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Will Tesla Semi decelerate quicker than ICE semis?

ZeApelido

Active Member
Jun 1, 2016
2,779
21,694
The Peninsula, CA
Is the rate of deceleration in regular semis limited by brakes or by tires?

If by brakes, then the regen could help decelerate the vehicle more quickly than normal.

This is will be more important coming down from mountains. Overheating brakes causing runaway semis could be significantly reduced, correct?
 
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SSedan

Active Member
Jul 24, 2017
2,948
2,309
Greenville Wisconsin
Rate of deceleration would be a consideration for accident avoidance, then you talk about long decent and overheating friction brakes that is a very different thing even though the end result can be an accident.

Tire/brake rate of decel would suggest basically a panic stop situation, tires are usually the limiting factor, Traditional semi also have compression release braking they can use, so while it is not capturing energy like regen does it is a supplemental way of slowing the vehicle without the friction brakes if the location can allow the noise.

You ever stop to think that maybe the regen system could get overheated by the same prolonged grades that can overheat a regular semi's brakes? In which case hopefully the friction brakes are still cool enough to do the work in which case it would be those evil old archaic failure prone friction brakes that save us from the limits of regen.

I trust the semi regen system will be well engineered and reduce friction brake wear but it is still going to have limits like everything else, what those limits are we don't know, in all reality inside Tesla they are probably still working that all out.
 

mongo

Well-Known Member
May 3, 2017
12,911
38,147
Michigan
Given that holding speed on a downhill takes less energy than maintaining speed on the uphill, I do not forsee the regen system overheating. Of course, that will depend on how it is implemented.
Hopefully, the operators get sufficient training (or software buffer) to not fully charge the battery when at the the highest elevation on the trip.
 
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WannabeOwner

Well-Known Member
Nov 2, 2015
5,758
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Suffolk, UK
Hopefully, the operators get sufficient training (or software buffer) to not fully charge the battery when at the the highest elevation on the trip

I wondered that too, however how much capacity is needed to store Regen from a long / steep descent?

If charged to, say, 90% would the 10% remaining be enough for "Regen on any, realistic, hill"?
 
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mongo

Well-Known Member
May 3, 2017
12,911
38,147
Michigan
I wondered that too, however how much capacity is needed to store Regen from a long / steep descent?

If charged to, say, 90% would the 10% remaining be enough for "Regen on any, realistic, hill"?

Might not, semi may use 1.6kWh/mile. That is equal to the potential energy of 80k pounds moved 53 feet vertically (basically it can go forever on a 1% slope). At a 2% slope, it gains 1.6kWh per mile. 3,300 feel of elevation is 100kWh or 60ish miles. Very route dependent.

Then, depending on pack set up, regen may or may not be limited at 90%.
 

iwannam3

Member
Aug 8, 2016
902
1,273
Washington
Cabbage or Deadman Hill on I-84 eastbound in Oregon is 7 miles long and drops over 3,000 feet at 6% and has truck speed limits based on weight. Also has 2 runaway truck ramps!
75,000+ = 18 mph!
70,000 = 22mph
65,000 = 26mph
50,000 = 37 mph
Could regen alone keep an 80,000 truck at 18 mph? Will regen be adjustable? I wonder how fast it could go uphill? Can regen absorb the same power that the motors can deliver on the uphill segment? Can you charge and discharge a battery at the same rate or is one limited?
 

ccutrer

Active Member
Sep 3, 2015
1,348
1,228
Eagle Mountain, UT
Cabbage or Deadman Hill on I-84 eastbound in Oregon is 7 miles long and drops over 3,000 feet at 6% and has truck speed limits based on weight. Also has 2 runaway truck ramps!
75,000+ = 18 mph!
70,000 = 22mph
65,000 = 26mph
50,000 = 37 mph
Could regen alone keep an 80,000 truck at 18 mph? Will regen be adjustable? I wonder how fast it could go uphill? Can regen absorb the same power that the motors can deliver on the uphill segment? Can you charge and discharge a battery at the same rate or is one limited?
Going by experience in a Model X:

* Dunno, maybe. But probably not.
* On S and X, it is adjustable, but most people leave it on regular, instead of weak.
* No. I can deliver about 320kW, but only absorb about 75kW.

I doubt regen getting limited because of too much downhill grade will ever be a problem. You basically have to start with a 100% charge at the top of a mountain to do it. I’ve done it, once, with exactly that. I started in Duck Creek Village, UT at 100%, expended about 5% climbing out of town, had full regen, and then 30 minutes later regen started to get a bit limited as I approached 97% charge, driving 40mph the whole way downhill (no brakes until regen was limited) to Cedar City, UT. I’ll let the enterprising reader look up the distance and altitude change. But in normal driving, a semi will never charge to 100% during a long drive, and definitely not at the very top of a huge long hill. And if it does happen, it’s just a minor annoyance.
 
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WannabeOwner

Well-Known Member
Nov 2, 2015
5,758
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Suffolk, UK
I’ll let the enterprising reader look up the distance and altitude change.

EV Trip Planner says :)

Net Elevation Change -2670 feet
Journey 30 miles
Total Energy Used 4.1 kWh / 13 RM
Average Efficiency 136 Wh/mile

A Better Route Planner (doesn't show elevation change) agrees on 30 miles and 136 Wh/mile

definitely not at the very top of a huge long hill

... but naive / newbie / idiot might ... :(

if it does happen, it’s just a minor annoyance.

Brakes would get hot on a semi with no regen?
 

mongo

Well-Known Member
May 3, 2017
12,911
38,147
Michigan
Could regen alone keep an 80,000 truck at 18 mph? Will regen be adjustable? I wonder how fast it could go uphill?

Elon said it will do 60MPH up a 5% grade. That's around 1.6 kW for moving and 477 kW to climb. If semi pack is 800kWh, that is 8x a normal pack with 2x the drive units. If normal regen limit is 75kW, say it can do 200kW of regen.

Heading downhill, 200kW is 1.85 feet per second decent rate (ignoring aero/rolling for the moment). 60MPH is 88fps, so if the grade is 7%, the truck speed would be 18 MPH.
For a 6% grade at 18MPH, regen would be 172kW.

Rolling resistance would be a third of the 60MPH number and aero 1/9. So that adds in about 350W of buffer (non factor).

Handy calculator, setting output to kJ and height to feet, while inputting feet per second climb/ descent rate, gives KW output (J=W×s).

Brakes would get hot on a semi with no regen?

Very, if the truck lost regen, it would need to stop. All the energy from the above calculation goes staright into heat (spread across 10 brake units). Normal trucks use engine braking for the bulk of speed control.
 

Big Earl

bnkwupt
Jul 12, 2017
5,089
9,120
Springfield, VA
Most modern diesel semis can provide between 300 and 450 horsepower (225 - 335 kW) worth of engine braking power. That's in addition to the stopping power provided by the friction brakes. I expect the Tesla to be able to match that or provide more regenerative braking, depending on what the battery packs can handle.

If we assume that regeneration is limited to 50% of the battery's maximum charging power, we get something close to 500 kW of regenerative braking. Model S and X both regenerate at a maximum of 60 kW, which is half of their maximum charge speed of 120 kW. I think the Model 3 is similar.
 
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Brando

Active Member
Sep 27, 2016
2,865
1,990
Bainbridge Island, WA
You ever stop to think that maybe the regen system could get overheated by the same prolonged grades that can overheat a regular semi's brakes? In which case hopefully the friction brakes are still cool enough to do the work in which case it would be those evil old archaic failure prone friction brakes that save us from the limits of regen.
oh right. Generators burn up all the time from overuse.
That is why Nuclear reactors melt down, right?
I guess wind generators don't burn up cause of the wind cooling effect. Maybe we should put windmills on trucks to help stopping?

OK, tell the truth, you write for the Onion. Coffee out my nose and now I'm crying. I just love your comment.
great, really funny.

thanks.
 

brucet999

Active Member
Mar 12, 2015
2,676
1,485
Huntington Beach, CA
oh right. Generators burn up all the time from overuse.
That is why Nuclear reactors melt down, right?
I guess wind generators don't burn up cause of the wind cooling effect. Maybe we should put windmills on trucks to help stopping?

OK, tell the truth, you write for the Onion. Coffee out my nose and now I'm crying. I just love your comment.
great, really funny.

thanks.
No need for sarcasm. Not everyone here has the same level of technical knowledge.
 
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Silent Ludicrosy

Supporting Member
Mar 14, 2018
801
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Phoenix
9257C518-8111-45DD-8C0F-F110A46D76C4.jpeg
 
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APotatoGod

Member
Jul 31, 2018
133
81
California
oh right. Generators burn up all the time from overuse.
That is why Nuclear reactors melt down, right?
I guess wind generators don't burn up cause of the wind cooling effect. Maybe we should put windmills on trucks to help stopping?

OK, tell the truth, you write for the Onion. Coffee out my nose and now I'm crying. I just love your comment.
great, really funny.

thanks.

Although the generator might not overheat, the battery getting hot or filling up is a real concern. Although I suppose with the rumored 1MW charging rate it’d take quite a bit to overwhelm that battery!
 
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The Duke

Member
Nov 17, 2016
498
411
OK, mountains have been covered, but if the OP is talking single stop I still say yes.
Most trucks still use drum brakes because they are lighter and cheaper than discs. The Tesla semi will have discs and superior anti skid braking. That should translate to shorter stops than most semi's.
 
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Brando

Active Member
Sep 27, 2016
2,865
1,990
Bainbridge Island, WA
The TIRE is the limiting piece for traction to stop. {could add air brakes?? that is an idea)
Drum brakes can lock up the tires. You've all seen the skid marks on the Freeways.
Disc and re-gen are both better to fight brake fade. AND are more controllable to prevent lockup/skid of tires which is bad.

hope this helps
 
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Tezlanian

Member
Apr 24, 2018
214
92
North America
Most modern diesel semis can provide between 300 and 450 horsepower (225 - 335 kW) worth of engine braking power. That's in addition to the stopping power provided by the friction brakes. I expect the Tesla to be able to match that or provide more regenerative braking, depending on what the battery packs can handle.

If we assume that regeneration is limited to 50% of the battery's maximum charging power, we get something close to 500 kW of regenerative braking. Model S and X both regenerate at a maximum of 60 kW, which is half of their maximum charge speed of 120 kW. I think the Model 3 is similar.

The battery packs handling it isn't a concern. You see, there will be a giant Tesla brand Tesla Coil to handle the discharge of excess energy preferably generating music.
 

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