Adaptive cruise control uses brakes to regulate distance to next vehicle

[Birdowin]

Recently on a road trip while using Adaptive Cruise Control I noticed that the truck was using the brake pedal to regulate the distance between vehicles rather than regulating the speed with the accelerator. Seems to me that this could cause a lot of extra brake wear. You can test this yourself by setting adaptive cruise and getting behind another vehicle and then gently place your left foot under the brake pedal. You can feel the brake pedal being depressed as the truck comes up behind the vehicle ahead. One of the great things about a truck with such aggressive regenerative braking is that the brakes should last forever. (my wife's Tesla has 140k on it and we have not had to do brakes). Makes me wonder if Rivian could come up with an OTA fix for this.

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[Prime]

Tesla used to do this as well. They rather recently (w/in the year) started using regen to slow down the car when auto cruise control is enabled. Rivian will likely follow suit in a future software update though seems like with everything else going on that might not be a high priority for them atm.

 

[Birdowin]

Thanks for the reply.

 

[jjswan33]

Didn't realize that. Using friction breaks like this is a waste of energy especially in stop and go traffic, good reason not to use it.

 

[SoCal Rob]

There is a serious advantage to this. My Ford Fusion Energi did NOT move the brake pedal but DID rely on regen to slow and then the brake booster ABS hardware (I think) if more braking was required. If someone stopped suddenly in front of you and you had to take over braking manually there was an unintended consequence to the standard behavior of disengaging cruise control (or ACC) when the driver steps on the brake pedal. As usual stepping on the brake pedal caused the ACC to disengage which meant that whatever braking was being applied by the vehicle was canceled. Between the time that disengaged and the time the friction brakes started to make a noticeable difference, the lack of deceleration made it feel as if you were speeding up.

It was really disconcerting the first time it happened since I was used to a prior vehicle which did move the brake pedal. In that case the ACC disengaged as soon as you used more brake than the vehicle so you were always decelerating.

 

[NachoRivian]

Another issue with using the brakes (as opposed to coasting) is the ACC not taking into consideration the vehicles behind you.. but that's a generalized problem with most ACC's.

Does anyone know if the brake lights come on when letting off the accelerator/ coasting during normal operation?

 

[crashmtb]

Does anyone know if the brake lights come on when letting off the accelerator/ coasting during normal operation?

There is a deceleration threshold at which point brake lights must come on specified in FMVSS. Can’t remember what it is.

 

[DJG]

Didn't realize that. Using friction breaks like this is a waste of energy especially in stop and go traffic, good reason not to use it.

depends on speed of "stop and go", but slow enough and friction is the only option anyway. I don't think regen is/can do anything below 5-10 mph.

 

[Ravenron]

Haven't checked actual brake lights but the tail lights on the animated R1T in the dash display show brake lights during regen coasting. These come on once the regen starts aggressive decel. If it's minor decel, the animated brake lights don't come on right away. If brakes are applied, animated tail lights come on immediately.

Not sure if this helps but short of having someone follow and chat on the phone, this is what I've noticed.

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