Thursday, October 29, 2009

Vehicle Dynamics, Vol. 1 - Thinking in 3 Dimensions

At the outset, I promised that this blog wouldn't be a vehicle dynamics tutorial. Still true. Get out your Milliken, Gillespie, and even Carroll Smith's timeless "Tune to Win".

But, here's an observation.

When many race engineers and chassis tuners think about handling, they focus heavily on roll moment and roll stiffness. That's a vital topic, but it's only part of the total handling equation.

The car operates in 3-dimensional space, with forces and moments acting on it along the X, Y, and Z axes (longitudinal, lateral, and vertical to regular mortals). At any point in time, its handling is dependent on how it reacts to ALL SIX of those forces and moments. Limiting your thinking to roll only addresses lateral forces and longitudinal moments.

For example:
1-Savvy oval track engineers have long been focused on the vertical forces from both banking and aerodynamics.
2-When a car is cornering, it is rotating around its Z (vertical) axis. The are a number of very significant sources of moments around that axis, some of which are under the control of the engineer.

There is much to be learned from data acquisition. Install a steering travel potentiometer calibrated in spindle degrees. Install a 3-axis accelerometer and a yaw rate sensor. Just in the Z-axis, you can calculate understeer gradient (see Gillespie), yaw gain (yaw rate divided by steering), and stability index (see Danny Nowlan's excellent recent series in Race Car Engineering magazine). Use your imagination. There's more.

Apologies for the lack of illustrating diagrams. I'm still too busy with real work to explore the tools that Blogspot offers. Eventually.

The proverbial word to the proverbial wise should be sufficient. Go for it!

1 comment:

  1. Ah, you have Carrol Smith's book too! I'm willing to bet that our personal libraries may have a lot in common!