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.

Neat vid, however, he's showing the bearings outside of the axle - if I did that with the bearings on my truck, hey'd show a lot more than 0.001" of play (more like a 1/4"). Funny thing is that it was visually a lot more than that - I'd get 0.1" :)

Anyhow, I can't see it being a caused by the rotors pressing the pads away from them physically by flexing, at the hub, at the bearings, etc. Sorry man. I just don't see how it would happen.

I don't think you understand how the front wheel hubs on our cars work. The entire wheel hub assembly bolts onto the front spindle, and the bearings are contained within the wheel hub assembly. There are no bearings inside of the axle! These aren't front-wheel drive cars, after all. Here is a diagram from the Service Manual:

http://i618.photobucket.com/albums/t...09-25611PM.jpg

As you can see, the bearings are contained within the wheel hub assembly. The entire assembly bolts on to the steering knuckle and stays stationary against it; the only moving part is the wheel hub itself, via the bearings contained within. Now, the brake caliper bracket mounts up to these two holes:

http://i618.photobucket.com/albums/t...tingPoints.jpg

As you can see, the calipers also stay fixed in position. The wheel hubs have some flex. In fact, the Service Manual even has a spec for how much flex is acceptable:

http://i618.photobucket.com/albums/t...09-31653PM.jpg

Definition of Axial end play: "Most ball bearings are assembled in such a way that a slight amount of looseness exists between balls and raceways. This looseness is referred to as radial play and axial play. Specifically, radial play is the maximum distance that one bearing ring can be displaced with respect to the other, in a direction perpendicular to the bearing axis, when the bearing is in an unmounted state. Axial play, or end play, is the maximum relative displacement between the two rings of an unmounted ball bearing in the direction parallel to the bearing axis. Since radial play and axial play are both consequences of the internal geometry of the components in a ball bearing, they bear a mutual dependence." Source: nhbb :: hitech engineering :: radial play

So what happens when they flex is that the rotors (which are bolted up against the wheel hub via the lug bolts) can push up against the pads and push the pistons in slightly, resulting in more pedal travel the next time you step on the brakes. This is a basic and well known phenomenon that is common to all fixed-caliper systems. I really don't understand why you can't get your head around this. It is so elementary. Truth be told, I'm beginning to think that maybe you don't want to get your head around this, and that for whatever reason you've decided that you want to be argumentative.

StopTech has made available a technical paper explaining what causes pad knockback better than I ever could, yet you refuse to accept it because you think it's a marketing gimmick, which makes absolutely no sense because standard braking systems on passenger cars use floating calipers and don't have pad knockback. In other words, it makes no sense for StopTech to supply this write-up as a marketing tool because if anything, it would give people reason to not upgrade from a floating-caliper system to a fixed-caliper system (which all of theirs are). This paper would do more to hurt their sales than help them!

But let's see if we can approach this from a different angle. Maybe you and I are going around in circles because I don't understand exactly what it is that you're trying to argue. So, what exactly is the position you're trying to take on all this? That pad knockback is a myth and doesn't happen? Or that it happens, but it's not caused by wheel hub flex?

OK, thanks for the visuals. Seems like with the hub, spindle, and bearing being one unit (and non adjustable, ie, there's no nut on the end of the spindle as in with classic spindle/rotor+hub setups?), then I can see there how that could cause some play there and relate to the issue.

My position, previously, was that it wasn't caused by wheel hub flex...I can see where I was wrong now.

Thanks! :tiphat:

Correct, there's no nut on the end of the spindle. The entire wheel hub assembly is held in place with four bolts. You can see the bolt holes in the diagram.

i have this and i only have the base package brakes!

Really? That's very odd, because the base pkg brakes use floating calipers. I've never felt knockback with floating calipers before. Has it done this from day 1?

No, It hasnt done it since day one.. It just started happening about 2 weeks ago. I noticed it while i wasnt driving hard, I hit the brakes to stop and when i let off i got a little clunk noise and i could feel it in the brakes. I dont get it all of the time, just every so often, i would say out of about 20 braking tries maybe 4 or 5, but it is still very annoying. I have exhaust and rims on my car.. and BANKSTON nissan of dallas happens to have the WORST CUSTOMER SERVICE in their service dept i have ever come across. The car has wheels and exhaust, and apparently this is the cause all of my problems??

Hmm...I don't know what that would be, but pad knockback isn't characterized by any clunking sounds or sensations. Sucks that Bankston's customer service is so poor. You might consider taking it to a brake place (like Midas) to have them diagnose the root cause. Yes, it sucks because it'll be money out of pocket. But if it's a choice between spending some money or risking sudden and total brake failure (i.e., the problem could suddenly get worse), then I personally would spend the money. But that's just my opinion. Not trying to tell you what to do or anything.

yes, it isnt a loud clunk, but there is a sound. I think im going to take it to the dealership. Tell them the problem and ask them to diagnose what it is. At that point they will tell me, it is ______________ probably caused by the wheels you put on... like usual, then i will take it to a brake place to get it fixed. Im hoping on upgrading to someones used set of akebonos soon so maybe this will just get me more motivated.

I dont have this problem in my 370z.

On floating caliper setups like on the base model this can sometimes happen if the grease on the brake pads dries up. The pads hang-up for just a bit and then skip as they jump back out into position. A little lube on the backing plates and on the side edge usually does the job.

Great tip = +rep!

And don't forget the slide pins!

http://images34.fotki.com/v1150/phot...MG_0649-vi.jpg

Thanks! But you have the sport model don't you? The issue is a little different with the fixed caliper setup, but I have to say the 370 has much less pad knockback than the 350 did.

Yeah, I know your tip doesn't apply to my car. But I was repping you for helping out corbin09.

Are you feeling the knockback on track or on the street? I hope we are talking track, or else you must be driving like a nut to experience knockback on the steet. LOL.

Street. It's not huge. I think I just notice it because I had a Stoptech BBK on my 350, and it was really bad on that car. So I kinda 'looked for it' on the 370. It's not like the brake pedal drops to the floor or anything like that.