superswiss

Perhaps for very minimal speed adjustments, but not in general. The rear brakes are not very effective as the weight of the car shifts to the front. All Audis nowadays have Electronic Brakeforce Distribution, which automatically applies the right amount of brakeforce to each wheel for every situation. It wouldn't make much sense for ACC to control the brakes outside of the EBD parameters.

ACC limits the rate of deceleration to 4m/s or something like that and if more is required it demands the driver to apply additional brake force.

That's for very different reasons. Cruise control in general shouldn't be used in slippery conditions, because when the wheels start losing traction, cruise control gets confused, because the wheel speeds do not reconcile with the rate of acceleration anymore and then most systems start to accelerate and make hydroplanning etc. worse.

Go123

Wouldn't Presense or brake guard take over anyway if you don't apply any additional brake force?

superswiss

Yes, it would, but even brake guard relies on the driver to assist for full braking.

snagitseven

That's incorrect, at least on my 2013 A6. My manual states that the braking guard applies the brakes in stages up to full force if there is no intervention by the driver.

superswiss

That's not what it says. This is what it says. I double checked the user manual for your model year. I know the system has evolved over the years, but the limits are still there.

- If you do not react to the acute warning, braking guard can brake with increasing force within the limits of the system. This reduces the vehicle speed in the event of a collision.
- The system can initiate maximum braking shortly before a collision (with footnote: this is not available in some countries, your guess is as good as mine which countries that is)


The key part here is "within the limits of the system" and full braking only shortly before a collision if available in your country. My point is full braking is not performed by braking guard until shortly before the collision. That's because the ABS pumps are not strong enough on their own at higher speeds. The main goal of braking guard is to reduce the driver's reaction time and start braking with partial force until the driver reacts. If there is enough space then it could bring the car to a full stop by applying full braking force near the end just before impact.

snagitseven

You originally wrote "...brake guard relies on the driver to assist for full braking". The second bullet you quoted supports what I wrote: braking guard will "initiate maximum braking shortly before a collision". Full force would be applied with no assistance by the driver. The pre-warning portion of the system does indicate to the driver that braking intervention should be done but the sinple answer is it does not "require" driver intervention for the brakes to engage with full force.

The "within limits of the system" would also be true if the driver himself applied the brakes with full force. Certainly, it would be preferable for a driver to apply the brakes when warned before braking guard is engaged but then there would be no need for the braking portion of the system if this were always the case. However, the purpose of the system is to first act as a warning and finally, as a braking back up for an inattentive or slow reacting driver in an emergency situation.

Go123

If the car knows it needs to stop by giving the driver a heads up and has more stopping power than the driver (able to react quicker and brake harder), perhaps the programmers should have just let the AI prevent a collision ASAP whether the driver intervenes or not.

the_duke

Of course there is. They have to cover their ***. But then why does my ACC work exceptionally well in the city, stop and go traffic, and on twisty roads? I use it all the time. I havn't had the opportunity to drive in snow with my D4 but I did in my D3 and it was definitely applying the brakes on all 4 wheels. It's possible the system in the A6 is different but I find that highly unlikely.

snagitseven

As a side thought, I would think if braking guard were to activate on slippery roads, i.e. ice or snow, it's going to be way too late to avoid a collision. I seriously doubt the system would detect that there is a much longer stopping distance required in those conditions.