This week: Chassis

According to SAE (Society of Automotive Engineers) rules, each Baja team may use a frame no more than two years.  This is mostly for safety reasons, as the main part of the frame is the roll cage, which protects the driver in a rollover.

By our team’s definition, chassis and frame are somewhat interchangeable.  Technically the chassis is a broader reference to the vehicle, including the frame, suspension, and wheels.  When designing the frame, you are also designing the chassis, since small changes in the frame hugely influence chassis.

This year in our chassis design, the main goals were stiffness, a 50/50 weigh distribution, and a shorter wheelbase.  I will go more in depth into each of these design goals.  Below was the final design of our frame.

Source: Author

Stiffness, not to be confused with strength – which defines how hard the frame is to actually break – is the ability of the frame to resist deformation, such as twisting or bending.   When your car hits a bump, you want your shocks to be taking the hit.  Because of this, the stiffness of the frame should be ten times the stiffness of the shocks, to make sure that the shocks are actually doing their job.

Weight distribution is a very import aspect of any vehicle, especially a small Baja racer.  Weight distribution simply is where the weight of the car sits on the tires.  Some of the car’s weight will be on the front tires and some on the rear.  A 50/50 weight distribution is an even distribution of 50% of the weight on the front tires and 50% of the weight on the rear tires.  Pushing more weigh towards the rear will help you get more traction in your drive wheels, which could mean better acceleration.  More weigh in the front will help you get more traction with your steering wheels, which could mean better turning performance.  Finding the balance between these is important, and in our testing at the beginning of the year, we found that we could go from our previous 40/60 front/rear ratio to a 50/50 without losing much traction in the drive wheels.

Wheelbase can play a factor in stability and turning radius.  A longer car will be more stable, but will sacrifice a smaller turning radius.  One of the goals for our entire car this year was to get a smaller turning radius to improve our low speed handling, so we shortened the wheelbase of our car by 9 inches.  Additionally we determined that we would not sacrifice stability by shortening it as long as we made no changes to the width of the car.