Using EPB for emergency braking

[Leor604]

At a recent track day (where there were a couple unfortunate incidents, one a possible brake failure) a few of us started wondering exactly what happens if you have to use the EPB in an emergency. Being a mechanical AWD system, does engaging the EPB result in 4 wheel braking?

If we were to engage the EPB while cornering at high speed, will it result in instant snap oversteer (like a traditional 2WD parking brake turn) or does the quattro system transfer some of the braking to the front wheels, resulting in somewhat more balanced braking. I would think it would still be highly rear-biased.

Also, by attempting to lock the rear wheels only, isn't this putting a LOT of stress on the drivetrain, as the front wheels are still trying to turn? Where do the sport diff, torsen diff, stability control and traction control come into the equation. With so many systems in play, it's hard to figure out exactly what might happen (and I don't feel like being the test subject, lol.)

[JimmyBones]

I had a customer engage the EPB while on a test drive and I almost ate the dash with my seat belt on. It stops the car fast!

I can only assume that the EPB and ABS control modules takes priority over the other systems to stop the car. The Germans are all about safety.

[moose_head]

Don't have the manual in front of me, but I think it said it's the same thing as a full "emergency stop" by pushing the brake pedal to the floor.

From Audi's website: http://www.audi.com/com/brand/en/too...ilter_i_e.html


The electromechanical parking brake has little in common with a conventional handbrake. It is resistant to wear, its braking effect is permanently constant and it cannot lock up.

The electromechanical parking brake comprises the parking brake, starting-off aid and emergency braking functions. (...) In the event of an emergency braking manoeuvre with the parking brake, the vehicle can be braked considerably more effectively than with a conventional handbrake because the system communicates with the anti-lock braking system (ABS) and electronic brake force distribution (EBD) via the electronic stabilisation programme (ESP).

[ahcow]

Hmm interesting... so I guess if I ever need to, I just pull the EPB like I would to a traditional handbrake to get the same result?

[Stubek]

I've tried it on my A4 at about 60mph and the brake engages for about 1/10 of a second then disengages.

[Ggreek]

On another note, I had a 2013 S4 loaner while my 2010 was in for service a couple of weeks ago. I was at the end of my driveway waiting to pull out, opened my door to grab my newspaper while still in drive and the EPB engaged automatically! I thought that was pretty cool. Lol. My 2010 doesn't do that.

[PeerS4]

I've tried it on my A4 at about 60mph and the brake engages for about 1/10 of a second then disengages.

Did you hold it up, or just pull once? I heard that only if you hold it up it does it do a emergency brake for the period you hold it up for.....

I engaged mine a few times on my previous B8 A4 (with just one pull) at highway speeds by accident, and like you said it only engaged the brakes for a 1/10 of a second. I've been to afraid to try hold it up in the S4, as my previous B8 eventually started to get a E-brake malfunction, And I thought this was due to me accidentally engaging it at high speeds.

[JimmyBones]

On another note, I had a 2013 S4 loaner while my 2010 was in for service a couple of weeks ago. I was at the end of my driveway waiting to pull out, opened my door to grab my newspaper while still in drive and the EPB engaged automatically! I thought that was pretty cool. Lol. My 2010 doesn't do that.

A lot of the newer cars do this. It is a nice safety feature for most people but it is really annoying when techs are trying to move cars around the shop and get lined up on a lift.

[MrFunk]

I'd rather mash a brake pedal with my foot which I've "practiced" a million times in reactive situations rather than have to decide then try and pull up a tiny lever with my finger reactively.

Edit: I'm guessing we're talking about a brake failure... does the electronic hand brake engage mechanically or is it just linked through the brake system?

[AznSensation87]

On another note, I had a 2013 S4 loaner while my 2010 was in for service a couple of weeks ago. I was at the end of my driveway waiting to pull out, opened my door to grab my newspaper while still in drive and the EPB engaged automatically! I thought that was pretty cool. Lol. My 2010 doesn't do that.

Did you have an automatic or manual 13 S4 loaner? I have a 13 6-MT S4 and mine doesnt seem to do that...maybe it depends on transmission?

[Bomiz]

Did you have an automatic or manual 13 S4 loaner? I have a 13 6-MT S4 and mine doesnt seem to do that...maybe it depends on transmission?

I have the dsg and it will engaged of you open the door while being in drive.

[AznSensation87]

I have the dsg and it will engaged of you open the door while being in drive.

Hmm maybe it is based on transmission then...I've opened the door too while in first gear so I can arrange myself perfectly with the lines while parking lol and no e-brake comes on =/

[skiracerblah]

The real question is, can you "flip a bitch" with the EPM?

[Toecutter]

I engaged mine a few times on my previous B8 A4 (with just one pull) at highway speeds by accident...

LOL, how do you do that "by accident?"

[westwest888]

To the OP, the parking brake does not use the regular brakes, the regular calipers, or the quattro system. It uses a drum brake on the rear wheels, which is out of plain sight. Essentially the electronic part pulls a cable which causes two pads to move outward and press against the inside of a drum. Apparently, it can also release the cable in conjunction with the wheel speed sensors in the hub to ensure the rear brakes aren't locking up.

So it is rear biased, which is safer (straightens the car out even if you are turning). I don't know that it would help you if had a brake failure on the track, which usually happens at a castrostrophic point like the start of a turn. Essentially your best bet at that point is just to focus on steering safely off track (you're going off anyway), and avoiding immovable objects once you're 4 wheels on the dirt.

[Toecutter]

If we were to engage the EPB while cornering at high speed, will it result in instant snap oversteer (like a traditional 2WD parking brake turn) or does the quattro system transfer some of the braking to the front wheels, resulting in somewhat more balanced braking. I would think it would still be highly rear-biased.

Also, by attempting to lock the rear wheels only, isn't this putting a LOT of stress on the drivetrain, as the front wheels are still trying to turn? Where do the sport diff, torsen diff, stability control and traction control come into the equation. With so many systems in play, it's hard to figure out exactly what might happen (and I don't feel like being the test subject, lol.)

It would be fun to play around with this on a snowy, slippery, empty parking lot this winter in order to get an idea of how the car would behave.

[MrFunk]

To the OP, the parking brake does not use the regular brakes, the regular calipers, or the quattro system. It uses a drum brake on the rear wheels, which is out of plain sight. Essentially the electronic part pulls a cable which causes two pads to move outward and press against the inside of a drum. Apparently, it can also release the cable in conjunction with the wheel speed sensors in the hub to ensure the rear brakes aren't locking up.

So it is rear biased, which is safer (straightens the car out even if you are turning). I don't know that it would help you if had a brake failure on the track, which usually happens at a castrostrophic point like the start of a turn. Essentially your best bet at that point is just to focus on steering safely off track (you're going off anyway), and avoiding immovable objects once you're 4 wheels on the dirt.

Very interesting! A drumb brake for the parking brake. I'm assuming it's tiny and if used in an emergency situation it would probably need servicing or replacing - it can't be made to withstand that...

On another note - I have a Q5 loaner. Was pulling out of the parking garage and had to stop on a steep inclide. The parking brake automatically engaged when I took my foot off the brake to keep it from rolling backwards... kinda cool...

[vtheman]

I know BMW uses a drum brake but I was under impression that Audi uses rear brakes, I need to look in to that. As far as EPB, per manual if you pull and hold it it'll work just like a regular hand brake.

[ilspazzaneve]

There is no parking drum rear brake on the B8 (or B5, B6, B7). There is one like that on my e39 M5, it's a real pain, extra unsprung weight, and a general bother to deal with when trying to do a rear brake job. Cheers to Audi for not going this unnecessary route. Instead, there is a small servo/motor on the rear calipers that clamps the brakes when pulled at a stop (you can also hear it from the outside when you lock your car).

As far as when moving, if the EPB is pulled, it seems to use heavy (likely hydraulic) braking force on all 4 discs - I'd imagine this is generated from the ABS module. I don't think it uses the rear servo, based only on my observation that it reacts and releases a lot quicker than the servo seems to (listen to it when you lock your doors).

[Leor604]

To the OP, the parking brake does not use the regular brakes, the regular calipers, or the quattro system. It uses a drum brake on the rear wheels, which is out of plain sight. Essentially the electronic part pulls a cable which causes two pads to move outward and press against the inside of a drum. Apparently, it can also release the cable in conjunction with the wheel speed sensors in the hub to ensure the rear brakes aren't locking up.

So it is rear biased, which is safer (straightens the car out even if you are turning). I don't know that it would help you if had a brake failure on the track, which usually happens at a castrostrophic point like the start of a turn. Essentially your best bet at that point is just to focus on steering safely off track (you're going off anyway), and avoiding immovable objects once you're 4 wheels on the dirt.

West,

I think you have Audi confused with BMW. Our Audis definitely use the rear disc caliper for the emergency/parking brake, hence why it has to be opened with VCDS to do any work. One of the incidents we had at the track was an off at the end of the straight. Lots of runoff room, but not much use with no brakes. I think cranking on the EPB *may* have helped in that particular situation or at least minimized the impact at the end.

So the question remains: is there any braking action at the front wheels due to the mechanical braking of the rear wheels? I don't think it uses any of the hydraulics which is a good thing. If you've had a hydraulic failure such as a broken line, boiled fluid, etc then any braking system trying to use the hydraulics would also fail.

We are taught on the track that, if we go off, both feet in. Don't try to correct, don' try to power through the slide, just bring the car to a halt as quickly as possible. Hopefully the EPB is a good backup in case of hydraulic failure. The drum parking brake in my BMW was truly useless whereas the EPB seems to have quite a bit of power.

I guess someone is going to have to test this out on a snowy lot. Get up to speed, pull the EPB while going in a straight line and have an outside observer see if only the rear or all four wheels lock. Anybody driving a service loaner right now, lol?

[PeerS4]

LOL, how do you do that "by accident?"

Did a stint of long driving to a site and back every early morning and Late night. A mixture of being bored and tired my fingers were playing around with all the MMI buttons and knobs next thing I knew I had pulled the EPB. LOL. I quess thats where the saying Idle hands comes from. It happened a few times, now I'm always conscious about what I'm doing...

[S4_Phantom]

There is no parking drum rear brake on the B8 (or B5, B6, B7). There is one like that on my e39 M5, it's a real pain, extra unsprung weight, and a general bother to deal with when trying to do a rear brake job. Cheers to Audi for not going this unnecessary route. Instead, there is a small servo/motor on the rear calipers that clamps the brakes when pulled at a stop (you can also hear it from the outside when you lock your car).

As far as when moving, if the EPB is pulled, it seems to use heavy (likely hydraulic) braking force on all 4 discs - I'd imagine this is generated from the ABS module. I don't think it uses the rear servo, based only on my observation that it reacts and releases a lot quicker than the servo seems to (listen to it when you lock your doors).

Interesting... I always wondered what the heck that noise was when I locked my doors. I always thought it sounded like a servo (back from my RC car days as a kid) but could never figure out what it was. So does this engage whenever the car is stopped or only when placed in park and you lock the doors?

[A4 Centaur]

The old 2.5 Ton trucks in the Military had a drum brake on the drive shaft. Ancient technology.

[MrFunk]

Very interesting! A drumb brake for the parking brake. I'm assuming it's tiny and if used in an emergency situation it would probably need servicing or replacing - it can't be made to withstand that...

On another note - I have a Q5 loaner. Was pulling out of the parking garage and had to stop on a steep inclide. The parking brake automatically engaged when I took my foot off the brake to keep it from rolling backwards... kinda cool...

I'm pretty sure the electronic brake is NOT a drum brake -

[Stubek]

The old 2.5 Ton trucks in the Military had a drum brake on the drive shaft. Ancient technology.

The parking brake is a miniature caliper/pad style brake. I think on the A4/5/6 it is part of the regular rear brakes. I know the R8 it is a separate little brake like the one on the right in the picture below.
[img]http://lh3.ggpht.com/-ENiB17uY38Q/SSO8IfgOzkI/AAAAAAAAEqo/_OY8WI-4MQM/W8_cutaway_large04.jpg {/img}

[MrFunk]

The parking brake is a miniature caliper/pad style brake. I think on the A4/5/6 it is part of the regular rear brakes. I know the R8 it is a separate little brake like the one on the right in the picture below.
[img]http://lh3.ggpht.com/-ENiB17uY38Q/SSO8IfgOzkI/AAAAAAAAEqo/_OY8WI-4MQM/W8_cutaway_large04.jpg {/img}

I'm quite sure the B8S4 does not have a separate brake for the parking brake - it's integrated with the main rear brakes - which is why you need to VAGCOM to release the rear calipers due to the parking brake system in order to service the rear brakes.