I was hoping the revisit would clear this up, but it didn't, so here we go.

So, they illustrated that if you go faster over bumps, less vibration occurs. But one variable was different every time. The duration of these vibrations.
They didn't really test whether or not speed does influence vibration. They just proved that if you go faster, the rough road you're driving over goes away faster.
They didn't test each speed for the same amount of time. I think this could be a critical flaw in their experiments.

I believe what matters here is 1) the mass of the car, 2) the velocity of the car, 3) the spring rate of the springs, and 4) the frequency of the bumps. At a certain velocity the inertia of the body in motion horizontally is balanced by the spring rate at the frequency of the of the bumps and the body resists vertical acceleration. If you've ever driven on a washboard dirt road, you'll have noticed that your car was smoothest at a certain speed.

Just to let you know that I live five miles down a dirt road. I almost stays a washboard. I can tell you from doing this every day. the faster you go, the smoother the ride. But there is a drawback. The faster you go the less control you have. Try turning just a little bit and it is a fight. You can float off the road if you are not careful. Now the slower you go the more damage you'll do to your car or truck. Trust me. I Replaced the front end three times and an axle on two trucks in the past two years. If they really want to do this Myth again get me a call. Like I said I live five miles down the red clay dirt road.

Gorilla, the man who knows.
There are several issues to deal with here. The first is a standard highway vehicle coping with a washboard dirt road as described by Mr gorilla, The veriations here are usually the stiffness of the springs and the shocks. Your dads old buick with the soft, mushy ride usually did pretty fair whereas Mr gorilla pickup with the stiffer springs tended to float, very much like hydroplaning. Make sure you slow down before that turn.
The other situation is the truly offroad, or "unimproved dirt road" as describedon some maps. These are the ones with the randomly spaced potholes and large rocks. At this point we should defer to the experts such as those that run the Baja 1000. All I can add is that they usually have long travel, soft suspensions, softer tires, and a lot of shocks.

Another thing that affects the ride is the "wavelength" of the impressions. I had a range rover... was taking it to the beach one day and the ruts were of a decent size but were formed in such a way that te crests were exactly in snyc with my wheelbase. Maybe three apart but nonetheless each time my front wheels were on the crest the backwheels were to. So driving I had to go very very slowly or the car actually bounced like teslas rig to bring down the building. This problem didnt happen to a mate just ahead of me (at first he was just ahead of me then a long way in front of me) in his land cruiser. He could float over the top just like the myth says. We worked it out to be the wheelbase being on a multiple of the "wavelength" of the ruts.

Very interesting, masterofstanley, I used to run into the same problem in my 18 wheeler on concrete highways where they had the seems too close together.

One time while meeting my parents on a camping trip in mid-eastern Idaho I encountered the PERFECT washboard dirt road. The washboards were very uniform and small, maybe a foot or so peak-peak and maybe 6 inches deep(all estimates of course). I was driving my stock 1967 Impala (back in 1988 I think). It felt like driving over a cattle guard, mostly a tooth rattling vibration and modest bucking. The faster I went the smoother it got. The ideal speed basically required Dukes of Hazard control on turns, but what the heck I was young!

This is a question for gorilla, since you live in washboard country. It is my belief the the washboarding of the road is caused by the road grader not having any springs and that the wasboard cuts are caused by the grader bouncing on its' big tires. Can you confirm this or, perhaps, give me an alternate explanation?

Sorry it took too long to reply. Internet down for a month.(living in the boondocks) I can answer the Question that mrg567 ask because I drive it every day. The road grader does not have springs but when he does make a run it is smooth as a baby's bottom. Now what cause the washboarding is when someone drives down the road at any speed the dirt moves. The down draft and out draft of the moving car,truck,SCHOOL BUS, and horses along with wind makes it look like a washboard. Have you ever been in the desert and seen how the wind forms the ripples. Same thing on a dirt road, But you have everyone and everything moving down it.

As far as the wine glasses filled with water, wouldn't the acceleration and deceleration, start up and braking parts make a significant amount of water spill?

The accelerometer was recording acceleration, both positive and negative. With more verticle displacement, the car would have more room to accelerate. With less displacement, the car would have less room to accelerate, have to accelerate faster, and thus the accelerometer would have a higher displacement.

mrg567 is correct in that at a certain speed, resonance is achieved. I don't know much about resonance and cars, but what about this idea? The tires hover for a small interval, before descending. If the time elapsed between the tires hitting the bumps is sufficiently less than that small interval of time, the ride will be smooth. If the time elapsed between the tires hitting each bump is much higher or lower than that will have a much rougher ride.

Any thoughts? Andy V

The method discribed for washboarding was way off from what we in the desert are used to seeing. Out here washboarding is caused by wind blowing across the dirt road causeing the errosion. This makes the washboarding less than uniform as shown on the show with some peaks being bigger and some being smaller than the norm. If you really want to test this the right way. Head to the desert and find a road that is exposed to alot of crosswinds, in California that should not be to hard to do. I believe you will get a better test result that making one out of metal.

One second of bumps at 5mph vs. One second of bumps at 40mph.

How hard can it be?

You would have to build the road they used in the show halfway through the experiment though.

I've also lived many years on dirt roads and I can say that it is a combination of the local drivers and the Road Commission(road graders and such).

The average secondary road does not have a crown on it (high in the middle). Michigan secondary roads have the same speed limit as the highways, so people don't slow down.

The Road Commission does not put down gravel to protect the road. If we had gravel, there wouldn't be the ruts in the road that MB talks about.
my 2c

The whole test was worthless, they measured suspension deflection which is INDIRECTLY related to the 'smoothness' of the ride. The wine-glass test is horrible science, too many variables aside from the roughness of the ride. Maybe use some accelerometers to measure forces perpendicular to the road over time or something similar, like, you know, real science...

I grew up learning to drive on dirt roads and you are correct, driving faster on washboard roads does make it feel smoother but your test was to short of a distance. Long, high speed washboard drives,especially over 50 mph, will cause your vehicle to go sideways and force you to slow down if you don't want to wreck!!!

I have a couple of suggestions.
1. It seems to me that the question isn't what happens to the car but what happens to the person. To that end, put the three axis accellerometor inside a human analog dummy, strap it into the passenger seat and take it for a ride.
2. Constant high acceleration feels like extra weight, not like a bump. Bumps are are changes in accelleration. Its the first derivative of acceleration that should be examined (a quantity that engineers refer to as "jerk").

I think someone sort of hit upon this, but not in detail. The wine glass test was...well bad and very uncontrolled. Although I'm guessing it was used more for the fun value. The big issue, mainly with the corrugated metal, was that the faster you go, the bigger the inital force when going from smooth pavement to the metal. Simple wooden ramps with a small angle at either end would have solved this. Watching the incar cam you can see the water dance upon the initial entrance to the metal and then die off quickly once the car is on.

The test was all wrong. They used a moto cross track to do the long run and a 10 foot set-up for the final test. edgehos said it right. on a real road it will be a lot differnt. The peaks will be at differnt heights and distance every time. A real dirt road does not and will not be the same everytime. The wind blows the ripples in there so everyday the road changes. You will get a smoother ride at faster speed but you will lose control a lot faster.

I want to explain my opinion on why their measuring devices were an issue on their own.
whew, here goes.

1.)the wine glasses

Alright, so the bsic idea makes sense. Shake a glass and water will fall out. However, there are, like others mentioned, too many variables on its own. Let's look at inertia. When a person or article is in a vehicle, the original acceleration of the vehicle will not be equal to the acceleration of the water in the glass. The need for the water to stay still make's its mass move backward in the glass, spilling it. However, after the car is moving for a while, the water is moving at a constant rate with the car. It will want to move the same velocity, no matter what happens. Technically, the water, if bumbed, whould remain basically vertical compared to itself. So what makes the water spill out of the glasses? Forces moving in other directions. As their camera shows, there is more forward/backward motion, causing more water to fall out, though the faster car might have had more water "jump" out of the glass that didn't actually spill. One more point, calmy swirl a glass, without spilling any water and continue to swirl gently. any change, though calm, could spill a lot of water.

2.) Shock monitor

The easiest way I can explain this is that just because there is shock movement, doesn't mean there is a direct relation to the cab. Quick analogy: jumping on a trampoline. If someone were to shake the trampoline, you mass on the trampoline would jiggle, giving you quite the shaking. However, the more you bend your legs everytime the trampoline moves (here your legs are your shocks) the less impact of the vibrations on your core or the rests of your body. Here, just because your shocks are working in a wide range of motion, you aren't feeling the effects yourself. So, just because we saw a lot of shock movement in the car, doesn't mean it will be uncomfortable. Back to the example of inertia, the car will generally want to stay in one velocity; one position. If your shocks were to not move out of the way of crests in the road, the whells would force the vehicle to move in an unpleasant direction. That is why we have shocks, so that they do the work, and not our cushions! Just try the same example on a tricycle.

I don't Have a lot of problems with the accelerometer. It is going to measure the movement up and down in the car. It is measuring what the CAB feels, meaning what you feel to. The more changes in direction are going to affect your comfortability.

Finally though, I agree with earlier posts that the intial banging into the washboard is going to affect the shaking as well. Moving into a washboard on a road is not usually the same affect as RAMPING onto one. It changes your inertia, and again will affect how much you feel in the experiment.

Sorry if this post is confusing or rambling.

A comment on the accelerometer and suspension deflection measurements:

Anyone who has taken an undergraduate course in vibrations or has a grasp on dynamic mass-spring-dashpot systems will realize that, unlike what the hosts said during the show, BOTH of the measurements confirmed the myth, they were not in opposition.

For a dynamic system (consisting of the two reference points of the wheels and the body moving relative to each other) with the same input displacement applied at different frequencies, the best shock absorption will occur at the resonance frequency.

Now at the resonance frequency there will be almost no force, and thus no acceleration (F=m*a), transfered to the car body from the input displacements (rough road), yet the displacement of the car from the tires will at that point also be the largest (because all the displacement input motion is being traversed by the wheels while the body rides steady).

It can be easily looked up, this is standard behavior of a second order dynamic system. Indeed, both of these systems say the same thing. . .

MYTH CONFIRMED