Drag force analysis

[bwilson4web]

The EPA shares the Test Car Data which has metrics for all vehicles. Of particular interest are the roll down coefficients:

• Target Coef A (lbf)
• Target Coef B (lbf/mph)
• Target Coef C (lbf/mph**2)

By doing the math using different speeds, we can get the expected drag force for any vehicle. The drag force and speed has a direct relationship to the drive train energy needed. To simplify the metrics, three speeds give an accurate drag force in common operational ranges:

• 35 mph - an expected urban only, home, work, and errands vehicle
• 50 mph - expected urban usage plus some high-speed, cross town expressways
• 70 mph - cross country speed, what the YouTube team uses, @Domenick

CAUTION: drag force is not electric drive train efficiency! For example, silicon power electronics are less efficient than silicon-carbide power electronics.

So here are the top six vehicles at these three speeds from best to worst.

35 mph

50 mph

70 mph

Discussion

The Lucid and Ioniq 6 show up in all speed ranges, 35, 50, and 70 mph. The Teslas show up in the 50 and 70 mph range. The i4 shows up in the 35 mph, urban range. Given the reported price differences, the lower cost Ioniq 6 should take sales from the more expensive Lucid. The Teslas need to reduce their low speed drag which can be done by better tires and lower weight.

I've attached the OpenSource spreadsheet in the attached ZIP file.

Bob Wilson

 

[Domenick]

The EPA shares the Test Car Data which has metrics for all vehicles. Of particular interest are the roll down coefficients:

• Target Coef A (lbf)
• Target Coef B (lbf/mph)
• Target Coef C (lbf/mph**2)

By doing the math using different speeds, we can get the expected drag force for any vehicle. The drag force and speed has a direct relationship to the drive train energy needed. To simplify the metrics, three speeds give an accurate drag force in common operational ranges:

• 35 mph - an expected urban only, home, work, and errands vehicle
• 50 mph - expected urban usage plus some high-speed, cross town expressways
• 70 mph - cross country speed, what the YouTube team uses, @Domenick

CAUTION: drag force is not electric drive train efficiency! For example, silicon power electronics are less efficient than silicon-carbide power electronics.

So here are the top six vehicles at these three speeds from best to worst.

35 mph

View attachment 20381

50 mph

View attachment 20382

70 mph

View attachment 20383

Discussion

The Lucid and Ioniq 6 show up in all speed ranges, 35, 50, and 70 mph. The Teslas show up in the 50 and 70 mph range. The i4 shows up in the 35 mph, urban range. Given the reported price differences, the lower cost Ioniq 6 should take sales from the more expensive Lucid. The Teslas need to reduce their low speed drag which can be done by better tires and lower weight.

I've attached the OpenSource spreadsheet in the attached ZIP file.

Bob Wilson

Is there another variable involved in calculating this beside coefficient of drag and frontal surface area (besides air density)?

 

[insightman]

Thanks @bwilson4web for those illustrations, but such a tight bunch of lines! I added my reference brick just to see some separation. I'm sure my 2000 Honda Insight would have done much better, but it was nowhere as much fun as my MINI Cooper SE.

 

[Domenick]

Is there another variable involved in calculating this beside coefficient of drag and frontal surface area (besides air density)?

I guess weight might be a factor here too, right?

 

[insightman]

I guess weight might be a factor here too, right?

Also, wheels make such an amazing difference--both dead weight and, even more importantly, rotional weight, which affects acceleration even more than dead weight (something bicyclists who have experienced super-light wheels can appreciate).

 

[bwilson4web]

Is there another variable involved in calculating this beside coefficient of drag and frontal surface area (besides air density)?

The roll down coefficients are not 'coefficient of drag and frontal surface area.' These EPA coefficients come from getting the vehicle to speed and then accurately measure how the vehicle decelerates.

It is an involved, detailed process but gives trustable results. For example, at least 10 roll down tests are run and the results averaged. I tried it once using an iPhone GPS App and it is a bear!

I was trying to find a way to quantify tire rolling resistance. Although real, I could not get metrics that came even close to the EPA coefficients. I still want to do tire rolling drag testing.

Bob Wilson

 

[Domenick]

The roll down coefficients are not 'coefficient of drag and frontal surface area.' These EPA coefficients come from getting the vehicle to speed and then accurately measure how the vehicle decelerates.

It is an involved, detailed process but gives trustable results. For example, at least 10 roll down tests are run and the results averaged. I tried it once using an iPhone GPS App and it is a bear!

I was trying to find a way to quantify tire rolling resistance. Although real, I could not get metrics that came even close to the EPA coefficients. I still want to do tire rolling drag testing.

Bob Wilson

Ah, I see. I know manufacturers run these tests themselves and give the EPA their numbers. I had assumed they used computer modeling to produce the results. The EPA does, of course, do its own tests, but not on every model.

 

[insightman]

The roll down coefficients are not 'coefficient of drag and frontal surface area.' These EPA coefficients come from getting the vehicle to speed and then accurately measure how the vehicle decelerates.

It is an involved, detailed process but gives trustable results. For example, at least 10 roll down tests are run and the results averaged. I tried it once using an iPhone GPS App and it is a bear!

I was trying to find a way to quantify tire rolling resistance. Although real, I could not get metrics that came even close to the EPA coefficients. I still want to do tire rolling drag testing.

Bob Wilson

Oh, if there's no acceleration involved, then heavy wheels would produce better results? And the lighter weight of my aerodynamically challenged MINI EV also contributes to its shorter roll down distance.

 

[bwilson4web]

Actually the roll-down test is so demanding, there are firms who do this work. About 10 years ago, a Korean firm paid a fine for misunderstood testing.

Bob Wilson

 

[Puppethead]

Which wheels are being tested? Almost every car maker has multiple choices, MINI being an extreme example with nearly a dozen factory choices. And then there's the question of tires. MINI ships their SEs with at least three different tire styles, even though the customer doesn't get to choose which one they'll get. We've seen in the Cooper SE forum that the wheel/tire combo can have a huge impact on range.

 

[insightman]

Which wheels are being tested? Almost every car maker has multiple choices, MINI being an extreme example with nearly a dozen factory choices. And then there's the question of tires. MINI ships their SEs with at least three different tire styles, even though the customer doesn't get to choose which one they'll get. We've seen in the Cooper SE forum that the wheel/tire combo can have a huge impact on range.

View attachment 20390

I'm sure every car maker chooses the best tires and wheels for the test. Power Spokes would be the obvious wheels if the test involves only a roll-down, without acceleration where heavier wheels pose a disadvantage. I wouldn't be surprised if there is no requirement that the tires used in the roll-down test must be the same tires actually sold on the vehicle. These tests may be performed long before the final tires are selected and the vehicle goes on sale.

 

[bwilson4web]

I knew the OEM tires on my 2019 Model 3 were a poor choice. So I replaced them with Bridgestone ECOPIA and the wheels with light weight ones. Benchmarks showed a significant, 4-3%, improvement at speeds below 40 mph. It crossed at 45 mph and the absence of aero covers show more drag, 2-3%, at 70+ mph.

Bob Wilson

 

[papab]

Also, wheels make such an amazing difference--both dead weight and, even more importantly, rotional weight, which affects acceleration even more than dead weight (something bicyclists who have experienced super-light wheels can appreciate).

For normal driving the affects of rotational inertia is in the noise. For bikes too. It's like putting on a louder muffler, it make you feel faster.

 

[insightman]

For normal driving the affects of rotational inertia is in the noise. For bikes too. It's like putting on a louder muffler, it make you feel faster.

This article says "Shaving a pound from your tires is equivalent to shaving at most 2 pounds of non-rotating weight. That's PER TIRE, so a pound off each tire could worth close to 8 pounds of weight reduction. For wheels, the multiplier is closer to 1.6, so saving 5 pounds per wheel (20 total) would feel like a static weight reduction of 32 pounds."

Empirical proof is that larger wheels on an EV provide less range than smaller wheels.

I believe the effects of rotational inertia on lighter bicycle wheels are easy to detect because you feel faster. In the 70s, I was amazed at how much easier it was to accelerate with 200-gram rims and 200-gram silk sew-up tires, but that delicate combination that wasn't for everyday riding. Decades later, I found another way to reduce the rotational inertia by getting a Moulton Speed bike with 20-inch wheels (Alex Moulton helped design the hydrolastic suspension for the first Mini).

 

[papab]

That only matters when you're accelerating, not much effect when cruising.

 

[bwilson4web]

That only matters when you're accelerating, not much effect when cruising.

Source?

My benchmarks before with the Tesla OEM wheels and tires compared to the lighter, open spoke, and lighter Bridgestone ECOPIA tires showed reduced power needed under 45 mph and more power needed above 45 mph. Also, the EPA roll-down coefficients reveal a low-speed performance impact:

The blue line shows the Tesla drag force at lower speed has a higher than expected value compared to the high speed drag force.

I have plan to measure the effects of open, spoke wheels versus: (1) 'hub caps', (2) wheel well covers, and (3) aero 'mud flaps'. The hub caps will block the fan effects of the spinning wheel and interference with side body air flow. The wheel well covers, easier on the rears but harder on the front, reduce airflow in and out so the body side air has reduced turbulence. The aero 'mud flaps' will aero shape reduce the blunt tire effects. Easier done on the rear than the front.

Bob Wilson

 

[papab]

Source?

F=Ma, or in this case trq = inertia * acceleration.
mass or rotational intertia only come into play when accelerating. When we're not changing speed, we're not accelerating.
Mass does affect rolling resistance. Aero drag is the biggest energy loss at highway speeds. The shape of the wheels or wheel covers affect aero drag.

Also, with a hybrid or BEV we recover a large amount of the energy it takes to accelerate when we use regenerative deceleration.