I’ve been excited about the Car9 project ever since Horizonjob started his “Messy Shop” thread. So I offered to help Marcus by making Wishbone runs to support his Car9 design effort. For what it is, Wishbone is a great tool at the perfect price ($0). But setting up Wishbone to run requires building a matrix of twenty-four X,Y,Z dimensions which represent the eight suspension pivot points. And these rather esoteric coordinates change with changes in track, wheel offset, tire size, ground clearance and a bunch of other pragmatic, designer-oriented factors. When evolving a new design like Car9 (or when tweaking an older book-derived design) it can become a huge PITA to re-figure the required X,Y,Z’s every time a design change is contemplated and Wishbone is needed to be re-run. I’m sure many of you have experienced this.
What was missing was a tool to automatically calculate the Wishbone X,Y,Z dimensions directly from the various design parameters. Among the spindle options being considered for Car9 are the Wilwood Mustang II units, which fortuitously have detailed engineering drawings available on the Wilwood website. So, a Wishbone Input Generator was conceived to combine the known Wilwood spindle specs with twenty-two chassis design/set-up parameters and automatically calculate the X,Y,Z’s required. The high school trig books got dusted off
and the Excel spreadsheets included below came to pass.
Here’s how the Wishbone Input Generator is used:
On page 1, the desired design parameters are entered into the Generator: Track width, tire size, wheel offset, ball joint length, rotor hat thickness, etc., etc. Most of the inputs are general design parameters; a few are specific to Wilwood and/or Car9.
On page 2, the twenty-four X,Y,Z dimensions and other items needed to run Wishbone are automatically calculated by Excel. Coordinates for both the standard height and the 2 inch dropped Wilwood spindles are generated.
The coordinate data on page 2 is manually keyed into Wishbone, where one makes multiple roll, bump, steer iterations to analyze a specific geometry in the normal fashion.
In addition to the normal printing, the results of the Wishbone runs can also be recorded (manually again) onto the tables included on pages 3 and 4 of the Generator. As a by-product, swing-arm length is calculated from Wishbone's IC "Z" results and track width. Multiple tables are included so results from different settings of the upper chassis pivot can be more easily compared. These tables are useful in keeping track of (and filing) inputs and results. Otherwise, it’s easy to lose configuration control of what inputs resulted in what outputs. Don’t ask me how I know…
One could go further and use Excel to plot the various camber curves, etc., but that’s left to the user. And, yes, it would be nice to have the Wishbone input and output automatically transferred to/from the spreadsheet, but that's not implemented.
As mentioned above, the Generator is programmed based on the dimensions and geometry of the Wilwood uprights. The outboard X,Y,Z's are calculated specifically for these units. The inboard X,Y,Z’s are built off these outboard dimensions and are specific to Car9’s chassis and a-arm design. Nevertheless, the concept is valid for other spindles and chassis designs, and the Car9/Wilwood version can be useful in other situations if the differences are taken into consideration. That said, this tool has been a huge help in using Wishbone to analyze multiple Car9 design iterations and suspension configurations in a relatively short period of time.
Two downloadable copies of the Wishbone Input Generator are included in the zip folder below. They are identical, except one has the input parameters and results for a possible Car9 “Track Car” configuration, the other for a possible Car 9 “Street Car”. Both configurations are works-in-process and the design inputs and Wishbone results shown, while getting close, are not final. A Wishbone screen shot for each are also shown below.
Many thanks to Marcus Barrow for his guidance in putting this together, selecting the input data and interpreting the results. And for all his efforts on Car9.
I hope the Generator is useful to other Locosters. Comments and feedback are welcome. There’s always room for improvement.