Making A Under-tray

[The Bogan]

I'm going to make a under-tray for my 33 and was wondering how for back i should go, I was going to tuck it inside the standard front bar and have it cover the whole length of the bar then taper it back mounting it with the points on the sub-frame that the standard one uses finishing just behind the sump plug where another 2 mounting points are, ill be cutting out a section to get to the sump plug.

I was going to use 20 gauge alloy.

Any tips or suggestions

[The Bogan]

Now using 3mm polyethelyne sheet. $63 for a 2.4 x 1.8 sheet. Impact resistant.

[Craved]

you think that polyethelyne will form under heat?

if you used big washer and bolted it up as a flat sheet in multiple place, then heated it up it might stretch to reduce the tension on ridges then cool to keep the shape.

i would take it back to the firewall, putting in vents like naca ducts to help the air exit that area. air must be able to leave the engine bay efficiently to stop the tray creating lift on your front end at higher speeds.

[boxheadmr]

you should be able to get the sheet cheaper, there is a place in seven hills that sells all sorts of sheet plastics they are on station road.

To give you an idea i got the same thing for $20.

[W0rp3D]

Why not do the whole length of the car?

[Keeper]

go back to almost the gear box, then again from behind the rear axle

[funkymonkey]

One of the guys in Canada (eh?) did this to his S14.

not what you're thinkin of?

[Roy]

May be ok for a lap or three, but hate to think what that would do to gearbox, engine, and diff temps

[funkymonkey]

he seemed to do OK with it while coming 1st in their local touring car championships he did run diff and gearbox coolers with NACA ducts.

[Medium Dave]

Here's a link to an autospeed article which covers a bit of theory, building one and testing on a toyota prius mostly for fuel consumption, but they do describe some handling benefits - Worth a read

[benm]

Interesting article on that Autospeed page.

[Carbon 34]

Im in the process of making a CF tray for my GTT, but some ass hit my car in a car park last week and did a runner, so its delayed for a bit while I fix that first. photos when its done. full tray front to rear for the most part with room for engine, trans, rear axle to breath still. kevlar aramid fibers near tail pipes to keep the fire risk down. built in brake ducts and lower lip.

[The Bogan]

Thanks for the input.

Ive decided to not cover the whole front and just cover under the engine bay following the sub-frame, and ill stop at the gear box.

The auto speed article was good, but, I've raised my bonnet with washers and tested it by placing wool inside the engine bay which when driving along showed good airflow coming out the raised rear section ?, also when I'm stationary you can see hot air pouring out the gap.

[daisu]

I'm going to make an undertray for my cefiro, but my main concern was that air was entering the mouth of the bumper and then going underneath the radiator support instead of through the radiator. I'm not too concerned about the aerodynamic effects of having the tray extend further back.

[dontfeelcold]

flat bottom all the way is what you want.

Like it has been said before, put ducting in to allow air out under the car, even out via vents behind the wheels. You can also put ducting in to cool the gearbox, just make sure you allow the air to exit. A controlled airstream may be better than the standard airflow under a skyline to cool the gearbox and diffs. Look at undertrays from other cars (R34 GTR's and the new GTR, porsche etc) for inspiration and ideas.

You can always check gearbox and diff temp by putting a engine temp sensor in the drain plugs then hooking up a gauge to it, be creative.

[ellie]

I've been looking into this a fair bit also. Mainly because I love fooling with stuff that most others don't have the patience for (The 1 percenters if you will) because it all adds up. This was my first attempt just prior to going for a nice drive.

After this I modded a little by moulding it to the raised centre section of my front bar, a long evacuation gill just before the engine to aid evacuation of radiator heat, then slightly lipped the rear egde of the tray to help create a low pressure area just after to help pull the heat of the engine bay under the car (As the rear of the engine bay graduates air flow in that direction generally but can suffer from swirl/backpressure etc.). This section will hopefully help with cooling also as it dierects air to flow in through the radiator and straight out under the car in one motion. cars with out an under tray tend to have air travel under the car and swirl into the engine bay at certain speeds causing a pressurisation, theoretically opposing good flow through the radiator.

I've got my self a new infra red temp gauge but haven't played much yet because my standard radiator is to be replaced soon with a triple core because it is quite simply pathetic for anything over commuting in a tropical state.

I know on a lot of high end cars, they tend to have a very smooth under body to just before the the rear carrier. To cool the fuel tank, but from what I have read, moat cars go backwards in fuel efficientcy when the rear is trayed. But in general they do make sure there are no parrachut areas like the rear bar by extending them further than usual which also reduces wake by pulling excess air from under the car into the low pressure zone.

No expert here! Just looking to learn.

[GTR-N1]

...and if you want some NACA Ducts Mark, you can google

www.kenlowe.com.au > KenLoweRaceCars > Raceglass Composites > scroll down to 4" Naca Ducts in black plastic. Based in Qld.

or larger Naca Ducts in clear plastic from Blacktown.

[SLY-25T]

u could see if GKtech could send u a pic of the one they had for the 33. sadly dont import anymore i dont think.

[skylinecouple]

I have made a front tray from the leading edge of the front spoiler to the standard under tray. Used 1.6mm alloy sheet which sits inside the lip in the bottom of the spoiler and covers the entire width of the car. This is bolted in using existing holes and clip nuts as is often used for sheet materials. At the rear it is bolted using 10mm bolts in existing bolt holes. I don't know about drag but I can assure you there is less wind noise at anything above 80kph. In fact, I can carry on a normal volume conversation now since this mod and removing the rediculous sun visors from the windows.

[boxheadmr]

One thing to think about with the poly is the heat from the exhaust and engine/drivetrain. You might have to use something else then the std plastic around these areas as most will melt at higher temp's. Spending some time making up a flat under tray on a saloon car in theory will give some benifit to downforce and also with the aid of a rear defuser will allow for better exist of the air from under the car. You should get less drag as well.

You would want to incorperate a front spliter if you are going to the effort to channel the air under the car and generate downfore and make sure that the front is lower then the rear or possiabally face some lift as the tail catches the air.

Matt