Model 23 with roof rack - testing wh/km

[shonline]

It's been a while since I posted but I wanted to ask a question. If anyone can help me get my head around this I'd appreciate it.

- I have a model three long range with aero wheels.
- I have the tesla roof racks installed.
- I have a Yakima cargo carrier and I've read many articles here and online about turning it backwards to see if there is better efficiency.

I did a real world test today of 17 km as best I could in a perfect circuit, using the same start band end point.

With the roof rack mounted backwards I was saving 3 W hours per kilometre vs the pointing end forward.

My question is, over a 4000 km road trip does this really translate into a saving worth having it flipped backwards? I know it's just math but I really can't get my head around it and would appreciate any help or any links that could explain if this is worth it.

Thanks in advance for your help.

Math was never my best

 

[3sr+buyer]

Assuming you mean mounting the cargo box (rather than the roof rack) backwards, since the Tesla roof racks can only be mounted one way...

3Wh/km is small, but why not take it if there is no disadvantage with the cargo box mounted backward?

 

[RayK]

In my mind it's not really math but a question of fluid dynamics. Without sticking your car into a wind tunnel (with a smoke generator) and seeing the resulting airflow and measuring the drag with your cargo carrier both "forward" and "backwards", one would have to guess at the reason why you (and others) have seen better efficiency with a backwards mounting but it may be due to an explanation that I will talk about later. It does go against the conventional wisdom that a pointed, curved end will have air flowing around it cleaner, than a blunt, flat end. It may come down to how the air goes around the front end of your car, up the windshield, over the roof and exits at the back of the vehicle.

It's the same thing why some people think that leaving the tailgate of a pickup truck down results in better gas mileage. Or using one of those mesh nets instead of the original tailgate. There's been scientific measurements done that shows that removing the tailgate results in worse drag on the vehicle since the airflow over the bed is disrupted. That causes an increase in drag and therefore worse gas mileage. The same thing can be happening to cargo carrier orientation.

 

[Ital]

lol no one can answer the question?
well 3w per kilometer is 300w every 100km, or 3000w every 1,000 km, or 1000w every 333km
Where I live in CT that would translate into $.25 every 333km when charging from home

 

[ewoodrick]

It's been a while since I posted but I wanted to ask a question. If anyone can help me get my head around this I'd appreciate it.

- I have a model three long range with aero wheels.
- I have the tesla roof racks installed.
- I have a Yakima cargo carrier and I've read many articles here and online about turning it backwards to see if there is better efficiency.

I did a real world test today of 17 km as best I could in a perfect circuit, using the same start band end point.

With the roof rack mounted backwards I was saving 3 W hours per kilometre vs the pointing end forward.

My question is, over a 4000 km road trip does this really translate into a saving worth having it flipped backwards? I know it's just math but I really can't get my head around it and would appreciate any help or any links that could explain if this is worth it.

Thanks in advance for your help.

Math was never my best

3W per km isn't a big deal, that's a little more than 1%.

But, I have to question your test. How many times did you drive the route? Was the wind the same each time, was the temperature the same?
How much did the roof rack use pointing backwards?
How much weight was in the rack when testing?

 

[shonline]

I did the same route four times within an hour. So, all things considered, I would expect the meteorological conditions to have been pretty much the same. But of course you never know 100%.

The roof rack was empty.

I used 222 within pointed forward, and 219 with it pointed backwards. 3 watthours per kilometre difference. It did not make sense for me to put it backwards for this road trip give it these results, and the fact that I'm sure I would get a lot of questions or raise some attention for myself on the trip. Had it been been larger than that, then I certainly would have done it.