Making your own undertray

I have been thinking about making an undertray for my 164 (or having one made). I don't believe it ever came with one from the factory. I would like it for undercarriage protection and for better aerodynamics. I know I can use cardboard to make a pattern, but my concern is with actually fabricating it. I have two problems:

1) I have zero metalworking skills.

2) The front downpipe runs directly under the oil pan. I'm concerned about the undertray turning into an oven if I cover the downpipe.

What alternative materials are out there for undercarriage protection that are easy to work with? Would I be better off biting the bullet and taking it to a race shop to have one fabricated?

You could put some louvers in it under the oil pan, that would work while the car is moving at least.

If you can make it out of cardboard, you can make it out of aluminum. Just rivet it together.

My prizolla has an interesting setup, it has a factory undertray that is sorta formed to the front of the engine and transmission. This tells me most of the aero gain can probably be realized in a transverse engine application just by getting clean air up to the bottom of the oil pan. Plus it doesn't get in the way very much when working on it. Try that and let the exhaust hang down below to some extent (clearance the plate for it).

In my 924s the under tray comes up to the oil pan. The bottom of the pan is finned and is in the air stream behind the pan. The header pipe drops down next to the oil pan and is open as well. Something to consider is to make sure air can get out of the engine bay. Seal it off to well and cooling can be an issue as can "packing" of air in the fount of the car.

A sheet of 3/16" UHMW polyethylene from McMaster Carr, some aluminum angle stock from the hardware store & pop rivets worked for me. Single sheet for the bottom, angle & skirts along the edges with a radiator fence in the middle. Held up well for years now.

I would leave the exhaust uncovered and do the tray on everything else. My Camaro has the exhaust in a tunnel above the pass side floorboard. Man does it get hot in there, even with a lot of insulation. Just something to think of.

Yeah look at how sportbike bodywork has cutouts near the exhaust. You could do something like that.

The other option is to wrap the exhaust and put aluminum tape on the undertray near it...you'll still get more heat buildup though.

Depending on how wide you need to go, I have found that several local rallyx vehicles made lower skid plates out of old road signs. They are also fairly inexpensive on CL.

I vote for louvers.

I would add a small door with a boxed interior large enough to accomodate a small meatloaf and a baked potato.

fornetti14 wrote: Depending on how wide you need to go, I have found that several local rallyx vehicles made lower skid plates out of old road signs. They are also fairly inexpensive on CL.

or call the local DMV and ask where you might be able to pick up some retired road signs. Some places have piles of them, others scrap em almost immediately.

brich plywood with a resin coating over it is the only real answer. Its got the strength, its got the flexibility to not break when impacted, its got the lightness, and its orders of magnitude cheaper than everything else that meets the other criteria.

I wonder how the birch ply would do under a pickup. That's a big span without a lot of supporting points. I'm thinking I want to aero mod a full size truck and see how it does MPG wise. Undertray being that starting point I was thinking about. The resin coat would keep it from absorbing moisture. If you wanted to go completely nuts with it encapsulate it in a layer or two of glass, drill your mounting points oversized and fill with thickened resin the same way you would through holes/hulls in a boat hull.

the wide the span the thicker it needs to be. For a normal car splitter/tray with 2 main attachment points and 2 front points 3/8" ply is more than sufficient. The bigger span would probably need 1/2" or 3/4". You can also make your own ply in 5.5mm increments of thickness by bonding lauan together with wood glue. I glassing the wood can go both ways. It will make it stiffer but it'll also make it more likely to break rather than bend if you hit something.

I like these ideas.

Dashpot, do you have any pictures of your setup?

I will probably leave a cutout for the downpipe whatever I do.

Spinout007 wrote:

fornetti14 wrote: Depending on how wide you need to go, I have found that several local rallyx vehicles made lower skid plates out of old road signs. They are also fairly inexpensive on CL.

or call the local DMV and ask where you might be able to pick up some retired road signs. Some places have piles of them, others scrap em almost immediately.

A local scrap yard used to sell them by the pound. They're thick, though, and I'm not sure the alloy works well for bending. More skid plate material than under tray.

It's election time. Coroplast and zip ties.

dean1484 wrote: In my 924s the under tray comes up to the oil pan. The bottom of the pan is finned and is in the air stream behind the pan. The header pipe drops down next to the oil pan and is open as well. Something to consider is to make sure air can get out of the engine bay. Seal it off to well and cooling can be an issue as can "packing" of air in the fount of the car.

How do the race car builders get heat out with a flat bottom?

I'll bet a lot of it goes out the wheel wells, and the exhausts aren't run down underneath the car.

Keith Tanner wrote: I'll bet a lot of it goes out the wheel wells, and the exhausts aren't run down underneath the car.

wheel wells are high pressure though. But yeah the exhaust on flat bottom racecars doesnt go the full way back. There's still tranny and diff heat to worry about but I'm confident that they've tested and determined if tranny and diff coolers are required or not for their length of race.

Leafy wrote:

Keith Tanner wrote: I'll bet a lot of it goes out the wheel wells, and the exhausts aren't run down underneath the car.

wheel wells are high pressure though.

Not with louvers on the top and/or rear of the wheel wells. Some do vent this way, but most race cars with flat bottoms and rear-exit exhaust vent out of the hood.

Tranny and diff coolers are usually rear-mounted to vent above the diffuser.

Wheel wells aren't high pressure if they're not there

Keith Tanner wrote: If you can make it out of cardboard, you can make it out of aluminum. Just rivet it together.

Seriously this is the best way. If you can rivet two pieces or metal together you can hack one together. The biggest issue is making it easy to remove for maintenance.

Do not just cut slots in it to remove heat louver it. Also race cars vent it out the back you can build a heck of a lot of drag if you are not careful. Or just hack the top of the fender out to give the air somewhere to go.

Some folks on AlfaBB pointed out a factory alternative to a skiplate:

I doubt it does much for aero, but I was wondering if anyone else has tried a similar solution and if it does anything to protect the oil pan.

Find/ buy/ borrow an aluminum road sign; the large ones that they use for construction zones work best. Get a sawzall, cutoff wheel, hack saw, bandsaw, whatever and get to work. People may wonder why you have a road sign on the bottom of your car.