Mechanical Solar Car Design Philosophy

High-level constraints for mechanical design are mostly set by the mechanical team

We can pull some requirements and constraints from the nature of designing a car and competing:

• The car can perform its basic functionality(driving, braking, surviving wear and tear, turning, protecting the driver, etc.)
  • Mechanical failure
  • Car tipping/flipping
  • Fitting the driver inside
  • Egress timing
  • Packaging all the required components(Battery Box, Electronics enclosures, Driver)
• we have to follow all of the regs set out by the competition
• we have to be able to manufacture or acquire and assemble everything that goes into the design

These are the requirements, aka the only things we absolutely must have for the car. Everything else is technically optional but if we have these things, we have a functional car. A lot of these things for mechanical are mainly structural and geometry. As long as through sims, our mechanical components survive and our geometry works out such that nothing interferes during normal car ranges of motion, the car would work. However, notice that a super heavy bare frame with wheels directly attached on the back and a basic steering system with a 6-point harness wrapped with plastic wrap would technically pass only these requirements.

This is because, unlike most engineering groups, we do not have a customer or market analysis feeding us all our requirements and constraints. We have to set our own performance metrics and design goals

Next up is our goals for optimization. These are things that we would like to have to maximize performance but if we don't meet these goals/don't have these features it's not the end of the world. The car still does its job

• We want to go as far as we can in a set time(FSGP) or go a set distance as fast as we can(ASC) therefore, we want our car to have the highest average velocity it can. Let's look at the car from a thermodynamics perspective:
  • Power In:
    • Solar Array - this is limited by the size of our array and the efficiency of our cells and MPPT boards. This is Powergen's job to maximize
  • Power Out:
    • Drag - this is a big one, it's roughly proportional to velocity squared so it's typically the biggest component of energy loss at reasonable speeds. This is Aero's job to minimize by optimizing aeroshell shape and minimizing cross-sectional area and surface roughness.
    • Rolling Resistance/Tire Hysteresis - These are both mechanical losses that can be lumped into a rolling resistance that comes from bearing tolerances, unsprung friction, and losses from our tires. These are roughly proportional to velocity and mass so they are also important to get rid of. These are in dynamics' wheelhouse but are primarily handled by unsprung mass design and manufacturing
    • Steering Losses - non-ideal(aka all) steering geometries create lateral slip on tires for any turning angle which bleeds energy out of the car as well. These are a lot harder to predict but are also roughly proportional to velocity squared
    • Wheel Slip - This is kind of a corner case but any time our powered wheel is not in full contact with the ground and the wheel slips we are essentially wasting energy through kinetic friction. This is typically handled by suspension design
    • Electrical Losses - This is extremely minor for most of our electrical team since they are very good at minimizing power draw. Unfortunately, that isn't quite true for our cooling systems which do draw a relatively significant amount of power.
  • Internal Energy:
    • From our goals, we know we want the highest average velocity we can get. 
      • average velocity →  average kinetic energy ~= average internal energy → maximize power in and minimize power out
    • We know that internal energy will be primarily stored as potential, kinetic, and electrical. To maximize the amount we have in kinetic, we should reduce the other 2*
      • Potential - Pe = m*g*h. g and h are set by Earth's gravity and the track, 2 things we can't do a ton about. However, the mass of the car plays a role here so we see that we once again are incentivized to reduce car mass
      • Electrical - in theory reducing this means constantly dumping all of our power into the motor but reality is a little more complicated. The exact power draw we get from the array is variable based on conditions, and recall that most of our losses are a function of our car's velocity. This means that by dumping all our power into the motor all the time, we're increasing losses which increases net energy leaving the car which decreases the overall internal energy. Thus, there is a balance on how much power we want to dump into the motor which is based on a bunch of factors like upcoming track layout, battery level, time of day, weather and predicted weather, etc. Deciding how much power to use when is Data Acq's job.

Those are all the factors that go into designing the car. From all of those we can pull a set of design goals

• Minimize weight
• Minimize cross sectional area
• Minimize steering losses
• Minimize rolling resistance
• ect.

After all of that we have the nice to have things such as driver comfort, dynamic performance, ect. But the important thing to remember is that these are secondary to the design requirements and goals for performance. 

In essence:

• If you don't satisfy your NEEDS, your design is not workable
• If you don't meet your GOALS, your design has poor performance
• Satisfying your needs always comes before optimizing your goals