitude, g = your downward acceleration due to the force of gravity, and t = the amount of time you're airborne.

TOM: So the faster you're going, the less time you remain airborne, and the less altitude you lose. You with me so far?

RAY: Sure. So according to that formula, at 60 mph, for example, between the two-foot washboard peaks, you would only fall about 1/10th of an inch. Not bad, right?

RAY: Tell your wife to go for it, Ron!

TOM: No, don't tell her to go for it. That assumes that a) all of the washboard bumps are precisely the same height, b) they're all exactly two feet apart, and c) you don't kill yourself doing 60 on a gravel-and-dirt washboard road. And our theory also doesn't account for the action of the springs, which push the wheels down toward the ground in addition to gravity.

RAY: And if we're wrong, you'll be doing the equivalent of bashing into a series of curbstones at 60 mph. But I think it might work.

TOM: Our lawyers are insisting that we tell you in no uncertain terms to go slow, Ron. But we'd love to hear from physicists and washboard-road drivers. Does every washboard road have an optimum speed? Write us and tell us what you think. I'll be glad to test any and all theories with my brother's car!

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