How to fabricate a skid plate?

Time to start thinking about putting a skid plate under the car. Have some question about how to properly mount the skid plate. My biggest concern is transferring the load from an impact and damaging a cross member or frame rail. I am also hesitant to mount it to the bottom of the core support because they are typically very weak.

Here are a couple pictures looking from front to back under the car.

You can see the oil pan is hanging way down and would be the first thing to hit. At least it's sheet metal and not cast. The problem is in the front there is really nothing to mount to. The boxed frame rails end at the weak core support.

Damn accidentally posted.

I'm thinking I could kill two birds with one stone and build a brace to tie the ends of the frame rail together that would also provide a mounting point for the front of the skid plate. Looks like I could mount it using the sway bar mounts.

Moving back the cross member is just u shaped stamped steel. Not really strong enough IMO. Maybe I could use one of the cross member mounting bolts on each side to mount the skid?

Also how much clearance should I give between the oil pan and the skid plate?

I'm wondering if I should design it very rigid and use lots of mounting points to try and minimize the risk of pin point loading and subsequently tweaking areas of the frame/chassis that where not designed to be loaded, or if I should design it flexible using few mounting points in an effort to dissipate the energy from an impact so as not to damage to the frame/chassis.

Or maybe I'm over thinking it?

Paging Woody.....

How did your homebrewed one work out?

I still like the fact that it used to be a sign
http://grassrootsmotorsports.com/forum/grm/bending-aluminum/47117/page1/

On the e30 my first one was mounted to the core support in front and to the front subframe at the rear edge (using two holes already in it, through-bolted to the top of the subframe). For rallycross you're unlikely to take any super-hard hits (like you would in stage rally going over jumps, etc), so its unlikely that you'll damage the core support. The skid will likely push in at the bend before it breaks any part of the chassis.

My current plate bolts to the core support in front (reinforced with a metal bar running across it, and at the rear it uses the steering rack bolts.

For max support, some guys use a U-shaped bar or DOM tubing running from the frame rails on the sides and down in a "U" supporting the bend in the skid. e30motorwerks makes a setup like that for e30s, but google it and you'll see what I mean.

Our buddy who rallycrosses an e28 has a 6-point "frame" above the plate welded to the car frame rails and it looks like it can take a hit from falling off a cliff. And also looks heavy.

Your main goals for rallycross are to deflect any direct hits from sharp rocks, etc on the oil pan (especially with us M42 guys with the cast pan, but you're gonna be a lot safer with the sheet metal pan). I'd say 12ga or even 14ga would work fine, and will deflect enough so that it won't damage the core support.

Could you run 2 slider bars a half inch lower than the oil pan on either side of pan. It would not stop a rock or chunk of cinder block but it would skid over a drop off. May also give you a mounting point for an easily removed sheet steel skid plate. You must be able to access the drain plug.

my clearance is about 1/2" under the pan. I'm not worried about the skid hitting the pan, as it would be distributed across the whole pan. The real key is just to keep single-point impacts (like a rock or something) that can dent the pan easily.

There is some good reading on the subject in this thread on specialstage.com.

In the last year I've seen more damage caused by skid plates than prevented by them. Cars tend to have a big hit, dig the skid plate it, rip the front off and fold it under the car and smash the hell out of stuff. I know every region is different, but a poorly built plate is worse than none at all.

irish44j wrote: For rallycross you're unlikely to take any super-hard hits (like you would in stage rally going over jumps, etc), so its unlikely that you'll damage the core support.

I severely mangled my skidplate's front mount by hitting a dirt berm without lifting.

Given that the thing protects my oil cooler, that 70-odd pound chunk of steel paid for itself in spades that day.

Driver is key. I tend to commit to a line and then overshoot what I was going for.

In the last year I've seen more damage caused by skid plates than prevented by them. Cars tend to have a big hit, dig the skid plate it, rip the front off and fold it under the car and smash the hell out of stuff. I know every region is different, but a poorly built plate is worse than none at all.