Your specious staff report seems to be at odds with important facts.
The washboard pattern exhibited on dirt or gravel roads usually has a period [distance between peaks or valleys] smaller than the footprint area of most tires.
Suggesting that the washboard pattern is created by tires is like saying that a horse could master the piano.
The theory that the impact oscillations of a number of different vehicles with different weights, tire footprints, and shock absorber oscillation characteristics would create a uniform pattern seems absurd.
Yet the pattern only exists in high traffic areas and usually where brakes are applied. So, given that the traffic appears to have something to do with the washboard pattern, how do we account for washboard valleys smaller than the tire footprints?
Welcome to the Straight Dope Message Boards, compunuke, I guess we’re glad to have you with us.
When you start a thread, it is helpful to provide a link to the Staff Report upon which you are commenting. Helps keep everyone on the same page, makes it easier for others to understand what you’re saying and why, etc.
But the explanation supplied is not affected by the footprint area of the tires. It purely relies on the bounce of the suspension, and drivers maintaining the most comfortable speed.
I can vouch from personal experience that the effect does NOT only occur in braking areas.
If you are so sure this explanation is wrong, what is the correct one?
I’m not sure. It just seems unreasonable that a tire can make a washboard pattern with valleys smaller than it’s own footprint. Back to my analogy of the horse playing piano. How can the horse play the keys smaller than its own hooves?
I am completely at a loss to understand the issue of “footprint” in the OP. Tires roll continuously. Look at tire tracks on smooth sand. Long “footprints” that go on and on.
What do you mean by “Footprint” in terms of a moving vehicle???
Keep in mind that we are also talking about vehicles. Which is why washboarding extends across most, if not all, of the width of a road. What one vehicle causes and what 1000 vehicles cause are entirely different things. For example, you get averaging of effects. Not all of the vehicles have to produce the exact same effect. But the distribution of effects will have a nice big peak.
I believe a tire’s “footprint” is a sensible concept. Suppose a vehicle weights 2400 lbs and has 4 tires, each inflated to 40 psi. The average tire load is thus 600 lbs, which implies a “footprint” of 15 square inches.
But that in now way invalidates the Staff report. A 15 sqin footprint is compatible with rather closely spaced washboard “ribs”.
Washboarding is most noticable when the ground is softish and a succession of cars exert horizontal force: braking, accelerating (e.g. climbing a hill) and cornering.
With the edges. Not that your analogy is remotely relevant.
The tire footprint does not have to fit into the valleys to make the waves. Consider a blanket lying flat on a table. Push the blanket edge with your hand, it makes waves in the blanket that are unrelated to the size of your hand.
Having grown up in the desert, I can assure you that dirt roads washboard in all sorts of different situations, not the just cases of brake application or acceleration. I have ridden over roads which are washboarded (let’s make that a word if it isn’t already) for miles without any break. Especially annoying when you are on a bicycle!
I grew up in an area that was prone to the washboard effect - lots of poorly-maintained dirt and gravel roads, very little rain to re-carve the surface.
It’s always been a fascinating phenomenon to me - basically a self-reinforcing cycle. Car tire hits random bump on the road, tire bounces and immediately hits the ground after the bump, digging a very slight hole. The far end of this hole is now essentially a bump (since it’s higher than the hole that precedes it) and the next car’s tire will bounce over that. Given enough time and enough vehicles, this phenomenon results in a whole series of bumps and holes - the washboard road.
If you’ve ever driven on a washboard road, you find out that it forces you to drive at a certain maximum speed, otherwise it seems like your car will shake itself apart. This naturally enforces a specific frequency of tire bouncing, re-enforcing the washboard effect (tires bounce directly off the bumps and into the holes, diggint the holes deeper). Interestingly, there seems to be a point when this effect de-stabilizes itself - the distance between the holes and peaks gets too large for fast movement, so people either creep over them (allowing natural errosion to take over) or continue to bank over the bumps, at which point the tires don’t seem to fall into the holes at all, but start knocking the top off of the next bump. This eventually brings the series of bumps and holes back to the natural oscilation frequency of the standard car suspension, and the cycle starts again. In my experience, the only way to get rid of the washboarded sections is to run a grader over them, or wait for the rainy season to wash the top of the road off.
Sorry for the long post - I didn’t have much else to think about on long road trips, and it’s hard to read when the truck is going bouncebouncebounce for hours on end.
Is the washboard effect similiar in any way to moguls on a ski run? I know moguls happen independently of grooming, but I can’t recall if it’s from the act of skiing or from wind or other natural factors.
It seems that studying one may help in figuring out the other.
Scientific American had an article on this a long time ago (maybe the 60’s?). The researcher set up a tire on a circular path . He quickly created the washboard effect. As has been mentioned above, the first irregularity got magnified and depressions and hills got bigger. The article pointed out that while automobiles come in a wide range of sizes and shapes, their suspension frequencies are all very similar.
I would have sworn they were caused by blades or dozers before I read this article.
Hello jimmythefly.
Your question doesn’t really belong here, but I’ve had the chance to see moguls being made. Of course a lot of places they are made by machines, but that is a different matter.
If you take a nice steep slope right after it has snowed, most people will choose a path where noone else has been, to enjoy the virgin snow. After a while, this is impossible. Most people then choose to turn where others have turned before, the snow is less “chunky” there, plus there is a flat area for you to land on, if you are the kind that turns by jumping. So after a few thousand people have been down the slope, you have the moguls, simply because people tend to turn on the same spots.
ras2000, why would jimmythefly’s question not belong here? The topic of discussion is the cause of washboarded roads, and jimmy asked if the phenomenon might be related to another phenomenon of bumpiness on travelled paths. Regardless of whether the answer is yes or no, the question at least is relevant.
One thing that puzzles me.
If the washboarding is caused by the tires of the car, how come the washboards reach right across the road? I can imagine some of the factors already mentioned causing hills and troughs (i.e. washboards) in the part of the road where everybody drives (i.e. where the wheels touch the ground), but that does not explain why they almost always extend right across the road.
For example, imagine a narrow dirt road in the middle of nowhere, just wide enough for a single car and steep banks, big rock or something else on the side of the road that prevents you from off to the side of the road. This will force everybody to drive roughly on the same spot of the road, but the washboards will extend right across it, even in the middle part of the road, where it is more or less impossible to drive (or more accurately: put the wheels).
I don’t think motorcycles will be the cause of the WB there, so think of another explantion.
IMHO if the WBs were some kind of wave pushed in front of the wheels, then they should be curved and the WBs from either side should join and form a m-shaped ridge across the road, but since washboards are straight (or at least appear straight) that explanation does not seem to work.
The more I think about it, the more confused I get. Need to go out and buy some lubricant for the brain (i.e. beer, the more you drink, the less you think and confusion evaporates:) (B)
My theory is that on a soft surface, a small amount of loose material is pushed ahead of the tyre. As it is being pushed ahead, some of it becomes compacted by the weight of the vehicle. A point is reached where compaction prevents further forward movement of the material - and the tyre rolls up and over the compacted area. This creates a small ridge at right-angles to the forward movement of the vehicle - followed by (in effect) a small dip. Repeated actions of other tyres would enhance the effect.
Material would also be squeezed or extruded sideways during compaction, so the small ridge would very gradually lengthen so that it would be wider than the tyre - and eventually, the road. The compacted ridge would also act as a trap for windborne material - similar to what you see when wind-blown sand builds up behind a small obstacle.
Traffic which travels mainly at slower speeds forms ridges which are at wider intervals. Fast travelling traffic forms ridges which are much closer together.
If you drive too slowly, your vehicle is subject to considerable vibration as the tyres (apparently) roll into each small valley and then hit the next small ridge. If you travel faster, the tyres appear not to have time to deform to enter each valley - but roll smoothly from ridge-top to ridge-top.