why bluff body

Translation into a more standard English:

"Why are trucks and buses so boxy and unaerodynamic? Can't trucks and buses be shaped more like cars?"

**

I used to drive an "aerodymanic" minivan (Chevrolet Lumina APV). A large vehicle with a more rounded shape often can have reduced visibility. Many buses these days do have a slightly more rounded front.

The "forward" cab type of tractor isn't as common these days. Many tractors even have aerodynamic fairings on top.

Also, I believe many (european) truck have a legal length limit, encouraging designers to make the tractor as short/flat as possible and maximize the payload. That's why there are so few Mack type trucks in europe.

Martijn

jas wrote:

First. The job of a bus or truck is to carry stuff inside, lots of stuff. A boxy shape is most efficient for this primary job. Second Rounding the front corners of the bus will reduce the drag significantly. It does not need to be all swoopy shaped to minimize the drag. After rounding the front you get a benefit by tapering the rear of the bus. But tapering takes away interior volume. Volume that is used to make money. Maybe with$3/gal and up fuel it will change the equation but I doubt it.

I'm still trying to figure why the original poster chose to use such an obscure word as "bluff" as a synonym for flat.

Aerodynamics aren't as big an issue at the lower speeds of a typical city bus route. Aerodynamic drag goes up exponentially with speed, such that it will be a much bigger issue at highway speeds.

A longer "articulated" bus has the benefit of two sets of drive wheels powering about the same "profile". I remember tandem bicycles have the benefit of marginally higher drag than a standard bike, but powered by two sets of legs.

Actually there has been some R&D attention paid to these issues, sporadically. Punch "heavy truck aerodynamics" into your favorite search engine and you'll see. One of the hits will probably be this paper, which gives a good historical survey and explains some of the complexities:

The field seems to be enjoying a renewed level of activity these last couple of years, for obvious reasons.

The "aero cab" look has really gained a lot of favor with over-the-road tractors in the last several years, as have aerodynamic fairings at the top of the tractor that can be extended when pulling a tall boxy trailer. Unfortunately some of the aerodynamic complexities occur at the tractor-trailer gap and at the rear of the trailer, areas over which the designers and selectors of the tractor might not have much influence (especially the latter).

For vehicles like city buses and local-delivery trucks it probably doesn't matter quite as much. Their efficiency is dominated by other issues at those speeds, and compromising either maneuverability/parkability (and driver vision) or load/people-hauling capacity isn't worth it. Looking closely, though, you will observe that some more attention is being paid to their subtleties in many cases -- they're rather bluff bowed but not always as square and sharp-cornered as in days of yore.

Passenger cars benefit a lot more because their gas consumption is less utterly dominated by weight, and the designers literally have more room to work with. And they have control over the whole thing from bumper to bumper. And no worries about marrying up with a standard loading dock ...

All in all, it's an excellent question with much at stake, economically and in energy conservation, in the answers, and still a lot of room for improvement; but not at all a trivial one!

Cheers,

--Joe

what is u'r opinion if we lower the seat of driver and taper the front of truck, will it reduce air drag

Where you gonna put the engine, 1st trans, & 2nd trans?

You need to google wind tunnel studies on vehicle bodies.

If I remember right from a Discovery Channel TV show, an oval front and rear with a flat top and bottom has one of the lowest drag numbers going. In other words a city bus is really close for really low drag.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail >

And more to the point, trucks that are intended for highway use ARE getting fairly highly optimized aerodynamically. Look at a Kenworth T2000 or a Freightliner Century Series. Compared to a car, they still have a huge frontal area, but the overally PROFILE is aero tailored. A sharp pointy nose is not the best design for low subsonic speeds, nor is too gradual a taper. Something that "looks" sleek is not necessarily a good aerodynamic shape- best example I can think of is the 1968 Dodge Charger. That body looks slicker than anything built before or since, but has a whole lot of drag at 200 mph, so the Daytona was born to address those problems.

For city busses, it just flat doesn't matter. They never operate at a speed at which aero matters a whit, so things like interior space and visibility are paramount.

I would think they would need a reasonably sized grill just for the huge radiator, as well as the engine.

They're getting slightly rounded. Over the years I've seen the same busses used by my local bus system on lower speed streets as well as inter-city service over highways. It may simply be a purchasing and maintenance decision.

They seem to be getting LESS rounded to me, at least compared to the old GMCs of the 70s:

My local bus service is AC Transit here in the Eastern part of the San Francisco Bay Area. They've been using Van Hool buses from Belgium with a bubble shaped front window. These also have slightly slanted fronts. They're also using some hydrogen fuel cell buses which definitely have a rounded shape. Strangely enough, Gillig is headquartered in the AC Transit service area, although I don't think they've bought a new bus from them in over 10 years.

Today I was waiting for something so I browsed an Ace hardware store across the street. They were unloading from a rig with a big 'ol Volvo tractor. The hood was slanted, the fenders were low, and they had a rounded fairing over the cab that was matched to the height of the trailer. Volvo has a several similar looking models like the following:

On the way home I noticed another tractor with a flap over the cab that looked like it could be adjusted for height.

Austin, where I live, buys almost exclusively from New Flyer now (barring a batch of really nice looking Optima Opus coaches) including the New Flyer hybrid busses with the enormous battery boxes on the roof (looks like a rollover waiting to happen). The used to buy almost all Gillig, but when they bought a batch of low-floors, as opposed to the old Phantoms with a REALLY flat nose, they had a rash of major elecrtrical issues and I think that pushed them to New Flyer and Optima. The Optima Opus has to have the biggest one-piece windshield I've ever seen on ANY vehcile of any sort:

Of course it doesn't matter much because no one rides the d*mn things- they're tax subsidized so ridership isn't an issue . I hate transit authorities.