Brakes fail - how do you stop?

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ahagge

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#1
Hypothetical scenario: You're coming down a mountain road in your Model 3 and when you press the brakes, they go to the floor (hole in the brake line or something similar). How do you stop?

As most folks know, ICE cars have at least 3 options at their disposal:
  • Use the emergency brake
  • Shift into a lower gear
  • Turn off the ignition and use compression to slow to a stop
But the Model 3 has none of those options available. From what I can see, the only way to engage the parking brake is to shift into Park, and you can't do that if the vehicle is moving (I think).

I realize it's an unlikely scenario these days, but I'd really like to know how to react BEFORE it happens.
 

DR61

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#3
All modern vehicles have dual circuit brake systems. If there is a leak in one, the other still operates. So a major brake fluid leak will leave either full front or full rear brakes functioning.

Also, I think I read that pressing and holding in the parking brake button will activate the emergency (parking) brake, regardless of speed. Can anyone confirm for Model 3?
 

Bokonon

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#4
Does this answer it?
I'd assume the behavior in the Model 3 would be similar. Idk for sure tho. And idk if it's actually called the "parking brake button". :confused:
Makes sense.... Come to think of it, the first-generation Volt did something similar if you shifted it into reverse at speed.

 
4

4701

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#6
It applies to all modern vehicles, including Model 3:
Use parking brake. In case of manual lever (operated with hand or foot), operate slowly.
In case of electronic button, force is adjusted automatically.

Of course, in case of EV, maximizing regen is step 1.
In case of extreme electronic brake failure (12V system failure) it is likely that electronic parking
brake will not work. In this case, push brake pedal hard to the floor and do not let go for few seconds.
If vehicle is slowing down, keep holding, if not, let go and push again. If brake booster dies (no power)
brakes will operate with foot force. Which requires extreme pressure to the pedal. No less than "standing on one foot" pressure.
There are exceptions (Nissan Leaf).
Hydraulic lines are redundant in criss-cross logic.
 

garsh

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#7
Hypothetical scenario: You're coming down a mountain road in your Model 3 and when you press the brakes, they go to the floor (hole in the brake line or something similar). How do you stop?
  1. All brake fluid master cylinders have been required to be dual-reservoir since 1976 in the US (I'm sure other countries required this around the same time). If you break a brake line, you'll notice a problem because you'll have to mash the brake pedal to the floor. It won't be great, but you'll still have two of your wheels providing some braking.
  2. Regen. You're in an electric car. Turn on heavy regen.
  3. Yeah, there's probably a way to activate the parking brake in an emergency. But I couldn't find anything in the manual. If someone doesn't mind being a guinea pig, try holding down the "park" button while driving at speed to see what happens. That would seem to be the obvious way to activate it to me.
If things are truly desperate, go off the road into a field to add a lot of "friction" and help prevent your car from running into another.
 

ahagge

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#8
Does this answer it?


I'd assume the behavior in the Model 3 would be similar. Idk for sure tho. And idk if it's actually called the "parking brake button". :confused:
Perfect! As long as it works the same way in the Model 3 (AND someone can confirm that the parking brakes are activated with a mechanical linkage instead of the same hydraulic system used for the main brakes), we're golden! :)
 

MelindaV

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#9
  1. Yeah, there's probably a way to activate the parking brake in an emergency. But I couldn't find anything in the manual. If someone doesn't mind being a guinea pig, try holding down the "park" button while driving at speed to see what happens. That would seem to be the obvious way to activate it to me.
If things are truly desperate, go off the road into a field to add a lot of "friction" and help prevent your car from running into another.
There is a “now you know” video (I think in their 12 days of Model 3 series) where they did just this. It brakes until it hits a low speed, then puts it in Park.
 

teslarob

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#10
Perfect! As long as it works the same way in the Model 3 (AND someone can confirm that the parking brakes are activated with a mechanical linkage instead of the same hydraulic system used for the main brakes), we're golden! :)
Yes, the parking brake is an electric motor on the back of the rear caliper.
 
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#11
  1. All brake fluid master cylinders have been required to be dual-reservoir since 1976 in the US (I'm sure other countries required this around the same time). If you break a brake line, you'll notice a problem because you'll have to mash the brake pedal to the floor. It won't be great, but you'll still have two of your wheels providing some braking.
Just a note here. I had the brakes go out on my 1996 Toyota Corolla Wagon a couple of months ago. The brake lines under the car feeding the rear brakes (not the front) sprung a leak (car used to be on the east coast). It drained the reservoir. When the fluid had emptied the brakes failed... on a hill... pedal went right to the floor.

I did not notice that the car has dual reservoirs. There is certainly only one fill hole. And I have seen dual reservoirs. If this car has one it is not apparent.

None the less my note here is that anything is possible. Know how to stop you car if the brakes fail. Test it.

I was lucky in my case, as the car has a readily accessible hand brake on the console. Pulling the handle stopped the car, even on a hill (that's why with conventional brakes (not Model 3) it is wise to check the adjustment in the emergency brake periodically). But I had just bought the car from my neighbor (literally days before). She has a baby, and was clueless about the hand brake when I told her the story. We both felt our guardian angels must have been on the job. The hill only stops when you reach the Pacific Ocean.
 

garsh

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#12
Just a note here. I had the brakes go out on my 1996 Toyota Corolla Wagon a couple of months ago. The brake lines under the car feeding the rear brakes (not the front) sprung a leak (car used to be on the east coast). It drained the reservoir. When the fluid had emptied the brakes failed... on a hill... pedal went right to the floor.
When that happens, pump the brakes. There should still be pressure to two of the wheels, but it may require a couple of full-force, to-the-floor pumps of the pedal to get them to work.
I did not notice that the car has dual reservoirs. There is certainly only one fill hole.
Like I said, all cars since 1976 have them. Some have two completely separate reservoirs, but most OEM ones have a single tank with a partial internal divider, and only a single fill hole.
 

PNWmisty

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#14
I guess it's good to know how to bring the car to a stop if both regular brake circuits failed simultaneously but that would be extremely rare on a car that was maintained in any reasonable manner. It would require neglecting to fix the problem of one brake circuit failing or the even more rare case of both circuits failing simultaneously.
More of a theoretical concern than a practical one.
 

DWhatley

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#15
There is a “now you know” video (I think in their 12 days of Model 3 series) where they did just this. It brakes until it hits a low speed, then puts it in Park.
I enjoyed going back to the Now You Know 12 days of Model 3 playlist but the braking demo was not among the things they showed.
 

NO BRAKS

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#16
All modern vehicles have dual circuit brake systems. If there is a leak in one, the other still operates. So a major brake fluid leak will leave either full front or full rear brakes functioning.
Correct, except that in passenger cars each circuit is diagonally linked, i.e. if one circuit fails you’ll be left with front left plus rear right or vice versa. This prevents the oversteer associated with rear-only braking.
 

Karl Sun

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#17
If someone doesn't mind being a guinea pig, try holding down the "park" button while driving at speed to see what happens. That would seem to be the obvious way to activate it to me.
Didn't have my g-meter installed for these tests so I'm using (guessing with) my semi-calibrated butt-o-g-ometer.

40 MPH level road, engage (hold) parking button for one second. Vehicle immediately slows at about 0.4g as long as the button is pressed, stops decel (back to normal regen braking level) immediately when button is released.

Holding the Park button longer (perhaps more accurate when speed gets below ~20 MPH) braking decel rate increases significantly, perhaps to 0.8g. I did not come to a complete stop.

Methinks the Park button engages the elec motors on the rear calipers to which initially are not strong enough to engage heavy braking or lock the rear wheels (small pad area, single piston, 4300 Lb vehicle, ...). Then, as the pads/rotors heat up and as speed decreases, friction in the rear [brakes] increases enough where it will stop the car more quickly .

I ddi not take IR temp readings of front and rear rotors to see if any front friction action was occurring. My laziness again.

This method can't be [easily] modulated to make a smooth stop.
 
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Karl Sun

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#19
So.....not a useful alternative to the (functioning) brake pedal. :)
Accurate statement. Unless you are trying to get that loose filling out?

More data: Pushng the Park button at speeds lower than 20 MPH just makes the car beep at me. It does NOT activate the same braking function that happens at 40 MPH. I have not [yet] tested where between 40 and 20 MPH this change occurs nor have I tried allowing the car to come to a complete stop using the Park button from 40 MPH.

So many things to test, so little miles driven!
 
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#20
>You're coming down a mountain road in your Model 3 and when you press the brakes, they go to the floor
As mentioned above - the *first* thing to do in this scenario is to *pump* the brakes. Often there's enough fluid still in the system you can build up pressure by pumping.

That's always your first line of defense. Before trying any of the other more radical techniques.