Undertrays on cars

Discussion in 'Fuel Economy' started by Smile-n-Nod, Aug 14, 2008.

  1. Smile-n-Nod

    Smile-n-Nod Well-Known Member

    I've read that some people have bolted sheets of aluminum or corrugated plastic to the undersides of their cars to improve the aerodynamics. In general, what effect does that have apart from increasing FE? Does the plastic sheet accumulate oil and other fluids which can no longer drip on the street or driveway? Also, the sheet probably needs to be removed to access certain parts of the car; how much of a problem is this? Would it cause maintenance problems if car designers streamlined the undersides of cars?
  2. Takashi

    Takashi FUD

    Make sure you can access important parts e.g. the oil pan bolt and oil filter. You also don't want to cover any parts of your car's exhaust system.

    Heck I won't border adding panels to the undercarriage to your car unless it comes with it. The advantages are minial and at the same time you add more dead weight to your car.
  3. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    It's good enough for Lotus and Ferrari from the factory, and for Honda to add to their hybrid models. You do need to be able to access some parts, like the oil drain and filter. Other parts can be fully covered, like my gap between the rear axle and the parachute bumper.

    Side view looks something like this, with ] being the bumper:


    There are no serviceable parts there, just empty floor space. I could cover that up permanently.
  4. JusBringIt

    JusBringIt Be Inspired

    If you do highway more than city, then undertray would work in your favor. How you install your undertray will determine the difficulty in accessing/maint.
  5. basjoos

    basjoos Well-Known Member

    Other advantages of an undertray is that it reduces "road noise", part of which is generated by wind turbulence under the car. Also you can hit deep puddles on the road at speed without worrying about drowning the engine with water splashed up into the engine compartment, ditto driving in deep snow and getting it piled up against the firewall inside the engine compartment. You can cover your exhaust system if you design it to allow for sufficient cooling air flow, especially around the catalytic converter. As far as maintenance goes, just design it with quick access panels to allow access for oil/filter changes and so you can easily remove parts of or the entire undertray for those rare occasions where you need to take it into the garage for nonscheduled undercarriage repairs (CV joints, etc.).
  6. pumaman

    pumaman Well-Known Member

    The Mazda 6 comes with a large tray that pretty much covers the entire underside of the engine bay . You have to remove the whole thing to change the oil and filter, but it's 7 easily accessible bolts and only takes a couple of minutes to remove. Not much weight. I'm sure it helps with FE and noise.
  7. scissorhands

    scissorhands Well-Known Member

    I wish I had some wheel ramps to do this job
  8. pumaman

    pumaman Well-Known Member

    Yeah, shortly after getting the 6 I got ramps. Makes the oil change so much easier.
  9. Markmysite

    Markmysite Active Member

    Adding the underbelly helps with road noise, and mine doesn't block access to the oil filter or oil pan. I also made a grill block to reduce aero drag on the car (the radiator grills account ofr over 30% of a cars drag).

    Here's a link to a post I just put about about how I did mine:

  10. Chuck

    Chuck just the messenger

    Missed a Golden Opportunity!

    I just realized that all those corrugated plastic campaign signs I could probably have taken for free. Then the underside of my car would look like a polling place. :D

    EXPIOWA Man of Leisure

    Smile-n-Nod, I just finished my front undertray and have a temporary one in place on the back. I love it. It blocks nothing mechanical because my oil filter is on the back of the block.

    BENEFITS of an undertray:
    -Lower drag, better mileage, longer drift-down distances
    -In my case, increased stability, especially in turbulence from gusts or passing semis
    -In my case, better control of airflow through the radiator causing lower operating temps
    -I have noticed maintaining a constant speed is easier on the freeway
    -As mentioned earlier, water won't splash up onto your engine, belts, and pulleys

    I must disagree with you Takashi. You have obviously never added one to a car before. Here is your homework assignment. Modifying Under-Car Airflow, Part 1 and Modifying Under-Car Airflow, Part 2

    The BAD:

    -Slightly reduced passive airflow with the cabin fan OFF and windows up. Easily corrected by finding out where the cabin air is vented overboard and providing a vent hole in the undertray. Not really a big deal anyway. Plenty of airflow still exists.

    -Added approx 2 lbs. of of "dead weight" to the car. - Easily compensated for by not overeating. (As often) :) Cleaning the Snickers, and other assorted, wrappers out of the back seat would probably be an overall weight reduction for a lot of people.

    scissorhands, ramps are cheap at Walmart. They are probably the safest and easiest way to get underneath your car.
    Last edited: Nov 10, 2008

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