This week: Composites

This week in Baja, I learned about composites.  Composites can be used instead of many different structural members.  On the Baja car, we use composites to replace what would usually be large sheets of aluminum or steel.  Instances of this include body panels, the floor, and a front skid plate.  The main benefits we look for here are weight savings while at the same time improving the strength and stiffness of the components.  Strength of a material refers to the ability to be hit without breaking, while stiffness refers to the ability to get hit without bending (or deflecting).  For the floor and skid plate, we used a foam core to greatly increase stiffness.  In the past, the floor has been a solid plate of either steel or aluminum.  The steel skid plate at the front of the car from last year weighed 8 pounds where this year’s composite plate weighs less than 2 pounds, and can still take the same impacts without deflecting.

The advantages of the composites are clear, so why isn’t everything made of composites?  The cost of materials as well as the difficulty to manufacture the molds required to form the fabric-like carbon fiber or fiberglass into the desired shape make composites less feasible for most applications.  The manufacturing process for composites, known as a lay-up, can be a lengthy process.  First you must make a mold, which for our low budget, could be shaped foam, or in an industry setting, a machined metal mold.  Next, you have to determine what the layers of your composite are going to be, and what order they will be in.  Fiberglass and carbon fiber are the two main components in the composites we use.  Carbon fiber is much stronger and stiffer, but tends to be vulnerable to rubbing and friction, so it is usually surrounded with layers of fiberglass.  Additionally, composites are stronger in some directions than others – this is determined by the direction of the weave.  The fibers are woven in a perpendicular pattern; layers should be offset by 45 degrees to obtain the maximum possible strength from the composites.  The layers are impregnated with resin which ultimately hardens to give you the final product.

Below is an image of a sheet of carbon fiber that has just been layered with resin.  You can see the weaves of the fibers as well as get an idea for how fragile composites are around the edges before they harden with resin.

Source: http://volvospeed.com/~volvo/Pics/Mods/carbon_fiber_sheet.jpg

This week, my task was to make a cover for the steering rack to keep a barrier between the driver’s feet and the moving parts of the rack.  This image shows the steering rack when it is uncovered.  The steering wheel attaches to the part in the middle to turn and slide the horizontal rod side to side, turning the wheels.

Source: Author

I shaved a foam mold to cover the rack, leaving a hole for the steering column which will stick out.  Here the mold is, covering the steering rack.

Source: Author

Next, I layered the fiberglass and carbon fiber onto the mold, coating each layer with resin before proceeding.  After all the layers were laid up, a vacuum bag was put over the whole thing to help force the resin into the fibers and get rid of air bubbles.

Source: Author

Finally, after 8 hours of setting, the final product could be removed from the mold.  All that remains to do is cut off the excess around the edges and drill the holes needed to bolt it on.  The picture below shows the finished piece next to the mold which was wrapped in aluminum tape to keep the resin from sticking.

Source: Author

This week: Suspension

This week in Baja, we had to get the car ready for technical inspection.  What that means is that all of the rules of SAE, the society of automotive engineers who sponsors the competition, must be met by our car.  The remaining items for us include a lot of small things, including car numbers, as well as various other aesthetic parts.  All of the sub team leads are double checking all facets of their systems for any way that the car will not be acceptable according to SAE rules.  The first thing that happens at competition is technical judging, not only to make sure that the car is safe to drive, but also to score our car based on our design decisions.

On the note of design decisions, it is easy to get lulled into the idea that our car is the only concept of vehicle that can be realized.  Upon arriving at competition, you rapidly realize how many design variations there can be.  Variations in suspension alone make no two cars alike.  The most basic option is double A-arm suspension, as pictured below.

Source: http://www.carbibles.com/suspension_bible.html

The part with the circular hole on the right side is called the upright.  The upright is the part that attaches the suspension linkages and also where the wheel attaches.  The idea of any suspension is to have the wheel move along a predictable arc when the suspension articulates from hitting a bump.  Bearings attach the A-arms to the upright so the wheel can stay somewhat vertical as the suspension moves up and down.  This introduces the concept of dynamic suspension geometry, where an angle of the tire, called camber, can help dig in during turns.  Suspension theory can get very complicated; the list of contents on the Wikipedia page for suspension is intimidating enough to make a young engineer question his career choices.  Thought that is the case, the basic idea is relatively simple: to have the wheels able to cushion the impact of bumps on the road.  Additionally, the suspension should react desirably to weight transfers during turning, accelerating, and braking, as should sound familiar to anyone who has driven a road car.

Wikipedia Suspension Article
http://en.wikipedia.org/wiki/Suspension_(vehicle)

This week: Transmission

This week in Baja we managed to keep the car mostly intact.  With three weekends remaining to get the car ready for competition, we are starting to feel the pressure.  Weekly testing has been going well: we drive until something breaks.

Each week we take the car out to Pozo, an off-road recreational vehicle area about an hour northeast of San Luis Obispo.  Recently, we have been testing the car for the endurance race at competition, which requires that the car be able to drive for 4 hours straight, where points are awarded based on total distance traveled.  Our goal for each session at Pozo is to complete 4 hours of uninterrupted drive time.  We have completed the 4 hours a few times, but not with the consistency we want to see for the actual competition.

The car currently utilizes numerous parts from last year’s car, which has caused reliability issues.  One in particular, the CVT, or continuously variable transmission, has been failing recently.  The CVT acts similarly to the chain on a bike, except it varies the sizes of the gears based on the current rpms.  You can see this action below – also note that it can run at every ratio between the highest and lowest possible.  The idea is, a well tuned CVT will make your car permanently in the optimal gear for the current speed.  On our car, this makes for a very smooth acceleration to a respectable top speed of 35mph, without the need to shift gears.  We already have the new CVT; we just need to make a few small modifications to it to work with our current engine setup.

Source: http://www.howstuffworks.com/cvt2.htm

Last week, we had a huge crash during testing, requiring repairs to our frame as well as both the front and rear suspension.  The team worked hard and managed to get the car back up and running within 5 days: quite the impressive feat partway through the first week of a new quarter.

As the brakes system lead, my job this week was to finally install the rear brakes.  For several weeks, we had been testing with only front brakes on the car.  This is not as unsafe as it may sound, as about 70% of braking is done by the front wheels because of weight transfer forward during deceleration.  With my official duties all but completed, next week will undoubtedly have something new in store for me.  There are always more tasks: the to-do list never ends.