Regen or Friction? How Highway Assist Actually Slows You Down

[mpshizzle]

I’ve always wondered this, thank you @mpshizzle for scratching my curiosity’s itch with all your experiments! hope you continue to do more!

Haha I'm just glad I'm not the only one who gets these curiosity itches. And don't you worry, I've got lots of plans!

I was wondering this recently as well. Thanks for taking the time to really investigate and come up with a reasonable answer!

By the way, I noticed that the "braking" force when in either cruise mode is much, much higher when you override the safe distance setting with the accelerator pedal, and then lift off the pedal while too close to the vehicle in front. I suspect friction brakes may be used when that happens.

My assumption, which you seem to have proved, is that with normal driving the regen is the same regardless of the mode. But in that situation, when the slowing force is so much greater, it would be interesting to find out if it really is the friction brakes or just much more regen. The motors definitely have the potential to provide higher regen, so it could just be that.

What do you think?

Hmm that's a tough one. Definitely a couple of things at play here, one of which being that the Highway Assist ML model certainly has a ways to go when it comes to reacting smoothly when there's not enough buffer between you and the car in front.

One thing that I do know is that when you're manually driving, it has a slow roll on for the regen force to help smooth out pedal inputs. The motors and inverters certainly have the ability to give more quick stabs of regen with less roll on. That said, I'm not entirely sure if it employs that ability during highway assist. (I didn't really look much at sudden braking situations in my testing).

Just an educated guess, though - I would assume that when sudden braking is required it does a quick calculation if regen can supply the decleration needed, and if it can do so quickly enough. If not, it will then squeeze on the already pre-charge brakes. Again, just a guess based on what I've seen.

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[Captain JB]

WEll done and thanks for the great info.

 

[Pappamoto]

Coming from a Porsche 911, one thing I noticed about my Rivian was the lack of brake dust when washing it.

 

[ThisIsMyHandle]

I can only say this anecdotally, but if I am in slow highway traffic driver+ (R1T G1 ) I get horrible numbers in the efficiency graph (like 1.5 M/Kwh). But if I drive manually I may even gain a % or two and the number goes upwards of 5.

 

[mkhuffman]

I can only say this anecdotally, but if I am in slow highway traffic driver+ (R1T G1 ) I get horrible numbers in the efficiency graph (like 1.5 M/Kwh). But if I drive manually I may even gain a % or two and the number goes upwards of 5.

When using Driver+, do you find it brakes heavily as traffic slows and other vehicles jump in front of you due to the gap? The gap is extremely annoying, and I will close the gap when in Driver+ with the go-pedal, so others don't keep jumping in front of me. That reduces the excessive slowing.

I suspect excessive slowing when in Driver+ does use the friction brakes. Moderate slowing, like when using regen without Driver+, only uses the motors. Just a theory.

 

[hudesigns]

My educated guess is for every friction braking there’s regen involved. It’s an integrated process by design. That explains driver+ (or highway assist) braking happens inside braking booster where regen also happens. It’s not an either/or issue but various percentage of the two. Friction part is final resort to take care of whatever force required to stop, either initiated manually by pedal or automaticly by ADAS.

 

[mkhuffman]

My educated guess is for every friction braking there’s regen involved. It’s an integrated process by design. That explains driver+ (or highway assist) braking happens inside braking booster where regen also happens. It’s not an either/or issue but various percentage of the two. Friction part is final resort to take care of whatever force required to stop, either initiated manually by pedal or automaticly by ADAS.

Rivian has stated it does NOT have blended braking (friction mixed with regen). This is beyond dispute. Lucid follows the same approach.

When you push down the brake pedal, the friction brakes are engaged. Regen is whatever you selected (high, medium or low) and it is totally unrelated to how hard you press on the brake pedal.

When using adaptive cruise, the discussion is about what happens when the stopping force needed is more than what the selected regen provides. I think it is likely friction brakes are added to the regen to slow the truck. Nobody has definitively proven if that happens or not.

 

[godfodder0901]

(Hope ChatGPT is wrong in this case, but it’s kind of interesting it explains with such detail.)

“Does Rivian braking system not involve regen function”


Good question — the Rivian braking system absolutely does involve regenerative braking, but it also integrates with traditional friction brakes. Here’s how it works in detail

1. Regenerative Braking (Regen)

• When you lift off the accelerator, the Rivian’s electric motors reverse torque to slow the vehicle.
• This process converts kinetic energy back into electrical energy and stores it in the battery.
• It’s a major contributor to efficiency and extended range — similar to Tesla and other EVs.
• The regen strength is adjustable in Rivian’s drive modes (“Standard” vs “High” regen).
• In “High” regen mode, Rivians can achieve true one-pedal driving — you can often stop completely without using the brake pedal.

2. Hydraulic (Friction) Brakes

• Rivian trucks (R1T, R1S) also have conventional hydraulic disc brakes on all four wheels.
• These come into play when:
  • You press the brake pedal beyond what regen can provide,
  • Battery is fully charged (no room to store regenerated energy),
  • Or when very low-speed stopping is needed (regen is less effective below a few mph).

3. Blended Braking System

• Rivian uses brake blending — a computerized system that seamlessly transitions between regenerative and friction braking.
• The driver just presses the brake pedal; the system decides how much regen vs. friction to apply for a smooth, consistent feel.

4. Extra Features

• The Rivian platform includes ABS, EBD, and Brake Assist, integrated with regen for stability.
• Software updates can tweak regen behavior — for example, newer updates improved low-speed smoothness and increased regen consistency.

Definitely some AI slop in there.

 

[hudesigns]

Definitely some AI slop in there.

You’re probably right. I deleted my posts to avoid confusion.

 

[hudesigns]

Here’s google Gemini summary for Rivian which is more to the point:


In summary:

• Normal Deceleration (Lifting off the accelerator): Controlled by regenerative braking (one-pedal driving).

• Limited Regen Conditions (Battery too cold/full/downhill): The system automatically blends in friction brakes to maintain the expected deceleration rate (Regen Assist).

• Brake Pedal Input: Primarily engages the friction brakes.

 

[portdirect]

Here’s google Gemini summary for Rivian which is more to the point:


In summary:

• Normal Deceleration (Lifting off the accelerator): Controlled by regenerative braking (one-pedal driving).

• Limited Regen Conditions (Battery too cold/full/downhill): The system automatically blends in friction brakes to maintain the expected deceleration rate (Regen Assist).

• Brake Pedal Input: Primarily engages the friction brakes.

What’s the objective of these posts?

 

[godfodder0901]

Here’s google Gemini summary for Rivian which is more to the point:


In summary:

• Normal Deceleration (Lifting off the accelerator): Controlled by regenerative braking (one-pedal driving).

• Limited Regen Conditions (Battery too cold/full/downhill): The system automatically blends in friction brakes to maintain the expected deceleration rate (Regen Assist).

• Brake Pedal Input: Primarily engages the friction brakes.

The brake pedal doesn't provide any regen. Regen happens when you lift off the throttle. The only thing the brake pedal does is engage the friction brakes. Rivians do not have blended braking in this way. This was exactly what I was referring to as AI slop.

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