Anyone have 800 Lbs of clay handy? New mythbusters episode

I'd be curious to know the expert's thoughts on streamlining bicycles. For those of you who don't know, the current record is over 80 mph - that's for one rider on a flat, straight road with no wind and no power other than their own. The fastest speedbikes, or Human Powered Vehicles as they are usually known, are completely enclosed and as smooth and slippery as possible. The riders are taped in, and typically launched and caught by hand. Some of the more radical innovations include riding head first, and using cameras for vision to eliminate the drag of a head bubble. So far as I know, none of them have been tested or developed in a wind tunnel, though computer modelling is often used. Any ideas on shapes or surface treatments to make these awesome machines even faster?

Here's a link to the most recent event, so you can get a better sense of what I'm talking about.

 

If a theory disagrees with experiment, it's wrong. We can discuss and chat and theorise all we like, but it won't mean anything compared to "Let's try it to find out." Sure, it may not be the greatest experiment ever devised, but you have to give them kudos for trying.

 

I'm all for "Let's try it to find out." Kudos to them for trying...it's the way most new technologies and trailblazing discoveries come about.

But, when you put it on TV and profess it to be a scientific experiment with rock solid 15% FE gain results to be true (aka "Myth Confirmed") all the ignorant and no-nothings (no offense) take it as fact. It's not even CLOSE to a solid scientific experiment although it is portrayed to be so and you will not get a 15% increase in FE by adding dimples (clay or otherwise) to your vehicle.

So now, until the "sheep" watch a Mythbusters episode that denounces their recent results the sheep believe that dimples on a car increase FE. You will not find one educated person that will confirm this using science-accepted methodolgy.

 

The myth buster's experiments are hardly scientific but the theory is proven. The Lexus LS 600h L and rx 450h have dimpled air flow dams underneath. There is also a bridge in china that has all of the cables (for a cable stayed bridge) covered in a dimpled covering. I saw that on "build it bigger" and the chinese scientists demonstrated the theory in a wind tunnel and showed how the smooth cable would cause turbulence and vibrate excessively. While the dimpled covered cable remained mostly still.

 

Putting dimples on a bluff car body that already has laminar flow is counterproductive. However, putting dimples on air flow dams on the underside of the car is productive...I take no issue with that. The underside of a car does not produce it's own laminar air flow or allow for laminar air flow (i.e. the air flow is not attached like on other areas of the car) and dimples added to the underside help to create a turbulent wake to streamline the air off, under and away from the rear of the car.

Does adding dimples to a car increase FE by 15%...no chance...not even close. Does dimpling have some positive qualities...it sure does...where laminar air flow is not present already. VW does it on some of their cars...so does Lexus as you stated...both on the underside.

Cars transition to a turbulent boundary layer at or about 20 mph so exterior body dimpling is not productive for cars. Where it might be useful is on wheel covers (i.e. racing discs). And no, not the ones that go on cars traveling for maximum speed records...rather on the ones that go 40-70 mph for everyday travel. Dimpling could also be helpful on "some" areas of the shell of the car where air flow is detached or separated.

To get back to the entire point of this thread...the Mythbusters "experiment" of dimpling an entire car with hundreds of pounds of clay did not increase FE by 15%.

 

I am a scientist and watched the show. I thought their methodology was fine (the fuel switch and weighing the fuel used was good). They mentioned that this was the FE of a car already at 65 mph. They only measured FE once the car was cruising at 65 and admitted that they did not include the added fuel required to get up to 65 with 800 extra lbs. So if you don't think this is "real" you should provide your explanation of how they got these results? What mistake in experimental design is causing an erroneous result?

 

I'm no expert, but I have a few thoughts. With bicycles weight is an issue, so that enters into the equation. Streamlined bicycles are long and thin, which is excellent for laminar flow. A very smooth foil section is most likely the best for drag and weight. Ideally the faring should extend as close as possible to the ground.

There isn't a lot of need to test these foil sections, as NACA did that extensively many years ago. Given their work, you can select an optimized foil section for any standard set of conditions. When we were designing the keel for our most recent boat, we used NACA's research to get the characteristics we wanted.

Perhaps micro scales could help a bit, but they would need to be light. Most likely they would be embossed into the covering material.

Tom

 

so when do we head for the nearest hailstorm for our free dimpling?














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I put dimples in my front license plate, and for the last 3 tanks, I am getting a steady 8% improvement on fuel economy...

 

How can you prove this? Any pics...data?

 

I built a wind tunnel in my backyard, placed the Prius in with an apparent wind of 73.245MPH, then used a digital strain gauge to measure force.

I also found that different states license plates (without dimples) can change the MPG by up to 3.1% (we measured California vs Alaska). Interestingly, a plate form Mexico gets 1.2% worse mileage.

 

Thanks to you...we are all collectively dumber now.

 

I'd not assume anything - you likely are - but you cannot state others are - unless you have done some scientific research over the last hour.

 

Well we all are anxiously waiting for the pics of your wind tunnel, dimpled license plate and statistical data provided by the hi tech equipment you have in your backyard wind tunnel.

When you are ready to produce those items...check back in. Until then...I'll assume everything.

 

A neighbor drove his car back from Kuala Lumpur - we found its license - after dimpling - provides the best mileage increase - a whopping 12.24%. Of course - the plate is smaller - but mileage is mileage...

 

I do not dispute your improvement in FE...but there are about 50 different variables that could have increased it by 12.24% (of which COULD have been the dimpling). But "a neighbor" driving his car back from Kuala Lumpur is hardly scientific.

With your example we should then conclude that all of the dimpling that Mythbusters did...12.24% came from just dimpling their license plate...unlikely! And here is the problem with forums like this when it comes to mod testing...most don't DO the testing. They just assume because they changed their car in some way that it's the reason for improved FE.

Again, when you do some legitimate testing...let us all know. Until then, this conversation is a waste of time. Talk to you then!

 

Science is an art... For grins, we just placed the California version of dimpled license plate on a Pinnacle series Mack truck (one of their larger truck cabins available).

It turns out there is a 3.14% improvement on fuel economy. Think about the scale of things if all trucks could instantly increase their mileage by 3.14%. The savings over a year would be tremendous - and they could have some spare change for a piece of pi for desert every day....

 

Science is hardly an art...hence the reason there are numbers to qualify results with procedures to ensure their validity.

Here's an experiment you can do. Put the license plate on your head and see if you run faster...looking forward to those results!

 

To verify the validity of our tests, we tried the dimpled plates on the rear of the car, and showed no net improvement in mileage.

I do not get the idea of placing a license plate on my head - I would not be able to run fast enough to measure any improvement.

 

I agree with CT. To those of you that did watch the episode, could you find anything wrong with their methodology? I could not. Some of their methodologies in other episodes are sometimes pretty weak, but I thought that this one was one of the more experimentally sound ones.

The only part that I though may not have been covered were differences in barometric pressure and wind speed/direction between the runs. I didn't catch whether they covered these or not. They may have, and I just missed it.

They did logical control runs, and even put the "removed dimple material" back into the vehicle to keep the vehicle weight constant between the "pre- and post-dimpled" runs.

Matt Herring: did you watch this episode, or are you just dismissing their results on principle alone?

If more experiments were done, and the effect was validated, I would guess that dimpling would still not make it to the mass market. Cosmetically it would probably be too objectionable to most people.