Test: comparing Blackfly vs. ForkenSwift rolling resistance (Goodyear Invicta GL)

[MetroMPG]

For quite a while I've had the suspicion that the dinosaur burning Blackfly (Pontiac Firefly) is a better roller/coaster than the wind & hydro powered ForkenSwift (electric Geo Metro).

But! I don't put much faith in Butt-o-Meter readings in general, even my own.

So today I ran a simple, controlled-as-possible comparison to gauge the cars' relative rolling performance to see if one was really better than the other.

The winner? In a way, both cars are winners! But in another a more accurate way, the Blackfly was the winner.

And in an even more accurate way, the Blackfly's tires were the winners.

Preparation:

• I made sure both cars had the same tire pressure.

• I equalized the weight between the two cars, which meant adding 240lbs to the Firefly (~1830 lbs stock) to equal the heavier electric Metro (~2070 lbs post-conversion). (I actually ended up adding 260 lbs, since at 65 lbs each, 4 flooded golf cart batteries were a quick way to do it.)

• I washed, waxed and Armor Alled each car (OK, no.)

Methodology:


The distance of the "course" between the ForkenSwift (blue) at the start, and Blackfly (black) where it rolled to a stop.

• I drove each car the same distance (from a cold soak start - overnight) to a quiet dead end road with a small downhill followed by an almost level "run-out" (very slightly uphill).

• Start: at a marked position on the hill, engine/motor was shut off, transmission placed in neutral and the brakes were released (from a stop).

• Finish: after rolling down the hill (maxing out at approximately 21-22 km/h) and coasting to a stop, the "end" position was marked on the road with chalk for each run.

• 3 runs were done for each car

• Then the wheels/tires were swapped between the two cars

• 3 more runs for each car were measured

• There was also waving at and/or talking to several neighbours who were either curious about or suspicious of the repeated slow driving up and down the hill on their quiet street.

Conditions:

24 C / 75 F - clear & dry
wind west 6 km/h (~4 mph)
humidity 34%
pressure 101.4 kpa / ? in. hg

Results:

First set of runs (tires in their "proper" places)

219.9 m / 721.5 ft - Average Blackfly distance

201.6 m / 661.4 ft - Average ForkenSwift distance

18.3 m / 60.1 ft - difference in coasting distance

Second set of runs (wheels/tires swapped between cars)

205.3 m / 673.4 ft - Average Blackfly distance

221.1 m / 725.5 ft - Average ForkenSwift distance

15.9 m / 52.1 ft - difference in coasting distance

That's not insignificant.





The tires:

Firefly:
Goodyear Invicta GL M+S (79S)
155/80/13
Max Load 436 kg / 969 lbs @ 44 PSI
Treadwear 260
Traction A
Temperature B
Tread: 3 plies (1 polyester cord + 2 steel cord)
Sidewall: 1 ply polyester cord
Made in Canada

ForkenSwift:
Magna Grip Radial HT M+S (82S) (Walmart winter tires)
175/70/13
Max Load 470 kg / 1036 lbs @ 240 kpa / 35 PSI
Tread: 3 plies (1 polyester cord + 2 steel cord)
Sidewall: 1 ply polyester cord
Made in Canada

Experiment, part two!

The next part isn't at all scientific, but after doing the controlled comparison, I took the ForkenSwift for a drive - still wearing the Blackfly's Goodyear Invictas (and borrowing its smooth wheel covers) - to see if there was a measurable difference in energy consumption compared to recent driving.



I won't argue that this is in any way a valid comparison. All I can say is I tried to do similar driving to previous cycles - ie. all sub/urban use, in light traffic, while trying to maximize efficiency.

The car is on the charger now - should be done in about an hour. So while waiting for the numbers, here are some (subjective) differences I noticed:

1) the Goodyear tires seemed quieter (makes sense when you look at the blocky tread pattern of the snow tires)

2) even at the same inflation pressure, they seemed to absorb bumps better (which makes sense, since the narrower tires should deform more on impacts)

3) not surprisingly, I found myself coming in "hot" when coasting up to several familiar stops & turns. I had to adjust my driving as the "test" went on.

Power consumption results to come!

[MetroMPG]

WOW.

OK, I know it's still not at all scientific, but today's ForkenSwift test drive with the Invictas on it was an all time record and beat my recent similar driving by a big margin:

27.6 km / 17.2 miles @ 3.43 kWh

199.7 Watt hours per mile / 124.1 Watt hours per km or 168 mpge (US)

My two best recent results are 224 and 228 Wh/mile. My previous all time best was 207 Wh/mile.

Maybe I should be putting together a pit crew to swap the tires back and forth between the two cars for me.

[RobertSmalls]

So the Forkenswift outrolls the Blackfly by 1.3m on Invictas, but the Blackfly beats the Forkenswift by 3.7m on WalMart snow tires. Is this a statistically significant difference? If not, then both cars are, near as we can tell, equally good rollers, with unequal tires.

[NeilBlanchard]

Hi Darin,

That's a lot of (good) work there! I was surprised to figure out how important tire pressure is (duh!) on my wife's xD. And yes, part of the gains to be had from better rolling can only happen after you (re)adjust your driving habits. After installing my video mirrors, it has taken me a few weeks to begin to realize the improvement. You keep getting surprised at how fast you come up to a stop, and so after you start to adjust, then you get the higher FE...

It's nice to confirm that your Butt-O-Meter wasn't lying!

[MetroMPG]

If all else were equal except the tires, the distance between them would have stayed the same after swapping the wheels/tires.

But the gap between the cars shrunk slightly. When the ForkenSwift was wearing the snow tires, it came in 18.3 m / 60.1 ft behind the Blackfly. The Blackfly, wearing the snow tires, rolled to a stop 15.9 m / 52.1 ft behind the ForkenSwift.

I wasn't surprised to see both cars roll further in the second set of runs. Likely because the tires were warming up throughout the testing. (In almost all cases, each successive run was a little bit longer than the previous one by a couple of feet. I took an average for each set.)

Overall the tests took nearly 4 km of driving (both cars going exactly the same total distance).

[NeilBlanchard]

Hi,

Did the air temperature change? If it got warmer, then the aero advantage of the Blackfly would have been reduced...

[MetroMPG]

Neil - definitely. And that additional coasting distance can have a seriously big impact on your numbers if the engine is off for those extra car lengths. The power of infinity on your fuel economy.

That's why I suspect the EV did so much better even with a modest increase in rolling ability - of course the car stops using energy every time the accelerator is released.

[MetroMPG]

Aero would have been a small factor: peak speed in the coastdown testing was 22 km/h (13.7 mph).

The average speed would have been significantly less (but I didn't measure that).

EDIT: and yes, the ambient temp was probably also going up during the testing. Looks like 1 degree C change, or from 73.4 to 75 F. Though the temp of the asphalt was probably going up faster than the air temp.

[MetroMPG]

Originally Posted by NeilBlanchard

It's nice to confirm that your Butt-O-Meter wasn't lying!

Yeah, but I still won't trust it!

[MetroMPG]

Also: I would have done a steady speed MPG comparison between the two sets of tires on the Flea, but I still don't have a functioning cruise control. I need to replace it (again).