Tesla Semi Must-Haves: Trailer with Regeneration, Full Aero Treatment

Tesla Semi


Both Tesla Semi trucks from the reveal event

Tesla Semi – Cd = .36

A custom Tesla trailer with regeneration and full aerodynamic treatment is a must-have option for Tesla Semi truck buyers.

To be clear, Tesla hasn’t made any mention of selling a custom trailer for their semi truck but based on our analysis, they probably should and we prove why.

Tesla has said the tractor will come in two basic flavors: a 300-mile version for $150,000 and a 500-mile version for $180,000. Obviously, those prices don’t include the trailer. Tesla also announced some very impressive performance specs. They’ve told us their truck has a very low aerodynamic drag with a Cd of .36 and then showed us a sleek tractor-trailer rig with full aero treatment.

Check out our exclusive 4-part Tesla Semi series here

So, perfect aerodynamics is key to the equation. Also, missing from any discussions is the regeneration of wasted brake energy in the trailer.

Based on our detailed modeling (see section 2) trailer aero treatment and regeneration of waste braking energy in the trailer is worth 15% in fuel consumption.

That’s a big number, worth $20,000 in fuel costs (ref 4 at the bottom of the article) over one million miles. There are more reasons why offering a custom trailer makes sense. Perhaps there is more profit potential in a custom trailer. Our previous analysis showed little or no profit margin in the 500-mile tractor (ref 9). Maybe Tesla could compensate by offering a custom trailer (to make up for profit margins through options).

Tesla Semi custom trailer Slide 1

Figure 1 – Five reasons why

During our research for this article, we uncovered a company that is already marketing just such a product. The company is Hyliion and they make two very interesting products similar to what we are discussing here. For those interested, see references 1-3 to check them out.

Tesla Semi custom trailer Slide 2

Figure 2 – Hyliion retrofit

One system is a retrofit of a motor/axle assembly (e-axle) with regen capabilities to the trailer. The e-axle replaces one of the two axles on the trailer. Batteries capture the braking energy normally lost to friction. The other system replaces an axle on the tractor with an e-axle with motor and adds batteries to the tractor. The system costs approximately $38,000 for the e-axle with motor, the battery pack, and the controller. The e-axle can also provide additional power on steep hills. Hyliion is also claiming fuel savings in line with our analysis.

Tesla could easily implement regen capabilities in a custom trailer. No additional batteries would be needed as the main tractor pack can easily take the energy, and only a single Model 3 motor would be required, based on our analysis. Tesla could get the job done at a fraction of the cost.

Tesla Semi custom trailer Slide 3

Figure 3 – e axle


For those Tesla truck aficionados that missed it, Keith and I wrote four detailed articles on the Tesla semi truck you may like to revisit (see ref 6-9). In these articles, we showed that the semi truck can meet all of Tesla’s performance specs, including acceleration, speed on a steep grade, battery weight, and whole truck weight. The last of the four-part series was a cost analysis. Results of the cost analysis showed the 300-mile tractor had some profit margin but the 500-mile range tractor with the larger battery was a challenge in the cost department. In order to make profit margin, that truck would require Tesla to make battery packs (not cells) for 100$/kWh.

In order to verify performance specs, Keith developed an excel model of the semi-truck. The model takes into account hill climbing energy, aero losses, rolling resistance, accessory power, acceleration energy, and drivetrain losses.

In order to do the analysis described in this article, I added waypoints and route following capability to the model. That way, the route can be broken into segments and little details, such as stopping at a stop light and re-accelerating. Loading and unloading through the route can be accounted for as well.

Tesla Semi custom trailer Slide 4

Figure 4 – Computer model description

The goal of the analysis was to determine what fuel penalties are associated with not having any regeneration in the trailer and for not having full aero treatment in the trailer. The Tesla semi can pull any kind of trailer so we need to know the penalty for this if a carrier decides to use his/her own fleet of older trailers.

We ran in-town routes and highway routes for three cases

  1. Baseline truck
  2. Truck with no regen in the trailer
  3. Truck with no regen in the trailer AND poor aero drag characteristics (see figure 8 below).

The in-town route consists of multiple 30-mile legs between loading docks. Before the freeway stretch, we simulated five start-stop cycles. After the freeway stretch, we modeled four start-stop cycles.

As one would expect, all the stop lights, as well as a heavy load make regen a must-have option for this in-town route. Bottom line: a solid 15% influence for regen and full aero treatment. We would think most buyers of the short-haul version for in-town deliveries would want to opt for regen in the trailer.

Tesla Semi custom trailer Slide 5

Figure 5 – In-town route

The highway route we modeled was from Fremont, CA to the Tesla Gigafactory near Reno, NV over Donner Pass. Again, we saw a solid 15% influence for regen and full aero treatment.

Tesla Semi custom trailer Slide 6

Figure 6 – Highway route

The only case where regen is not that critical is in the case where you have very few stop lights and a constant 55 MPH freeway speed. In this ideal case, the influence of regen and aero is only 8%. So, some carriers that have this ideal type of route might not want to opt for the custom trailer with regen.

Tesla Semi custom trailer Slide 7

Figure 7 – Ideal highway

Aero loss assumptions are detailed in figure 8.

Tesla Semi custom trailer Slide 8

Figure 8 – Aero loss assumptions

After reviewing the reveal videos it appears that the trailer showed in the reveal was very close to a standard trailer….ie a flat front and partial skirting. The gap between the trailer and the cab is closed via some extensions on the cab. It also appears that perhaps those extensions may hinge in order to facilitate better aero when the trailer is not perfectly straight in line with the tractor.

Therefore it appears that Tesla has indeed gone to extra efforts to make this truck aerodynamically correct with “near standard’ trailers.

It is also possible that more braking could be done in the tractor than was assumed in the analysis. We assumed that the trailer would provide 40% of the total braking effort. However, the dynamics of the situation during a stop would result in more braking effort being done by the cab, thus reducing the effect of regen in the trailer.

I reran the numbers with and lowered the braking in the trailer to 25% from 40%. This reduced the effect of trailer regen from 13% to 8.2% in figure 5.  So regen in the trailer still helps but not by as much.

Therefore there should be no implication in this article that Tesla’s stated specs cannot be made without regen and added trailer aero improvements.

*This analysis and article was a joint effort between myself (George) and fellow Mechanical engineer Keith Ritter.


  1. Hyliion trailer e-axle
  2. Hyliion tractor e axle and battery system
  3. Hyliion web site
  4. Tesla Semi fuel analysis
  5. Tractor Aero Data
  6. Tesla semi performance specs verified
  7. Tesla Semi battery lighter than you think
  8. Tesla Semi meets weight Bogey
  9. Tesla Semi cost targets a challenge

Categories: Tesla, Trucks

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60 Comments on "Tesla Semi Must-Haves: Trailer with Regeneration, Full Aero Treatment"

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A semi + autonomous + SpaceX satellites = something like Uber and a lot of truck drivers staying at home World Wide!

You are saying it like it’s a good thing … you realize this could replicate not only in this industry, right?

Not sure where you work, but ….

We have to accept change. History is full of examples. But I agree with you it is going to be a disruption that will affect nearly all industries. In a way it is a good thing. We will have peace and no war for oil. People just have to learn new skills, as they did before.

Are serial-killer former/unemploed/replaced truck drivers better?

This is why some semblance of financial equality is required. As automation eliminates jobs its critical that the benefit of such is spread around, instead of only going to the 1%.
Some kind of universal basic income will be required. Otherwise, society will collapse.

Technology eliminating the need for labor is a great thing. If you disagree throw away ever thing that uses electricity you own, and buy a horse.

Simple thing in my opinion. Great worldwide policy seeking volunteer women:
1. Insert blood sampling thingy in person’s arm. It reports measurements wirelessly.
2. Insert long term birth control (already exists) into arm.
3. Measure blood. As long as blood tests show birth control still in place, person gets free and 125% of median income (not minimum…median) in the US free. In developing countries, person gets 300% (or appropriate big incentive). They are free to earn additional money as desired.

Aren’t trailer brakes only necessary for hard stops, especially for a Tesla truck cab that has extra battery mass to keep it planted?

If driven well, the cab should be able to take on 80%+ of the regen duties, even in stop and go traffic.

You are assuming that all kinetic energy is captured all the time. This is not the case with any EV. That logic might apply if you were moving in a straight line all the time.

Seems about right to me.

“The cab should be able to take on 80%+ of the regen duties, even in stop and go traffic.”

Right. Trailer regen would only be useful when the tractor-trailer rig used its air brakes, which is only when making a sudden stop. That shouldn’t be very often; not often enough to make a significant impact on overall energy efficiency.

Sad to say it, because the authors of this article clearly put a lot of work into it, but this article is based on a series of premises which are simply wrong. Value of “information” in this article = 0.

We researched this.

Current-generation Class 8 semi-trailer trucks use the same type of sophisticated ABS and Electronic Stability Control systems as current-generation passenger vehicles – on ALL axles, both tractor and trailer. It isn’t just “hard braking” that kicks in the trailer brake system.

With a loaded trailer weighing about 30 tons, the trailer axles are carrying 42% of the total rig load and contributing their share of braking throughout the route. The result is that jackknifing events have dramatically dropped over the last decade.

O.K. but what if it’s because normal diesel engine gearing and brake doesn’t have the controllability of 4 motorized electric motor at the main axle acting smoothly and within few millisecond?

It sure is a lot of weigh to brake, but if the trailer can pull it, it can stop it.

It will only be better in low traction condition or in prone jackknifing situation.

On a daily basis, I don’t understand why the main axles setup won’t be enough.

It is truth that: “Current-generation Class 8 semi-trailer trucks use the same type of sophisticated ABS and Electronic Stability Control systems as current-generation passenger vehicles – on ALL axles, both tractor and trailer.” However: “It isn’t just “hard braking” that kicks in the trailer brake system.”, is at leas misleading. Electronic Stability Control (ESC) can activate trailer brakes, but: 1. this is not often (if it goes more than once a month that driver is really dangerous) 2. ESC do not use brakes for more than few seconds 3. Regeneration can not break quick and powerful enough to be used with ESC (ESC require quick, powerful and short breaking to stabilize vehicle) For almost all truck breaking except for full stop or in case of emergency drivers use engine breaking. There are simply no brakes that will withstand going down mountain with 80.000 pounds gross weight So if current trucks almost all times break with only engine driven axles. Why shouldn’t Tesla also only use regeneration on existing engines, and keep things simple and weight down. I simply did not understand this part: “With a loaded trailer weighing about 30 tons, the trailer axles are carrying 42% of the total… Read more »

If the drive axles can get the rig to 60 in 20 seconds, they sure as hell can stop it in 20 seconds. A faster stop than that sounds like an emergency stop to me.

I hear semi trucks using air brakes when approaching a toll road entrance. They also use them when approaching a tight turn on a freeway exit.

So air brakes are used for more than just emergency stops. They are used whenever hard braking is required. But they aren’t used that often, and when they are used, it’s only for a few seconds. So, logically and realistically, there just isn’t that much opportunity to recapture regen energy from such events, contrary to what HVACman is saying.

I commend HVACman and his writing partner for previous articles in this series, but facts are facts, and the assertions here are simply wrong.

Nicely detailed article, thanks.

Yeah, It is pretty obvious when an engineer composes an article. Not knocking on the rest of the wonderful staff here. Just refreshing that they incorporate articles written by actual engineers from time to time.
More George Bower articles!

Thanks, guys! We really love and appreciate articles like this and want to feature them. We’re happy that you notice that. More to come, as always!

Makes you wonder why full aero is not a current requirement, similar to europe. Guess we just like wasting fuel here.

Aero trailer requirements are slowly creeping into the regulatory environment. CARB (as usual) is leading the way. There are now “California Compliant” trailer regs:


There are also some Federal regs that are approaching, but I don’t know if they are in-place yet. Trump EPA considering revision or delay.


I certainly hope that Tesla is not so foolish as to base its economic model only on some sort of next-generation trailers. If trucking fleets can’t use the Tesla Semi Truck with existing trailers — both their own trailers and their customers’ trailers — then they simply won’t buy Tesla trucks, period.

Now, admittedly, Tesla did show one of its Semi Trucks pulling a trailer with “skirts” at the Reveal (see image linked below). So it’s entirely possible that Tesla’s official numbers assume trailers with skirts. But Tesla did not show a trailer with a “boat tail” or, so far as I can see, any other of the more extreme modifications seen if you use Google Images to search for “aero semi trailer”.

Tesla had durn well better not be expecting its customers to use “aero” trailers. If it does, then the response it’s gonna get is… crickets. From what Tesla showed at the Reveal for the Tesla Semi Truck, it seems pretty clear that Tesla knows this.


Interesting if you consider the expected 2kWh per mile that everyone is using and the difference in the range prices of $30,000 then when you do the math it comes out to $75 per kWh of battery.
200 miles = 400kWh battery size difference for $30,000. 30,000 / 400 = 75

So Tesla is planning to be able to sell batteries at a profit for only $75/kWh? That would be huge.

We figured the 300-mile-range semi would require a 510 kWh battery – so the difference is about 390 kWh for $30,000. $77/kWh. That is why we figured the 300-mile-range urban hauler semi will be sold first and the 500-mile-range line haul semi will await future battery developments to reduce cost.

Note that a smaller battery means a lighter rig and fewer kWh/mile. The 300-mile range Semi would use about 1.7 kWh/mile.

From article: “Do Tesla’s specs assume that trailer-braking energy is captured? …”

Answer: No

More specifically…

Trailer regen braking is assumed in the spec but only for that whitch is captured by the Tesla Semi cab… the trailer brake controller (located in the semi cab)
will have the +/- gain adjustment set for the Tesla Semi cab to take on the majority of braking and thus action most of the trailer braking from the cab regen braking system.

Good article analysis from OP in terms of the $ value (fuel wise) of regen capturing the trailer braking back into to the battery and also less wear-and-tear maintenance cost for the trailer brakes…

BUT what the article misses is that the majority of trailer braking for the trailer will be captured by the Tesla Semi cab regen braking system… no need for custom trailers outfitted with onboard trailer regen braking.

Not being involved with trucking, what this is essentially saying is that the trailers will not have brakes on them at all. Is that a reasonable assumption? Can a tractor do ALL the braking for an entire semi?

No, standard semi trucks have brakes on the trailers. Brakes powered by compressed air, called “air brakes”. It’s almost certain that Tesla Semi Trucks will have hookups to power trailer air brakes. Not doing so would be very dangerous. You do need those brakes for situations where the truck needs to slow its speed quickly. But one of the things which the authors of this article seem to have missed — and even after they revised the article in response to some comments, they’re still missing — is that Tesla has probably engineered the tractor to take up most (perhaps nearly all) of the braking that’s normally done by the trailer. Note especially Tesla’s claims for “eliminating” jack-knifing with improved dynamic control of the 4 motors used to power the drive wheels individually. In one of the comments here, George/HVACman said: “Current-generation Class 8 semi-trailer trucks use the same type of sophisticated ABS and Electronic Stability Control systems as current-generation passenger vehicles – on ALL axles, both tractor and trailer. It isn’t just “hard braking” that kicks in the trailer brake system… The result is that jackknifing events have dramatically dropped over the last decade.” Now, I don’t believe it’s… Read more »

From article update: “I reran the numbers with and lowered the braking in the trailer to 25% from 40%…”

Wild guess here but perhaps trailer braking lowered to 2-5% may be closer to the mark… yup I know that seems crazy low.

Trailer braking from the trailer braking system (located on the trailer) I imagine would be mostly fringe case events… such as a Tesla Semi with a near fully charged battery going down a long steep mountain grade in winter.

Today, Musk: “This is stupid”…
12/31/20, Musk: “We have delayed the Semi as we decided to make the e-Trailer”…

Sorry to see such an absurd assertion on the part of the authors of this article. In previous articles, I though their analyses were very well researched and very well thought out. No trucking company is going to lock itself into using non-standard trailers. Not even retrofits to standard trailers. The biggest practical and economic advantage of a tractor-trailer rig is that (at least in theory) any tractor can pull any trailer. Once you start requiring non-standard trailers, you have just thrown the biggest advantage that semi tractors have out the window. No trucking fleet which at least sometimes uses customers’ trailers, is going to buy any tractor which requires the use of non-standard trailers. And few trucking companies of any size would consider buying such tractors even if they use their own trailers. Requiring that some tractors in a fleet be matched to only certain trailers, would be a logistical nightmare for the company. Bottom line: In general, this ain’t gonna happen. Tesla might be able to sell to a small niche of small companies willing to convert all or nearly all their trailers, but there is no way large trucking fleets would even consider saddling themselves with a… Read more »

Thank you well said. The trucking industry is all about cost. Tesla make a trailer that is only compatible to the tesla truck then you can say goodbye to those orders

The authors are not stating that the Tesla rig will require a specific trailer, only that the performance specs may be linked to a specific trailer.
A trucking company will absolutely be able to use their own trailers, just that performance may not be as good.
Much like efficiency in Colorado is less than in Kansas.

Your criticism of my assertion is valid, and I thank you for that. I did overstate my case, in the opposite direction from how George (HVACman) and Keith overstated their case in the article, when they said: “A custom Tesla trailer with regeneration and full aerodynamic treatment is a must-have option for Tesla Semi truck buyers.” It’s not a must-have, and in fact in the update, George added at the bottom: “…there should be no implication in this article that Tesla’s stated specs cannot be made without regen and added trailer aero improvements.” This update directly contradicts the more forceful assertion quoted above — in boldface — near the beginning of the article. Yet that earlier boldface statement remains in place. So yes, trucking fleet drivers would get better energy efficiency using “aero” trailers than standard trailers. But that’s true no matter what kind of semi tractors the trucking company is using. Lower drag would benefit diesel semi tractors every bit as much as BEV semi tractors. If trucking companies have not already spent money on “aero” trailers to be hauled by their diesel tractors, then clearly they don’t think those non-standard trailers are worth the additional cost. Buying Tesla… Read more »

I like this kind of articles of deeper thought.
But in this case I’m pretty sure the answer is no.

I think the stated range is calculated on the Tesla tractor and a ‘Typical skirt’ trailer without regen, no need for special trailers. The scale module wind tunnel test basic model has a vastly different tractor shape and no gap skirts between tractor and trailer.

I do agree that Tesla may use special trailers for moving their own batteries, just not a requirement for buyers to get the stated range.

The only real difference in the tractor shape is the nose shape, which only contributes about 0.05 to the Cd at most. Both the wind-tunnel and Semi tractors have full-skirting and air dams.

Note that Tesla gave a “500 mile” range estimate at the reveal, which we confirmed in a previous article but only in ideal flat constant-speed conditions which do not require regen. But trucking is not ideal at all. We are now modeling reality – which is what the trucking industry faces every day – not ideal.

We do not know why Tesla has not yet clarified what trailer characteristics their range estimates and 0.36 Cd is based on. Our numbers say full-aero-trailer and, in mountains or urban haul, trailer regen. Perhaps if enough people hound them about it, they will clarify.

“I think the stated range is calculated on the Tesla tractor and a ‘Typical skirt’ trailer without regen, no need for special trailers.”

That is the most reasonable scenario, for multiple reasons. First, that fits with what Tesla showed at the Reveal for the truck. No trailer with extreme “aero” modifications was shown; just a trailer with pretty normal skirts. Secondly, that scenario fits with what Tesla would want to advertise to existing trucking fleets, which is its target market. No way is Tesla going to try to sell trucking companies on having to modify all the trailers used with Tesla’s Semi Trucks. Even “temporary” mods would be a logistical nightmare for any fleet scheduler.

Tesla marketing isn’t going to shoot itself in the foot like that.

For urban haul with start-stops and in the mountains, trailer brake regen will be vital to meet specified range with the installed battery size. For flat-land line-haul on the Interstate, not so much. Full Aero is vital to all applications.

We don’t make up the numbers or pre-judge Tesla or their potential customers. The bottom line is: Customers whose networks are national and who wish to take full economic and performance advantage of the Tesla Semi’s benefits will use a full aero-trailer WITH regen. And I suspect their field trials will show them that. Perhaps Wabash or other existing trailer manufacturers will jump in and handle these details.

We can be reasonably sure that PACCAR, Volvo, Daimler, Mack and others will have equals to the Tesla Semi also rolling into the market soon if field-trials show Tesla’s onto something. So it will behoove all the major trailer manufacturers to evolve an EV-specific trailer w/regen axle set that can plug into the EV tractor. Aero and regen retrofit kits would also be good. This really doesn’t require a whole new trailer. Trailer axles are easily-changed. Just slide the powered axle on and away you go.

As a reminder, we are told by actual semi truck drivers that trucking fleets typically have 5 to 7 trailers for every semi tractor.

The premise here that the trucking fleet will buy or modify just a single trailer to fit the Tesla Semi Tractor, and use that for every load the tractor pulls… well, it seems to be one of multiple false assumptions/premises in this article.

That simply isn’t how trucking fleets operate. Tesla’s truck must fit how fleets actually run their businesses, not fit an unrealistic “green tech” ideal.

Remember who Tesla’s potential customers are for this truck. Tesla must sell to them, or find itself with only a tiny niche market.

You are right about the 5:1 trailer/tractor ratio – and that is an issue for Tesla and their potential customers to sort out – and why the conversion to an all-electric Class 8 trucking future will not be an “instant” revolution, but an evolution. There is a whole lot of capital tied up in the current structure that has to be amortized before being abandoned.

Missed a few pluses on the e-axle.
1) lower maintenance due to less braking.
2) ability to move fully loaded trailer around lots with a small mule.

A trailer that has motorised axles – i.e. motors on all the axles, not just one – would be awesome for grip in the wet, and snow and ice. They would have to completely re-think the “Ice Road Truckers” TV show because it wouldn’t be as hazardous any more!

Would also be able to reclaim loads of energy on downhills and not wear out the brakes.

Would also even out tire wear as tires on the regen axles suffer more than those that are just rolling axles.

I’m not sure they need to provide more then the technology (units), that different trailer manufacturers can buy, and use- to build their custom trailers. Just like there are a lot of specialized trucks, there are a lot of specialized trailers. If you go to a truck company and ask what a truck cost, they will at best smile at you, since there are so many options, cab models, 2×4, 4×4, 6×6, 8×8 and so on. You order a truck with the axels you want, due to price, weight, load capacity, tire wear, area of use, and so on. What do you want to transport? Heated goods, refridgerated goods, hanging goods (like meat), live animals, dangerous goods, liquids, powders, cement mixer, cars, shipping containers and what not. At work, we have day cabs for local transport, sleeper cabs for longer trips, and even larger cabs for international long haul. We have some goods that weighs next to nothing, but the volume is huge. Since it is a contract over several years the truck, and trailer is custum ordered for this job. Extra heavy goods require a lot of extra wheels to handle the weight. Tesla could off course try to… Read more »

It’s cool he can shoot a car into space but haul a load of strawberries from Oxnard,CA to Hunts point market in 72 hours with a Tesla semi impossible..
Stay charged

I like to see how they are going to handle the great canadian climate ice and snow and dont forget the salt and calcium

As someone who actually designs this stuff for a living, I worry about having the ‘e axle’ at the back of a 48 foot trailer feeding power to the tractor battery.

An 80,000 lb tractor/trailer could easily generate 400V at 1kA during load. That’s 400kW. (the vehicle I worked on was a single vehicle and easily generated 500A during braking, so I’m being conservative here. I can’t IMAGINE the power generated from a full trailer descending the Sierra Nevada– which is the mountain range between Reno (Gigafactory) and Fremont (Tesla Factory)

Not sure I’d be happy running 400kW through 50′ of high-voltage cable– especially if there were issues such as arcing, or insulation breaks, or pinched cables which won’t show up until you’re under load.

I dunno, that’s why they pay engineers the big bucks– but it still makes the hairs on the back of my neck stand up.
Not sure

Re update to the article:

“Therefore there should be no implication in this article that Tesla’s stated specs cannot be made without regen and added trailer aero improvements.”

Hey, shout out to George and Keith: Thank you for updating the article for better accuracy!

Now, this is what happens when you get engineers rather than reporters to write articles. 🙂 They actually respond to positive criticisms of their analyses, and revise the article to reflect them!

Too bad we don’t see more of that from reporters writing on technical issues.

For reference with a current truck at 80,000 lbs going down 6% grade for like 3 miles you start at about 25mph even with the jake, you snub brake ( so you don’t overheat your brakes) on the way down.
When you run that heavy, you seriously do not want that to get away from you.

I’m a truck driver,we have to run a lease 600 miles a day to make money and a 300 mos truck is not work a penny, plus another big problem is charging ,they have to test that truck in very cool Condition like below 0 and were they go to get lithium is not that much in the planet be realistic

Quit hauling cheap freight, you wouldn’t have to do that