A skid plate is an important addition to any dual-sport bike. For relatively little weight and cost it protects your engine case from stones kicked up by the front wheel and harder impacts like when you bottom out on a big rock or a log. I've installed a Flatland Racing skid plate on my WR250R and am quite pleased with its performance. As you can see from the peened underside, it's prevented a lot of gravel from hitting my crankcase.
The downside is extra noise. This comes from three sources: (1) engine noise reflecting back up to the; (2) resonance effects caused by engine and drivetrain vibration; (3) impact noise from gravel and trail debris hitting the skid plate. Fortunately it's fairly easy to attenuate these effects.
One method is to insert a foam sheet between the skid plate and engine case. (There are motocross products designed for this purpose.) Filling this volume makes it harder for the air to vibrate (translating to less noise), and the foam itself also transmits vibration poorly. This foam has the added benefit of preventing mud from accumulating in the gap.
A second method is to bond a dense, flexible material onto the skid plate. This serves to damp vibrations, particularly in the higher frequencies that are more noticeable (and annoying) to most ears. I opted for the mass-damping approach.
Online there are many suggested methods of applying mass-damping materials. I was most curious about what worked best for the lowest cost, and somewhat skeptical of conflicting claims. At one extreme are special sound-proofing materials such as Dynamat or Brown Bread which incorporate a kind of dense, rubberized inner layer backed with an aluminum foil constraining layer on one side and a self-stick adhesive on the other. This stuff works great, but it's expensive and not readily available to me. At the other extreme are various spray treatments like rubberized undercoating or truck bed liner. These appeared to work fairly well, although several coats are required to build up enough thickness to work effectively as a mass damper.
Should be easy, right? Yeah, here's what I learned to save you the hassle.
Option 1: Dupli-Color Truck Bed Liner
There are various undercoating/liner sprays available. Most seem to have a high bitumen content and I wasn't sure how that would hold up under engine heat and exposure to engine oil and gasoline. The truck bed liners seemed to favor either a vinyl or polyurethane chemistry. I opted for the latter, since in my experience PU is quite durable and fuel resistant. At almost $20 for this can, I wish I'd seen other reviews saying to stay away from the Dupli-Color!
After carefully cleaning the skid plate (shop hand cleaner with pumice works surprisingly well, followed by degreasing with rubbing alcohol), I carefully applied two layers of the spray (allowing drying in between) to build up a solid coat on both sides of the skid plate. It looks spattery when it first goes on, but as the material dries it pulls together and forms a matte sheet that looks pretty good. Here it is after one coat.
Unfortunately, this stuff proved to adhere really poorly to the aluminum despite 24 hours of drying time. I was able to scratch it off with a wooden stick. More important, although I'd only applied two layers and created a noticeable rubbery texture, this made little difference in attenuating sound. I figured two more layers would be a waste of time and money and decided to strip it off and try something else. Rubbing alcohol was amazing for stripping. A few sprays followed by light scraping with a metal spatula, and I had the whole skid plate shiny and looking new in about 20 minutes. There's no way this spray would withstand trail abuse.
Option 2: Rubber mat with contact cement
Home Depot sells these recycled rubber floor mats that seemed perfect for my application. There's a thick version (maybe 6mm) but I opted for the 2mm material. It's fine rubber particles bonded in a tough, flexible sheet. This roll cost only $10 and will come in handy for many other projects.
Contact cement seemed like the obvious way to bond the rubber to the skid plate. Unfortunately, the cement adhered poorly to the rubber and I was able to peel off the sheet easily. Fortunately it was also easy to peel the contact cement off the aluminum. Back to square one.
Option 3: Rubber sheet with Elmer's Spray Adhesive
I'd forgotten how sticky this stuff is:
After a couple of quick sprays I clamped the edges of the sheet with strips of wood and weighted down the center to ensure a good bond. It was easy to see when I trimmed out the areas for the holes that the rubber had bonded well.
Later I found that only the clamped area bonded well, so I resprayed and used a chainmail hauberk to weight down the rubber while it dried. (What, you don't have 13th century armour in your workshop?) This worked great and all appears to be bonded well.
Sound damping was also impressive. Instead of a ringing "ding" sound when you tap on the plate, it now makes more of a dull "thunk." Looking forward to seeing how it performs and holds up in riding.
Update March 17, 2015
Despite what appeared to be a good initial bond of the spray adhesive, just sitting in my workshop the rubber started to peel away after several weeks. Very frustrating! The problem is that solvent-based glues just aren't going to work where metal and rubber are bonded in a large surface area, because there's no easy way for the solvent to evaporate from inside the bond. Out of desperation I lathered some cyanoacrylate glue around the edges and this appears to have bonded well. I also added some aluminum-faced Dynamat sound-proofing to the "wings" of the skid plate.
The assembly is quite acoustically dead, and judging from a few rips around the block, it does an impressive job of reducing reflected engine noise. The exhaust note sounds deeper when you remove all the reflected high-frequencies.