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Paging Physicists, Bubbas... and everyone in between

edited September 2013 in The Show
This week on Car Talk, Tom and Ray pondered one of rural America's great head-scratchers: How best to drive on an unpaved, washboarded dirt road? That is, those country roads that feature annoying cross-wise ruts.


Should you go very slowly, so as to follow the peaks and valleys of the washboard... or fly along atop the crests of said washboards?

Tom thought it makes sense to take it slow, even if it means getting carsick en route. Ray thought there was an ideal speed, at which one could cruise along the top of said washboard. But, of course, he had no idea what that speed might be.

You can hear their pontifications on the topic, including a discussion of Albert Einstein traveling at the speed of light, right here.

What's your theory? We want to know!


  • I usually just try to put my wheels on the smoothest part of the road, sometimes the shoulder and the middle has smooth dirt.
    What I want to know is what causes the washboard effect. It always seems to happen, sort of like moguls on ski slopes. they aren't artificially created, they just eventually happen from everybody skiing down the hill.
  • Country roads, nothing! That looks just like the highway running from where I live at least the first 4 or 5 miles of it. After putting up with this for several decades I've come to the conclusion there is no "best method" as both lanes are like that. It's mostly wallow around, hoping to hit less offensive spots due to sheer luck, and beating every profane word, along with the car, to death.

    That washboard is caused by poor construction practices; mainly grader blades chattering from taking too big a bit or going too fast.
    Some graders are even running chatter bands on the blades but it doesn't mean anything if they're digging too deep and/or have the machine at top speed.

    A brake rotor or drum being machined on a lathe will also chatter and have a coarse texture for the same reasons; too much being cut off at one time, cut too fast, and no chatter band being used.

    The powers that be finally resurfaced the highway here within the last few days and it accomplished absolutely nothing. The roadway as of today still reminds one of driving down the railroad tracks; sans rails. :-(

  • You can sometimes go fast enough to stop the suspension from bouncing you around in every single valley. The speed you need varies with the size of the washboarding. The problem is, the size varies constantly. Also, you're essentially floating on just the peaks of each bump, at speed, so it's not terribly safe.

    Washboards suck.
  • I understand that even if you start off with a perfectly smooth road, wheels rolling over it will eventually cause the washboarding. Smooth gravel and sand is the worst. Crushed gravel that's a mix of coarse and fine will interlock and resist this effect.
    It even occurs on railroad tracks, called "roaring rails" and I have seen corrogated surfaces develop on roller bearing races.

    If I had several miles of this road that I had to drive on daily, I would consider getting an on-board air compressor and deflate my tires to about 15 psi to drive that stretch of road and then use the onboard compressor to reinflate my tires back to normal when reaching the paved road. That way, the tires and not the suspension system absorbs the bumps.
  • I drive very slowly OR very fast! Nothing in-between. In the 60's (gasp) that was my theory driving my British racing green MG until the day an axel broke and I ended up in a field. Now I am in a mini-van and slow is the way to go. Plus there is a lot more traffic.
  • Asked and answered by the geeks from Mythbusters

    Driving faster can make it smoother. Let the suspension absorb the bumps.
  • @B.L.E. IMHO, I would be a little wary of dropping to 15 psi to let the tires absorb the bumps. They share that function with the suspension now. Letting air out takes that away from the shocks and springs. The springs can take it. Shocks are cheaper than tires, 4 struts however....
  • edited September 2013
    I used to take my pickup truck tires down to about 15 psi before driving in deep sand. That way, the tires float on top of the sand instead of sinking in and I don't get stuck. Naturally, I reinflated them back to normal before hitting the paved roads.
    Most tires don't even begin to look like they are low until you get down to 15 to 20 psi and there are probably a lot of cars going down the highway with tires that low and the drivers don't even know it because they never check their tire pressure.

    Another plus for soft tires is that they contribute less to making the washboard worse than it already is.
  • edited September 2013
    The best speed is dependent on the stiffness of your springs mostly. I grew up in the country with those washboard roads. With a normal US sedan, 30 mph was a livable speed, although 50 mph gave a smoother ride, but that would be 20 mph over the speed limit.

    Here in the mountains we have open range grazing, and there are many "Texas gates" which are gaps in the road filled with round pipes far enough apart to discourage cattle from crossing them. There are narrow strips of solid steel to drive over with the average car. Small compacts with narrow treads have to take the bumps
  • For washboard roads there is only one way to drive on them. There is one speed that is optimum. You have to go fast enough not to jar your molars, but not too fast that you fishtail and lose control. It's a balancing act that is a lot of fun (like a demolition derby) unless you have a nice car and not a truck. Usually you have to go fast enough that the dust cloud doesn't overtake you. LOL !
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