# Why aren't tractor-trailers more aerodynamic? **URL:** \ **Category:** Factual Questions\ **Created:** [August 18, 2009, 2:21pm UTC](https://boards.straightdope.com/t/why-arent-tractor-trailers-more-aerodynamic/506841 "2009-08-18T14:21:21Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![gonzoron](https://avatars.discourse-cdn.com/v4/letter/g/a6a055/32.png) [@gonzoron](https://boards.straightdope.com/u/gonzoron)\ **Post date:** [August 18, 2009, 2:21pm UTC](https://boards.straightdope.com/t/why-arent-tractor-trailers-more-aerodynamic/506841/1 "2009-08-18T14:21:21Z")
I know they’re a huge, heavy machine, and probably never going to have great fuel-economy, but since they’re on the road pretty much 24/7, wouldn’t it make sense to squeeze out every little bit of savings for the shipping companies? Why is the front of the cab flat (like old-school Optimus Prime) or stepped (like movie Optimus Prime), and not a smooth, tapered shape like a minivan, to direct the air up and over the trailer? The biggest concession I see to aerodynamics is that little angled metal flap you sometimes see on top of the cab. Is that the best they can do? ---
**Author:** ![SanDiegoTim](https://avatars.discourse-cdn.com/v4/letter/s/85e7bf/32.png) [@SanDiegoTim](https://boards.straightdope.com/u/SanDiegoTim)\ **Post date:** [August 18, 2009, 2:42pm UTC](https://boards.straightdope.com/t/why-arent-tractor-trailers-more-aerodynamic/506841/2 "2009-08-18T14:42:46Z")
Over the last several years tractors destined for over-the-road use have become more aerodynamic and “spoilers” have been used to direct the air over the trailers. However, such changes aren’t all that common for tractors destined more for “city” use. Equipment you might see on Interstate 80 in the middle of Kansas is quite different than what you’ll see on city streets. ---
**Author:** ![Mr.Moto](https://avatars.discourse-cdn.com/v4/letter/m/9de0a6/32.png) [@Mr.Moto](https://boards.straightdope.com/u/Mr.Moto)\ **Post date:** [August 18, 2009, 2:44pm UTC](https://boards.straightdope.com/t/why-arent-tractor-trailers-more-aerodynamic/506841/3 "2009-08-18T14:44:32Z")
Believe it or not, a lot of people seem to be working on this. The problem is that the savings you can achieve on the tractor side are fairly modest - roughly 70% of the drag comes from the trailer. And while things like fairings, skirts, vortex strakes and the like show great promise, they will be slower to implement on trailers. There are lots more of them and companies won’t change them out until they wear out. ---
**Author:** ![SanDiegoTim](https://avatars.discourse-cdn.com/v4/letter/s/85e7bf/32.png) [@SanDiegoTim](https://boards.straightdope.com/u/SanDiegoTim)\ **Post date:** [August 18, 2009, 2:46pm UTC](https://boards.straightdope.com/t/why-arent-tractor-trailers-more-aerodynamic/506841/4 "2009-08-18T14:46:36Z")
[http://www.maytrucking.com/images/trucks/pete387a.jpg](http://www.maytrucking.com/images/trucks/pete387a.jpg) ---
**Author:** ![elmwood](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/elmwood/32/3060_2.png) [@elmwood](https://boards.straightdope.com/u/elmwood)\ **Post date:** [August 18, 2009, 2:55pm UTC](https://boards.straightdope.com/t/why-arent-tractor-trailers-more-aerodynamic/506841/5 "2009-08-18T14:55:53Z")
There are very few flat-faced cab-over tractor-trailers on the roads in the United States now. I see them on rare occasions, usually repurposed for low-speed “dirty” uses such as hauling gravel trailers. I believe cab-over trailers are far more common in countries with truck length restrictions. With such restrictions, it’s difficult to make a vehicle very aerodynamic. ---
**Author:** ![Philster](https://avatars.discourse-cdn.com/v4/letter/p/13edae/32.png) [@Philster](https://boards.straightdope.com/u/Philster)\ **Post date:** [August 18, 2009, 2:57pm UTC](https://boards.straightdope.com/t/why-arent-tractor-trailers-more-aerodynamic/506841/6 "2009-08-18T14:57:05Z")
I think there has been a huge push for aerodynamic cabs, but the service life of cabs and trailers is so long that it will take a long time for the trucking industry image to be that of a swept-back aero look. It will appear before your eyes gradually. Look around at the new/newer Kenworth and Peterbuilt tractors: Slick. [http://www.kenworth.com/2100\_vir\_t660.asp](http://www.kenworth.com/2100_vir_t660.asp) Volvo, too: > **[Team BGB Motor Sports](https://teambgb.com)** > > Welcome to Team BGB! We operate as a customer oriented business, servicing and supporting Porsche street, track, and race cars. We build, and provide engineering support for cars that run under IMSA, the Porsche Club of America, Sports Car Club of... ---
**Author:** ![Saltire](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saltire/32/553_2.png) [@Saltire](https://boards.straightdope.com/u/Saltire)\ **Post date:** [August 18, 2009, 3:58pm UTC](https://boards.straightdope.com/t/why-arent-tractor-trailers-more-aerodynamic/506841/7 "2009-08-18T15:58:41Z")
I’m not an engineer, but looking at **Philster** ’s links makes me think of another point–Those huge, powerful engines need more cooling than a minivan does. So they tend to have the radiator grill flat to direct air through it. If the whole front were aerodynamic, the engine would overheat due to lack of airflow. This is why even the shapely tractors in the links have a flat or almost-flat grill. ---
**Author:** ![gotpasswords](https://avatars.discourse-cdn.com/v4/letter/g/c57346/32.png) [@gotpasswords](https://boards.straightdope.com/u/gotpasswords)\ **Post date:** [August 18, 2009, 4:03pm UTC](https://boards.straightdope.com/t/why-arent-tractor-trailers-more-aerodynamic/506841/8 "2009-08-18T16:03:15Z")
There is a big demand for more efficient trucks, but as Philster notes, the things last so darn long that it’s not going to be a rapid changeout - with proper maintenance, you’re looking at 15-20 year lifespans. Somewhere around 600,000 miles, you’ll describe it as finally broken in, and at 1 million miles, you’ll be wondering if it’s about time to have the engine rebuilt. Trucks are expensive, and truckers are going to drive what they have until it takes more than one roll of duct tape per run to hold all the parts on, or until it’s otherwise financially not worth keeping. OTOH, for those who can upgrade, they can expect significant improvements in fuel economy. Peterbilt says their new lines [can save up to 12%](http://www.peterbilt.com/eco/navigator0819.htm). If you can keep a truck alive for 15 years, (not hard to do, really) you’re looking at an overall fuel savings approaching $100,000. ---
**Author:** ![ElvisL1ves](https://avatars.discourse-cdn.com/v4/letter/e/5daacb/32.png) [@ElvisL1ves](https://boards.straightdope.com/u/ElvisL1ves)\ **Post date:** [August 18, 2009, 4:57pm UTC](https://boards.straightdope.com/t/why-arent-tractor-trailers-more-aerodynamic/506841/9 "2009-08-18T16:57:53Z")
> [@Saltire](#): > > So they tend to have the radiator grill flat to direct air through it. Right, it’s a misconception to see the front as being flat and unaerodynamic. It’s chock full o’ holes. Air mostly doesn’t hit it, stop, and try to make its way around the hood and fenders instead. Mostly air hitting the front goes through the grill, through the radiator, around the engine, and eventually down under the truck. Slanting or recontouring the radiator wouldn’t do much for drag, it would just reduce cooling efficiency.