Paging Physicists, Bubbas... and everyone in between

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.

Washboard road


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. :frowning:

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…

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.

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 !

I have a lot of experience driving a 1962 Fiat 500 on washboard roads in West Africa, mostly in northern Nigeria, in the 1960s. My answer is: forget the math.

The best speed depends on two factors. One includes the mass of the wheels and the spring rate of the suspension. This combination of mass and spring rate determines how fast the wheels can move up and down, and it varies by make and model of vehicle, by tire size and inflation pressure, and by vehicle loading. You probably could find some kind of mathematical formula, but it would be so complex that you’d never bother with it.

The other factor is determined by the consistency of the road material and the. speeds and weights of the vehicles that travel on the road. Again, looking for a formula isn’t practical.

The wavelength (bump spacing) of the corrugations on most laterite (a reddish clay) dirt roads in Northern Nigeria back in the '60s was very roughly 2 feet. The amplitude (the peak-to-valley distance) was usually about half a foot, but was a factor only during acceleration and deceleration.

Any attempt to drive slowly over those corrugations quickly resulted in lost lunch. Besides, the slow speed was limited mainly by tire diameter, smaller being slower. For the Fiat 500, it was about 4 mph.

The trick for a smooth ride was to accelerate as rapidly as possible, toughing out the intense oscillations, until reaching the critical speed, at which the ride became perfectly smooth. In the Fiat 500, this usually occurred at 45 mph (indicated) and persisted up to 48 mph. Any faster and you started hitting the slopes of the corrugations again. No doubt double or triple the critical speed would have worked, too, but the 500 wouldn’t go faster than 58 mph on smooth pavement, so I never tested that idea.

As I said, the ride was perfectly smooth. But it was like driving on ice–because the tires were airborne most of the time, resulting in very little traction. Fortunately, the road builders had banked the turns nicely. Panic stops were a good cause for panic, although it was usually better to remain calm and keep on going. I only ever killed one chicken by adhering to that policy.

As an old moto-crosser I’m very familiar with whoop-dee-doos - the lingo of the day for washboards. Of course the faster you go the smoother they get - depending upon their spacing and your weight. Then if the spacing changes you need to adjust the speed accordingly. Often the required speed is much faster than is safe. Helmets, knee pads, chest protectors and gloves are required accoutrement. Faster is always better!

Amphibflyer is right on. I have come to the same conclusion, and would like to add two things; one, a good way to think of this simply is, the best speed for ironing out washboard is the speed at which the “suspension rebound response” precisely matches the periodicity at which the tires contact the washboard-tops, producing a condition of “harmonic cancellation” of tire contact over the entire range of road surface elevation variations. Of course, all bets are off on curves, where the synergy of high speed, centrifugal force and reduced tire/road contact dwell time causes a pronounced proclivity for four-wheel drift to ensue which is a thrill to the experienced driver but may cause unspecified stress reactions from the passenger(s).
Two, the original cause of washboard is the plasticity of pneumatic tires which, when encountering any deviation from flatness in the road surface, such as a pebble, deform to accommodate the inconsistency, resulting in a temporary shortening of the functional wheel radius ahead of and behind the point of flexion, i.e. there’s a bump in the tire ahead of and behind the “pebble dent”. The leading bump hits the road surface behind the pebble with more force than an unstressed tire would, and causes a slight depression in the road surface directly behind the pebble. That’s the beginning of washboard formation. The repetitive insult of multiple vehicles over time exacerbates any slight unevenness of the road surface, the washboard eventually conforming to the wheel radius of the majority of vehicles using the road. This is why very large tires also smooth out the washboard. If that large tire size were the norm, the washboard would be spaced out are you paying attention? It is worth noting that nonplastic wheels such as steel-rimmed buggy wheels, produce ruts instead of washboard.


Fortunately, the road builders had banked the turns nicely.

It’s possible, maybe even probable that the traffic created the banked turns by pushing the dirt to the outside of the turns.
I rode on a Snowcat once and the operator explained that skiers tend to push the snow downhill and to the sides of the runs and they have to move the snow back uphill and to the center.

An oversimplified spring-mass system would suggest a single degree of freedom response curve. Something like this: (note the log-log compression).

So, at slow speed the wheels and body follow the bumps, no suspension action. The peak in the curve is the natural frequency of the spring-mass system (vehicle weight and suspension springs). Here the body moves more than the wheels which follow the bumps, bad news for the passengers. But, if you survive this unfortunate excessive bouncing and keep going faster the response is lessened. If one goes faster than the natural frequency the motion of the body becomes less and less with more and more speed.

I believe the spacing of the washboard matches the natural frequency of the “standard” vehicle suspension and speed. Thus, a speed above the average drive’s is needed for improvement. The faster the better, well at least for the washboards. Running off the road is another concern, there’s always conflicting trade-offs. You’ll have to go faster on a rural road driven by crazy farmers (high average speed) than a tourist route drive by city folk (slow average speed).

Changing the suspension should help, i.e. off road racing vehicles with soft long travel springs. Thus a lower natural frequency.

Could be wrong, but that’s my story and I’m sticking to it. My wife, however, doesn’t agree.

Bubbas rules for driving washboard roads:

If your truck tires are the same size as your tractor tires, many of life’s problems go away

If the road you are driving on is flat and straight, check the sides of the road for stone walls, ditches and large trees, floor the accelerator and enjoy the ride - so what if the car fish tales like a trout out of water.

If the road curves and its at night, realize the washboards will help the car rotate easily. Floor the accelerator at the start of the curve and enjoy the ride - headlights will tell you if a car is around the curve. Washboards do affect traction on both front and rear wheels so make sure your speed going into the curve is not so much as to cause the car to drift off the road - unless its winter and there are snow banks. If it’s daylight and oncoming cars may be present, honk horn loudly before the turn

If the road goes up hill, floor the accelerator - you do after all have to get up the hill.

If you’re going down hill, washboards will make it hard to slow down, but the brakes on the bubba machine are poor to begin with so floor it and hope for the best.

Any time you should slow down for washboards? Yes, when you are arguing with your wife and she is drinking a hot cup of tea. Find the deepest set you can and roll them nice and slow - make sure the tea doesn’t hit the inside of your window.

Doesn’t it depend what you’re driving and what kind of shape it’s in?

If I was driving something made for off-road use, like a Jeep Wrangler, a pickup truck, or a truck-based SUV, I’d assume that because it was made for off-road use, it can handle reasonable speed. Like others have said, the best speed would be based on feel. What speed feels smoothest? I’m thinking something like 25-30 MPH (maybe even 40 MPH).

If I was driving a passenger car, or any vehicle that was not designed for off-road use, I’d drive as slow possible to minimize wear and tear. When I say “as slow as possible” I don’t mean at a snail’s crawl. Again, it would be based on feel, probably 15-20 MPH.

If either of these vehicles were in a poor state of repair, I’d drive slower.

Back in the early 1950’s there was a tremendous movie, The Wages of Fear. Part of the premise was the need to deliver a load of nitroglycerine over a corduroy road. The drivers were told that they had to drive at a specific constant speed to keep the nitro from sloshing and blowing up. At least in the movie, that proved to be true.

It is truly amazing the things which stick in ones’ mind regardless of probable utility. Fifty years ago, I came across an article in Scientific American on this very subject. The part I remember is there are two modes which provide relatively smooth travel on typical washboard roads. Namely low speed and high speed. Low being like five to ten miles per hour and high being around seventy miles per hour.
I actually had an occasion to test this when my wife and I were visiting Bryce Canyon and camping a few miles away down such a straight corrugated dirt and gravel road. I wasn’t going to take a half hour to get to the campsite at low mode so I hit the road full tilt seventy and voila, it actually worked. Of course at that velocity you are airborne at least half the time and are well on your way to becoming a ground effect floating vehicle and any time the corrugations were not relatively orthogonal to the direction of travel and/or the roadbed tilted right or left, we might as well have been skating on ice. It was phantasmagorical to careen down the road cocked at an angle and engulfing the environs and other vehicles in a haboob of trailing dust, obliterating vision.
I don’t know how far back SA archives go, but someone with access might be able to dredge it up.