dicktahoe

"I would also like to add that my car has the collision avoidance system. Yet despite this and the S6's massive brakes, my wife still ended up rear ending someone. "

I don't have my manual handy, but I'm pretty sure it says that the Brake Guard does not work if the object is staionary. So if the vehicle ahead had been stopped for a while, outside the detection range of the brake guard to detect while that vehicle was still moving, then the brake guard would not have intervened.

snagitseven

That is correct, with or without ACC engaged.

DeerHunter

I should also point out that massive brakes are rarely an advantage (or disadvantage) for the first panic stop. They're big to allow continuous (within reason) spirited street or track driving without fade. Even your average econo-car can lock up the wheels (or invoke ABS) at city speeds and, once you're at that point, more braking power doesn't do any good. Stopping distances are primarily determined by your tires followed by static weight distribution, with minor factors being (in no particular order) unsprung weight, suspension design, rolling moment of inertia (larger/heavier wheels being a detriment), brake hardware (caliper type, pad choice) and software (ABS and brake force distribution algorithms).

A collision avoidance system and/or brake assist can maximize the effectiveness of your setup and, as you found out, can minimize the seriousness of incidents.

jdong

The radar portion of Brake Guard, which is the portion that you rely on for long range stopping action and preventing collisions altogether, indeed cannot identify a vehicle that was stopped when it entered the sensor's field of vision and never moved. (e.g. rounding a curve to find stopped traffic)

But the vision portion of the sensor would still be able to detect a stopped car. However, by the time it initiates braking action, it's too late to prevent a collision. It will, however, reduce its severity.

DeerHunter

I think it's a choice made by Audi engineers to ignore static objects. Several other cars (e.g. current Volvos), with similar equipment, will brake for stopped objects in the road.

snagitseven

Your first paragraph is correct. Braking Guard (and ACC) do use radar and, as you stated, will not react to a stopped object unless it was already detected as a moving, followed vehicle. However, re the second paragraph, there is no camera/vision interaction with Braking Guard/ACC. Lane Assist on the other hand, does rely on a camera which attempts to monitor lane markings.

Here's a detailed explanation from Audi:
http://www.audiusanews.com/newsrelea...id=1113&mid=44

gatorguy7

I think thickness of the rotors and whether it is ventilated or not has more to do with heat displacement and thus brake fade more than rotor diameter. The larger diameter definitely allows for greater braking force. That increased force is present all along the braking spectrum so greater force earlier will lead to a lower stopping distance. Most people do not instantly go to 100% braking power. The S6 is not a track car and thus wouldn't have such large diameter rotors if it was one about reducing fade. Definitely agree about the other factors involved though . If you have crappy tires, the best brakes in the world won't help.

DeerHunter

Mass and cooling are pretty much the major factors in preventing brake fade. Cooling comes from air flow and proper ducting, coupled with rotor ventilation design. Mass is purely a function of rotor diameter and thickness. Regardless, I think we're on the same page on this front.

Larger-diameter rotors can provide more braking torque, that is true, but that's somewhat offset by having to overcome greater rotational inertia. Regardless, even a puny 10.5" rotor with a single-piston sliding caliper can instantaneously lock up a wheel at city speeds. Once you have enough torque to achieve lock up, more torque is completely useless.

The key component, as it so often is, is the nut behind the wheel. Most people are hesitant to brake hard in the early stages of an emergency. Braking distances are significantly increased due to half-hearted braking in the early stages of the stop. Brake Assist is a godsend in these situations, as it takes the decision away from the driver and applies maximum braking as soon as the brakes are stabbed (even if foot pressure isn't maximized). I would place this feature above even ABS in terms of braking distance reduction, in the real world.

What were we talking about again?

jdong

Well, the Audi/Hella radar sensor is LM-FSK based, which is primarily a doppler shift principle radar. A lot of the other cars on the market with more sophisticated forward collision systems either use stereoscopic front cameras or a hybrid of K-band radar and a sweeping LIDAR sensor. Both of those systems would give you more info about whether a stopped object is likely to be a car or not.

It seems to me more that Audi didn't set out to make a car that stops for obstacles, the way that Mercedes or Volvo did. They primarily set out to make adaptive cruise control, and they rigged the ACC sensors to also provide limited forward collision mitigation to the best of their abilities.

DeerHunter

You learn something new every day. Thanks for the tech update - very interesting.