Bubba is not the best person to answer this question. Also a car is not the best vehicle to provide the answer. The answer comes from a half starved professional cyclist that needs to go as fast as possible over the most treacherous cycling terrain in the world. I’m talking about the dreaded cobble stones in the bike race Paris-Roubaix. Rather than guessing at equations for this or that, the equation is simple. Win the race (ala the fastest) win the money. To make this event even more interesting, lets remove the dirt, and replace that with boulders. Also, instead of a car, lets use a bike, a carbon fibre bike at that. Then lets hold this race when there is a 99.9% chance that it will rain, and the temperature won’t get much above 40 and do it in a cyclist kit, also know as underwear.
When you are pedaling as hard as you can, underwear in 40F tempuratures is about all you need to stay warm.
You may also notice that XC skiers wear little more than leotards, the first time I tried XC, I learned why.
Back in the 80’s I lived in a part of South America where gravel roads were like super highways. Everyone traveled fast over these roads. At the time I was driving a Toyota Land Cruiser pick-up truck which had a very rigid suspension and was very hard on the kidneys. As the scientists in this discussion have suggested, there are several variables that effect the harmonic of the vehicle’s ride. I found that the easiest to control was the weight of the cargo. Since hitchhikers abounded and traveling in the back of pick-ups was not illegal, I gave people free rides in a exchange for a smoother ride. Great discussion!
I think the problem here is that people tend to think their butt is the deciding factor - and I’ll contend they aren’t thinking about what is happening at the wheel end.
It’s clear there is a speed where the affect “at the butt” results in minimal forces due to the natural resonant frequency of the tire/suspension system - BUT - at the wheel end, the tire and the suspension are absorbing all those forces - and my contention is that is where the discussion ought to be revolving around. Ergo, slow is better.
I can’t believe there is even discussion about going fast on washboard roads. There is no speed is which it smooths out, that is also safe. Driving too fast is what created the washboard in the first place, and it only makes is worse.
On the dirt road I drive on to get to my house, there is a narrow part with a steep edge. Over the edge and down below, smashed into some trees there is and old milk truck, which has been there for about 50 years. The driver had gone off the road while trying to plane out over the washboard. No one ever had milk delivered after that.
I live in Alaska now where there are a lot of gravel roads and where even paved roads can be made rough in winter driving conditions for 6 months of the year.
I have found that you have to find an intermediate speed. Too slow and you hit every bump. Too fast and you will bounce into the ditch. the speed will depend on the roughness of the road.
This topic is central to the plot of the classic French movie “Wages of Fear”. In it, untrained truck drivers are hired to deliver a cargo of nitroglycerine over a washboard road to an oil well that is burning out of control. The drivers must maintain a constant speed of about 40 mph in order to minimize jostling the highly unstable nitroglycerine. In the movie the theory is that the trucks will jump from crest to crest but I question that assumption.
We have them on our dirt road hills and it’s caused by solid axle wheel hop and poor driving practices. Trust me; the faster you drive, the worse it will be for the next guy. A grader with the blade set deep enough will fix it but frequent truck traffic brings it right back. There should be very little of it near the edge and hopefully less on the opposite side of the axle. If you can’t avoid them, slow down. You just make it worse when going faster as wheel spin just makes the ruts deeper.
@CapriRacer: “It’s clear there is a speed where the affect “at the butt” results in minimal forces due to the natural resonant frequency of the tire/suspension system - BUT - at the wheel end, the tire and the suspension are absorbing all those forces - and my contention is that is where the discussion ought to be revolving around. Ergo, slow is better.”
I couldn’t agree more. However, suppose you’re driving on a remote washboard dirt road in some kind of truck-based SUV or pickup truck that is designed for such use. Some of the stock suspension components have been upgraded (heavy duty shock absorbers and springs). At the speed that is best for the tires and suspension, it will take you four bumpy uncomfortable hours to get where you are going. If you drive at the speed that is best for your butt, you’ll get there in two hours in relative comfort. Are you really going to choose four hours of discomfort over two hours of relative comfort? I’m not sure I’d have that kind of restraint.
Fortunately, most of the driving I do on washboard dirt roads is for short distances, like after I get off a main road to get to a secluded fishing spot or a campsite. Besides, my 1998 Civic isn’t designed for off-road use, so choosing the speed that is best for the tires and suspension is a no-brainer. I’m not sure the best answer to this question is one-size-fits-all.
Mythbusters did this on TV 2010,11,12 with champagne glass spillage and faster had lessspillage i believe
But as others have said a smooth ride isn’t necessarily a safe one. Skating across the peaks won’t give a lot of directional control.
This topic is central to the plot of the classic French movie “Wages of Fear”. In it, untrained truck drivers are hired to deliver a cargo of nitroglycerine over a washboard road to an oil well that is burning out of control. The drivers must maintain a constant speed of about 40 mph in order to minimize jostling the highly unstable nitroglycerine. In the movie the theory is that the trucks will jump from crest to crest but I question that assumption.
I question the entire premise of that movie because liquid NG is on the DOT’s list of “forbidden” substances, it can’t be legally transported without being desensitized.
Before the invention of dynamite, NG was made on site and it was the reagents needed to make it that was transported to mines, railroad tunneling sites, etc.
The interesting thing about fictional movies is that they are not beholden to science or reality. I’m not sure how a fictional story is relevant to this discussion.
Washboard dirt roads occur because all car manufacturers design the natural frequency of their suspensions to oscillate at 1-10 cycles per second. This is the natural frequency of walking and a “comfortable” frequency of vibration. Since all the car suspensions have the same frequency, once one car goes down the dirt road and compacts the the dirt at that frequency, all the other cars traveling down the road will also vibrate at that frequency, adding to the compaction of the dirt. (Given, we all drive about the same speed down dirt roads.) So, each car contributes to the compaction of the road and the wash-boarding of the road gets worse over time.
The last thing you want to do is travel so fast that your tires just touch the top of each crest. Think about it. The tires will be in contact with the crest of the bump for just a microsecond. If you want to brake or turn, will you be able to? No, because your tires are not in contact with the ground. I’ve seen empirical evidence of such attempts lead to sad results.
Riding the crest of the bumps on a washboard road is analogous to having no shock absorbers on a car. The tires vibrates at it’s natural frequency and barely touches the ground. There’s no traction for steering and braking.
Nevertheless, there is a critical speed at which a cars suspension cannot react quickly enough for the tires to maintain contact with the washboard road. This speed will depend on the characteristics of the spring and shock absorber of the car and the frequency of the washboard road. If you’re traveling at the speed of light, then you’ll just touch the crest of the washboard. If you traveling very slowly, the tires will completely conform to the road. Somewhere in between, the tires will lose partial contact with the ground. At these speeds your teeth rattle.
All this may be mathematically modeled using a simple mass - spring - dashpot system to describe the automobile dynamics. I’d do it, but I gave up such nonsense years ago and am now trying to learn to play the guitar.
@GetReadyMan Right on! This is a classic physics problem and takes a lot of calculations to include all the variables. Serves no useful purpose.
@Docnick, Thanks! Something like a second order differential equation with a sinusoidal forcing function ought to do it. I used to eat those for breakfast, but that was a long time ago.
There are two completely distinct issues here: actual control of your car, and the feeling of control. Driving quickly on a bad, washboarded road may give you the feeling of better control, but it’s an illusion, just like driving after a few beers gives you the illusion of being more in control, and we all know how risky that one is.
The problem with driving quickly on bad roads is that the road can/will suddenly change unpredictably, and the faster you go the more likely you are to lose whatever control you think you have. The change can be a grade change, a curve, a pothole or washout, or a large animal (or person or vehicle) wandering into your path.
You have many other problems with driving fast on a poor road: your dust lowers visibility, you increase the chances of putting a rock through your windshield (or someone else’s), you may overstress your tires, you are probably damaging the road more, and braking distances are likely to increase on washboards.
Anyway, your life (and that of your truck) is too important to trust to free advice or speculation you hear on the internet. Slow down to a reasonable speed.
Drive slow ,speed is what kills the road to start with- as the happy owner of a public right of way ,through my scant “pencil scratch” of land,I can tell you that inconsiderate people ruin the road for everybody,the dust chokes the people up and the dust is actually the road surface drifting away,my driveway is now down to the larger aggregates now ,due largely to this behaviour(and the people that do this dont fix it either) plus tandem trucks will create washboards in short order-SLOW DOWN! everybody doesnt own an off road vehicle or Baja racer-Kevin (axle tramp contribute too)
The true answer is the same as for driving on ice. Drive at a speed that allows you, with your driving skills, with your vehicle’s capabilities, to maintain control.
I have navigated wash boarded roads for some 35 years now. The vehicle, including how stiff the frame is, how the suspension is designed, i.e. Stiff vs soft ride, the tires you have, plus how bad the washboard is all enter into the equation. You have to try several speeds to find the best speed.
My Dodges all handle washboard very well. So does the wife’s Subaru. I can go very fast on washboard. On the other hand I have ridden three Ford trucks on washboard, all of which will vibrate your molars out if you go faster than 10 mph. The worst was a super duty. I thought the truck was going to vibrate apart.
So the answer is that there is no answer.
I know you guys supposedly “retired” from new shows last year… but with the current promos and inserts, I can’t tell whether I’m listening to a “new” or “used” show… at least during my casual listening.
IN CASE you’re still doing “NEW” stuff, regarding your “current” request for Washboard Road driving feedback - YES YOU CAN DRIVE PEAK TO PEAK. A few years back I was driving the ex-wife and two of our friends to Chaco Canyon in the Southwest, and there are about 30 miles of washboard to get there. When I first got up to the ideal speed in the Pathfinder (about 50 MPH), I scared the crud out of my passengers. So at their request, I slowed down to the point where the washboard was otherwise tolerable… I think between 15-20 MPH. AND it was not nearly as comfortable. After less than 5 minutes of this, the passengers requested I return to “flying” over the peaks.
CAVEAT: THIS IS GOOD ONLY FOR STRAIGHT UNIFORMLY WASHBOARDED ROADS. Since you’re spending so little time with the tire actually contacting the road, you have to realize that anything but gradual turns are NOT advisable. And stops need to be gradual.