Coefficient of drag as a function of vehicle profile

[1998_K1500_Sub]

I stumbled upon this graph which illustrates the coefficient of drag as a function of vehicle profile, at least in crude terms.

The utility body style is illustrated, but the pickup-truck body style isn't.

No real surprises here. Crappy front end design means there's less Cd sensitivity to the rear end design, and vise versa, generally.

The negative slope evident in at least one of the curves does come as a bit of a surprise, although the segment(s) with negative slope is shallow and brief.

$0.02

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[Intragration]

That's interesting. I've always heard that the most aerodynamic shape was a teardrop, which makes a lot of sense, and is what your chart seems to show if I'm reading it correctly. What they don't show on the chart is the Cd of a small barn.

 

[boy&hisdogs]

Interesting about the more squared SUV front means that the back matters less. I always wondered about those goofy sloped camper shells you used to see years ago. I always figured there wouldn't be any real world difference, looks like this chart suggests the same.


Funny though, I had always heard the teardrop shape was the best as well, since that's the shape that the drop of liquid naturally "chooses" for lack of a better word when it passes through air.

 

[1998_K1500_Sub]

I had always heard the teardrop shape was the best as well, since that's the shape that the drop of liquid naturally "chooses" for lack of a better word when it passes through air.

^^^ Agreed, and yet the "teardrop" front end seems to provide about the same Cd as (what I'll call) the "dolphin nose" front end... shown 2nd from the left.

I'm reminded of the bulbous front nose on the front of many (all) modern ships, but perhaps that's a completely different operating paradigm (big word alert).

Bulbous bow - Wikipedia

en.wikipedia.org

 

[Spareparts]

I can't remember the name of the front "Bulb" on ships but i believe it has something to do with the frequency of the waves flowing down the side of a ship underway. The bow creates one wave and the Bulb is designed to create a cancelling wave. Basically the Bulb creates more speed and less fuel used.

 

[Orpedcrow]

That’s a cool chart. The research that I’ve seen done shows the bed area of a pickup as a low pressure/turbulent zone but also causing the air that’s going over the top to form a teardrop shape. Showing that if a truck and suv weighed the same, the truck would still have a better Cd. Then there’s also the tailgate up or down debate showing the tailgate greatly increasing the turbulent air in the rear and canceling out the tear drop shape in the back.

 

[Trio]

That’s a cool chart. The research that I’ve seen done shows the bed area of a pickup as a low pressure/turbulent zone but also causing the air that’s going over the top to form a teardrop shape. Showing that if a truck and suv weighed the same, the truck would still have a better Cd. Then there’s also the tailgate up or down debate showing the tailgate greatly increasing the turbulent air in the rear and canceling out the tear drop shape in the back.

You are correct about the tailgate up improving airflow. And for those that have read the tests, I don't think it's a debate anymore - definitely a widespread myth about the tailgate down being an improvement. But whoever invented and sold those tailgate nets sure benefitted from the misinformation.

 

[1998_K1500_Sub]

Then there’s also the tailgate up or down debate showing the tailgate greatly increasing the turbulent air in the rear and canceling out the tear drop shape in the back.

You are correct about the tailgate up improving airflow. And for those that have read the tests, I don't think it's a debate anymore - definitely a widespread myth

Speaking of "myth", I remember Mythbusters doing a segment on the tailgate up/down myth:

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[alignman88]

Years ago I was determined to build a Bonneville Salt Flats car and get myself a red hat. That’s the 200mph club but that pesky math, no matter how ya figured it was too many cubic dollars. 200 mph is where the men and boys separate IMO.

If the graph kept going it begins climbing at a staggering rate. I forget the number but north of maybe 160ish the horsepower and aero get costly just like the exponential increase in drag. Straight up. First thought was a super speedway Cup car but HP you can keep pinned for 5 miles and minimal traction are crazy!

I got to hear from the man himself Frank Currie about his 200 mph attempt in one the hotrods that captured my eye decades ago, ‘32 Ford with a real deal Boss 429 that was no joke! He couldn’t keep it straight, spun out well over 150 mph and decided it was a professionals game in that car. Low frontal area + HP was like driving an icy road is what he said. The pro surpassed 200. There’s a good read out there somewhere on their trip to Bonneville. The Currie family (Currie Enterprises) are fine people and real hot riders.

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[boy&hisdogs]

Years ago I was determined to build a Bonneville Salt Flats car and get myself a red hat. That’s the 200mph club but that pesky math, no matter how ya figured it was too many cubic dollars. 200 mph is where the men and boys separate IMO.

If the graph kept going it begins climbing at a staggering rate. I forget the number but north of maybe 160ish the horsepower and aero get costly just like the exponential increase in drag. Straight up. First thought was a super speedway Cup car but HP you can keep pinned for 5 miles and minimal traction are crazy!

I got to hear from the man himself Frank Currie about his 200 mph attempt in one the hotrods that captured my eye decades ago, ‘32 Ford with a real deal Boss 429 that was no joke! He couldn’t keep it straight, spun out well over 150 mph and decided it was a professionals game in that car. Low frontal area + HP was like driving an icy road is what he said. The pro surpassed 200. There’s a good read out there somewhere on their trip to Bonneville. The Currie family (Currie Enterprises) are fine people and real hot riders.

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Low frontal area like the car was too skinny? And thus not stable?