Semi takes its first business trip

[scaesare]

My original aero calcs were only based on tractor frontal. No idea what the trailer factor is.

Well THIS is interesting... it suggests the trailer (and the gap it creates with the cab) accounts for 75% of aero load:

So given the aero treatment we saw with the semi event, at least the gap might be less of a factor. However, with the low Cd the semi itself has, that means a standard non-optimized trailer may still account for the majority of the aero load.

Given the range figures were for a fully loaded truck, including trailer, does that mean it may be necessary to revise the 1.6KWh/mi estimate?

 

[mongo]

Well THIS is interesting... it suggests the trailer (and the gap it creates with the cab) accounts for 75% of aero load:
View attachment 286368

So given the aero treatment we saw with the semi event, at least the gap might be less of a factor. However, with the low Cd the semi itself has, that means a standard non-optimized trailer may still account for the majority of the aero load.

Given the range figures were for a fully loaded truck, including trailer, does that mean it may be necessary to revise the 1.6KWh/mi estimate?

Cool pic.
The tail drag would still exist with a tractor only, so that should not be a differentiating factor. The gap drag is minimized with the wings. The sides and top are fairly smooth (maybe laminar?), so the big one would be the rough trailer bottom structure.

If Tesla's Cd is tractor only, then the aero number I came up with made is optimistic. 1.6 kWh/ mile was 80k load and low resistance tires + 6% extra buffer.

 

[scaesare]

Cool pic.
The tail drag would still exist with a tractor only, so that should not be a differentiating factor. The gap drag is minimized with the wings. The sides and top are fairly smooth (maybe laminar?), so the big one would be the rough trailer bottom structure.

If Tesla's Cd is tractor only, then the aero number I came up with made is optimistic. 1.6 kWh/ mile was 80k load and low resistance tires + 6% extra buffer.

OK cool... I do agree the gap may be a smaller factor.

I do suspect that the Cd was semi only. If that's the case, then even with minimizing the gap, etc... that the total trailer aero load may be at least equal to the tractor.

 

[mongo]

OK cool... I do agree the gap may be a smaller factor.

I do suspect that the Cd was semi only. If that's the case, then even with minimizing the gap, etc... that the total trailer aero load may be at least equal to the tractor.

Here's my thinking.
Cab only: pressure at the front, vacuum at the back, turbulence on sides/ top bottom
Cab+trailer: pressure at front is the same, the vacuum that was behind the cab in now behind the trailer, so no change there
The sides and top are smooth, so I'd expect less drag off those, some turbulence at the gap.
Bottom is bumpy.

So I'm thinking the extra length and bottom are much less of an effect that the original frontal / trailing cross-section of the tractor was.
Maybe 25% hit?

 

[scaesare]

Here's my thinking.
Cab only: pressure at the front, vacuum at the back, turbulence on sides/ top bottom
Cab+trailer: pressure at front is the same, the vacuum that was behind the cab in now behind the trailer, so no change there
The sides and top are smooth, so I'd expect less drag off those, some turbulence at the gap.
Bottom is bumpy.

So I'm thinking the extra length and bottom are much less of an effect that the original frontal / trailing cross-section of the tractor was.
Maybe 25% hit?

You mentioned earlier that your "original aero calcs were only based on tractor frontal". Based on that I assumed you did not factor the rear of the vehicle, be it just cab, or cab + trailer? Yet in the last couple of posts you mention that the vacuum behind is same regardless of either cab or trailer. Maybe that's part of the disconnect?

My (perhaps inaccurate) thought process is that if that above graphic holds roughly true, then then distribution of aero load on a standard tractor trailer is 25%/75%. Now if Tesla has reduced the Semi's aero load, then proportionally the trailer contributes an even GREATER percentage of the overall load, even if that absolute load value has been reduced. Maybe now it's more like 15%/85%?

Using round values, let's say a standard rig experiences 2,000 lbs of resistive force @ 60MPH. That's 500 for tractor, 1500 for trailer. If Tesla has reduced the cab Cd so now it experiences 40% less drag, that's now 300lbs tractor, 1500lbs trailer.

Even if you reduced the trailer drag by 50% (gap flaps, etc...), that's still 750lbs trailer... or 2.5X the drag of the truck. Even if you reduce the trailer drag by 80%, it still accounts for 50% of the overall aero load. I'm not sure that much optimization for a standard trailer can be assumed.

Thus if the calcs are for semi frontal only, I suspect they might be low.

Thoughts?

 

[mongo]

You mentioned earlier that your "original aero calcs were only based on tractor frontal". Based on that I assumed you did not factor the rear of the vehicle, be it just cab, or cab + trailer? Yet in the last couple of posts you mention that the vacuum behind is same regardless of either cab or trailer? Maybe that's part of the disconnect?

My (perhaps inaccurate) thought process is that if that above graphic holds roughly true, then then distribution of aero load on a standard tractor trailer is 25%/75%. Now if Tesla has reduced the Semi's overall load, then proportionally the trailer accounts for an even GREATER percentage of the overall load, even if the overall load has been reduced. Maybe now it's more like 15%/85%?

Using numbers for an example, let's say a standard rig experiences 2,000 lbs of resistive force @ 60MPH. That's 500 for tractor, 1500 for trailer. If Tesla has reduced the cab Cd so now it experiences 40% less drag, that's now 300lbs tractor, 1500lbs trailer.

Even if you reduced the trailer drag by 50% (gap flaps, etc...), that's still 750lbs trailer... or 2.5X the drag of the truck. Even if you reduce the trailer drag by 80%, it still accounts for 50% of the overall aero load. I'm not sure that much optimization for a standard trailer can be assumed.

Thus if the calcs are for semi frontal only, I suspect they might be low.

Thoughts?

Gotcha
The aero drag equation is based on speed, air density, Cd, and frontal cross-section. So the drag from the cab rear is already accounted for.
When you add the trailer, the cab rear drag goes away and the trailer rear drag gets added (largely the same being non-tapered).

In the pic, the trailer is 75% because it includes the rear drag. Keeping forces the same, the tractor alone would be >50% (25 front 25 back, some top/side/bottom) so the trailer addition is less than the cab.
So I'm thinking the side and top friction are pretty low, and the bottom adds in 20% from the cab alone value. Double that to 40% additional, that adds 300Wh/mile to energy usage getting close to the 2kWh/mile upper limit.