The only way to tell you’re listening to old calls is the age of the cars being discussed. Listen for the model year, and that will tell you whether you’re listening to a recycled call. I’ve given up on the show, but my girlfriend still insists on listening to it, and they seem to be discussing cars made in the early 1990s, so those are probably old recycled calls.
The cause of washboards is the same as what is called “the butterfly effect”. The butterfly effect – described by chaos theory – is that it is possible a butterfly flaps in wings in Algeria, and a rain storm a month later hits Hawaii. The claim of chaos theory is that it is perfectly possible that if the butterfly in Algeria wasn’t there flapping and disturbing the local wind currents, which would then disturb Algeria’s wind currents, then Europe’s wind currents, then the world’s winds current, well, no butterfly in Algeria, than no rain in Hawaii a month later.
Same with the washboards in the roads. Starting with a graded road, smooth for the moment, a small pebble, bump, or depression causes the first car on the road to bounce on its suspension, forming slight depressions and bumps down-road. The next car encounters the intial disturbance and the effects of the first car, causing its suspensions system to bounce, reinforcing the effects of the first car. etc etc.
There’s a similar description of how windrows form in fields and snow.
In my opinion and experience it is essential to speed up when driving on unpaved washboarded roads. My experience is in driving highways in the high Andes, the old Cooktown Highway from Cairns to Cooktown, North Queensland, Australia (has now been largely paved), and the Leeup Road to Grand Falls Arizona among others (in 4WD vehicles). The reason for higher speed is to prevent ruining your shocks and getting boinged out of existence and increasing the stability of the ride. It’s a bit scary getting up to speed and slowing down and you find a speed that is most comfortable but maybe not much more than 30mph. At lower speeds the tendency is to slide and fishtail. Under some conditions it is near impossible to drive safely even in 4WD; e.g. driving over washboarded “Bull Mud” in Aussie once my wife demanded that she get out and walk rather than risk the dangers of going over the edge in parts of the Great Dividing Range
and talking about The Wages of Fear, what about the scene when truck #1 blows? all you see is the flame that Yves Montand is lighting his smoke with suddenly extinguish! Great Cinema!
The fast/slow question is kind of a no-brainer. Stopping distance on a dirt road is much greater even if you don’t do something stupid. If you don’t want to significantly increase the risk of having an accident, slow down. You need to have all four (or both when on a motorcycle) wheels on the road surface with constant pressure as much of the time as possible.
But the next obvious and related question is: how can washboarding be reduced or prevented? As with any other problem, understanding the actual cause is critical to finding a solution.
Washboarding is not caused by poor quality road grading - but it can be reduced by corrective driving and road grading.
There are a couple of significant symptoms of washboarding that help us understand the problem.
First is the fact that they tend to be in the uphill lane. Second is on a given road, they consistently appear with the same distance between peaks in the same locations. The uphill relation is because they are related to the torque applied to the driving wheels, which is greater when going uphill. A smooth freshly graded road is not perfectly smooth. There are slight irregularities that are not evident at first. When the drive wheels of a car going uphill go over an almost imperceptible bump (possibly just a stone below the smooth surface of the graded mix of clay/san/gravel), the wheel lifts slightly and spins forward very slightly before it settles down again. The wheel will also tend to bounce once or twice while the shocks are reducing the amplitude of the bouncing. Each time the wheel is returning to its normal pressure on the road, a little bit of the road surface is kicked to the rear creating a slight rise. Thus the first almost imperceptible bump is transformed into two or three similar bumps that are as wide as the footprint of the tire. The next car approximately in the same track encounters this series of bumps and the similar behavior of its drive wheels extends the series, widens, and amplifies the bumps with the same distance between peaks because the speed and bounce frequency of most cars on that part of the road is similar.
So – prevention: Washboarding can be significantly reduced by all drivers always slowing down on the uphill portions of dirt roads. And that’s not just slowing down driving over washboards – it also means slowing down to stop causing washboards to appear. The faster you drive uphill, the sooner washboards will return and the faster they will get worse.
Now – about corrective grading: Existing washboards have to be totally destroyed with grading to prevent and/or slow down their reappearance. This means grading the washboards deep enough to break up their structure in the road mix and thoroughly re-mix the clay/sand/gravel. If the normal grading practice of pulling the road mix back in from the sides to build up and put a smooth crown on the road is followed, the existing washboarding is lurking just below the graded surface and will begin to reappear as soon as the mix between the washboard peaks is packed down by the normal traffic.
A related dirt road problem is potholes – which I like to call vertical porosity anomalies. Porosity is the quality of the clay/sand/gravel road mix that lets water drain downward to make it less muddy. If there is a small area in a well graded dirt road that retains water slightly more than the surrounding road mix, the moisture makes it slightly softer so that a tire rolling over it creates a slight depression and every other passing tire packs it down just a little more thus making it even less porous. The next rain fills the resulting depression with water and every tire passing through it makes mud and forces some of the fine (clay) part of the mud downward into the mix below making it even less porous - kind of a waterproof bowl. Some of the mud in the bowl splashes out and deepens the bowl so it holds even more water in the next rain, continuing the growth of the pothole. The next superficial grading fills the bowl with some fresh road mix which also turns to mud with the next rain and the pothole reappears in the same spot even larger than before. The way to end this pothole persistence is similar to the corrective grading for washboards – i.e. grading deep enough to destroy the less porous bowl and replace it with a uniform road mix to eliminate the vertical porosity anomaly.
Good road graders are equipped with ripper/scarifiers – rows of long claw-like teeth at the rear of the grader that can be lowered below the depth of washboards and potholes to destroy them and facilitate re-mixing the road mix to prevent or at least slow down the return of these problems in the same spots.
(And BTW – the picture used at the top of this page is obviously not washboarding on a dirt road – it looks like farm vehicle tire tracks over a field with ridges made by some farm implement – possibly a disc harrow.)
I was listening to your discussion about driving on a road with even but continuous bumps. You are absolutely right about driving fast but at a particular speed. I am not sure about the equation you conjured up. But I can tell you from practical experience it works.
I was working in civil engineering construction in Sri Lanka many years ago as a young engineer, where the temporary roads were just gravel covered surfaces. When the heavy trucks carrying full loads on these roads they created evenly spaced ridges on the road almost a sine wave (just like the picture above). We traveled on Jeeps to the work-sites etc. and our driver knew exactly what speed to drive at so as not to shake our bones off the flesh and of course the Jeep suspension.
So, I don’t know if you can derive an equation with so many variables, but you can certainly prove your theory by experiment.
Hope this helps, love your program.
With all due respect to the physicists and Bubbas in the audience, the best person to answer this question is someone who has been driving on said washboards since before false teeth cement was invented. Having lived on a gravel (pronounced “dirt” in Minnesota) road for 35 years where even our half mile driveway turned to washboard in midsummer, I can only hope that I come close to qualifying. Everyone who has ever driven on a washboard will quickly tell you that the “normal” speed for that road will tear the car apart in a New York minute (which Johnny Carson once said is the time between a Manhattan traffic light turning green and the guy behind you honking his horn). Simple logic would seem tell you that driving slowly up and down on the accordion ridges and valleys is the safest alternative and easiest on your car. However, the collateral damage from the resulting road rage that develops from having to travel at such a snails pace makes the high speed option the much better choice. The slowest possible high speed is mainly determined by the distance between the ridges. When one is in doubt I have found that anything over 80 miles an hour works just fine. The main thing to keep in mind is that this is somewhat akin to hydroplaning in a heavy rain storm except that there is normally nothing soft and wet to cushion your stop when you leave the road. I should probably add that this should only be attempted by mature drivers. One family in our rural neighborhood has had two (of their five) youthful drivers end up upside down in a marshy field on our road (which, as luck would have it, was soft and wet). To our credit this happened to only one of our children and we had nine so our per capita rate was much better.
Having both a PhD in Physics and considerable experience with washboarded roads, I have evolved a theory which covers their formation and the best way to drive over them. Most of my washboard driving has been in Lassen County, CA, which has firm Basaltic soil. The ridges on roads here are typically 8”- 10” crest-to-crest, and the pitch (or wavelength, if you prefer) is very uniform. I noticed some years ago that there would be a particularly poor speed choice for a given road which would maximize vibration of the vehicle suspension. Driving either slower or faster than this speed gave decreased vibration. It occurred to me that the suspension system of a vehicle is a mechanical oscillator whose resonant frequency is determined by the spring constants of the springs and the unsprung mass of the vehicle (wheels/tires, axles, differential(s), etc.). Although the shock absorbers provide damping, the resistance of the shocks is chosen to provide critical damping of the vehicle body/payload, the suspension, having the same springs but less mass, has a higher resonant frequency and is probably not critically damped. The “bad speed” for a given road is the one which applies vibration to the suspension at its resonant frequency. Most modern vehicles probably have similar suspension resonant frequencies, since manufactures what the handling to feel familiar to drivers.
Suspension resonance also suggests a reason for the uniform pitch of the washboarding. The vibrating suspension will produce washboarding with a pitch P = V/ F where P is the pitch of the washboarding, (let’s use inches as units), V is the vehicle speed (inches/second), and F is the suspension resonant frequency (cycles/second). Ironically, the vehicle speed which formed the washboarding is also the worst speed at which to drive over it.
Some consequences of this theory:
- Washboard pitch increases with the speed of the vehicle.
- If vehicles on this particular road drive at a wide range of speeds, pronounced washboarding will not occur. In practice, I suspect that most drivers stop accelerating when vibration becomes pronounced, hence most drivers drive at the same speed.
Some things I have not figured out:
- Does the wheelbase of the car make a difference?
- Since the tires are striking only the tops of the ridges, why do the ridges get bigger rather than being worn down? At least on our roads here, the washboard pitch is too narrow for the tires to settle into the bottom of the washboarding.
Just slow down,thats all you have to do.( as the unfortunate sharer of a right of way on my miniscule property) drive slow it works-Kevin
The answer to this question is in how the washboards form on a road, from multiple vehicles driving FAST on gravel roads, is how the washboards form. SO BY DRIVING FAST OVER THEM YOU ARE MAKING THEM LARGER AND THEREBY WORSE!!! I have spent my life driving, plowing, and grading gravel roads in Montana. If you slow down on gravel you will have both steering and braking control, if you drive fast you have neither. A gravel road is made up of sand, gravel and large aggregate or rocks, many have been crushed and ground to be rather round in shape and by driving fast you make the larger rocks pile into rows and the wind from the vehicle blows the finer sand off of the road creating the washboards. when a road washboards you are essentially driving on 1/8 of the road surface and that 1/8 of road surface your tire is contacting is the same as driving on marbles. in conclusion THE FASTER YOU GO THE WORSE YOU MAKE THE ROAD AND THE LESS CONTROL YOU HAVE TO AVOID AN ACCIDENT. If you must drive fast on these roads do not ask me for a tow out of the ditch when i come by in my grader.
I’m a mechanical engineer and I live on a dirt road in Michigan. There is a long section of this road that gets a washboard pattern quite often, and the worst part right in front of my house.
When you drive a car over a washboard, as you speed up from a stop, the oscillations that you feel, increase in magnitude until the speed of the passing waves matches the natural frequency of the suspension and wheels under the car. At this speed, the oscillations of the car body are the most intense. As your speed increases beyond that speed, the ride gets smoother the faster you go. The waves are not perfectly perpendicular to the road, and they’re not all exactly the same size. If you really wanted to, you could measure them, find the average wavelength, and calculate the resonant speed, but only if you know the mass properties and spring rates of your car, which few people do. It’s not worth the effort. You can just drive at different speeds as see what works. If your only concern is to get the smoothest ride possible, just go as fast as you can.
There are complicating factors though. When your tires are skipping over the tops of the ripples, on a dirt road, you have very little traction. In my case, I need to slow down to turn into my driveway on the worst part of the road. So if I hit the washboard going fast, I need to slow down through the very rough speed range to get slow enough to make the turn. I have a Honda Insight, which has a pretty high natural frequency because it’s very light and hugs the road pretty well. I also have a Toyota Tacoma, which is heavier and has a lower natural frequency. I can slow through this rough speed range in my truck, but if I try it in my car, there is a range where the ride is so rough that I have no steering control, and the car moves in bad directions. I’ve never ended up in a ditch, but it is conceivable.
So my overall recommendation is that if you’re driving a heavy vehicle with a nice suspension, like most SUVs and trucks, go ahead and drive 50 mph or so over a washboard road if you have good visibility on a straight road and little risk of animals or kids running out in front of you. If you have a small car that’s fun to drive on smooth roads, you just need to slow down. It’s not unusual for me and other people driving by my house to go less than 5 mph when it’s really bad.