Recently I tried to measure the roll-down coefficients and found they are impractical for my studies. But I did find a 'good enough' approach using three benchmark points and better still, four points.
EPA ROLL DOWN PROTOCOL (SAE J1263)
The EPA publishes the roll-down coefficients for:
- A (lbs/velocity**2) - corresponds to aerodynamic force
- B (lbs/velocity) - corresponds to rolling drag
- C (lbs) - corresponds to vehicle overhead
So following the protocol of SAE J1263, I tried to locally replicate but was disappointed. The full protocol requires 16 bi-directional passes on a sufficiently flat track. Using the closest road to the airport:
For my first tests, I used "Scan My Tesla" and "Speedometer" GPS records:
- There were problems getting the same section of the test track. I'll have to re-run to identify the specific start and stop spots after doing a dash cam plot.
- GPS data is notorious for having altitude challenges.
- The South bound runs were consistent because I'd mapped the starting position accurately.
- The North bound runs were less consistent so their runs, especially below 40 mph, had significant variance.
- There is no altitude metric in "Scan My Tesla".
I'll have to survey the test track and get a better start positions identified on both the North and South bound routes. I also have a high-resolution, 6-axis accelerometer that can provide altitude changes as well as acceleration and derived velocity. Combining the data is doable but not trivial. Still, I should be able to get credible A, B, and C coefficients.
BENCHMARK SOLVING QUADRATIC
My faster, alternate is three point, benchmarks:
- Perform at least three different speed, benchmark:
- < 15 mph - Prius benchmarks show the maximum is in the 15-20 mph range. The low value sets the maximum range part of the parabolic curve to real world values.
- > 20 mph - Set the range of the minimum power, curve, which in the Prius is 15-20 mph. We need the minimum power value to evaluate the effect at ordinary speeds.
- 40-45 mph - A middle range speed used for urban travel. Not necessary BUT in this case, accidentally having the heaters on result in a significant increase in Wh/mi.
- 65-75 mph - The high-speed benchmark gives a way to evaluate aerodynamic changes.
- Use a "Three Point Parabola Calculator" or "Three Point Quadratic Calculator"
- Use at least three benchmark points, more helps identify problem (see bad data point)
Unlike roll-down benchmarks, these can be performed in 1-2 hours. Also, the sensitivity of roll-down appears to be poor at slow speeds. Regardless, make sure to use a checklist to avoid problems like my unplanned heater test. Hold as many environmental parameters constant.
Bob Wilson