Power Brakes

My Nissan Leaf Forum

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These features are listed:

4-wheel Anti-lock Braking System (ABS)
Electronic Brake force Distribution (EBD)
Brake Assist (BA)

Chances are all of these, and any "Power Brake"
(it that the "Brake Assist"?) function, use the
hydraulic braking system to achieve their function.

Also, there is no source of vacuum on the EV.

So, I think a vacuum Power-Brake is unlikely.
 
My understanding is the Leaf is completely drive-by-wire, so there is no mechanical connection between any of the controls (steering, accelerator, brake, parking brake) and the actual mechanisms. So rather than a booster and master cylinder, it probably uses an electric hydraulic pump/ram of some kind.

There was some leaked info regarding maintenance a few weeks ago that mentions brake fluid flushes, so it's almost certainly hydraulic. The Service and Maintenance Guide (PDF) is a great flip-through and gives some nice details about the vehicle, like the fact that the HVAC system uses antifreeze coolant (Replace every 75,000 miles!)
=Smidge=
 
Smidge204 said:
My understanding is the Leaf is completely drive-by-wire, so there is no mechanical connection between any of the controls (steering, accelerator, brake, parking brake) and the actual mechanisms. So rather than a booster and master cylinder, it probably uses an electric hydraulic pump/ram of some kind.

Wow, that would be a departure. I thought the US required direct mechanical steering and brakes on all cars. If this is the world's first steer-by-wire, well, I have think hard about whether I'm ready for that. :)

There's probably an electric brake pump that provides boost pressure, like a Prius.
Likewise, an electric steering pump for that overboosted steering feel.

Next time someone does a drive, try cranking the wheel over while the Leaf is off. I'd bet it feels pretty normal -- heavy, but mechanically connected.
 
The steering in normal and has electric assist, the P brake is an electric solenoid that pulls a cable. the brakes appear to be normal hydraulic electric-assisted, the accelerator is most likely a standard HEPI (Hall Effect Pedal Sensor), that's a magnetic sensor. There are e few hydraulic brake distribution blocks in the motor compartment.
 
I believe the power steering's electric assist (to the normal mechanical steering) has no hydraulics involved (in the steering function). It could just be an electric motor coupled "directly" (through gears) to the steering shaft (and gear).
 
Steering and brakes are both mechanically connected. Steering has an assist motor on steering column. The brakes have a motor mounted in a booster assembly. There is a stroke sensor in the booster assembly to read how fast and hard pedal is applied. From the video I watched at work it shows a "cooprative" brake system. It starts with mechanical brakes switches to regen then back to mechanical brakes at final stop.
 
It applies the energy-wasting disc brakes BEFORE the Regen?

So, with a LONG Regen-Braking run you are (perhaps slowly) heating the brake pads to ... "very hot"?

Is that done so that the brake pads will already be touching (lightly) the brake rotors, ready for increased disc-braking pressure?
 
garygid said:
It applies the energy-wasting disc brakes BEFORE the Regen?

So, with a LONG Regen-Braking run you are (perhaps slowly) heating the brake pads to ... "very hot"?

Is that done so that the brake pads will already be touching (lightly) the brake rotors, ready for increased disc-braking pressure?


Classic example of poor regen to make the car "normal" The brakes wont drag on ECO.
 
GroundLoop said:
Wow, that would be a departure. I thought the US required direct mechanical steering and brakes on all cars. If this is the world's first steer-by-wire, well, I have think hard about whether I'm ready for that. :)

There's probably an electric brake pump that provides boost pressure, like a Prius.
Likewise, an electric steering pump for that overboosted steering feel.

Next time someone does a drive, try cranking the wheel over while the Leaf is off. I'd bet it feels pretty normal -- heavy, but mechanically connected.

Pretty much my position on it. I know drive-by-wire cars exist as prototype/concept vehicles, and that drive-by-wire systems are common on industrial equipment (construction vehicles, forklifts, etc). But I just had the impression that the Leaf was also drive-by-wire. I'd be a little :| about that.

But from what others have said it seems I was mistaken about that, which is good IMHO! Part of what convinced me of this was the electrically operated parking brake instead of a normal cable pull arrangement. (I wonder if that has anything to do with the battery pack taking up the entire underchassis?)


wgs1912 said:
From the video I watched at work it shows a "cooprative" brake system. It starts with mechanical brakes switches to regen then bake to mechanical brakes at final stop.


I would love to see the video you're referring to. That just seems like unnecessary wear on the brakes.
=Smidge=
 
garygid said:
Perhaps this brake dragging and the resulting heating is the reason that "frequent" brake-fluid flushings are specified in the LEAF maintenance manual?

It is not going to drag the brakes through regen switches from 100% mech. to 100% regn back to 100% mech. With a variety of % inbetween each. The longer the time slowing I assume the longer it will stay in regen.

As far as brake fluid services it is for moisture and copper in the fluid not just heat. This site has some good info on brake fluid.

www.aa1car.com/library/bfluid.htm
 
For what it is worth, the VW Golf Citystromers have a vacuum pump providing vacuum (can you provide vacuum ??? ) to a completely standard brake servo.

Diesels also don't have a natural vacuum in the inlet manifold, and have to have a vacuum pump. Normally this is mechanical, but on some diesels (e.g. some Volvos) they use an electric pump.
 
Now that the Leaf is out, can anyone tell me how much regen is applied with slow braking (not using ECO mode), and whether this is a reasonable alternative to ECO mode? Do you see as much regen using the brakes as you would using ECO mode starting at same speed and using same rate of deceleration?
 
The topic of peculiar "brake assist" behavior was discussed elsewhere, but I can't seem to find the thread.

Anyways, I continue to notice a slightly odd inconsistency in the brake pedal action at very low speeds.

A typically scenario is like this:

I am pulling up to a stop light, using the brakes to slow down gradually and get ready to come to a stop behind the car in front of me. As I am doing this, the cars in front bunch together a little more closely so I ease back off the pedal a bit to "roll" forward a bit more, then ease back on... But now it seems to "grab" and slows me down suddenly (but just from like 5mph to 1mph), so I back off the pedal again since I wasn't quite ready to stop... Now this time the brake pedal seems numb, and I have to push it a lot harder than before to finally come to a stop.

When starting out slowly from a stop and "feathering" the brake pedal it seems OK. It is more when I have just slowed down a bunch (from say 40mph) and then try to modulate the pedal carefully to get into position that I find that pedal can sometimes seem overly grabby or overly numb.

I guess I attribute this to the "brake assist" function turning off or on.

http://en.wikipedia.org/wiki/Brake_Assist" onclick="window.open(this.href);return false;

It seems like it is turning on in some cases which are no where near a "panic stop" situation.
 
wgs1912 said:
garygid said:
Perhaps this brake dragging and the resulting heating is the reason that "frequent" brake-fluid flushings are specified in the LEAF maintenance manual?

It is not going to drag the brakes through regen switches from 100% mech. to 100% regn back to 100% mech. With a variety of % inbetween each. The longer the time slowing I assume the longer it will stay in regen.

As far as brake fluid services it is for moisture and copper in the fluid not just heat. This site has some good info on brake fluid.

http://www.aa1car.com/library/bfluid.htm" onclick="window.open(this.href);return false;


That is not the reason for more frequent changes over other cars, it is because of the design of the LEAF electronic brake system and the added stress to the fluid. THe system is not like a traditional system and that is why it requires more frequent changes. All systems are subject to the same environmental issues.
 
I don't believe this to be true. The actual braking system is a completely standard hydraulic and ABS system, and should see no more stress than any other such system, EV or not. The only real reason for brake fluid changes in street vehicles is because brake fluid is hygroscopic and absorbs moisture. Assuming that the system has not been abused and the fluid boiled, we change the fluid every two years or so because of this to prevent the moisture from lowering the boiling point of the fluid and, more importantly, causing corrosion of the brake components.

There is nothing in the design of the brake system in the Leaf that requires a yearly change. I simply see it as a profit mechanism...

EVDRIVER said:
That is not the reason for more frequent changes over other cars, it is because of the design of the LEAF electronic brake system and the added stress to the fluid. THe system is not like a traditional system and that is why it requires more frequent changes. All systems are subject to the same environmental issues.
 
TEG said:
...I am pulling up to a stop light, using the brakes to slow down gradually and get ready to come to a stop behind the car in front of me. As I am doing this, the cars in front bunch together a little more closely so I ease back off the pedal a bit to "roll" forward a bit more, then ease back on... But now it seems to "grab" and slows me down suddenly (but just from like 5mph to 1mph), so I back off the pedal again since I wasn't quite ready to stop... Now this time the brake pedal seems numb, and I have to push it a lot harder than before to finally come to a stop. ...
Yep, I get that quite often. I've learned NOT to creep ahead in those situations, but just sit still. Only completely releasing the brake seems to clear it, and since I'm stopped at a light, I can't do that.
 
When it comes time for brake service (fluid change, pad change, rotor turned, etc), I wonder if you can take the Leaf to any normal brake shop, or whether you should stick with Nissan dealerships for brake service.
 
Anyone who is used to bleeding ABS brake systems can do the fluid change. The rest is standard old disk brakes that anyone who does brakes can also do.

Volusiano said:
When it comes time for brake service (fluid change, pad change, rotor turned, etc), I wonder if you can take the Leaf to any normal brake shop, or whether you should stick with Nissan dealerships for brake service.
 
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