When we built our project "Ain't It Grand-er" Grand Cherokee, Bob Levenhagen of T&T Customs (www.tntcustoms.com) put a lot of thought into the skidplate/crossmember combination to ensure more than adequate protection, and it has served us well. Chris Overracker of Code 4x4 (www.Code4x4.com) convinced me that I really needed a skidplate on the underside of the Ford 8.8-inch rearend, as the housing design was a real rock catcher, so I fabricated one up. A Superlift diff guard went over the differential cover plate to ensure its safety. Up front, the high-pinion Dana 30 got an ARB steel cover that not only protected the gears but added some strength to the housing.
But I also made a big mistake. I never built anything to protect the lightweight stock lower control-arm brackets. I didn't put it on my high-priority list, so it just kept being put off. Yep, they have been bashed pretty badly, and to the point where the lip was bent so badly that I couldn't even get the control-arm retaining bolts out. While a bit on the late side, it was time to correct this problem and prevent it from happening again. Skid Row Off Road (www.skidrowoffroad.com) had just what I needed (plus lots of other Jeep-based skidplates). Instead of welding on directly to the brackets, these control-arm skidplates bolted on using both the control-arm mounting bolt and some U-bolts. A nice, easy, clean installation. However, after a few trips, the powdercoated black finish is no longer nice and shiny, proving that they are doing their job.
This brings up the subject of skidplates in general. Skidplates on an off-road vehicle are quite necessary. However, as far as I am concerned, you can pretty much forget about the ones that come from the factory. Generally speaking, they are too light in weight, ill-supported, and generally don't cover enough of the area that they are trying to protect. In some instances, they are more of a hindrance than a help. Yes, I have also seen people get carried away with trying to protect the undersides and end up with a lot of unnecessary weight that hinders access to under chassis components.
So what makes a skidplate a "good" skidplate? First, it must provide protection to an area of the vehicle that is vulnerable to damage if it should come into contact with a solid object that's harder than the part that it is trying to protect without hindering the operation of other components. Okay, that makes a bit of sense. But what are these areas?
Let's start up front with the differential. I went for years with just a standard cover on my front differential. Maybe I didn't four-wheel as hard as I presently do, or just got lucky. One day I smacked a big rock at a faster speed that I should have been traveling, and put a nasty dent in the cover-so bad, the cover was up against the ring gear. The gear teeth were like cutters. Luckily, it was near the end of the trip, so I just unlocked the front hubs and drove home. Now I run a heavy-duty cover or at least some type of a diff guard.
Tie rod and steering components? Well, for the tie-rods, it's just something that gets in the way and causes more problems that it's worth. Now, the steering box on some vehicles, like some J-series Jeeps-where it sits out front and is exposed-is something that I highly recommend. Well, at least on the end. That big round cover on Saginaw steering boxes is just stamped metal held in place with a snap ring. Bang it on a rock just right, and off it comes, and there goes your steering.
Engine oil pan? Probably not needed. The pan sits behind the front axle or the IFS unit and is pretty well protected on most vehicles.
Transmission and transfer case? These are pretty important components that generally really do need to be protected.
Some rearends need one, like the aforementioned Ford 8.8, since due to its design it works as a rock-catcher.
The spring bolts that attach leaf springs to the axle really need attention. Having the bolts hang down below the spring make them quite susceptible to catching on rocks and even getting broken off. At the very least, the bolts should be cut flush with the nuts.
What is the best material for skidplates? Well, steel far outweighs aluminum (pun intended) and is the most common choice in more ways than one. It's easy to weld, cut and form, along with being a fairly inexpensive material. Its major drawback is its weight. Aluminum seems at first to be the ideal material in that it comes in a wide variety of alloys, depending on the application, that offer strength with a much lighter weight. The most common to find and to use is 6061, which offers a good combination of strength, corrosion resistance and machinability. However, it really doesn't like to be bent in a really tight radius and impossible to bend if heat-treated, and it's not as easy to weld as some of the other alloys. Being somewhat softer than steel, it can gouge a whole lot easier, and in some cases this "gouging" may not let it slide over rocks as easily as steel.
Stainless steel, Teflon, HDPE, UHMW, nylon, and Delrin are just some of the other materials that have been used for skidplates. The popular choice with a lot of rockcrawlers is a plastic referred to as UHMW or HDPE which have high impact resistance and a slippery quality. Commercial-quality cutting boards are made from HDPE, and sometimes you can find it close to the dimensions you need from a commercial kitchen supply house. UHMW, while quite a bit more expensive, is six times more abrasion-resistant than steel, has no cold embrittlement problems, and works from minus-155 to 200-plus degrees Fahrenheit. However, you're pretty much limited to using flat sheets of either material. Some people have built their skidplates out of welded steel or aluminum and then covered them with the "plastic" material.
Whatever you use for a skidplate, just remember its function is to not only protect a component but to allow the vehicle to slide over an obstacle and not get hung up on it. Oh, and don't forget to allow for access around the skidplate to things like the oil drain and fill plugs and lube fittings.