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Model X Coefficient of Drag?

ratsbew

Active Member
Mar 3, 2012
1,289
957
O'Fallon, IL
Any guesses on the Cd for the Model X? I'm sure that aerodynamics were the highest priority for range purposes. Do you think the coefficient of drag is lower than the Model S? I'm sure the CdA is higher.
 

BerTX

Supporting Member
Supporting Member
May 2, 2014
3,505
3,568
Texas/Washington
I expect the Cd is higher. The rear window is too steep of a slope, to allow 3rd row seating headroom. The spoiler was added to attempt to correct the problem, and it will help, but if it were that simple to correct such an issue, then all cars would have a spoiler, since the rear of almost all vehicles has that particular design problem.
 

LargeHamCollider

Battery cells != scalable
Jan 10, 2015
981
1,837
United States
I would not be surprised if it was the same as or very close to the MS, drag will be higher due to increased A but cd could be the same or even lower, I guess .25...
 

Saghost

Well-Known Member
Oct 9, 2013
8,217
7,010
Delaware
Any guesses on the Cd for the Model X? I'm sure that aerodynamics were the highest priority for range purposes. Do you think the coefficient of drag is lower than the Model S? I'm sure the CdA is higher.

Well, let's see. :)

We think we know that the car has ~10% less range on the same battery (Sig XP90D @240 EPA vs SP90D @ 253 EPA "+6%" per Tesla.)

We think we know that the car is 1200 pounds heavier (a tech gave the weight as 6100 pounds to someone on the forum (Post#543 here,) and that's consistent with the promised 0-60 times vs the model S 0-60 times.)

We think we know that the X is 198" long, 66.3" high, and 89.4" wide with 7.2" ground clearance vs the S's 196" length, 56.5" height, 86.2" width, and 5.6" ground clearance. The S is 77.3" with the mirrors folded. (Post#447 here:)

We assume the same powertrain efficiency and the same tire/drivetrain rolling resistance per pound.

With a 24% heavier car, the rolling resistance should be 24% higher - but rolling resistance is typically a small factor at freeway speeds compared to aerodynamics.

The X is (66.3-7.2) 59" tall vs the S's (56.5-5.6) 50.9" - 16%% more cross sectional area. Assuming the same mirrors, it's also 3.2" wider = 4% more cross sectional area.

If rolling resistance had no effect, the Cd has to be ~9% lower to get the demonstrated range with ~20% more frontal area. If rolling resistance is 50% of the freeway number, Cd has to be at least 20% lower to get the range shown - because the losses from rolling resistance exceed the demonstrated range loss, and the car has to have a lower overall CdA.

The reality is presumably somewhere in between these two cases assuming our baseline "facts" are correct.
Walter
 
Last edited:

richkae

VIN587
Jan 15, 2008
1,917
29
The fact that they managed to get rid of the side mirrors means that the Cd could be lower than the Model S. Their absence will also reduce the frontal area gain a little bit.
 

Saghost

Well-Known Member
Oct 9, 2013
8,217
7,010
Delaware
The fact that they managed to get rid of the side mirrors means that the Cd could be lower than the Model S. Their absence will also reduce the frontal area gain a little bit.

That would make perfect sense - except the online configuration images and data (89.2" width...) are clearly still based on having mirrors, so I assume the estimated EPA range is also based on that.
Walter
 

ScepticMatt

Member
Nov 5, 2014
453
10
Austria
The fact that they managed to get rid of the side mirrors means that the Cd could be lower than the Model S.
wrong.

No way around it, as the current US/Canada regulations require a unity magnification mirror. The UN Regulations used almost everywhere allow cameras, but also require mirrors. Side Mirrors can be flat, convex or aspheric.

"S5.2.1 Each passenger car shall have an outside mirror of unit magnification. [...]

Direct link: http://www.gpo.gov/fdsys/pkg/CFR-2011-title49-vol6/pdf/CFR-2011-title49-vol6-sec571-111.pdf

"S.15.2.1.1.1. Compulsory: 1 on the driver's side and 1 on the passenger's side"

Direct link: http://www.unece.org/fileadmin/DAM/trans/main/wp29/wp29regs/r046r3e.pdf
 

Rashomon

Member
Mar 10, 2014
260
1,233
East Troy, WI
US Model Xs will have sideview mirrors, per US regulation. Tesla has petitioned for a change in that regulation, but the US system moves very, very slowly. Bureaucrats never lose their job for saying no, while a change to a "safety" regulation that ended up with unexpected consequences could be difficult for them. While UNECE regulations would seem to require that EU Model Xs have sideview mirrors, there is a precedent for a different result there. It is possible to petition for type approval with relief from a specific standard, if you can make a good case for that relief. An example of that in the past was the first Audi LED headlights, when UNECE standards didn't allow for LED lighting elements. Similarly, and on this specific issue, VW has already released the XL1, which is being produced with cameras instead of rear view mirrors. I don't know if Tesla has enough manpower to push something like this through, because it would indeed be extra homologation hassle.
 

richkae

VIN587
Jan 15, 2008
1,917
29
We'll know in 24 days.
What if they managed to provide a camera system that makes the mirrors unnecessary and then you can just leave the mirrors folded in all the time.
 
Last edited:

Rashomon

Member
Mar 10, 2014
260
1,233
East Troy, WI
Well, let's see. :)

We think we know that the car has ~10% less range on the same battery (Sig XP90D @240 EPA vs SP90D @ 253 EPA "+6%" per Tesla.)

We think we know that the car is 1200 pounds heavier (a tech gave the weight as 6100 pounds to someone on the forum (Post#543 here,) and that's consistent with the promised 0-60 times vs the model S 0-60 times.)

We think we know that the X is 198" long, 66.3" high, and 89.4" wide with 7.2" ground clearance vs the S's 196" length, 56.5" height, 86.2" width, and 5.6" ground clearance. The S is 77.3" with the mirrors folded. (Post#447 here:)

We assume the same powertrain efficiency and the same tire/drivetrain rolling resistance per pound.

With a 24% heavier car, the rolling resistance should be 24% higher - but rolling resistance is typically a small factor at freeway speeds compared to aerodynamics.

The X is (66.3-7.2) 59" tall vs the S's (56.5-5.6) 50.9" - 16%% more cross sectional area. Assuming the same mirrors, it's also 3.2" wider = 4% more cross sectional area.

If rolling resistance had no effect, the Cd has to be ~9% lower to get the demonstrated range with ~20% more frontal area. If rolling resistance is 50% of the freeway number, Cd has to be at least 20% lower to get the range shown - because the losses from rolling resistance exceed the demonstrated range loss, and the car has to have a lower overall CdA.

The reality is presumably somewhere in between these two cases assuming our baseline "facts" are correct.
Walter

A little surprisingly, rolling resistance at 70 mph with such heavy vehicles is significant, almost 40 percent of the total road load. It's easy to set up a spreadsheet to calculate that, but you can also just go to http://www.analyticcycling.com/ForcesPower_Page.html and plug in some ridiculous numbers for a bicycle! A typical automotive rolling resistance is in the 0.012 to 0.007 range -- I'd use .01 for a Model X. If the X is indeed 1200 pounds heavier than the S (and I hope not), and with a significantly higher A, the Cd has to be very good indeed to be getting the claimed EPA numbers.
 

Atlantis

Member
May 22, 2014
151
98
Portugal
As the drag of side mirrors will increase exponentially as the speed increase, we could imagine automatically folding mirrors at a speed superior of 45 miles/hour with cameras in their end points that would give side views at high speed. Maybe, the side mirrors could be mandatory at low speed (in urban traffic) but not at high speed.
 

ScepticMatt

Member
Nov 5, 2014
453
10
Austria
As the drag of side mirrors will increase exponentially.
As a mathematician I'm slightly annoyed with the frequent misuse of exponentially :tongue:
Rather, drag increases quadratically per distance and cubically per time.
Doubling the speed results in a 8 times as high drag power.


28560fb9ccae7b5f811de11f965d5478.png

e31430f0898268091f410282a89503b1.png
 

Vitold

Active Member
Aug 10, 2015
1,688
1,786
NM
Well, let's see. :)

We think we know that the car has ~10% less range on the same battery (Sig XP90D @240 EPA vs SP90D @ 253 EPA "+6%" per Tesla.)

We think we know that the car is 1200 pounds heavier (a tech gave the weight as 6100 pounds to someone on the forum (Post#543 here,) and that's consistent with the promised 0-60 times vs the model S 0-60 times.)

We think we know that the X is 198" long, 66.3" high, and 89.4" wide with 7.2" ground clearance vs the S's 196" length, 56.5" height, 86.2" width, and 5.6" ground clearance. The S is 77.3" with the mirrors folded. (Post#447 here:)

We assume the same powertrain efficiency and the same tire/drivetrain rolling resistance per pound.

With a 24% heavier car, the rolling resistance should be 24% higher - but rolling resistance is typically a small factor at freeway speeds compared to aerodynamics.

The X is (66.3-7.2) 59" tall vs the S's (56.5-5.6) 50.9" - 16%% more cross sectional area. Assuming the same mirrors, it's also 3.2" wider = 4% more cross sectional area.

If rolling resistance had no effect, the Cd has to be ~9% lower to get the demonstrated range with ~20% more frontal area. If rolling resistance is 50% of the freeway number, Cd has to be at least 20% lower to get the range shown - because the losses from rolling resistance exceed the demonstrated range loss, and the car has to have a lower overall CdA.

The reality is presumably somewhere in between these two cases assuming our baseline "facts" are correct.
Walter

Calculations would make more sense if highway range was known where Cd is the biggest factor. Using EPA range skews result since Cd is only one of many factors affecting range in EPA mileage test.
 

Saghost

Well-Known Member
Oct 9, 2013
8,217
7,010
Delaware
Calculations would make more sense if highway range was known where Cd is the biggest factor. Using EPA range skews result since Cd is only one of many factors affecting range in EPA mileage test.

I don't disagree, although EPA highway range is still a composite of a bunch of factors. Do you have the highway range? :)

In three weeks we should know a whole bunch more - possibly including knowing that one or more of the assumptions I stated at the top is incorrect. Until then, I think it's the best I can do with the data we (think we) have.
Walter
 

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