Pad knockback
 

Has anyone else noticed pad knockback with the sport brakes? This would be applicable to the sport pkg. brakes only, as the Akebono calipers are fixed vs. the floating calipers on the non-sport models. I could swear there's just a tiny little bit of knockback. Nothing major, mind you (although the effect would be magnified with larger diameter rotors). I'm used to double-tapping my brakes anyway because I had huge knockback on my 350 when I ran a StopTech BBK. I'm just wondering if anyone else has noticed this.

If you don't know what I'm talking about, read this -> StopTech : Balanced Brake Upgrades

Supposedly the pad knock back problem was fixed in the 370Z by strengthening the control arm and using a beefier wheel bearing.

I have noticed that sometimes on the street you get a little bit of extra travel that first time, and then it is fine after that. Haven't had any problem with that on the track (where it was a problem with the 350Z).

Right..that's exactly what I've noticed. That's pad knockback. Okay, glad I'm not the only one.

I haven't noticed any such issue, but I there's really only a couple of spots I regularly drive through that would have any chance of inducing enough lateral g-force to trigger such a thing.

Well it's not happening under lateral load anymore, which is a good thing. Seems to only happen when you're driving around like an old lady.

I have noticed the same thing on my Nismo...at first I thought it was suspension because it would make a noise each time I went over a speed bump here in my complex. Then I realized it was the brakes making that noise right as I let my foot off the brake pedal to ease over the speed bump... It goes away after the pads heat up but it still bothers me as this is a new car :(

I will just mention this to dealership when I take her in for 1st oil change..Meh!

Let us know if they do anything about it.

knockback as in jittering movement when braking? or delayed braking time?
sorry please refresh my memory thanks. ^_^

Well, when I posted this question originally (over 3 months ago), what I was referring to is that sometimes it feels like there's a little more travel in the brake pedal before the brake pads bite. So I guess you could call that delayed braking time, sorta. If I release the brakes then step down again, then it bites with barely any pedal travel at all. See what happens is that when there's a lot of lateral load (as during hard cornering), the wheel hubs can flex a bit, which makes the rotors push or knock the pads back into the calipers (hence the phrase 'pad knockback'). The result is a small gap between the pads and the rotors, which in turn accounts for why you have to push the brake pedal down deeper before the pads bite into the rotors. If you go up to my original post and click on the link, it'll take you to StopTech's page explaining it all, complete with a little graphic to illustrate. I've gotten into the habit of double-tapping my brakes now, which means then when I want the brakes to really bite hard, I don't just step down on the pedal, but do a quick step-release-step on the pedal.

I always thought that was a combination of two things.

1) Pumping the brakes will always build up pressure in the lines. In a sense, that's what you're describing (first hit of the brakes is "long" then if you release, it's quick) - every hydralic brake system I've had has done this, even my motorcycle which doesn't have power brakes.

2) Air pressure builds up between the rotors and the pads, creating a small gap, much like the heads on a hard drive float above the platter. I imagine this would be amplified by BBK's (more surface area) and slotted rotors (larger pockets of air to pump with)

I can't imagine the braking system "flexing" under any type of cornering - it doesn't make sense. There's not enough mass there to cause a centrifugal force to pull or push a rotor. There's not enough mass in the pads to push or pull them from the caliper to where they'd be any more or less engaged in either the rotor or the piston(s).

Anyone that has done their own brake job would know the amount of force needed to squish the piston(s) back into the caliper, would see what I'm referring to. There are too small of a force at play for cornering causing anything like what you (or StopTech) is claiming. StopTech is in the market of selling brakes. Of course they'll come up with some kind of phenominon to explain something as a marketing tool...

And here's the crux of the issue they present (stoptech's site) that I have a problem with.

http://www.stoptech.com/tech_info/img/knockback_1.gif

Why would the area where the caliper be angled...why would it flex that way? Brake rotors are spinning, if anything they would turn concave or convex...which we know isn't possible. Even in their diagram, they show the hub portion as being angled - last I checked, either the wheel bearings on a spindle (old school), or the lugs / wheel hub (new school) prevent that from happening. Either way, if what they show is happening, you don't have your tires bolted on right, or, your wheel bearings are failing.

Even if you suggest that it's lateral forces and that I'm blind because of XYZ reason - look where our calipers are mounted. They aren't on top. Not possible.

The diagram is exaggerated for illustrative purposes. Trust me, it happens. It was particularly bad on the 350Z. Wheel hubs flex. On my 350 I actually had to replace them semi-regularly after I switched to StopTech's BBK (annoyed the heck out of me too). It doesn't matter where the calipers are mounted, I don't see the relevance of whether or not our calipers are on top. That diagram could just as easily be a top-down view with the calipers mounted in the 9 o'clock position.

If it is a top-down view, then why would it flex in a front/rear direction unless it's a product of excessive toe in / toe out causing that kind of force / load - even then, those forces would still be at play at any speed. Sorry, I don't see it happening due to cornering. The hub itself can't flex enough...the rotors certainly won't flex enough...


Combine my point of view of why does it happen when not on a track?

It's not flexing front/rear, it's flexing side to side. Look dude, I'm not going to waste my time arguing this with you. If you don't want to believe me, fine. But do me a favor and do a Google search on "brake pad knockback."

Are you familiar with Modified Magazine? They experienced it pretty badly on their 350Z as well. Here's a link. Project Nissan 350z - Knock Back - Modified Magazine

But believe whatever you want to believe.

Check out this video. I don't follow everything this guy is rambling on about, but it demonstrates how much flex is in wheel hubs when he measures the flex he can get with pressure from just one hand.