Coast Down Test

Nissan Leaf coast down testing

How much benefit is gained from my 3D printed aero mods - the rim inserts and the front grill block?

(The 3D files are available free)

Results from my second series of coast down tests, showing better performance for runs with aero mods, both on north and south bound runs. (data here)

This second round of coast down tests is an improvement on my first test runs 2 weeks ago.

Improvements were:

• 1.5 km long straight road - enough to speed up to just over 100 kph then coast down to 30 kph
• GPS set to log every second (rather than every 5 seconds last time)
• engaged neutral at just above 100 kph on every run

Test conditions

• a straight road (just south of Featherston)
• no interrupting traffic - fewer than 10 other cars during my test runs between 4:50 pm and 5:27 pm on Saturday 25 May 2019
• 10 m net elevation delta over the 1.4 km length (from 20 m at the southern end to 5 m at at 0.9 km then back up to 10 m at the northern end)
• strong wind northwesterly wind - 20-30 kph, gusting to 50 kph. The impact of this is uncertain because the road runs from northeast to southwest - so the wind is close to a perfect tangent to the direction of travel.
• time and speed data was recorded with a Garmin GPSmap 62s set to record once every second. The GPS was clipped onto the drivers sun visor.
• I started the runs with about 40% State of Charge (SOC) and finished at about 10%

Both wind and SOC were changing throughout the 37 minute test period, creating unwanted extra variables

Due to the 10 m elevation rise going south, (and possibly the wind), all the south bound runs were significantly shorter duration than the northbound runs. The northwesterly wind was almost a 90 degree crosswind as the road runs from northeast to southwest.

Test schedule

I drove two warm-up runs to start with to get used to the road and learn the timing for slipping in to neutral. I accelerated up to 105 kph, then held the "gear stick" in neutral, whilst still accelerating until it actually switches in to neutral (it takes about a full second to switch). Then I coasted all the way down to 30 kph.

On the south bound runs I carried on to my designated turn around spot. Then I'd start my north run from there so that all runs started from the same south spot. All the south bound runs started from the same spot also.

From the start it floor the accelerator till reaching 105 kpg again - and repeat.

After the first two warm-up runs, I drove 6 runs (3 south and 3 north) with the aero rims and from grill mods in place.

Then I removed the aero rim inserts and the from grill blocks. (The front number plate has been lowered to cover the middle of the front grill. As it is not easily moved I left that in place.) I ran 8 runs without the aero mods. All in quick succession.

Next I replaced all the aero bits and ran another 2 runs. This was to counteract any underlaying environmental changed that may have occurred over the 37 minutes of the testing period (ie wind gusts and dropping SOC).

Data cleanup

The raw GPS data was then imported into Mapsource and sliced into 18 segments. The first two practice runs were discarded.

Next the start of each run was trimmed down to the last point that showed a "Leg Speed" of 100 kph (if there was no 100 kph entry then the next highest was kept, ie 101 kph).

Then the end of each run was trimmed back to the last point that showed a Leg Speed of 30 kph.

So at the end of all the trimming I had 16 Tracks which started with an entry of 100 kph (or in a few cases 101 kph) and ended with an entry of 30 kph) ranging in length from 52 seconds up to 74 seconds.

The data was then saved as a CSV text file. From the CSV file I extracted the Leg Speed data which was fed into this spreadsheet. Which allows me to make the nice graph shown above. [Yes - OK. We don't actually NEED a nice graph, but, but... we all WANT a nice graph, don't we? You're welcome.]

A result

I summed the times of all the runs (from 100 down to 40 kph) with aero mods, then summed the times of all the runs without aero mods. Then divide one by the other, and there's your improvement.

Improvement percentage = (aero runs / non aero runs) - 1

Then... 436.5 / 423 = 1.0319

And... (436.5/423)-1 = 0.0319

And for a percentage we go 0.0319 * 100 = 3.2 (rounded)

Which means my aero mods improve the efficiency of my Nissan Leaf by 3.2% (when travelling between 40 and 100 kph).

Which is not too shabby.

But wait... there's more:

Attach:Coasting-down-to-20199525.jpg Δ In this chart I'm calculating my improvement using the data from 100-90 kph, 100-80 kph, 100-70 kph and so on. The non-linear trend is probably due to wind gusts messing with my small data set. I was hoping to see a nice clean trend showing decreasing improvement as I include more data at slower speeds. Anoterh option would be to plot 100-80 kph, 90-70 kph, 80-60 kph, and so on.