Washboard Roads

[BobP]

Fascinating stuff

The phenomenon you refer to is known as "washboarding," a wave-like pattern on unpaved roads that might more aptly be called speedbump hell. As you've observed, the ruts occur with striking regularity, belying a chaotic event like erosion.

According to Tom Pettigrew, a Forest Service engineer, the cause is an unlikely source: your car's suspension. (Well, maybe not yours specifically, but it's not innocent in this matter, either.) A vehicle's suspension system distributes the shock and energy of road irregularities with a bouncing rhythm called harmonic oscillation. At each downstroke, the wheels exert extra force on the road, causing the particles in the road to either pack or displace at regular intervals. Once a pattern of ruts starts to establish itself, it becomes self-reinforcing due to what engineers call forced oscillation. The next car hits the same irregularities in the road and bounces at the same rate, causing the pattern to become more and more defined. Forced oscillation overcomes minor variations in oscillation rate that might otherwise arise due to differences in car weight.

Wouldn't variations in speed affect the washboard pattern? Sure, which brings us to another critical part of the feedback loop: you, the driver. Drive too fast on a washboard road and the downstroke exerted by the car wheels may meet the road at a point where a bump is ramping upwards. You know what that means: You bounce off the ceiling. Instinctively most drivers slow to a speed at which the downstrokes coincide with the troughs between bumps, reinforcing the pattern.

Washboarding is inevitable in any unpaved road that sees fairly heavy traffic. The only way to avoid it is to: (a) radically redesign how automotive suspensions are made, (b) give up suspensions altogether, or (c) keep off those dirt roads.

[chumley]

I don't have the links anymore (or time to look them up) but its a phenomenon that is caused by a spinning wheel on a dry, loose dirt surface.

I remember reading a study showing that a wheel that spins (while traveling) will always cause them to form given a loose surface and a change in direction. Over time, the effect is exaggerated. The study I read mentioned nothing of suspensions, though I suspect that it could also be part of the exaggerating effect.

One of the keys was that the effect occurs while turning. Washboards do not form as easily while wheels are traveling straight.

Picture this ... a merry-go-round on a beach or sand playground. Attach a bicycle wheel to the outside of the merry-go-round so that it is resting on the sand. Spin that merry-go-round so that the wheel is rolling in a circle around it, and washboards will form. If the ground is wet or firm, it won't happen. The more loose the surface is, the more quickly the washboards will form.

Roll the same wheel straight down the beach, back and forth, and the washboards won't form.

Can you imagine being the scientist who studies this stuff? Sets up experiments, etc.?

[joebartels]

In Nebraska we had about a twenty mile drive to the farm, ten on dirt roads. Seems everyone drove those roads like a bat outta hell, even grandma. When you'd slow down the vibration was worse. Well with the exception of '81 or '82 when the rabbits were as thick as the white fly epidemic we experience here years ago.

[chumley]

I question the suspension explanation above as well. Since vehicles have different wheelbases, nearly every vehicle that passes has one set of wheels that is a different distance away from the second set of wheels. That would mean that if the front wheels were acting on the washboards, the rear wheels in one vehicle may also be helping to build them, while in 9 out of 10 other vehicles with a different wheel base the rear wheels may actually be destroying the washboard.

So it makes sense if the same vehicle drives the same road, but if multiple different kinds of vehicles drive the same road, then the suspension hypothesis doesn't support the result. Am I over-thinking that?

[BobP]

I just cut and pasted it to "answer" Shawn's ?... and Joe started a new topic. I have no idea... but I've seen that explanation in several sources. So they must all be looking at the same study(wether right or wrong).

[BobP]

Some more "fascinating" info

Scientists Reveal Mystery of Bumpy Ride
Country Road, Take Me Home.... But Why Are Many Like Washboards?
By PHIL SCHEWE

WASHINGTON, April 9, 2009 —

Exquisitely engineered superhighways allow cars to fly along smoothly at high speeds, but many of the world's roads are surfaced with gravel that develops washboard patterns of periodic bumps.

This car-jarring phenomenon not only makes for an unpleasant ride but can be a safety problem, causing car wheels to lose contact with the road. Now, at long last, scientists have figured out why a smooth gravel road goes bumpy.

A group of scientists from France, England, and Canada performed a battery of tests to determine why any road covered with loose material, such as sand or gravel or snow, develops ripples. They set up an artificial road consisting of a suspended wheel allowed to travel around a rotating road surface.

No matter what the material covering the surface -- the researchers even used rice -- washboard bumps appeared after the test apparatus reached a certain combination of speed and wheel weight.

The scientists eliminated any suspension effects that would mimic the way a car can bounce up and down on its shock absorbers, but the bumps appeared anyway.

They even replaced a rolling wheel with a flat blade, and still a washboard pattern emerged.

Solid Waves

University of Toronto scientist Stephen Morris and his colleagues concluded that the development of washboarding is very similar to skipping a stone across the surface of a lake. The wheel feels a combination of forces that cause it to both push off and dig into the surface.

This force combination causes the wheel to jump off the surface when it is going fast enough, creating the ripples. The difference between a skipping stone and a washboard is that the road surface "remembers" its position and does not, like a momentary water ripple, fall back into place.

Country Road, Take Me Home....

Morris says that ripples similar to washboard appear in a number of settings, shaped by wind and water on sandy beaches or by skiers on ski slope moguls.

Washboard ripples even turn up on some solid surfaces, such as on train tracks and on the surface of magnetic hard drives, which develop slight ripples in response to the passage of a read-head, which acts like the "wheel."

[Jeffshadows]

There used to be a good write-up about the reasons not to drive like this on Bill Burke's site...

[chumley]

Thanks Bob-

I didn't have time to do the research today, but the reading is fascinating and corresponds in large part with what I had previously read.

Hadn't considered railroad tracks, but knew about the hard drives ... just hadn't linked it to the same effect. Cool!

Oh, and I also reconsidered my previous comment about wheelbases, since most vehicles have independent front/rear suspensions, its an irrelevant thought.

[PLC92084]

Mythbusters had an episode that covered this topic in depth... Their slant was more concerned (obviously) about the myth that going faster over a washboard road actually smooths out one's ride (it does... sort of). Look on Discover's web site or Hulu if interested.

[SuperstitionGuy]

Driving like hell on country gravel roads is highly recommended by body shops and new and used car dealers. ;) :STP: :whistle:

[Jeffshadows]

Oh, and I also reconsidered my previous comment about wheelbases, since most vehicles have independent front/rear suspensions, its an irrelevant thought.

That depends, of course. Some of us still favor the good 'ol solid axle and so does at least one specific modern four wheel company. ;)

[chumley]

Having been on FR300 last summer on a day that they were actively grading, as well as a week or so later, I can tell you that if you are not properly equipped, you will trade the washboards for the flat tires.

The grading process turns up the roadbed, which in the case of FR300, includes plenty of sharp rocks. After a week of getting driven on, those sharp rocks have been pushed down or toppled over, but when its fresh, only the better tires will withstand the obstacles in the freshly graded surface. (I will wholeheartedly endorse BFG A/Ts, but there are a select other worthy options).

[Jeffshadows]

I finally moved away from BFG with my most recent tire purchase. I think I'm going to be happy that I did, but they are great tires and I used them for over ten years with great results.

[PaleoRob]

I think the wheel spin thing has a lot of merit. Consider the fact that washboards develop first and more strongly on the right hand side of an uphill slope.
I drive relatively quickly on dirt roads.

[nonot]

One of the big causes of washboarding in AZ is the idiots who drive the forest roads in ATVs and SUVs/pickups with their diffs locked, when it isn't necessary. Locking diffs on gravel road causes washboarding, vs destroying your vehicle's 4WD components.

[Jeffshadows]

One of the big causes of washboarding in AZ is the idiots who drive the forest roads in ATVs and SUVs/pickups with their diffs locked, when it isn't necessary. Locking diffs on gravel road causes washboarding, vs destroying your vehicle's 4WD components.

Absolutely.

[joebartels]

I don't think that's the cause of the majority of washboard roads.

[nonot]

Next time you're driving pay attention to how the washboarding is concentrated on the parts of the turns and not that much before the turns. Maybe I just notice it more in my crappy car?

[joebartels]

I'm doubting it for two reasons

The first I'm not so sure about. I've only had one Tacoma with a rear locker. Really couldn't drive at speed with the locker or you'd studder off the road. Granted it studders but I can't image hordes of people driving like that. Perhaps over use of 4x4 but that shouldn't affect it.

The second is more of the clincher. We had private irrigation roads through the corn fields. They were among the worst washboard around. No lockers on the work trucks, no one else around for fifty miles. Believe they were graded yearly.

[Jeffshadows]

I'm doubting it for two reasons

The first I'm not so sure about. I've only had one Tacoma with a rear locker. Really couldn't drive at speed with the locker or you'd studder off the road. Granted it studders but I can't image hordes of people driving like that. Perhaps over use of 4x4 but that shouldn't affect it.

The second is more of the clincher. We had private irrigation roads through the corn fields. They were among the worst washboard around. No lockers on the work trucks, no one else around for fifty miles. Believe they were graded yearly.

The same holds of some of the roads out west of here around ranching country. Of course, everyone tore up and down those things like savages...