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Subaru Impreza 2.5rs Kevlar Skid Plate

Of the projects I am working on I expected this to be the simplest. Do not be fooled; simple does not mean easily. If you are looking for a good learning project I would suggest this as it provides broad composite experiance. The use of mulitple types of reinforcemnt, open ended molds, measuring, tollerance, and knowledge of product requirements are just a few of skills required to successfully build this part.

A skid plate is a piece of material that protects the underbody of a car or truck from damage. Under normal driving conditions, a vehicle would not require the protection provided by a skid plate. Skid plates can benifit any vehicle that is opperated in extreme weather conditions, taken off road, or where the driving surface is poor enough that damage from rocks or other debris being kick up from the tires or other cars is a possiblity.


Have you ever wondered what it is like to be a mechanical engineer? Ok so maybe you have not but you are about to find out anyway. For starters we need to define the product and customer specifications. The customer specifications are a qualitative description of what this skid plate should be while the product specifications are quantitative.

Customer Specifications:
1. Protect the undercarage of the vehicle from ice, rocks, snow, and any other debris that might be encountered
2. Must be able to be installed with basic mechanics tools and require little to no modification of the vehicle
3. Must be light enough to cause no noticeable reduction in performance
4. Must not restrict access to frequently used areas such as the oil drain plug
5. The vehicle must be able to clear a speed bump with the plate installed
6. The skid plate should be able to take mulitple hits while still maintaining strength
7. The parts should not be effected by the elements
8. The skid plate must cost under $300

As you can see the customer specs desribe everything that the skid plate must do. When building a part for another party, these specifications would be defined by the customer. When designing a commercial part, a marketing team will determin what the part must do by performing numerous market analysis. This is a very general form of a product specification document. The next step would be to assign a level of importance to each spec and then compair the spec against themselves to determin which specs interact with each other. For example Cost and material are directly related. But I digress on to the product specs.

Product Specifications:
1. The skid plate must be able to withstand a maximum force of 200lbf
2. There are to be no more than 8 attachment points to the vehicle
3. The plate should weigh less than 20lb
4. Ground clearance of the vehicle should not be reduced by any more than 1"
5. The skid plate must be withstand 20 repeated hits of 100lbf
6. The parts should be rust, corrosion, abrasion, and chemical resistant.

For this project, simple product/customer specs will be fine. Now that we know what we must build let's get started. There are many choices on the market as far as materials for skid plates from high strength steel, aluminum, and plastics. As we determined earlier and subsequently listed in the product specs, Weight is a factor so steel is not an option. Aluminum is a possiblity due to its light weight and corrosion resistance resulting from the aluminum oxide coating that forms when it is exposed to oxygen. It is however a very soft metal and dents easily. It may not survive the the repeated hits required of it.

I have decided to focus on a composite plate as we can combine several materials to achieve properties superior to that of steel or aluminum alone. Overall strength is not required so carbon fiber should not be used as a primary material. Instead I will be using a combination of S-glass for strength and abrasion resistance, and Kevlar for puncture resistance. The layers of S-glass will also provide excellent stiffness, good chemical resistance, good moisture resistance, and the ability to maintain its properties over a wide range of temperature conditions. Due to its properties the S-glass will be facing the road while the Kevlar will be bonded to the back of the s-glass for protection against major impacts.

As will all composites we will need to construct a mold but before we can do that, a shape design and accurate dimensions are required. One way of avoding a majority of the work determining the dimensions is to build a prototype out of a material you can use as a plug such as a thin sheet of aluminum or cardboard. I prefer aluminum as it is easier to mold to my desired shape, unfortunatly for me though my budget prevents its use so my tactic will be the creation of a mold using detailed measurments.

My shop is in the outskirts and materials are sometimes difficult to come by so I decided to use some sheets of MDF to make the mold. MDF is decently strong and readly available at most lumber stores. Making good use of the scrap bins can also help save on a project such as this. You can see the basic design (side view) to the left.I used some of scrap wood I had picked up to reinforce the MDF. There were gaps where the MDF butted up against each other so I used Bondo to fill in the cracks. In hind sight this was a bad idea to do Bondo at this point. After curing, Bondo becomes very brittle and I put several cracks in the Bondo while working on the mold. This step should be saved for later when you are ready to seal the mold with expoxy. This would be a good point to mention that less is more with Bondo. When working with bondo it is so much easier to add more then to sand/scrape extra off. I learned this the hard way. The best method to apply bondo that I have found is a razor blade.

The basic shape of the skid plate mold is now complete. This next part is where it gets tricky. I need to build a hump on the mold so that the skid plate will wrap up the steel plate and be sandwiched in between the plate and the frame. My choice of materials for this is some scrap wood and plaster of paris. I used the wood to obtain the desired height of the hump, built out the sides to the correct dimensions with plaster, and then gave each side a sizeable draft. I attached the wood to the base using standard nails. The plaster is tricky because it is only usable as a paste immediately before it cures. It does however, sand extremely easily. Plaster is porous so a sealant is needed to ensure parts can be easily pulled and also improves its stability during manufacturing. I mixed up a small batch of epoxy and painted it over the plaster. Below you can see my progress.

When using a sealant with plaster, be sure that the plaster is completely cured and dry. Epoxy will not stick to a wet surface. I also recommend using some type of fiber reinforcement for increased strength. When you pull a part off the mold it will stick slightly and a weak bond between the epoxy and plaster could cause the mold to pull apart. To test the shape of the hump, I covered it with a plastic sheet and then layed up a quick fiberglass part over the plastic. After making some minor adjustments, I reinforced the hump with s-glass and they applied some finishing layers of epoxy and wet sanded the mold until it was smooth.

The location of the mounting points and thin material will cause the skid plate to flex and possibly even vibrate at higher speeds. Therefore, I will add strips of foam core in the part which raises the moment of inertia and therefore increases rigidity. The layout of the foam is up to you, but I placed core along each of the edges, down the center of the part, and two diagonal pieces in the center to prevent flexing.

Because this is a flat part it is not necessary that you vacuum bag it, so if you do not have a vacuum pump this is still a part you can make. There are added benefits if you can vacuum bag: The Kevlar will bond better with the foam core when they are compressed together. The vacuum removes any extra air bubbles which result in a stronger part and a better surface finish.

This project is still under development.

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