Coefficient of Drag

Actually they are doing a better job the higher box sides, like on the F150 an the Silverado, are part of the reason. Trucks are designed to be trucks, pretty hard to get a bunch of passengers and cargo into something shaped like the Pruis that looks like a door stop.

Yeas ago GM and others tried designing streamlined trucks and even locomotives and railroad cars. The problem for trucks was the streaming and extra weight reduced load capacity that offset the economy gained

mike

I've read that the most aerodynamic shape looks more like a banana , with the ends of the banana pointing downwards. A truncated back end will create a low pressure area immediately behind the vehicle and increase drag.

The other area that people often forget about is the undercarriage. A shield under the engine compartment will help reduce drag.

Since the 2007 Trunda is not for sale yet, the Silverado probably is "best in class" for cD.

Bill, I agree that truck manufacturers could be doing things to improve the aerodynamics of some trucks, but most of those things would nat make the truck "look" more massive so they get put on the back burner.

Two comments on your input above:

1) Granted a tear drop is the most aerodynamic shape, but it is blunt end forward, not pointy end, and the tear drop works so well because the fluid is constantly being swept toward the back to be replaced by the next "layer" of fluid. It is a lot like the action of team bicycle racers. The lead racer peals off to rest in the back while the rest of the pack forges on. 2) I have always wondered why automotive manufacturers advertize CD (Coeficient of Drag) rather than D (Drag). In order to compute D for a vehicle, you multiply CD * Frontal area. It is Drag that your engine has to push through the wind, not CD. CD is the measure of resistance per unit area.

That is one of the reasons the exact same car with narrow wide tires will get better mileage than one with wide tires. The wide tires add to the frontal area.

Take two cars with the same CD that weigh the same. Drive them at the same speed over the same course. the one with the smaller frontal area will get better gas mileage.

Then the article disagrees with commonly available authoritative data sheets, that (no doubt while lying through their collective teeth) claim 0.26 -- not a big error but whattheheck.

If so, I suspect Toyota would have installed them. Overall fuel efficiency is a selling point on that model. T would be keen to improve the Cd wherever possible.

Maybe this has to do with whether aerodynamicity refers to a falling raindrop or a vehicle moving along the ground?

The teardrop is the reverse of my expectations: the blunt end is in front, with the tapered end at the back.

Aha, another use for skid plate! ;-)

That explains it, thanks.

A friend got 23 MPG from his 1999(?) Silverado small V8 on the highway, with camper shell and nothing on the roof. The EPA for that vehicle was 21 MPG highway. Pretty good for a pickup truck, eh? His mileage went down when he replaced the crappy factory tires. (They were crappy off-road anyway, but obviously had low rolling resistance.)

Aha! So Toyota managed to nearly equal the Insight without specifying rear-wheel fenders, which make tire changes more difficult.

The Honda Insight is also very noisy inside at highway speeds. I'd rather pay more for gasoline than lose my hearing.

Holy smokes! This is a huge revelation to me. Thanks.

Pickup trucks should have an advantage over SUVs because they are generally lower profile.

The reason I'm thinking about this is that a(nother) friend has a relatively new Silverado and reports that fuel economy is worse now than the one he bought 10 years ago, especially with a full load.

Just looking at the windshield wipers and underside, it looks like the manufacturers could make 10-20% improvements in fuel economy with only minor engineering changes.

There are many things that contribute to drag and make the propulsion system work harder. Anything that contributes to turbulence will increase drag. The spoilers and wings on an Indy car are smaller for high speed races than for road races, where the additional downforce to aid cornering is more important than reducing drag.

Having the pointy end at the back of the car reduces the turbulence and drag behind the car. A blunt rear end creates a low pressure area, which sucks the car backwards, but a vehicle following in the low pressure area doesn't have to work as hard to move through the low pressure area, the principle behind drafting in races.

could be!

The tailgate on a pickup can act as a large wind brake. Shells and tonneau covers keep air away from the tailgate and reduce drag, as does lowering or removing the tailgate or replacing it with a net or mesh.

There are lots of tricks to reduce drag and improve fuel mileage besides fender skirts. Reducing interior sound insulation saves weight. I expect to see exterior rearview mirrors go away, replaced by cameras that doesn't stick out into the airstream.

The Prius has some subtle design tricks that maintain decent interior room without major contributions to drag. Look at the roofline from behind, and notice that the centerline of the roof is lower than the left and right side.

Actually, some pickups have the same or a higher profile than comparable SUV's. The tailgage on pickups are a pretty big source of drag. The biggest advantage pickups have over an SUV bases on the same chassis is weight. Without the interior trim, carpets, seats, sheet metal, AC and heating capacity, etc., the pickup will weigh less.

There are many reasons your friend's new Silverado gets worse mileage. The new one is probably larger and weighs more and has more amenities that add weight. It may have a larger engine or be heavier-duty than the old one.

Both the 2007 Tundra and Silverado were compared by MT for the truck of the year award, The Tundra lost out in that comparison. ;)

mike

This gets me to thinking about drafting behind trucks on the highway to save fuel. Just how close does one need to be to make this effective (notwithstanding safety issues of being that close). How about for behind a pickup or a 'regular' car? Tomes

OTOH, a while back I posted this [trimmed]...

I'd be interested to hear whether the WW2 ship story is true. My source was an engineering-oriented schoolmaster, in my teens (ie, early 60s).

From what I have seen and read of the 2007 Tundra, it is a much better truck than the previous generation Tundra and getting closer to the F series Ford and Silverado in utility but have not passed them yet. I predict that a few more people will purchase it as a work truck than the previous Tundra but it will still appeal primarily to urban cowboys.

Notwithstanding safety issues, depending on speed and the shape of the leading vehicle, you probably need to be within 1 to 3 feet.

Is that why the Oscar Meyer wienermobile, with the ends of the hot dog pointing upward, has never broken any speed records at the Bonneville Salt Flats?

Yup, they should have flipped the dog over!

I think the chopped off rear end does not contribute to streamlining because of the low pressure areas created by the squared end. Cars have chopped off rear ends because a streamlined one would either make the car too long or the passenger compartment too low.

GM's Sunraycer solar powered car

is a good example of a low-drag design. On the damaged ship, the story I recall hearing is that the bow was damaged, patched, and towed stern-first to reduce the strain on the patches, and that towing it backwards worked well.

How, exactly, does a pickup with a camper shell look anything like a teardrop? The teardrop shape has to be fat end first and long thin tail after to be vaguely aerodynamic (so without the shell would be better if were going teardrop-analogous). But also a) We were always taught a teardrop shape isn't the most aerodynamic shape (urban myth), the rear end of the teardrop is ideal, the front is not. A standard kiddies fish shape is far better as a teardrop has too much of a bulbous front. The teardrop is a trade-off of water weight, surface tension and air resistance all of which are very important at the scale of a drop but not in the same orders when on the scale of a car. b)The camper shell helps the situation because it removes the giant step-down at the rear of the cab which causes turbulence and back-flowing of the air which creates low pressure behind the cab and attempts to slow the cab down. As well as creating messy flow after it. c) the drag co-eff of a car is determined by a lot more than just the overall body shape, as you mention the wheel arch areas create vast losses and the underbody is by far the worst and most messy area aerodynamically. It could be that certain models have a fuel tank in a different place which aids the flow below the back axle instead of through it all, for example.