[M Fifty]

That moment when you press the brake pedal and nothing happens as the pads aren't in contact with the disc.

So you have to press the pedal again...

Anyone with the 2NH brakes had this?

[nearwater4me]

Pad aren’t in contact?!
If you’re not talking about slipping brakes during rain, then no, I haven’t experienced anything like that.

[5.M0NSTER]

There are only 2 scenarios where this is feasible:

1) you just changed pads, and had to push the brake pistons back into the caliper to accommodate new, thicker pads

2) knock back, which can happen under high lateral loads during aggressive track use, or driving over rumble strips. This causes the pads to push back and retract the pistons. But one apply after all should be back to normal.

[M Fifty]

Originally Posted by 5.M0NSTER

There are only 2 scenarios where this is feasible:

1) you just changed pads, and had to push the brake pistons back into the caliper to accommodate new, thicker pads

2) knock back, which can happen under high lateral loads during aggressive track use, or driving over rumble strips. This causes the pads to push back and retract the pistons. But one apply after all should be back to normal.

Point 2.

I've just been reading about someone experiencing it on the road. It seemed surprising.

[nearwater4me]

Didn’t know these types of phenomena existed. (Never been to a track )
So, the lateral movement/flex of wheel bearing and brake discs push the pads in?

[chris719]

Was it raining? I've had this happen with aftermarket pads in the rain on my Mazda.

There seem to be an awful lot of complaints with 2NH relative to the blue brakes both on the forum and in magazine tests given the number of M2C that have been out there relative to M2 and M3/M4.

[5.M0NSTER]

Originally Posted by M Fifty

Point 2.

I've just been reading about someone experiencing it on the road. It seemed surprising.

Originally Posted by nearwater4me

Didn’t know these types of phenomena existed. (Never been to a track )
So, the lateral movement/flex of wheel bearing and brake discs push the pads in?

Indeed, this is somewhat common in fixed caliper design with 2-piece rotors. The fixed caliper doesn't move as much as a 2 piece rotor during high G corners. And the larger the rotor diameter the more flex there is, resulting in the rotor pushing on the pad.

Some OEMs have logic to compensate for it in the ABS/ESC unit. The unit detects high cornering forces and tracks how long the event lasts to judge if there will be push-back. Then when the car straightens out again ABS applies just a few bar of pressure to push the pistons back into place, so that the driver doesn't notice anything next time they apply the brake.

I'm guessing BMW doesn't use it if you can tell, but I don't know for sure. I do know that GM uses this algorithm in their performance products.

[nearwater4me]

Very interesting. Learn something new everyday!
Thanks for sharing the information.

[VIERsr]

It's quite common on motorbikes (all sport bikes use fixed calipers and 2 pieces rotors).

He braked too hard after knock back on first brake, but on 4 wheels it's not so dangerous

[COChris]

Originally Posted by chris719

Was it raining? I've had this happen with aftermarket pads in the rain on my Mazda.

There seem to be an awful lot of complaints with 2NH relative to the blue brakes both on the forum and in magazine tests given the number of M2C that have been out there relative to M2 and M3/M4.

Of all the negative press out there on the 2NH brakes, Chris Harris praised the kit during his initial M2C review. I think the press cars had issues, and it's also my experience that the stock pads and fluid aren't quite up to snuff for track use. They are more designed for aggressive street driving.

[omasou]

Originally Posted by COChris

Of all the negative press out there on the 2NH brakes, Chris Harris praised the kit during his initial M2C review. I think the press cars had issues, and it's also my experience that the stock pads and fluid aren't quite up to snuff for track use. They are more designed for aggressive street driving.

I love the 2NH brakes. I can feather them and they stop really, really, well.

My last loaner was a M240i X with the blue calipers. They were horrible, though I never remember an issue with them on the M2 when thought they were rather good.

[chris719]

Originally Posted by omasou

I love the 2NH brakes. I can feather them and they stop really, really, well.

My last loaner was a M240i X with the blue calipers. They were horrible, though I never remember an issue with them on the M2 when thought they were rather good.

I don't think those are the same brakes. They only look similar.

[chris719]

Originally Posted by COChris

Of all the negative press out there on the 2NH brakes, Chris Harris praised the kit during his initial M2C review. I think the press cars had issues, and it's also my experience that the stock pads and fluid aren't quite up to snuff for track use. They are more designed for aggressive street driving.

There feels like something is up with them given the sample size, but perhaps I'm just reading too much into it. I agree it could just be the pad. It may be an older pad since it's very close to the F10 M5 brakes. I would think with equivalent pads and fluid there is no way the blue brakes are better just because the thermal mass must be greater on the 2NH brakes and neither caliper design has any exotic features.

[mrdstr]

Knock back is a real issue with the 2NH brakes. After 14000 miles, the problem has only gotten worse, which makes sense given that pad wear can exacerbate knock back. I don't so much notice this when driving in a city/suburban setting, but on the rural windy roads it is more common for me. Aside from preloading the pedal prior to actual braking, has anyone found a solution to address this on the street? It's rather disconcerting to press the pedal a given amount and not get the stopping power you were expecting.

[jritt@essex]

Pad knockback is definitely an issue, and is most common after an S series of turns. In proper racing calipers designed for track use, we include an anti-knockback (AKB) spring behind the pistons. It pushes the piston back into contact with the disc face, so you don't have any pedal drop when heading into a brake zone on the track. All of our AP Racing by Essex Radi-CAL Competition Brake Kits come as standard with AKB Springs.

From our website:

Anti-knockback Springs

Not only are the pistons stainless steel, they are also fitted with anti-knockback springs. Springs in pistons you ask? Yes, springs. If you’ve ever gone through a series of S turns and then had your pedal drop when going into the following brake zone, you have experienced knockback. To say it is disconcerting is an understatement. You’ll often see pro drivers ‘pre-tap’ their brakes lightly when approaching a brake zone. They are fighting knockback.

Knockback is a phenomenon that is common with fixed calipers. Knockback occurs when your car’s wheel, hub, and bearings deflect during cornering, allowing your brake disc to move out of sync with your caliper and brake pads. The amount of knockback varies by vehicle, and depends on the amount of deflection seen in the parts listed above. As the brake disc deflects, it actually pushes the pads away from each other, forcing the caliper pistons back into their bores. The piston seals don’t have enough tension in them to completely return the pistons to their original location. That means there is slack in the system that needs to be taken up. When you press the brake pedal, it will continue to drop until that slack is taken up.

Anti-knockback springs help alleviate this situation by putting some tension on the back side of the pistons. When the disc deflects and makes contact with the pistons, the springs push the pistons back into their proper location, reducing slack in the system. That means less pedal drop and far fewer pucker-factor moments when going into heavy brake zones.

There are no major downsides to lightweight AKB spring as long as the caliper is designed to accommodate them. More specifically, AKB springs do not create any increased drag or wear on the pads and discs as long as the shape and material of the piston seals takes them into account.

As you're driving the suspension is constantly compressing, the disc is moving around laterally, and the pads are being pushed slightly away from the disc. Think of the seals in the caliper as a spring or hinge attached to the side of the piston, rather than just a ring through which the piston slides. In an AP Racing competition caliper, the groove in which the seal resides isn't a square cut groove.It has angles. When the pistons slide in or out there is friction between the outer piston wall and the seal, and the seal distorts a bit as shown in the illustration below.

A caliper piston sliding out to the left would

distort the seal in this manner (the slashes are the seals on either

side of the piston):

/
---
---
\

As the piston slides back in to the right, the seal does this:
\
---
---
/

There is a certain amount of tension or friction that needs to be overcome before the piston actually starts moving through the seal ring. That tension/friction keeps the piston from dragging on the disc once the pistons are pushed back into the bores by the disc/suspension movement.

When AKB springs are added, a little more force is required to push the pistons back into their bores than would be required without them. After the spring is compressed, it unloads and pushes the piston back to 'neutral.'

With the proper seal and spring the goal is to keep the piston in the 'neutral' position, not pressed against the disc. The piston is still able to slide freely in either direction, but a bit of friction or tension needs to be overcome initially to get it moving in either direction. The seal offers that first bit of friction to limit movement, and then the spring provides additional resistance. The end result is that the properly designed AP Racing calipers won't drag or create additional or unnecessary wear.