Why allow roll back on a hill ?

I use 2 pedal operation during slow speed negotiations and find having 'gravity pull' available at all times is a needed feature. Hold the accel while you are waiting with the brake ON until the coast is clear. Then let off the brake and LAUNCH. Do not mind the nanny chimes!!
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I like the 1+ second assist - helps in that one instance of go from hill'd stop. In many other cases (slight inclines into tricky parking etc), I find it useful to let gravity do the work and only use the fun pedal to fine-tune. So I like that the car will drift both ways. I guess for me the "good reason" is, there's no real reason to not allow rolling (except hill assist of course).

I use the gravity pull backwards when driving out of tight windy garage in NYC every day. its very very very useful. it would be a pain in the butt if i had to keep switching back and forth to/from reverse.

I sense a need for a software option here, may be "Incline Force Adjustment". It could be enabled by default.

instigator said:

I often use 2 pedal operation as well to hold it on a hill as well. The question is why allow the car to roll back so we don't need to hold the brake ? What is the benefit of rolling backwards ?
Why not just electronically prevent the car from rolling backwards while in forward gear ?

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instigator said:

The recent SW update to brake for a moment to while switching to the "gas" pedal is a slight improvement but is brings up the question.

Why not prevent rolling back always while in forward gear ?

My driveway is uphill to the road and I am always inching up and braking to prevent rolling back while waiting for an opening in traffic. I can't think of any good reason to allow the car to roll back while in forward. If I want to back up I put it in reverse. Now that the brakes and engine/transmission control are completely programable, I suggest eliminating rollback completely, at least as a configuration option.

Other thoughts ?

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If you have creep ON, it will perform a 1 second hill assist. If you don't, it'll just roll backwards -- I have no idea why that's the case, but I agree with you.

It's a waste of energy compared to just applying the brakes. You might exit the car by accident while in hill hold, thinking you're in park (well the seat sensor should prevent this but you never know)

tiblot said:

If you have creep ON, it will perform a 1 second hill assist. If you don't, it'll just roll backwards -- I have no idea why that's the case, but I agree with you.

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Have you ever driven stick?
Manual transmission cars roll back on hills and the driver expects to do a hill start with gas pedal and hand-brake.
Automatic transmission cars have creep as an artifact of the high idle of the engine.
It's just a matter of driver expectation. Essentially, Creep = AT mode.

Having learned to drive stick (I'm from the UK) it's natural to me to set the parking brake to hold my Prius and I like it because I don't have to sit with my foot on the brake at stops and there's no jumping from brake to gas pedal to get going.

Johan said:

It's a waste of energy compared to just applying the brakes. You might exit the car by accident while in hill hold, thinking you're in park (well the seat sensor should prevent this but you never know)

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That would be difficult to do in one second.

dsm363 said:

That would be difficult to do in one second.

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Exactly, and that's why sustained hill hold could create dangerous situations.

ItsNotAboutTheMoney said:

Have you ever driven stick?
Manual transmission cars roll back on hills and the driver expects to do a hill start with gas pedal and hand-brake.
Automatic transmission cars have creep as an artifact of the high idle of the engine.
It's just a matter of driver expectation. Essentially, Creep = AT mode.

Having learned to drive stick (I'm from the UK) it's natural to me to set the parking brake to hold my Prius and I like it because I don't have to sit with my foot on the brake at stops and there's no jumping from brake to gas pedal to get going.

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Yes, I drive stick and ride a motorcycle. I understand its the same behavior. Creep has NOTHING to do with AT. Creep is simply behavior they added via Software to apply a minute amount of forward power when you are at a stop.

Applying the BRAKES to provide hill assist is a completely different functionality, regardless if its an EV, AT, MT, car, motorcycle, etc...

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Johan said:

Exactly, and that's why sustained hill hold could create dangerous situations.

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Incorrect. The moment you exit your car the car goes into Park. Try it - release your seat belt, lift yourself off the seat, open the door.

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ItsNotAboutTheMoney said:

Have you ever driven stick?

Automatic transmission cars have creep as an artifact of the high idle of the engine.

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Also incorrect. Idle has nothing to do with AT/MT or EV.
AT cars roll forward simply because they are ALWAYS in gear, trying to move forward. The engine is applying tq through the transmission/drivetrain no matter what.
You don't roll in MT if , and only if you are in Neutral, out of gear.

Have you ever left it in first and found the friction zone on your clutch? You roll forward with zero throttle.

tiblot said:

Incorrect. The moment you exit your car the car goes into Park. Try it - release your seat belt, lift yourself off the seat...

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I know. I wrote it in my first reply. No feature especially software driven is 100% fail proof. My simple point is that mechanical brakes are 1) safer 2) do not use energy as opposed to using the motor to hill hold.

tiblot said:

Yes, I drive stick and ride a motorcycle. I understand its the same behavior. Creep has NOTHING to do with AT. Creep is simply behavior they added via Software to apply a minute amount of forward power when you are at a stop.

Applying the BRAKES to provide hill assist is a completely different functionality, regardless if its an EV, AT, MT, car, motorcycle, etc...

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You asked why it works as it does. Creep is paired with hill start because it's most helpful for people accustomed to AT who think their car has an emergency brake. If the Model S' creep is like my Prius it's weak, much weaker than in a conventional AT, and any reasonable incline will cause you to roll backwards.

MT drivers don't expect creep, expect rollback and use their hand brakes and biting points.

tiblot said:

If you have creep ON, it will perform a 1 second hill assist. If you don't, it'll just roll backwards -- I have no idea why that's the case, but I agree with you.

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Incorrect; I've never had Creep on in my car, but I still definitely get the hill assist. They are two separate functions.

tiblot said:

Also incorrect. Idle has nothing to do with AT/MT or EV.
AT cars roll forward simply because they are ALWAYS in gear, trying to move forward. The engine is applying tq through the transmission/drivetrain no matter what.
You don't roll in MT if , and only if you are in Neutral, out of gear.

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MT: Car in gear, clutch out, no foot on gas = stall.

You're right, the actual key is that in an MT you'll use the clutch. The clutch means that if you lift your foot off the brake you'll roll backwards on an incline. So, on an incline any MT will use the handbrake and the biting point.

An AT driver, as I found when I moved here, doesn't think about hill starts unless it's very steep because as soon as you lift your foot off the brake the car moves forwards. It does so pretty forcefully because the engine's already at a paranoid idle rpm. When we first got the Prius my wife was surprised by the weak fake creep and would roll backwards a bit on an incline.

The Model S Creep mode works as it does because it's there to meet expectations of AT drivers. Creep and hill start assist have EVERYTHING to do with each other.

My Mercedes SL63 AMG has an option that holds the brake on if the brake peddle is pushed a little harder than normal... then when it's time to drive you just hit the gas. I really like this option. I would love an option to allow something like this.

Now that i have trained myself to not use the brakes when parking on a hill, the Assist function isn't bad. It still doesn't always do the hold, so it's a kind of guess. As to why it doesn't just hold until you press the accelerator pedal--I have no idea. It shouldn't use any more energy than holding the brakes with your foot. Maybe this is just a step to hill hold. (e.g. We'll try it this way and if it works without problems we'll make it hold longer.)

I hear what Johan says about safety, but I'm not convinced as there is a warning beep when you open the door and the gear selector is not in park. It would be very rare to have both the hill assist and the door alarm fail at the same time.