Regen or Friction? How Highway Assist Actually Slows You Down

[mpshizzle]

Full test is on YouTube (Thunder Volt Auto): Click Here

As is usually the case for my posts - it is much easier to understand what I'm talking about with the visuals in the video (pedal cam + service screen), but here’s the written breakdown if you’d rather read.

This all started when I noticed something not long after getting my R1S. When using Highway Assist or even basic cruise control, the brake pedal moves. At first, I worried — does that mean Rivian is skipping or limiting regen and going straight to friction brakes? That would waste a lot of efficiency. So I set up some tests to find out. TL;DR - yes it does use regen (sort of a "well duh, of course it does" moment for me), but the way it works I found to be a bit surprising and very fascinating. So if you are as intrigued as I was, read on!

Before we get to my testing, though, we need a little background: How the R1 brakes are set up. The R1 uses a traditional hydraulic brake system, not a brake-by-wire setup. The pedal is physically connected to the brake lines.

The “magic” comes from the brake booster, which can be actuated two ways:

1. By you pressing the pedal.
2. By a motor inside the booster, which the car uses for things like AEB, Highway Assist, or ACC. Because it’s physically linked, whenever that motor moves the brakes, the pedal itself moves too.

⸻

Setting up the Test:

The parameters of the test were relatively simple. I set up a camera pointed at the pedals so I could see exactly how much the brake pedal would move in each scenario. I also pulled up service mode on the main display and watched the battery current display to see how much current (in amps) was leaving (or entering) the high voltage battery. Negative amps for discharging, positive amps for charging. When at a stop it seemed to hover between -1.5 and -3 amps. Certainly not the most precise, scientific setup - but hopefully enough to get a better understanding of how regen works.

I wanted to observe behavior in 3 scenarios:

1. Manual control with one pedal driving
2. Cruise Control
3. Highwaty Assist

⸻

Baseline Test: One-Pedal Driving

Before getting into highway assist, I wanted to get a baseline for how the system acts uder normal one pedal driving. For the testing I just used my normal driving configuration - high regen with regen assist turned off to prevent friction brake blending.

The results: The brake pedal didn't move at ALL while the vehicle was in motion under normal conditions. Acceleration, deceleration, no matter what it just stayed completely still. The only exception to this was when coming to a stop. At about 1MPH, just before rolling to a stop, the pedal would move down a fair bit to help stop the vehicle and lock into position for auto hold.

⸻

Cruise Control Test

This is where things got interesting. As I had noticed before, the brake pedal did move every time the vehicle decelerated. Each time the vehicle slowed, the pedal would move down just a TINY bit. What was surprising to me, though is that it was the same every time - no matter how much deceleration. Whether it was a heavy deceleration or just a light touch, it moved just a tiny bit to exactly the same spot - no more, no less. I also observed big spikes of energy going into the battery when the pedal would move to this position - indicating that there IS regen occuring. The only exception to this was again, coming to a complete stop. Just like with one pedal drive at about 1MPH the brake pedal would move down just a bit farther and lock into position for auto hold.

⸻

Highway Assist Test

Highway assist (as predicted) behaved identically to cruise control when it came to regen. I figured that would be the result but I wanted to test both, just in case. As with cruise control, the pedal would consistently and repeatedly move into the same position no matter how much deceleration was happening. And while it did this, the battery current meter showed energy going into the battery. And also just like cruise control, the only time the brake pedal moved any farther down was when coming to a stop and locking into auto hold.

⸻

So back to my root question: How can the brake pedal move if regen is still happening?

The big hint here was the fact that the brake pedal seemed to have a "favorite" position where it would just hang out, no matter how much deceleration was happening. This indicates that whatever the brake pedal is doing, it's not tied to slowing the vehicle. If it was, it would change and move throughout the deceleration. So what exactly is it doing, then? My best guess - it's just removing slack from the braking system so it can be at the ready at a moment's notice. A bit of slack in the first bit of the brake pedal's travel is pretty normal for hydraulic brakes. Getting rid of this slack means that if the driver or the emergency braking system feels they need to make an emergency intervention, the hydraulic brakes are ready to go instantly with some pressure already in the lines.

So what does that mean for regen? With the brakes simply in a state of being "pre charged" the brake pads aren't putting much (if any) pressure on the rotors. So that leaves all the rest up to regen. I could clearly see this by the big spikes of energy going into the battery when slowing down.

⸻

Takeaways

1. Highway Assist and cruise control do, in fact, rely on regen — efficiency is preserved.
2. The brake pedal motion is a side effect of Rivian’s traditional hydraulic setup.
3. It’s essentially the vehicle “taking up slack” so the brakes are ready if needed, without actually wasting potential regen.

⸻

Final thought: For me, this answered the concern that regen might be skipped or diminished under driver assist. It’s not — Rivian is maximizing efficiency while keeping safety in check. As always this was a very fun learning epxerience for me, and I hope it was for you too!

Now I will take this moment for a shameless referral code plug. If you feel like you learned something interesting here, feel free to use my code. You get 500 Rivian points, and it helps me out too! Win-win
Referral code here: https://rivian.com/configurations/list?reprCode=MIKE4580282

What do you all think? Anyone else notice the pedal twitch under cruise/HA?

Sponsored

 

[usulio]

Very cool, thanks for checking this out.

 

[iforbes]

I think the easiest way to check this would be infrared thermometer the calipers before and after assist and no assist on the same stretch of road, same ambient temps, etc etc etc.

Having said that…Nice writup!

 

[godfodder0901]

Full test is on YouTube (Thunder Volt Auto): Click Here

As is usually the case for my posts - it is much easier to understand what I'm talking about with the visuals in the video (pedal cam + service screen), but here’s the written breakdown if you’d rather read.

This all started when I noticed something not long after getting my R1S. When using Highway Assist or even basic cruise control, the brake pedal moves. At first, I worried — does that mean Rivian is skipping or limiting regen and going straight to friction brakes? That would waste a lot of efficiency. So I set up some tests to find out. TL;DR - yes it does use regen (sort of a "well duh, of course it does" moment for me), but the way it works I found to be a bit surprising and very fascinating. So if you are as intrigued as I was, read on!

Before we get to my testing, though, we need a little background: How the R1 brakes are set up. The R1 uses a traditional hydraulic brake system, not a brake-by-wire setup. The pedal is physically connected to the brake lines.

The “magic” comes from the brake booster, which can be actuated two ways:

1. By you pressing the pedal.
2. By a motor inside the booster, which the car uses for things like AEB, Highway Assist, or ACC. Because it’s physically linked, whenever that motor moves the brakes, the pedal itself moves too.

⸻

Setting up the Test:

The parameters of the test were relatively simple. I set up a camera pointed at the pedals so I could see exactly how much the brake pedal would move in each scenario. I also pulled up service mode on the main display and watched the battery current display to see how much energy was leaving (or entering) the high voltage battery. Negative amps for discharging, positive amps for charging. When at a stop it seemed to hover between -1.5 and -3 amps. Certainly not the most precise, scientific setup - but hopefully enough to get a better understanding of how regen works.

I wanted to observe behavior in 3 scenarios:

1. Manual control with one pedal driving
2. Cruise Control
3. Highwaty Assist

⸻

Baseline Test: One-Pedal Driving

Before getting into highway assist, I wanted to get a baseline for how the system acts uder normal one pedal driving. For the testing I just used my normal driving configuration - high regen with regen assist turned off to prevent friction brake blending.

The results: The brake pedal didn't move at ALL while the vehicle was in motion under normal conditions. Acceleration, deceleration, no matter what it just stayed completely still. The only exception to this was when coming to a stop. At about 1MPH, just before rolling to a stop, the pedal would move down a fair bit to help stop the vehicle and lock into position for auto hold.

⸻

Cruise Control Test

This is where things got interesting. As I had noticed before, the brake pedal did move every time the vehicle decelerated. Each time the vehicle slowed, the pedal would move down just a TINY bit. What was surprising to me, though is that it was the same every time - no matter how much deceleration. Whether it was a heavy deceleration or just a light touch, it moved just a tiny bit to exactly the same spot - no more, no less. I also observed big spikes of energy going into the battery when the pedal would move to this position - indicating that there IS regen occuring. The only exception to this was again, coming to a complete stop. Just like with one pedal drive at about 1MPH the brake pedal would move down just a bit farther and lock into position for auto hold.

⸻

Highway Assist Test

Highway assist (as predicted) behaved identically to cruise control when it came to regen. I figured that would be the result but I wanted to test both, just in case. As with cruise control, the pedal would consistently and repeatedly move into the same position no matter how much deceleration was happening. And while it did this, the battery current meter showed energy going into the battery. And also just like cruise control, the only time the brake pedal moved any farther down was when coming to a stop and locking into auto hold.

⸻

So back to my root question: How can the brake pedal move if regen is still happening?

The big hint here was the fact that the brake pedal seemed to have a "favorite" position where it would just hang out, no matter how much deceleration was happening. This indicates that whatever the brake pedal is doing, it's not tied to slowing the vehicle. If it was, it would change and move throughout the deceleration. So what exactly is it doing, then? My best guess - it's just removing slack from the braking system so it can be at the ready at a moment's notice. A bit of slack in the first bit of the brake pedal's travel is pretty normal for hydraulic brakes. Getting rid of this slack means that if the driver or the emergency braking system feels they need to make an emergency intervention, the hydraulic brakes are ready to go instantly with some pressure already in the lines.

So what does that mean for regen? With the brakes simply in a state of being "pre charged" the brake pads aren't putting much (if any) pressure on the rotors. So that leaves all the rest up to regen. I could clearly see this by the big spikes of energy going into the battery when slowing down.

⸻

Takeaways

1. Highway Assist and cruise control do, in fact, rely on regen — efficiency is preserved.
2. The brake pedal motion is a side effect of Rivian’s traditional hydraulic setup.
3. It’s essentially the vehicle “taking up slack” so the brakes are ready if needed, without actually wasting potential regen.

⸻

Final thought: For me, this answered the concern that regen might be skipped or diminished under driver assist. It’s not — Rivian is maximizing efficiency while keeping safety in check. As always this was a very fun learning epxerience for me, and I hope it was for you too!

Now I will take this moment for a shameless referral code plug. If you feel like you learned something interesting here, feel free to use my code. You get 500 Rivian points, and it helps me out too! Win-win
Referral code here: https://rivian.com/configurations/list?reprCode=MIKE4580282

What do you all think? Anyone else notice the pedal twitch under cruise/HA?

One note: the "Regen" section of the meter isn't purely regen... Use ACC or HA when a sudden slowdown occurs and the vehicle is using active braking and observe the meter. I think you'll be surprised.

 

[mpshizzle]

One note: the "Regen" section of the meter isn't purely regen... Use ACC or HA when a sudden slowdown occurs and the vehicle is using active braking and observe the meter. I think you'll be surprised.

That's a good call it! I know that meter definitely lies, I've noticed as much when regen assist is turned on as well. That's why I wanted to look at how much the brake pedal was physically moving

 

[kewlasu]

I had the same experience about the brake pedal like the first 6 months of ownership in 2022. I've always had my right foot resting on the brake pedal when I used to drive ICE cars as a way of being ready. I broke that habit after driving my T. I, too, was surprised when the brake pedal moved by itself but knowing it does it during auto hold made me feel safe.

 

[Nixapatfan]

Assuming you have regen brake assist off, you could just check the rust layer on the rotors after a rain storm and use cruise/highway assist and see if that rust layer is cleaned up, any meaningful braking should remove that layer quickly.

 

[portdirect]

Great write up as always - though a couple of things worth highlighting:

• This is all based around gen2 I presume?
• This is all based on post-mobileye driver assist:
  • In gen1 for sure the brakes WERE being used with highway assist in 2024 - the discs were hot after slowing down.
• Also in gen1 - there's a noticeable difference between the slowing behaviors from 50-30mph in one pedal driving at 80f when brake assist is turned on/off - implying that its being used for more than just 'low temp' or high SOC conditions.

 

[mpshizzle]

Great write up as always - though a couple of things worth highlighting:

• This is all based around gen2 I presume?
• This is all based on post-mobileye driver assist:
  • In gen1 for sure the brakes WERE being used with highway assist in 2024 - the discs were hot after slowing down.
• Also in gen1 - there's a noticeable difference between the slowing behaviors from 50-30mph in one pedal driving at 80f when brake assist is turned on/off - implying that its being used for more than just 'low temp' or high SOC conditions.

Great call outs!

Yes this testing was done on Gen 2 - and regen assist was turned off, just in case.

While I didn't do testing on the rotor temp I DID watch the service mode display and saw 40-80kw (~150-200A) of energy going into the battery under light to moderate deceleration. So while the pre-charged brakes may have caused a small amount of drag, regen is definitely doing the heavy lifing. (If you're interested in seeing that display in action it is in the video)

 

[mpshizzle]

@godfodder0901 - A clarification. I realized that the written version of my post didn't make it super clear. I wasn't using the meter on the driver's display at all. I was relying on the service mode screen that shows exactly how many amps are coming into or leaving the HV battery

 

[SwampNut]

Full test is on YouTube (Thunder Volt Auto): Click Here

As is usually the case for my posts - it is much easier to understand what I'm talking about with the visuals in the video (pedal cam + service screen), but here’s the written breakdown if you’d rather read.

This all started when I noticed something not long after getting my R1S. When using Highway Assist or even basic cruise control, the brake pedal moves. At first, I worried — does that mean Rivian is skipping or limiting regen and going straight to friction brakes? That would waste a lot of efficiency. So I set up some tests to find out. TL;DR - yes it does use regen (sort of a "well duh, of course it does" moment for me), but the way it works I found to be a bit surprising and very fascinating. So if you are as intrigued as I was, read on!

Before we get to my testing, though, we need a little background: How the R1 brakes are set up. The R1 uses a traditional hydraulic brake system, not a brake-by-wire setup. The pedal is physically connected to the brake lines.

The “magic” comes from the brake booster, which can be actuated two ways:

1. By you pressing the pedal.
2. By a motor inside the booster, which the car uses for things like AEB, Highway Assist, or ACC. Because it’s physically linked, whenever that motor moves the brakes, the pedal itself moves too.

⸻

Setting up the Test:

The parameters of the test were relatively simple. I set up a camera pointed at the pedals so I could see exactly how much the brake pedal would move in each scenario. I also pulled up service mode on the main display and watched the battery current display to see how much current (in amps) was leaving (or entering) the high voltage battery. Negative amps for discharging, positive amps for charging. When at a stop it seemed to hover between -1.5 and -3 amps. Certainly not the most precise, scientific setup - but hopefully enough to get a better understanding of how regen works.

I wanted to observe behavior in 3 scenarios:

1. Manual control with one pedal driving
2. Cruise Control
3. Highwaty Assist

⸻

Baseline Test: One-Pedal Driving

Before getting into highway assist, I wanted to get a baseline for how the system acts uder normal one pedal driving. For the testing I just used my normal driving configuration - high regen with regen assist turned off to prevent friction brake blending.

The results: The brake pedal didn't move at ALL while the vehicle was in motion under normal conditions. Acceleration, deceleration, no matter what it just stayed completely still. The only exception to this was when coming to a stop. At about 1MPH, just before rolling to a stop, the pedal would move down a fair bit to help stop the vehicle and lock into position for auto hold.

⸻

Cruise Control Test

This is where things got interesting. As I had noticed before, the brake pedal did move every time the vehicle decelerated. Each time the vehicle slowed, the pedal would move down just a TINY bit. What was surprising to me, though is that it was the same every time - no matter how much deceleration. Whether it was a heavy deceleration or just a light touch, it moved just a tiny bit to exactly the same spot - no more, no less. I also observed big spikes of energy going into the battery when the pedal would move to this position - indicating that there IS regen occuring. The only exception to this was again, coming to a complete stop. Just like with one pedal drive at about 1MPH the brake pedal would move down just a bit farther and lock into position for auto hold.

⸻

Highway Assist Test

Highway assist (as predicted) behaved identically to cruise control when it came to regen. I figured that would be the result but I wanted to test both, just in case. As with cruise control, the pedal would consistently and repeatedly move into the same position no matter how much deceleration was happening. And while it did this, the battery current meter showed energy going into the battery. And also just like cruise control, the only time the brake pedal moved any farther down was when coming to a stop and locking into auto hold.

⸻

So back to my root question: How can the brake pedal move if regen is still happening?

The big hint here was the fact that the brake pedal seemed to have a "favorite" position where it would just hang out, no matter how much deceleration was happening. This indicates that whatever the brake pedal is doing, it's not tied to slowing the vehicle. If it was, it would change and move throughout the deceleration. So what exactly is it doing, then? My best guess - it's just removing slack from the braking system so it can be at the ready at a moment's notice. A bit of slack in the first bit of the brake pedal's travel is pretty normal for hydraulic brakes. Getting rid of this slack means that if the driver or the emergency braking system feels they need to make an emergency intervention, the hydraulic brakes are ready to go instantly with some pressure already in the lines.

So what does that mean for regen? With the brakes simply in a state of being "pre charged" the brake pads aren't putting much (if any) pressure on the rotors. So that leaves all the rest up to regen. I could clearly see this by the big spikes of energy going into the battery when slowing down.

⸻

Takeaways

1. Highway Assist and cruise control do, in fact, rely on regen — efficiency is preserved.
2. The brake pedal motion is a side effect of Rivian’s traditional hydraulic setup.
3. It’s essentially the vehicle “taking up slack” so the brakes are ready if needed, without actually wasting potential regen.

⸻

Final thought: For me, this answered the concern that regen might be skipped or diminished under driver assist. It’s not — Rivian is maximizing efficiency while keeping safety in check. As always this was a very fun learning epxerience for me, and I hope it was for you too!

Now I will take this moment for a shameless referral code plug. If you feel like you learned something interesting here, feel free to use my code. You get 500 Rivian points, and it helps me out too! Win-win
Referral code here: https://rivian.com/configurations/list?reprCode=MIKE4580282

What do you all think? Anyone else notice the pedal twitch under cruise/HA?

this is exactly how it was explained to me by the actual Rivian engineers, but I spent some time with a couple months ago. And yet people keep on doing that it works differently in the engineers.

 

[CarGuyCarl]

I think these trucks use friction brakes a lot more than you would imagine. In my own experience, if you full charge your truck, you'll notice it won't let you smoothly roll between drive and reverse. It'll force a stop before letting you change direction. Also, the brake pedal motor is pretty strong, but you can lodge your foot under the brake and you'll hear the truck freak out and attempt to slow down using the ABS pump.

 

[godfodder0901]

@godfodder0901 - A clarification. I realized that the written version of my post didn't make it super clear. I wasn't using the meter on the driver's display at all. I was relying on the service mode screen that shows exactly how many amps are coming into or leaving the HV battery

Yeah, after your last reply, I realized that and was about to post an apology/clarification as well.

 

[HapticWagon]

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

 

[mkhuffman]

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?

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