Odd brake pedal behavior under heavy braking
02-09-2018, 01:59 PM
#1
AudiWorld Member
Thread Starter
Odd brake pedal behavior under heavy braking
I had occasion to make a max effort stop from 70-0 (well, maybe 70-5, as I didn't come to a complete stop) today, and encountered what I thought was odd brake behavior. The brakes were maybe slightly warm from regular driving, but not to the point that fade would be an issue at all. Circumstances required me to start braking "hard" (maybe 80%), with resulting good pedal feel and effort as normal. Around 40 mph I increased pedal force ("stop NOW"), and with enough force that the ABS was just starting to cycle), the pedal effort suddenly became very light and the pedal traveled more or less to the floor. Braking performance remained good throughout (ABS was cycling, and I stopped safely). Is this normal behavior? It's almost as though I pushed hard enough to go through a detent, and on the other side of the detent there was no pedal force at all. After the event, I drove off and the brakes were back to normal when being used in "normal" traffic conditions.
Later on in a deserted location, I tried stomping the brakes from ~40mph, and didn't encounter the odd pedal feel. I did it again from 60, and the "firm/nothing" detent effect occurred again, again with no reduction in braking performance.
Not what I'm used to in my other vehicles at all. Is something wrong, or am I just not used to brake behavior at the limit in the Q7? Discs and pads were replaced earlier this month, but they're pretty well bedded in and have not behaved oddly until now.
Later on in a deserted location, I tried stomping the brakes from ~40mph, and didn't encounter the odd pedal feel. I did it again from 60, and the "firm/nothing" detent effect occurred again, again with no reduction in braking performance.
Not what I'm used to in my other vehicles at all. Is something wrong, or am I just not used to brake behavior at the limit in the Q7? Discs and pads were replaced earlier this month, but they're pretty well bedded in and have not behaved oddly until now.
02-10-2018, 02:48 AM
#2
AudiWorld Member
Thanks for heads up
Since May when I bought my A8 I haven't had to brake hard like that, I suppose I should go out and do a few max performance stops like that, it is good to know that a car stops in a predictable fashion
02-10-2018, 12:26 PM
#3
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Assuming something was in front of you on the first stop then maybe the pedal feel changed when Audi Grake Guard kicked in and the car helped to slow down automatically by applying full force on its own when you got nearer to a collision? If it was something like this then it would explain why you couldn’t replicate it on subsequent test stops as I’m assuming you did your test stops in an open area with nothing in front of you as you slowed down.
Description - Audi braking guard - Adaptive cruise control (ACC) - Controls - Audi A4 Owner's Manual - Audi A4 - Audihelp.com
Safety based on radar technology ? Audi braking guard
This assumes that you have Adaptive Cruise Control which includes Audi Brake Guard.
If not, then it may have been Brake Assist where the system can sense the difference between the brakes being applied aggressively vs the brakes being applied in a panic. If it senses that it’s an emergency/panic situation then it kicks in to boost the system to full pressure in order to help you reduce the stopping difference. This would explain the difference in pedal feel in the initial scenario. Then in your subsequent test runs Brake Assist never kicked in because the system didn’t detect a panic stop - your tests may have been aggressive but not enough to have been picked up as being a full on panic stop, so it didn’t activate Brake Assist in your tests.
Description - Audi braking guard - Adaptive cruise control (ACC) - Controls - Audi A4 Owner's Manual - Audi A4 - Audihelp.com
Safety based on radar technology ? Audi braking guard
This assumes that you have Adaptive Cruise Control which includes Audi Brake Guard.
If not, then it may have been Brake Assist where the system can sense the difference between the brakes being applied aggressively vs the brakes being applied in a panic. If it senses that it’s an emergency/panic situation then it kicks in to boost the system to full pressure in order to help you reduce the stopping difference. This would explain the difference in pedal feel in the initial scenario. Then in your subsequent test runs Brake Assist never kicked in because the system didn’t detect a panic stop - your tests may have been aggressive but not enough to have been picked up as being a full on panic stop, so it didn’t activate Brake Assist in your tests.
I had occasion to make a max effort stop from 70-0 (well, maybe 70-5, as I didn't come to a complete stop) today, and encountered what I thought was odd brake behavior. The brakes were maybe slightly warm from regular driving, but not to the point that fade would be an issue at all. Circumstances required me to start braking "hard" (maybe 80%), with resulting good pedal feel and effort as normal. Around 40 mph I increased pedal force ("stop NOW"), and with enough force that the ABS was just starting to cycle), the pedal effort suddenly became very light and the pedal traveled more or less to the floor. Braking performance remained good throughout (ABS was cycling, and I stopped safely). Is this normal behavior? It's almost as though I pushed hard enough to go through a detent, and on the other side of the detent there was no pedal force at all. After the event, I drove off and the brakes were back to normal when being used in "normal" traffic conditions.
Later on in a deserted location, I tried stomping the brakes from ~40mph, and didn't encounter the odd pedal feel. I did it again from 60, and the "firm/nothing" detent effect occurred again, again with no reduction in braking performance.
Not what I'm used to in my other vehicles at all. Is something wrong, or am I just not used to brake behavior at the limit in the Q7? Discs and pads were replaced earlier this month, but they're pretty well bedded in and have not behaved oddly until now.
Later on in a deserted location, I tried stomping the brakes from ~40mph, and didn't encounter the odd pedal feel. I did it again from 60, and the "firm/nothing" detent effect occurred again, again with no reduction in braking performance.
Not what I'm used to in my other vehicles at all. Is something wrong, or am I just not used to brake behavior at the limit in the Q7? Discs and pads were replaced earlier this month, but they're pretty well bedded in and have not behaved oddly until now.
02-10-2018, 02:47 PM
#4
I can’t find documentation of it but I have experienced that once too. ONCE in over 100,000 miles. It was a long time ago and details are vague in my memory. I do remember that I wondered if the vehicle helped with a panic stop. I wondered if it was due to suddenness of brake application and or pedal pressure applied. Perhaps it is related to this.
https://www.euroncap.com/en/ratings-...-brake-assist/
https://www.euroncap.com/en/ratings-...-brake-assist/
Last edited by J. Patterson; 02-10-2018 at 02:52 PM.
02-10-2018, 02:51 PM
#5
If not, then it may have been Brake Assist where the system can sense the difference between the brakes being applied aggressively vs the brakes being applied in a panic. If it senses that it’s an emergency/panic situation then it kicks in to boost the system to full pressure in order to help you reduce the stopping difference. This would explain the difference in pedal feel in the initial scenario. Then in your subsequent test runs Brake Assist never kicked in because the system didn’t detect a panic stop - your tests may have been aggressive but not enough to have been picked up as being a full on panic stop, so it didn’t activate Brake Assist in your tests.
02-10-2018, 06:06 PM
#6
AudiWorld Member
Thread Starter
Since mine is a 2015, I don't have ACC (not available on any trim level that year, even my Prestige). The incident didn't involve another vehicle, so I doubt ACC would have done anything. Could have been brake assist, but the application was in 2 clear stages. A second or so of heavy, but not max, and then full push. Interesting if it kicked in with that situation. If it were all in one big push, I'd easily agree it could be Assist. Still not sure why the pedal went so soft, though. Does Assist make it easier to get full travel? If so, that was probably it.
02-10-2018, 07:04 PM
#7
AudiWorld Super User
I hadn't noticed that, weird I have ACC on my 2014 but it was not offered in 2015.. I love it and dont know if I can now live without it.
If the brake assist is taking over hydraulics in a panic stop I could totally see the pedal going limp, since it clearly stopped fine when this happened I would just wrap this incident up to the car outperforming the driver.. when safety systems take over things get wonkey, ABS, ASR, and ESP all have their own unique reactions when engaged and you can feel em all through the break pedal too.
I enabled European panic brake lights on my Golf, basically it turned flashers on if you panic brake.. I thought I didnt have it enabled because I tried several times to mimmic a panic brake and trigger it and never saw it engage.. but then I did have to just mash on em once for an emergency stop on the interstate and I could see the flashers activate.. guess I never got it fast enough in my testing.
If the brake assist is taking over hydraulics in a panic stop I could totally see the pedal going limp, since it clearly stopped fine when this happened I would just wrap this incident up to the car outperforming the driver.. when safety systems take over things get wonkey, ABS, ASR, and ESP all have their own unique reactions when engaged and you can feel em all through the break pedal too.
I enabled European panic brake lights on my Golf, basically it turned flashers on if you panic brake.. I thought I didnt have it enabled because I tried several times to mimmic a panic brake and trigger it and never saw it engage.. but then I did have to just mash on em once for an emergency stop on the interstate and I could see the flashers activate.. guess I never got it fast enough in my testing.
Last edited by dreadlocks; 02-10-2018 at 09:40 PM.
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02-12-2018, 07:59 AM
#10
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Since mine is a 2015, I don't have ACC (not available on any trim level that year, even my Prestige). The incident didn't involve another vehicle, so I doubt ACC would have done anything. Could have been brake assist, but the application was in 2 clear stages. A second or so of heavy, but not max, and then full push. Interesting if it kicked in with that situation. If it were all in one big push, I'd easily agree it could be Assist. Still not sure why the pedal went so soft, though. Does Assist make it easier to get full travel? If so, that was probably it.
After your initial application of the brakes the system (a) primes to be ready to apply all available brake pressure and (b) monitors to confirm that it’s a panic stop situation. After a predetermined time, if you’re still braking hard, then the Brake Assist system will take over - hence your experience with the pedal pressure changing. Had you eased up before it kicked in then it would have remained primed but stayed on standby.
It senses some some combination of how quickly you hit the brakes and how hard you’re braking to know when to kick in. This is why you didn’t replicate the feeling in your test runs.