DIY - Prototyping undertrays
 

Instructables Link

Just thought I should blow my own trumpet. It should prove useful.

Next month (if it all works out and the result is positive) I'll do ABS ones, so that they look good...

Very nice!

I think your "reasons" for the OEMs not doing it aren't quite right though- I would put the reason at not being able to service the car easily. That IS how the dealerships make their money- working on the cars after the sale.

No, my undertray doesn't extend past the bottom of the radiator, it covers no maintainance items except possibly the air-con gas reservior. The rear one covers only the fuel tank (as stated in the instructable).

Incedentally it appears to work. The pair have apparently dropped 0.3l/100km (according to factory display), and the car has noticably less wind noise....

Ah, ok. I was thinking that your tray covered the entire length of the car.

Even with a full tray they can always add access panels. That is what I intend do do.

You have to understand that while you drive your car on the road at a nice pace with abundant air and usually at short distances, when manufacturers make a car, they engineer them to survive extreme conditions, including 100+ degree weather with full load capacity with the AC on and in stop and go traffic. In order for a car to function with an undertray and survive this and more it would take much more engineering, exotic materials, and money. This is why cars do not come with undertrays, save for the ones that cost north of $100k. I saw a whole story with car engineers about the matter several yeas ago. Heat and longevity are the problems.

I did the testing. Saved 0.4l/100km...

I also tested the cooling system performance with and without the trays, no difference. The only downside is that anything that goes through the grill opening either goes through the radiator or gets stuck. There is no more option for birds to fall out under the car, they just stay there and rot if I don't know they are there

Oh I should have mentioned my driving during testing covered a very wide range. Hot, cold, wet, dry, city and country (and any combination of those). And a couple of mountain ranges too.

And just for good measure, some 150kph cruising in 35*c weather...

the 2011 Audi A8 has a factory underpanel
http://i678.photobucket.com/albums/v...underbelly.jpg

And my 02 Infiniti Q45 has factory under panel for the first 38 inches.....just at the transmission.
also angled deflectors infront of the front tires.

there is a service panel that opens to reach the oil filter and the nut.

http://i678.photobucket.com/albums/v...pgq45014-3.jpg

Had I not installed a full body kit which lowered the profile 2 inchs and created a gap to the panel, I would not have made a new one.

Most new cars are quite smooth on the underside and have at least partial undertrays...you can even fit one to most older cars as well. I was referring to complete belly pans like those found on the Mercedes SLR.

http://img12.imageshack.us/img12/1143/sspx0214.jpg
http://img12.imageshack.us/img12/2594/sspx0213.jpg
http://img518.imageshack.us/img518/6392/sspx0219.jpg
http://img259.imageshack.us/img259/8250/sspx0218.jpg

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That may be so, but when they make a car they design it so that say for example should you choose to live in the Mojave desert and drive around all day, your car will keep going and wont have its components break down due to thermal failure or parts start melting. Not just take a drive one afternoon. Even so, some otherwise well designed cars suffer with all their guts exposed. Take for example the earlier v12 S-class Benzes, great cars, great mechanicals...but they generate so much heat that over time they would literally melt all the plastic components underhood if you stepped on it regularly or drive around all day....this is why today they are not the most desirable of cars even if you do find one still running.

What I am saying is there are no free lunches, yes you might gain efficiency in the short term, but unless you are engineering in ways to protect and cool vital components, you are shortening the life of your car or the rate at which parts breakdown....it does not always happen overnight.

Worth noting on the Audi A8 undertray is that the exhaust system is fully exposed - pipes, mufflers, right down to the tailpipe. Take note! No dummies, those guys...

i have to say BMW for the win and rear diffusers are the bees knees if used correctly.

joking aside i am doing similar things to my 94 530i and 96 vw cabrio. cabby needs it since its not lowered. my bimmer is lowered almost 2 inches in front and 1.5 in back. has a slight rake which creates a low pressure area in the mid-rear area. with belly pans and a diffuser it would get really good downforce at highway speed (good during passing semi-trucks and turbulent windstorms) and less air under car = lower Cd.

One of the things that could be done to help with the heat issues and air build up under the hood would be to some how vent the engine compartment. One of the things I have done on some of my cars is to remove the weather stripping between the hood and the firewall. It's not a big gap, but some cars you can space the hood up in the rear to let the air out. Ever noticed the hood on the old Ford GT 40? Trans Am's had a vent in the front fender between the inner fender and the firewall to vent the engine compartment. All the air that goes thru the radiator is vented out over the top of the hood. I haven't done it on this car because of the oil smell from minor oil leaks and seepage. Those fumes go straight into the air intake for HVAC (low pressure area at the base of the windshield) if it is switched to flow thru ventilation. I could keep that closed, but sometimes I like to use the outside air pressure to bring in cool air depending on the seasons. I have some other places I may add vents too.
You could add some louvers to the rear area of the hood too. Just my thoughts on this Idea.

2005 BMW 3 series have undertrays that extend well behind the front wheels as well.

Dave

My understanding is that the area at the rear of the hood / bonnet at the base of the windscreen is a high pressure area, and air is more likely to travel into the engine bay from that area, than escape from it.

generally the best place for a vent is at the front of the bonnet (ok hood...). Here in australia a few 80'z sports models were made with reverse facing scoops to best utilise the high pressure area at the base of the windscreen.

At least that explains why my mates rb30 equipped r32 would overheat without the fans on. He had to raise the rear of the bonnet for inlet manifold clearance...

incidentally I never did do a before and after pressure drop test with these trays, just economy and indicated temperature