# Why don't we build cars with smooth surfaces underneath? **URL:** \ **Category:** Factual Questions\ **Created:** [February 22, 2011, 4:19am UTC](https://boards.straightdope.com/t/why-dont-we-build-cars-with-smooth-surfaces-underneath/572192 "2011-02-22T04:19:33Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [February 22, 2011, 4:19am UTC](https://boards.straightdope.com/t/why-dont-we-build-cars-with-smooth-surfaces-underneath/572192/1 "2011-02-22T04:19:33Z")
Why don’t cars have smooth body panels underneath? Seems it would keep things a lot cleaner. Of course, there would have to be vents and such, and maybe drain holes, and possibly hot components would have to be uncovered, but in general it seems to me that the current situation makes cars a lot more vulnerable to rust and crud kicked uo from the road. ---
**Author:** ![thelabdude](https://avatars.discourse-cdn.com/v4/letter/t/dc4da7/32.png) [@thelabdude](https://boards.straightdope.com/u/thelabdude)\ **Post date:** [February 22, 2011, 4:32am UTC](https://boards.straightdope.com/t/why-dont-we-build-cars-with-smooth-surfaces-underneath/572192/2 "2011-02-22T04:32:46Z")
You may be right. It is one more expense the consumer would see little value to, and not want to pay for. I would think it would help with gas millage too. ---
**Author:** ![Darryl\_Lict](https://avatars.discourse-cdn.com/v4/letter/d/7cd45c/32.png) [@Darryl\_Lict](https://boards.straightdope.com/u/Darryl_Lict)\ **Post date:** [February 22, 2011, 4:35am UTC](https://boards.straightdope.com/t/why-dont-we-build-cars-with-smooth-surfaces-underneath/572192/3 "2011-02-22T04:35:08Z")
It’s more expensive and has more weight. It restricts access. It’s more prone to failure and damage. Unless you are looking for ground effect advantages, it’s tiny amount of aerodynamic advantage is negligible. ---
**Author:** ![kenobi\_65](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kenobi_65/32/13178_2.png) [@kenobi\_65](https://boards.straightdope.com/u/kenobi_65)\ **Post date:** [February 22, 2011, 4:40am UTC](https://boards.straightdope.com/t/why-dont-we-build-cars-with-smooth-surfaces-underneath/572192/4 "2011-02-22T04:40:46Z")
Given how hard automotive engineers have pushed auto design towards more aerodynamic shapes over the past two decades, I’d have to conclude that, if placing a shroud (or whatever) on the bottom of a car improved mileage in a cost-efficient manner, it would have been done by now. In other words, I simply can’t believe that automotive engineers the world over would look at this thread, go :smack: , and say, “Smooth on the bottom!!! Why didn’t we think of that before now??” ---
**Author:** ![Pork\_Rind](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pork_rind/32/111_2.png) [@Pork\_Rind](https://boards.straightdope.com/u/Pork_Rind)\ **Post date:** [February 22, 2011, 4:48am UTC](https://boards.straightdope.com/t/why-dont-we-build-cars-with-smooth-surfaces-underneath/572192/5 "2011-02-22T04:48:03Z")
Until last year, I hade a Lotus Elise, a car that featured a full-length smooth aluminum undertray that incorporated a diffuser at the rear. Great for aerodynamics, a real pain in the ass when it came to oil changes. My current BMW has a flat plastic underbody that goes at least as far back as the passenger compartment. It has a flap for the oil drain, so I haven’t had to remove it and see if it goes further back. Recent Ferraris and Lamborghinis also feature sealed underbodies incorporating venturis and diffusers for aerodynamic purposes. I imagine most other high end sports cars have the same. ---
**Author:** ![Superhal](https://avatars.discourse-cdn.com/v4/letter/s/b4bc9f/32.png) [@Superhal](https://boards.straightdope.com/u/Superhal)\ **Post date:** [February 22, 2011, 4:56am UTC](https://boards.straightdope.com/t/why-dont-we-build-cars-with-smooth-surfaces-underneath/572192/6 "2011-02-22T04:56:59Z")
I’d say supply and demand. People will buy cars without it, so manufacturers feel no pressure to add it. ---
**Author:** ![Horatio\_Hellpop](https://avatars.discourse-cdn.com/v4/letter/h/e5b9ba/32.png) [@Horatio\_Hellpop](https://boards.straightdope.com/u/Horatio_Hellpop)\ **Post date:** [February 22, 2011, 5:00am UTC](https://boards.straightdope.com/t/why-dont-we-build-cars-with-smooth-surfaces-underneath/572192/7 "2011-02-22T05:00:28Z")
Volkswagen Beetles actually had that feature in the late 70s, and ceased production for most of the 80s and 90s. ---
**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\ **Post date:** [February 22, 2011, 5:22am UTC](https://boards.straightdope.com/t/why-dont-we-build-cars-with-smooth-surfaces-underneath/572192/8 "2011-02-22T05:22:12Z")
My 2010 Insight has some plastic trays on the underbody to help with mileage. It’s not a completely flat surface, but it’s an improvement over nothing. ---
**Author:** ![campp](https://avatars.discourse-cdn.com/v4/letter/c/c5a1d2/32.png) [@campp](https://boards.straightdope.com/u/campp)\ **Post date:** [February 22, 2011, 1:49pm UTC](https://boards.straightdope.com/t/why-dont-we-build-cars-with-smooth-surfaces-underneath/572192/9 "2011-02-22T13:49:48Z")
But… Then they couldn’t sell the undercoating at the dealership… ---
**Author:** ![Al\_Bundy](https://avatars.discourse-cdn.com/v4/letter/a/e79b87/32.png) [@Al\_Bundy](https://boards.straightdope.com/u/Al_Bundy)\ **Post date:** [February 22, 2011, 3:49pm UTC](https://boards.straightdope.com/t/why-dont-we-build-cars-with-smooth-surfaces-underneath/572192/10 "2011-02-22T15:49:36Z")
Many newer vehicles, especially upscale brands, have protective shields all over the underside of the engine compartment area and more. Sometimes it’s a pain to remove these things for service and inspection. Beyond the immediate front area there is little to gain from an air turbulence standpoint because air dams direct most of the air over the top of vehicles. I favor the “less is best” philosophy. Why carry the weight and expense? ---
**Author:** ![DrFidelius](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/drfidelius/32/3447_2.png) [@DrFidelius](https://boards.straightdope.com/u/DrFidelius)\ **Post date:** [February 22, 2011, 3:56pm UTC](https://boards.straightdope.com/t/why-dont-we-build-cars-with-smooth-surfaces-underneath/572192/11 "2011-02-22T15:56:20Z")
> [@campp](#): > > But… Then they couldn’t sell the undercoating at the dealership… You see, they apply that Tru-Cote at the factory. There’s nothing we can do about it… ---
**Author:** ![postcards](https://avatars.discourse-cdn.com/v4/letter/p/8e8cbc/32.png) [@postcards](https://boards.straightdope.com/u/postcards)\ **Post date:** [February 22, 2011, 3:58pm UTC](https://boards.straightdope.com/t/why-dont-we-build-cars-with-smooth-surfaces-underneath/572192/12 "2011-02-22T15:58:12Z")
> [@Krokodil](#): > > Volkswagen Beetles actually had that feature in the late 70s, and ceased production for most of the 80s and 90s. [Obligatory link](http://www.google.com/images?q=if+ted+kennedy+drove+a+volkswagen&oe=utf-8&rls=org.mozilla:en-US:official&client=firefox-a&um=1&ie=UTF-8&source=univ&sa=X&ei=09xjTa7UNcqs8Aazv6XfCw&ved=0CFoQsAQ&biw=1078&bih=590). ---
**Author:** ![joebuck20](https://avatars.discourse-cdn.com/v4/letter/j/74df32/32.png) [@joebuck20](https://boards.straightdope.com/u/joebuck20)\ **Post date:** [February 22, 2011, 4:20pm UTC](https://boards.straightdope.com/t/why-dont-we-build-cars-with-smooth-surfaces-underneath/572192/13 "2011-02-22T16:20:59Z")
> [@DrFidelius](#): > > You see, they apply that Tru-Cote at the factory. There’s nothing we can do about it… You lied to me, Dr. Fidelius. You’re a bald-faced liar. A…A…A fucking liar. Where’s my checkbook. Let’s get this over with. ---
**Author:** ![DrFidelius](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/drfidelius/32/3447_2.png) [@DrFidelius](https://boards.straightdope.com/u/DrFidelius)\ **Post date:** [February 22, 2011, 4:29pm UTC](https://boards.straightdope.com/t/why-dont-we-build-cars-with-smooth-surfaces-underneath/572192/14 "2011-02-22T16:29:32Z")
NOTE TO MODERATORS– joebuck20’s response is expected, and the absolute correct call-back to a movie quote I made. It is NOT a personal insult, and while it may be off-topic for the thread it hw is NOT being a jerk. Thank you. ---
**Author:** ![akira5822](https://avatars.discourse-cdn.com/v4/letter/a/53a042/32.png) [@akira5822](https://boards.straightdope.com/u/akira5822)\ **Post date:** [February 22, 2011, 4:32pm UTC](https://boards.straightdope.com/t/why-dont-we-build-cars-with-smooth-surfaces-underneath/572192/15 "2011-02-22T16:32:55Z")
Fargo, If i am not mistaken? ---
**Author:** ![Morgenstern](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@Morgenstern](https://boards.straightdope.com/u/Morgenstern)\ **Post date:** [February 22, 2011, 4:34pm UTC](https://boards.straightdope.com/t/why-dont-we-build-cars-with-smooth-surfaces-underneath/572192/16 "2011-02-22T16:34:40Z")
Another problem would be maintenance, and warranty work. Adding panels to the bottom would increase the time repairs took, as well as the cost of maintaining the panels. Besides, who actually looks under a car before buying it? So having those panels would probably not increase sales or profit for the companies. ---
**Author:** ![Yarster](https://avatars.discourse-cdn.com/v4/letter/y/7c8e57/32.png) [@Yarster](https://boards.straightdope.com/u/Yarster)\ **Post date:** [February 22, 2011, 5:38pm UTC](https://boards.straightdope.com/t/why-dont-we-build-cars-with-smooth-surfaces-underneath/572192/17 "2011-02-22T17:38:56Z")
My 2003 Infiniti FX35 (SUV) has a plastic ‘skid plate’ that goes from the front of the vehicle to the passenger compartment area. I have never understood the point of this since the SUV is already very UN-aerodynamic to begin with. All this serves to do is make changing the oil and anything else under the car a pain in the ass because it has to be removed with these dumb plastic pushpin things, half of which are now broken because they are so cheap, they self destruct after they’ve been remove and pushed back in place after about 10 uses. I assume the real purpose is to catch minor drips so they don’t end up on the garage floor. The handful of times I’ve bottomed out the car on a dirt road, it’s not like the 1/4" thick piece of plastic protects anything either other than making the “thud” sound a little less bad. I can assure it does nothing to prevent damage. ---
**Author:** ![joebuck20](https://avatars.discourse-cdn.com/v4/letter/j/74df32/32.png) [@joebuck20](https://boards.straightdope.com/u/joebuck20)\ **Post date:** [February 22, 2011, 5:50pm UTC](https://boards.straightdope.com/t/why-dont-we-build-cars-with-smooth-surfaces-underneath/572192/18 "2011-02-22T17:50:01Z")
> [@DrFidelius](#): > > NOTE TO MODERATORS– > joebuck20’s response is expected, and the absolute correct call-back to a movie quote I made. It is NOT a personal insult, and while it may be off-topic for the thread it hw is NOT being a jerk. > > Thank you. Yes, I did mean that in jest. Thank you for pointing that out before I got a chewing-out from the mods. ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [February 22, 2011, 6:30pm UTC](https://boards.straightdope.com/t/why-dont-we-build-cars-with-smooth-surfaces-underneath/572192/19 "2011-02-22T18:30:11Z")
I’m not concerned about the aerodynamics; it just seems to be that having panels down there would keep things cleaner. I guess it wouldn’t make that much difference? Or are we just used to the current state of things? ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [February 22, 2011, 6:38pm UTC](https://boards.straightdope.com/t/why-dont-we-build-cars-with-smooth-surfaces-underneath/572192/20 "2011-02-22T18:38:18Z")
there are two different things going on here. - cars with “smooth” undersides usually have air ducts and diverters to increase downforce at high speeds. - on road cars, under-car aerodynamic treatment is there to reduce drag and doesn’t require a completely shielded/smooth surface. oftentimes you’ll see little things like “ears” and baffles and such to direct airflow away from or around places that might cause a lot of drag. [Next page](https://boards.straightdope.com/t/why-dont-we-build-cars-with-smooth-surfaces-underneath/572192.md?page=2)