The Aptima Motors eCobra is coming along. We are in the final days of this project. It rolls and drives well.
One of the concerns with this car has been the amount of power required to move it. For a two seat convertible sports model, it is a bit heavy at 2961 lbs – no interior or paint yet at that. But as we have added pieces such as the hood and trunk, the aerodynamics have improved and the power consumption has fallen to more of the expected levels just over 300 wH per mile – about as expected for it’s weight.
One of the eyebrow raisers from receipt of this car was the large wide tires that came on it. Lots of rubber looks good and rides well, but it usually means higher rolling resistance. Very early in the program we ordered a new set of lightweight WELD wheels with a carefully calculated offset so we could run Michelin Energy Saver A/S low rolling resistance tires on this car. The issue was having the offset such that the tires still filled the wheel well and didn’t look entirely odd on a Cobra.
Recall from our mystery surrounding the Porsche 550 Spyder and the Porsche 356 Speedster that we got a significantly better range and energy use from the heavier and rounder Speedster. Despite installing expensive aluminum rotors, calipers, low rolling resistance tires, and even ceramic bearings, we never did get the Spyder even close to the Speedster’s ability to roll much more freely.
As kind of a joke we ran what we called the Soapbox Derby – simply rolling the two cars down the street in neutral to see which rolled further. True to our range results, the Speedster rolled dramatically further than the Spyder.
We can of course do normal range testing but it is quite time consuming and subject to variabilities out of our control. If a tractor trailer blows past you, cuts in front of you, and then slows to 7 miles per hour below your target speed, there’s not a lot you can do about it in a small convertible. And the tests take hours and are much more accurate over a significant number of miles – 15 to 25 typically.
So we liked the quick indication of the Soap Box derby. But it didn’t actually provide much data – just a distance on a hill. And you have to use the SAME hill. So YOU can’t compare YOUR results to ours.
The way this is actually done in automotive testing is with a coast down test. And so we adopted the pretty standard procedure used in such tests – with perhaps less instrumentation and rigeur than is commonly done at the Chrysler Test Grounds. But we think it renders quite accurate information, and is reproducible by anyone anywhere on any car.
Basically, we go to a flat stretch of road sufficiently long to allow an acceleration to 75 mph and a subsequent unpowered roll to a full stop. Ideally, with very little traffic on it. We then accelerate to 75 miles per hour, and then remove all throttle input and place the transmission in neutral.
As the car speed decreases and passes through 70 mph, you take a time mark. As the point where it hits 60 miles per hour, you note the time from the 70 mph start time mark. As it passes through 50 mph, again take a time. And so forth until the car actually comes to a stop, noting the time each 10 miles per hour.
There is a human element using a stopwatch, and the incline of the road will affect the results, no matter how flat. So we run the test THREE TIMES in each direction, giving us six time sets. Then we average the times. We actually had little variation there.
The chart below shows this coast down test for the Michelin Energy Saver A/S tires/wheels, as well as for the Stinger Radial GTS tires that originally came on the car.
As you can see, the results are pretty similar at the higher speeds, where aerodynamics comprises the predominant effect. But as the speed deteriorates, the two curves diverge pretty strongly. Total time was a difference of nearly 30 seconds. That’s quite a bit of time and quite a bit of distance differential for two sets of tires.
We also did some actual range testing. Excluding extraneous factors such as hoods and trunk lids, we really only have directly comparable data for 40 mph and 50 mph.
But the results are startling. At 40 mph our max range calculates to 152 miles with the Michelins and 124 miles with the Stinger tires. LRR tires typically provide a 3-5% increase in gas mileage. But in this case, starting with tires that are so BAD for an electric drive application, this was 28 miles further than the Stinger results – a gain in max range of 22.58%. This is frankly just huge. Not precisely apples to apples as they are entirely differently sized tires, but it’s a real gain and we’ll take it.
The results at 50 mph are less as there is slightly more of an aerodynamic component at the higher speed – predictably enough. But they are still substantial at over 14%.
A number of people have waned us to keep the eCobra cobra like. We don’t know precisely what this means. But we think it has something to do with burning rubber. So we installed a line locker on the front brake line. This allows us to spin the rear wheels while applying brake to the front wheels. It worked well enough as you’ll see in the video. As an added bonus, it makes a very handy parking brake.
We also did some very preliminary testing of 0 to 60 times using the pretty basics device provided on the GPS speedometer. It would appear we ran 0 to 60 mph in 6.77 seconds and 360 linear feet. We think we can improve on that with practice, probably down to about six seconds.
We hope to get the car over to Slingblade Racing this week for a full dynamometer test.