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PostPosted: Fri Oct 12, 2018 8:56 am 
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Light Pressure Turbo

Joined: Fri Jan 09, 2015 7:22 am
Posts: 449
Location: Bristol
Car Model: OG 9-3 SE
I wasn't sure if this was a workshop or coffee bar topic so please move if more appropriate.

I have been wondering if cruise control can operate a vehicle's brakes?
Suppose you have set CC to 60mph, you're in 5th gear and going down a steep hill, the car will stay at the set 50mph, this must mean the brakes are being applied mustn't it?

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PostPosted: Fri Oct 12, 2018 9:07 am 
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UKS Encyclopædia G-M
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Joined: Sun Aug 15, 2004 9:39 am
Posts: 39614
Location: SW Herts
Car Model: 9-5 Aero est. 9-3 Carlsson
No. Just drag.

Our cars will increase speed above the set limit when going downhill.

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PostPosted: Fri Oct 12, 2018 12:10 pm 
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Full Pressure Turbo

Joined: Wed Jul 24, 2013 12:12 am
Posts: 726
Location: Leeds, West Yorkshire
Car Model: 9-3ss 2.8t aero
Some cars can and do apply the brakes if required.


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PostPosted: Fri Oct 12, 2018 12:17 pm 
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Full Pressure Turbo

Joined: Sat Feb 28, 2015 8:03 am
Posts: 955
Location: North Norfolk
Car Model: 9-5 Linear Sport Estate Auto
Certainly I have to be careful, I sometimes put CC on when I am going through a long 30 limit and on my 2.2 auto it will speed up on a hill so I still have to monitor the speedo. A manual car has more engine braking so will not do it so much.

Modern cars with adaptive cruise control and queue assist will use the brakes if the car in front slows but will they also do so if the speed increases due to gravity? Someone on here will know.

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PostPosted: Fri Oct 12, 2018 12:39 pm 
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Active user
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Joined: Wed Sep 26, 2018 7:22 pm
Posts: 43
Car Model: OG9-3 S Auto Convertible
bpetersb wrote:
Modern cars with adaptive cruise control and queue assist will use the brakes if the car in front slows but will they also do so if the speed increases due to gravity? Someone on here will know.


Interesting question. We've got a 2018 S-Max that has those features and I really have no idea whether it applies the brakes going downhill. I'm going to have to try now.


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PostPosted: Fri Oct 12, 2018 1:16 pm 
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Talking Turkey
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Joined: Mon Apr 02, 2007 11:52 am
Posts: 23199
Location: not in a SAAB.
Car Model: One of the first RHD 9-5s
Modern adaptive cruise does use the brakes when needed but standard cruise only has control over the throttle so on a hill and without the ability to change gear there is only so much it can do to control speed. I remember driving a particularly undulating road once on cruise and it was all over the place trying to maintain speed.

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PostPosted: Fri Oct 12, 2018 1:34 pm 
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Saab Nut
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Joined: Thu Dec 24, 2015 8:25 pm
Posts: 1941
Location: Lingfield, Surrey
Car Model: 93 Aero 2.8T cabrio
Gaz38 wrote:
I wasn't sure if this was a workshop or coffee bar topic so please move if more appropriate.

I have been wondering if cruise control can operate a vehicle's brakes?
Suppose you have set CC to 60mph, you're in 5th gear and going down a steep hill, the car will stay at the set 50mph, this must mean the brakes are being applied mustn't it?


Our Mk7 Golf had active cruise control. It used radar to detect vehicles ahead and would apply the brakes if required to adjust the vehicle's speed and maintain a safe braking distance.
Any vehicle equipped with automatic braking and cruise control should, in theory, have it.

It works very well, though not as fuel efficient as the driver looking ahead and driving appropriately.

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PostPosted: Sat Oct 13, 2018 12:47 pm 
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Saab Nut

Joined: Tue Mar 25, 2014 5:41 pm
Posts: 1114
Location: Kippen STIRLING
Car Model: 9-5 Aero Sportwagen and more
Guys,

If you haven't got adaptive CC then change down until you have sufficient engine braking. In an auto you need to swap to 'manual' mode and use the wheel mounted switches. It's what you should do even without CC on a long descent so that you don't heat your brakes up unnecessarily.

Paul @ Kippen.


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PostPosted: Sat Oct 13, 2018 5:53 pm 
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Shotgun
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Joined: Tue Aug 08, 2006 8:51 pm
Posts: 11291
Location: 'Ull
Car Model: 95 Aero Auto Est Stg 1 Noobed
On a similar vein to the adaptive CC, I found that the collision avoidance system (on a Skoda) is VERY aggressive. It has operated on me twice in the last 18mts.
2nd time, on the motorway, a reasonable distance from the car in front which hit the brakes hard. About 1/2 second second before I got on the brakes to reduce speed in a suitable manner for the situation, the CA hit the brakes hard, to the extent of an emergency stop from 70(ish)mph down to a rolling speed. It was rather disconcerting to have the car decide it knew better than me!! Had I had control, it would've been a rapid deceleration, but with control and without upsetting everything in the car with the continued kinetic energy contained by all and everything!! :argh:

1st time, I was in a queue of traffic moving slowly towards a roundabout, behind an Artic. As the road started to split into 2 lanes, I took the opportunity to leap-frog around 4 lorries by heading up the outside lane. I pushed down on the throttle and started to steer right to pull across, and the bloomin car decided I was going to hit the artic, so hit the brakes hard, halting all progress I had planned. :wall: :wall:

Annoyingly, if you turn the system off, you get a bright orange warning icon right in the middle of the dash. :loco:

I suppose if it were my own car, and not the company pool car, I'd be trying to find out how to remove this "warning" is the system was disabled by choice, and only show if there was a system failure.

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PostPosted: Sat Oct 13, 2018 7:05 pm 
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UKS Addict

Joined: Sun Jan 26, 2014 9:41 am
Posts: 2607
Location: Royal Tunbridge Wells
Car Model: '03 9-5 2.3 Vector Auto Estate
There was a case in the paper last week (in Austria IIRC) where the automatic braking 'failed' at speed on a motorway, stopped the car very hard with no cause and the car behind ran into it, 1 killed and 1in a bad way. I know people go wrong but they tend to rely on these systems when they become the norm and not sure these automatic systems are really up to it yet.

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'03 Saab 9-5 Vector 2.3t Auto Noob Stg 1
'01 Saab 9-3 2.0t SE Convertible
'81 Triumph TR7 DHC


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PostPosted: Sat Oct 13, 2018 7:44 pm 
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Talking Turkey
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Joined: Mon Apr 02, 2007 11:52 am
Posts: 23199
Location: not in a SAAB.
Car Model: One of the first RHD 9-5s
TWSaab wrote:
There was a case in the paper last week (in Austria IIRC) where the automatic braking 'failed' at speed on a motorway, stopped the car very hard with no cause and the car behind ran into it, 1 killed and 1in a bad way. I know people go wrong but they tend to rely on these systems when they become the norm and not sure these automatic systems are really up to it yet.


People are equally prone to doing stupid stuff as well though, this isnt an issue unique to automation. How many times have you seen a car stationary in a live lane of a motorway because they want to join the front of a queue at a junction, not the back of it. Tesla are showing significant safety improvements when their cars are running autonomously than when they are being controlled by a person. Something like a 25% decrease in accidents and injuries. The reality is that driving will never be totally safe, too many variables, too many opportunities for something to go wrong. With autonomy though, everything is playing by the same rules, its a game we really arent very good at.

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05 9-5 Aero with all the options.


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PostPosted: Sat Oct 13, 2018 8:59 pm 
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UKS Addict

Joined: Sun Jan 26, 2014 9:41 am
Posts: 2607
Location: Royal Tunbridge Wells
Car Model: '03 9-5 2.3 Vector Auto Estate
Agreed regarding people, a point I included, but they were not designed to drive, cars were designed to use their attributes (including faults). I have no idea how these car systems are designed but one assumes they are designed to improve on people. For systems like braking that you are expected to rely on (as people will if they are fitted) I would expect them to be safety critical with some form of 2 out of 3 voting sensors to ensure as far as possible that they did brake when required and did not when not required. It would be just as bad if people relied on the on the automatic braking and it did not brake when needed. I am sure they are improving but should they be used on the road before they are perfected?

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'03 Saab 9-5 Vector 2.3t Auto Noob Stg 1
'01 Saab 9-3 2.0t SE Convertible
'81 Triumph TR7 DHC


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PostPosted: Sat Oct 13, 2018 9:32 pm 
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Talking Turkey
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Joined: Mon Apr 02, 2007 11:52 am
Posts: 23199
Location: not in a SAAB.
Car Model: One of the first RHD 9-5s
They have been letting us on the road for over 100 years, we havent pefected the art of driving yet. Nor will we ever achieve that level (despite some people claims otherwise). Once the cars start talking to each other you get a much more comprehensive level of unedrstanding what is going on so they all beave in sync with each other.

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05 9-5 Aero with all the options.


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