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      04-05-2011, 09:50 PM   #1
LiM3y
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Drives: 2008 E90 M3
Join Date: Jun 2009
Location: CT

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DIY: Under-tray skid plates

This is my first DIY, so bear with me...and any experts in metal working and technical writing better look away now!

I had to replace the whole of the front under-tray due to one thing and another. The middle section was cracked by a speed bump and the side sections have become worn through catching the road on inclines. I have Eibach springs with winter tires combined with a residential road resurface which exacerbated this and the driver side side section wore completely through at the corner. I had to replace all three sections, and I elected to generate home-made aluminium skid plates for the side sections. This is my story. The names are fictitious to protect the innocent.

Photo #1: The 12" x 18" aluminium sheet purchased from Lowes. I elected to use this grade to minimize weight and also be easy to work/cut with without special tools

Photo #2: Using the plastic/felt section I made an outline on the sheet, using the steel washer backers (see later) as a guide for distance. I aligned the piece a little away from the edge as I wasn't sure how the sides would line up when folded up. This also allowed the end to fold around to help get a good fit. I also cut tabs on the inside to fold over when attaching. Make sure you leave gap for the attachment screw to the middle section. The fitting with the centre tray is loose enough to allow the tabs to fit.

Photo #3: Using snips to cut out the shape. You can use the cut out shape as a template for the opposite side, you just flip the sheet over to get a mirror image. (This occurred to me after I repeated the templating process - Duh!)

Photo #4: The completed cut outs. I finished the sections after the photo to remove burrs and edges with a file and a Dremel. The cuts aren't great, but as my girlfriend acutely pointed out, what does it matter, no one can see them anyway! You can see my attempts at making sure the measurements were accurate on the metal!

Photo #5: The folded rear edge and the front section. I made small v-cuts in the front section to allow the metal to fold up without too much warping. The sections were bent upright a little bit (20-30 degrees) at a time with pliers gripping the marked line to create a fairly consistent fold.

Photo #6: The completed folded part mounted on the plastic/felt section. I have drilled 5 mm holes for rivets in the metal piece only. I drilled around the front and made a single hole on the underside near the outside edge. On the right side, you can see how I have folded the section at this acute angle. This small edge helps later with getting a good fit. You can see that I made slightly too wide v-cuts for the fit. [NB Be wary of the placement of the hole on the left (nearest the centre tray) as when the section is riveted, the rivet can interfere with the join with the centre section. Next time, I would place further to the right based on this view]

Photo #7:The sections prepped for painting. These were rubbed down with coarse sand paper and wiped down with Goo-Gone to degrease. I painted with a matte black engine enamel. I did have an attempt with rubberized chassis paint, but this did not stick at all on the aluminium (learning point!). I used several light coats

Photo #8: The painted parts.

Photo #9: The mounted skid-plates. I kind of went over board with rivets, but I didn't want them coming off at the wrong time! I backed all the rivets with a broad 5mm steel washer for extra support rather than rely on the plastic of the section as an anchor. I started by drilling the bottom hole first and riveting. I then folded the tabs, drilled through and riveted. The front and rear sections were riveted. I felt doing it this way afforded a much better fit of the skid plate on the felt section. I repainted the section to cover the rivet heads

Photo #10: The rivets I used and 5mm steel washer backers. These rivets were just the right length

Photo #11: This shows where I had to cut out the centre tray to accommodate the rivets on the tabs and the inside front section. This needs better judgment next time around.

Photo #12: The skid plates finally mounted.

Comments on mounting the under-trays. It is all pretty obvious when you are under there what needs to be unscrewed (8mm bolts around the front and wheel wells, 10mm (I think) where the front under tray meets the engine under-tray.

I mounted the middle section first. Do the Bowden A hangers first, and then the rear most screws by the engine undertray. When attaching the side sections, attach by the screw on the underside of the middle section and side section, some of the others attach to the wheel arch or bumper, so need to be lined up, then fix the screws furthest inside the wheel arch and work out. The screws need to reach through the wheel lining, the side section and into the fixed nut on the center tray. This needs a bit of tugging and is almost impossible when the outside screws are fixed (ask me how i know) but they will eventually fit. Work your way around making sure that all the sections (side, middle and underside of bumper) are buckled together.

There you go! Oh, and you will definitely know when they are working - the hollow under-tray amplifies the grinding of metal noise very well. It helps you learn where you habitually catch the undertray in your daily drive
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