1. INTRODUCTION AND OBJECTIVES
Buggy, formally known as Sweepstakes, is traditionally a sport full of secrecy. This secrecy is particularly paramount when it comes to how teams design and build their vehicles. Each team has spent years or even decades perfecting designs and fine tuning their manufacturing processes. This institutional knowledge is fiercely protected, as teams loathe to give up any information that may give them even a microsecond of advantage. Unfortunately, this creates a high barrier of entry for new teams hoping to try their hand at the most unique sport at Carnegie Mellon University.
The purpose of this guide is to bridge that knowledge gap by walking new teams through the process of building a basic buggy. There are countless variables in the design and fabrication of a buggy, and the following instructions endeavor to present an introduction to principles and methods with a simplistic buggy design. Each section contains:
- A brief describing why design decisions were made
- A how to guide for completing that portion of the build
- A considerations blurb for alternative designs
The final product will be a buggy that is intended to be reliable, low maintenance and long lasting. As such, it will likely be heavier than the winning buggies. Rather than competing with the top teams, the aim is to produce a buggy that will last many years as a new team grows, while being fast enough to compete for a top 10 finishing position alongside a strong push team, solid prep, and a good set of wheels.
There are five goals for the buggy detailed in this guide:
- Easy to build
- Durable
- Low maintenance
- Easy to drive without spinning or crashing
- Relatively competitive
To achieve those goals, the buggy will be a reverse trike format (two wheels in front, one wheel in the back) with wagon steering and a carbon fiber with baseplate composite shell. This guide will explain unfamiliar terms or concepts and ensure new teams understand why this buggy design is effective.
A comprehensive list of tools required for a typical buggy build can be found in Appendix X.
Questions regarding content or the build process can be directed to Connor Hayes (cmhayesny@gmail.com), Diya Nuxoll (diyanuxoll@gmail.com), or the BAA (BAA EMAIL). The BAA also offers a program that provides guidance and financial support to new teams.
Best of luck with your first buggy build!
2. DESIGNING THE SHELL
2.1. Shell Brief
Skills: Basic knowledge of Solidworks and measuring techniques
Tools: Solidworks software, measuring equipment and paper
Materials: Broomstick handle or equivalent, long board that driver can lay on
Safety Tips: None
This section of the build guide covers a lot of material, as the shell is the most labor-intensive aspect of the overall build. It is important that the shell is designed around the drivers’ dimensions, so they fit properly and the buggy will handle in a predictable manner through the turns. The mold design will be done in Solidworks, and this guide assumes a basic understanding of the program.
The first step in modeling the mold is to create a lofted 3D object of either the left or right side of the shell and mirror the object to create a symmetrical model. To make the loft, a “wire frame” model must be created for the middle of the shell. The wire frame will be used as the baseline for the loft function. Once this is completed, the process is repeated for the nose and the tail. This method will make it easier to use the loft function to create the mold design.
2.2. Measuring the Driver
Buggy shell designs typically start with measurements of a driver or drivers. Ensuring the buggy can fit not only your current drivers but a wide range of potential future drivers is key for a team’s first buggy. Measurements should include all driving gear (helmet, mouth guard, athletic wear clothing). Use a flat board or table large enough for the driver to lay on, a round tube strong enough to hold ~100 lbs (mop/broom handle or cardboard shipping tubes work well) and a small tube that mimics a steering handle (Dry erase marker works well). The driver should lay down on their stomach on top of the board with their hands stretched out in front of them and head looking forward as if they are driving with the fake steering handle in their hands.
Once in this position, measure the distance from the steering handle in the driver’s hands to a few critical points: the elbows, shoulders, crown of the head, hips, knees, heels and end of toes, as shown in Figure 1. Follow this by measuring the width of the driver at their hands, shoulders, hips, calves and feet while maintaining the driving position. Finally, record the height from the floor to the crown of the driver’s head/helmet, hips/rump, calves, and heels.
A sample measurement guide is provided below, and a larger copy for usage can be found in Appendix X.
The final step is to remove the driver and place the board on top of the longer tube or broom handle, so the tube is perpendicular to the driver’s body. Lay the driver back down in the middle of the board in driving position. Have one person stand by the driver’s head and hold the front of the board, then roll the board forward and backwards until the board and driver are balanced on the tube. Measure the distance from the driver’s hands to the center of the tube to capture where their center of gravity is located, as in Figure 2.
Place the measurements into an Excel table like the example below for easy reference during the next step. These measurements will be used throughout the instructions.
Advanced Design Tip: Cross Sectional Dimensions
To make the most aerodynamic and small shell designs, factor in the cross-sectional shape of the driver at the body points measured above (elbows, shoulders, hips, etc). A cross section in this case is the shape of the front fact of an object after it is sliced through at a certain point.
To accomplish this, have the driver lay on their stomach and grab two rulers. Lightly hold a ruler against the driver at the widest point of the elbows and record the height where the driver’s body and ruler first touch. Then measure the horizontal inset from the ruler to the driver’s body at each inch from the ground. The easiest way to accomplish this is laying the other ruler horizontally at each inch ticker and slowly sliding it towards the driver until it makes contact with their body. Record the measurement.
2.2.1. Alternative CG Measurement - Scale Method
Center of gravity (Cg) can also be measured using a board (like above), two scales (cheap bathroom scales are fine), and some simple force balancing (covered in most engineering intro courses). A thin bar/beam to rest between the board and scale to keep the weight centered is helpful, but not critical.
- Place the scales with their centers spaced such that one would be underneath the driver’s hands, and the other under the driver’s toes.
- Place the board evenly across the two scales
- If possible, lay a bar/beam across the center of each scale perpendicular to the length of the board. This will provide the most accurate measurement.
- With the board in place, zero the scales (tare), OR record the baseline weight reading so they can be subtracted for your final calculations.
- Carefully have the driver lay onto the board and check that her hands and toes are over the center of each scale. If not, repeat 1-4 until they are.
- Record the weight readings with the driver on the board subtracting the respective baseline readings as needed.
- Adding the scale measurements together should be roughly equivalent to the driver’s total weight
- Since we are looking for the distance from the hands to the CG, in a static system (i.e. nothing moving) all forces AND torques are equal, so treating the hands as a pivot point, we know that:
- Weight of Driver * Length to CG = Weight at Toes * Length to Toes
- Wdriver * Lcg = FToes * LToes
- Reorganizing that equation to solve for Lcg we get
- Lcg = WToes * LToes / Wdriver
- Typically, a person’s CG is somewhere around their navel or just below their waist.