Taycan CT vs Hummer EV

[Jrkennedy37]

Ground clearance is a low order influence on efficiency as long as the underside isn’t too rough, which is usually ok on an EV because it is a battery not drive shafts and exhaust.
The big thing ruining highway efficiency is the shape and also frontal area, which is dreadful and huge, respectively, on the Hummer - and almost all trucks and SUVs.

Very interesting. How do the massive wheels, suspension and brakes, which are exposed with higher ground clearance, play into the turbulence under the vehicle?

I’ve noticed the full size American trucks and SUVs are on the trajectory of larger front fascia with lower and lower front bumpers/splitters. They have terrible approach angles as a result and require optioning an off road package or diving into aftermarket lifts to fix this, which is the complete antithesis for buying an SUV or truck.

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[f1eng]

Very interesting. How do the massive wheels, suspension and brakes, which are exposed with higher ground clearance, play into the turbulence under the vehicle?

I’ve noticed the full size American trucks and SUVs are on the trajectory of larger front fascia with lower and lower front bumpers/splitters. They have terrible approach angles as a result and require optioning an off road package or diving into aftermarket lifts to fix this, which is the complete antithesis for buying an SUV or truck.

I have only considered the aerodynamics of racing cars and a small number of road cars.
Most SUVs and Trucks are are aerodynamically rubbish anyway so probably any difference caused by ride height will be a small proportion of the huge number that they inevitably have.
Do you use the expression "a bit like polishing a turd" in the US?


It is rarely mentioned in the non-techical press but the overall drag is proportional to the frontal area as well as Cd, so two vehicles with the same drag coefficient will not have the same drag if they are different sizes.

SUVs are fashionable here at the moment but very few trucks.
I have zero interest in either, I spent my career trying to make cars as efficient as possible so they are anathema to me

 

[Jrkennedy37]

I have only considered the aerodynamics of racing cars and a small number of road cars.
Most SUVs and Trucks are are aerodynamically rubbish anyway so probably any difference caused by ride height will be a small proportion of the huge number that they inevitably have.
Do you use the expression "a bit like polishing a turd" in the US?


It is rarely mentioned in the non-techical press but the overall drag is proportional to the frontal area as well as Cd, so two vehicles with the same drag coefficient will not have the same drag if they are different sizes.

Too funny. Living in Texas, I try to refrain from commenting on the topic of truck and SUV popularity. We are a two wagon household, partially for the reasons you state, and are often met with confusion and prejudgement about the practicality, specifically the cargo space sacrifice we’re making by not driving an SUV. It’s also occurred on the other popular porsche forum.

I love your comment on Cd - it’s amazing how this stat is touted by many carmakers and used as a comparison between vehicles but is not qualified with the corresponding frontal area measurement. I’m curious your take on balancing low Cd with traction/handling for cars like an EQS, Taycan, Plaid, eTron GT, Lucid, etc. For EVs touting their slipperiness and range but also appealing to Nurburgring times, what’s the right balance?

 

[Jhenson29]

It is rarely mentioned in the non-techical press but the overall drag is proportional to the frontal area as well as Cd, so two vehicles with the same drag coefficient will not have the same drag if they are different sizes.

In the context of a vehicle, is there any reason to reference Cd by itself and not as a value that includes area? Seems a bit like saying “miles per gallon….per pound”, ignoring that vehicles have a fixed weight and aren’t the same. Like an intentionally less useful number. Unless there are uses I’m unaware of as I ask about.

 

[Jrkennedy37]

In the context of a vehicle, is there any reason to reference Cd by itself and not as a value that includes area? Seems a bit like saying “miles per gallon….per pound”, ignoring that vehicles have a fixed weight and aren’t the same. Like an intentionally less useful number. Unless there are uses I’m unaware of as I ask about.

Good question, that I’m also curious about. I’ve seen references that a particular vehicle has improved its Cd - ie the new Cadillac Escalade has improved its Cd by 25% to 0.36. This clearly indicates improvement of the aerodynamic efficiency of the Escalade in the context of itself but it doesn’t include the fact they likely increased the frontal surface area with the new generation. The times I’ve seen it referenced is usually in the context of efficiency, but as you indicated MPG is ultimately what matters for efficiency.

There’s been so much press about the EQS Cd but my inclination is that it’s a marketing sound bite and not actually as important for comparing the practicalities across EVs.

 

[JimBob]

Cd includes the frontal area so is unique to the geometry of the car.

These kinds of dimensionless constants are common throughout physics and engineering.

 

[f1eng]

In the context of a vehicle, is there any reason to reference Cd by itself and not as a value that includes area?

No there is none.

 

[f1eng]

Cd includes the frontal area so is unique to the geometry of the car.

Cd does not include frontal area.
It is a dimensionless coefficient, as you write, and frontal area is one of the dimensioned parameters used in its calculation.
The drag force is proportional to it and multiplied by the "reference area" which is pretty well always the frontal area, which is the area of the front elevation.
Disappointingly the 6-wheel tyrrell F1 car of the 1970s was built because the designer misunderstood this and thought it was the area of the front and would have less drag. The frontal area of the wheels is dominated by the larger rear ones.

In my times working on Formula 1 aerodynamics I didn't bother to calculate Cd at all, I just used drag (and downforce) figures in lbs at 150mph (yes I started before we went metric ).
This gave me a good feel for the loads involved for component strength and stiffness and requirements for suspension stiffness to stop bottoming.
The hundreds of young guys doing it now usually calculate a CdA and refer to a 0.1 increment in its value as a "point" - no idea why or who started it, probably one of the University tunnel systems. OTOH these guys don't have to design the car or run it at the circuit like the old days.

A standard house brick probably has a worse drag coefficient than a Hummer (but maybe not...) but would produce a lot less total drag.

 

[f1eng]

Too funny. Living in Texas, I try to refrain from commenting on the topic of truck and SUV popularity. We are a two wagon household, partially for the reasons you state, and are often met with confusion and prejudgement about the practicality, specifically the cargo space sacrifice we’re making by not driving an SUV. It’s also occurred on the other popular porsche forum.

I love your comment on Cd - it’s amazing how this stat is touted by many carmakers and used as a comparison between vehicles but is not qualified with the corresponding frontal area measurement. I’m curious your take on balancing low Cd with traction/handling for cars like an EQS, Taycan, Plaid, eTron GT, Lucid, etc. For EVs touting their slipperiness and range but also appealing to Nurburgring times, what’s the right balance?

Producing the low drag without introducing lift is important for grip and handling. Generating downforce anywhere but from the underside always induces drag IME and not worth doing for a road car though it will make a big difference for a trck car, though probably not as much as sticky tyres.

Good question, that I’m also curious about. I’ve seen references that a particular vehicle has improved its Cd - ie the new Cadillac Escalade has improved its Cd by 25% to 0.36. This clearly indicates improvement of the aerodynamic efficiency of the Escalade in the context of itself but it doesn’t include the fact they likely increased the frontal surface area with the new generation. The times I’ve seen it referenced is usually in the context of efficiency, but as you indicated MPG is ultimately what matters for efficiency.

There’s been so much press about the EQS Cd but my inclination is that it’s a marketing sound bite and not actually as important for comparing the practicalities across EVs.

A low drag is always good for efficiency particularly cruising at higher speeds. Cd on its own doesn't tell you much though because of frontal area.

The reasons I would never use a SUV were always crap aerodynamics plus big frontal area which I find professionally offensive, and high weight.
I had to think long and hard about the weight aspect of the Taycan before I ordered it (weight was a main reason I hadn't bought a Panamera PHEV last time) It has a reasonably low Cd and much lower frontal area than a SUV though.

 

[Dee]

I once saw an episode of Myth Busters where they examined the efficiency of pick-up trucks and what influence the tailgate had: open or closed.
You would expect that open would be more efficient due to air getting out easily at the back.
But it turned out to be the opposite: with the tailgate closed you'll get better efficiency.
The air over the cabin creates a bubble of air in the bed which leads to better flow over the car and less drag behind the cabin.
With the tailgate down there's no air bubble, only turbulence which causes more drag.
Quite interesting to know.

 

[DL_AU]

Producing the low drag without introducing lift is important for grip and handling. Generating downforce anywhere but from the underside always induces drag IME and not worth doing for a road car though it will make a big difference for a trck car, though probably not as much as sticky tyres.



A low drag is always good for efficiency particularly cruising at higher speeds. Cd on its own doesn't tell you much though because of frontal area.

The reasons I would never use a SUV were always crap aerodynamics plus big frontal area which I find professionally offensive, and high weight.
I had to think long and hard about the weight aspect of the Taycan before I ordered it (weight was a main reason I hadn't bought a Panamera PHEV last time) It has a reasonably low Cd and much lower frontal area than a SUV though.

This has turned out to be a much more interesting discussion than expected, given the topic!

First, thanks @f1eng for sharing your knowledge on this forum. When the porpoising issue arose in F1, I wondered whether the teams had properly modelled the dynamics. Is it a case of small perturbations in pitch/roll/yaw (resulting from bumps, steering inputs, or wind changes) exciting larger changes in the ground plane effect?

Drag coefficients are often misunderstood. A 747 has a lower drag coefficient than an Albatros, but it seems clear which one needs more energy to fly! Frontal area explains the difference in the energy required.

Given sufficient energy, even a pig will fly. I guess that explains how a Hummer EV works.

 

[JimBob]

Cd does not include frontal area.

Yes it does. See below, especially the fourth paragraph.

Drag is measured experimentally. Substitute into the equation, rearrange and solve for Cd.

 

[f1eng]

Yes it does. See below, especially the fourth paragraph.

Drag is measured experimentally. Substitute into the equation, rearrange and solve for Cd.

Sorry I wasn’t clear enough.

A reference area is used in the calculation of Cd to make it dimensionless.

On a car (and aeroplane, I believe) the reference area used is the projected frontal plan area.

That means Cd is useless in comparing actual drag of 2 vehicles unless the reference area is known because the drag force is proportinal to both the Cd and the reference area so two vehicles can have the same Cd but very different actual drag and hence overall efficiency.

None of the practical aerodynamicists I know bother to calculate Cd from the data, it isn’t worth bothering with when calculating anything useful.

 

[JimBob]

Sorry I wasn’t clear enough.

The issue is that most people do not know the difference between the coefficient of drag and drag force and when and how to use them.

Frontal areas for different vehicles can be found along with their Cd's. A quick check finds the frontal areas for Tesla's. So I can easily put together a table of drag forces for Taycan vs Tesla's and probably for other EV's and then you can see who is the most efficient. Using the comparison of Taycan's to pickups and SUV's as a case against the usefulness of drag coefficients is a red herring.

And if you have to model then you need the Cd's.

 

[f1eng]

And if you have to model then you need the Cd's.

Model what?

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