Spotless myth or just clean fun?

Tell us what you think!

DCFM

I don't usually find fault with the Mythbuster's methods, but I am questioning the methodology for the Clean Car vs. Dirty Car. If I were to run this experiment, I would have used a car that was normally dirty, found the mpg then run it again after it was run through the car wash. I think perhaps the weight of the seriously and artificially dirty car caused the car to have extra weight.

Another reason I think that the dimpled car might actually have better mpg has to do with race yachts. My husband, who is a professional boat builder of race yachts (think America's Cup level) told me that in the '87 America's Cup, Stars & Stripes used a "dimpling" product on the hull to reduce water friction. They won the America's Cup and the method has since been outlawed.

the episode is fun. I noticed something, which is either an error in the clip placement, an error by Adam, or the narrator.


When they were testing dirty vs. clean car, the narrator said each run was made 5 times. When Adam was doing the math for the clean car he says (I'm paraphrasing because i don't remember the exact numbers), "Half of these numbers are ... and the other half are... i can do that in my head...." How are half of 5 runs one number and half a different number. 5 doesn't have a whole number as its half....

I was a little disappointed they didn't talk about the Reynolds number when they went to the wind tunnel. You can't just build things to scale in fluid mechanics and expect them to act the same. It didn't effect anything on the show, just would've been nice to hear for us nerds =)

Something about the dimpled car test bothers me. I should have been listening to why the weight of the clay doesn't matter as Adam was saying because I don't get why that wouldn't matter.

Clay has lots of water in it, so a few runs in the hot sun would dehhydrate it, lowering its weight, so by the time they make the dimples, the result is already confirmed. Plus, when they were making the dimples, they were cutting off pieces of clay which minuses the ammount of clay on the car, lowering it weight further......Wait a min, jut as I was writing that, Adam predicts my arguement, and cancels it out, so nevermind.

I agree - this was a fun episode.

At an 11% improvement, you can't really quibble about clay drying out or anything else. Besides, they showed that adding 800 lbs of weight didn't affect mileage once the car was up to speed.

I think it was really solid. It's silly and it works.

lexus did it first:

http://www.autoblog.com/galler...exus-ls460-l/full/#2

lol

The test on the hangovers was so clearly flawed that it's embarrassing for the show.

Why did they not simply use a blood alcohol level (BAL)? A breathalyzer is not a difficult piece of equipment to obtain. This would eliminate the issue of whether or not they were equally intoxicated.

After the beer only test, they should have measured their BAL and aimed for that target on the beer + liquor test. The reason the beer + liquor test seemed to have better results is due to a couple of factors. First, training effect. The guys had already preformed the tasks while hung-over and were accustomed as to what to expect, this would elicit better performance. Second, alcohol tolerance. When you consume enough alcohol to get intoxicated your body up regulates the production alcohol dehydrogenase which improves the bodies ability to convert ethanol to lactate, ketone bodies, etc.

Perhaps they'll try this one again some day under more rigorous conditions.

I was waiting to leap like a panther on a sick pig about the weight of the dimples and then at the last second, they put the dimples in the car.

I think Adam and Jamie are now experienced masters of the MB forums.

as far as the golf ball effect goes, i'm pretty sure they do use use in the automotive world already (racing though) with F1 cars (formula 1 cars) but on a much smaller scale with dimpeleing the paint itself)

also why did they not test that gold ball that was banned (in like the 80's or something) because it was so aerodymnamic (the one with the triangles instead of circles)

I hate to say this, but I became wary because they DID put the dimples in the car!

The weight of the car doesn't matter (26 mph either way).

But they put the dimples over the back wheels, potentially changing how much surface of each tire meets the road, perhaps giving better traction or less rolling resistance.

I'd like to see how that car does WITHOUT the dimples in back... just in case.

Still, I don't think that it will do any worse - great test!

It sure makes you wonder about how carefully auto makers have really been about testing cars in wind tunnels, doesn't it?

My how time flies. 40 years ago an up and coming racing team owner by the name of Roger Penske showed up for the Trans Am series with a Camaro that had a vinyl roof. Roger maintained it was simply so he could tell his car apart from the others, even though his was the only dark blue car with a big number 6 on the side. Everyone knew better, and boundry air flow disruption was quickly looked at for the reason. Vinyl roofs were banned and race cars have never looked so elegant since!

I would have like to see this again with dimples the size that are on golf balls. It could change the results and if it still works you might actually see cars made that way.

quote:

Originally posted by Mike the Biologist:
But they put the dimples over the back wheels, potentially changing how much surface of each tire meets the road, perhaps giving better traction or less rolling resistance.

You're guessing that you can get 11% better mileage by putting 40 lbs of weight over the rear wheels?

Really?

I thought the dimple results were interesting. At the scale test, I couldn't see the difference that Adam was supposedly seeing in the water test. Looked the same to me.

also i have an idea for them to actually test the dimples...
is know when people put full vehicle wraps with advertisments on them on vehicles and THEY COVER THE BACK WINDOWS WITH THE PERFORATED VINYL so you can still see out
well anyways the idea would be to wrap the WHOLE vehicle in the stuff with the holes acting like the dimples, AND it would proably only ADD 20-25 POUNDS MAX!!! so extremly lightweight compared to the clay (some people's weight proably very at least that by the much over the course of a year)

you could also vary the speed at which it be most effective by varying the thickness of the viyl, also the size of the holes ,and the number/spacing of them

The problem I have noticied with all the fuel effeciency myths is failure to compensate for wind and pressure. All the myths revolve mostly around aerodynamics so these are very important. Tests must be run in both directions (like speed tests at bonneville) to compensate for wind and barometric pressure has to at least be taken into cosideration. Even in this dimpling test it is easy to think the temperature (therefore the pressure) difference of the air would change the resistance more than clean/dirty/dimpled when the tests were more than a few minutes apart. Without these the tests are lacking a critical factor that is far more than any wieght gain/loss.
Thoughts?

Well this is what I learned from my physics teacher in my junior year of high school; The reason for the dimples on the golf ball is that each dimple will hold a pocket of air inside it, and this greatly reduces the overall friction of the ball, because there is much less friction of air on air, vs air on golf ball material.

The same thing probably happened with the car, and would explain why a dirty car wouldn't work, because the dirt was not creating those air pockets. This doesn't sound like it should be very significant, but there have been bigger surprises before.

quote:

Originally posted by Mwilkin9:
Something about the dimpled car test bothers me. I should have been listening to why the weight of the clay doesn't matter as Adam was saying because I don't get why that wouldn't matter.

They didn't spend a lot of time explaining why the weight matters so little, so I'll give it a shot.

The key part of the test is that it's performed at constant speed. A car's gas usage will be very nearly proportional to the force its engine exerts on it, especially at constant RPM. When the car is at a constant speed on a flat road, the net force on the car is 0. There is some inefficiency in the engine and transmission, but that would be very nearly constant in all the tests. The two forces that come into play here are drag and rolling resistance. A smooth clay shell does very little to change the drag (ie wind resistance), and weights in the back do less. The dimpled clay shell, as they showed, does affect the drag.

The rolling resistance is a smaller effect, but it is affected by weight, and thus the clay shell. When a car rolls, the tires do not stay the same shape - they get bent and squished as they rotate. This takes energy, which must come from the engine. The more the car weighs, the more the tires deform, and the more energy is used, which hurts gas mileage. However, this difference is very small compared to the wind resistance, which is why the smooth clay shell had almost no effect.

In real conditions, 800 lbs of clay would certainly affect the mileage. This is because real conditions involve accelerating, which requires a net force. The higher the car's mass, the more force is required to accelerate it the same amount.

On a side note, I was really impressed with how consistent their measurements were, especially going from the silly stickers-next-to-tube method (they couldn't even get graduated cylinders?) to weighing the fuel used.