karlooz

i was at laguna seca a couple of weeks ago. at first i though my brakes were fading. the pedal would feel soft and there was much more travel but i noticed that it happened at the same point on the track, turn 9 just after the corkscrew. there is heavy left hand loading as the car comes down and compresses as you turn right.

what does one do to counteract? tap the brake with the left foot to pump it up? get floating discs/strut tower brace/new wheel bearings?

Larry Herman

Do you still have the anti-rattle clip installed. I find that the added spring pressure helps prevent it to some extent, but with the solid rotors you really don't have that problem very often.

karlooz

i don't have the clip installed. are you referring to the anti vibration pads with the two posts sticking into the pistons? doesn't it make swapping pads hard, i.e must remove the caliper?

i have 2 piece stoptech rotors/front brakes. would they be more susceptible to deflection?

Larry Herman

No, I mean the spring clip that you must depress and push the pad retaining pin past. It looks like a fat "I" and it provides pressure against the top of the pads to keep them from bouncing around. There is one per caliper.

karlooz

ahhhh. i have stoptech's version of the spring clip. it holds the pads down but not as tightly as the oem spring clip.

earlyapex

Tap the brakes...

There was an article on Stoptech's web site on pad knock back at one time.

Come out and play at LS in January!

karlooz

i just saw the article on stoptech's site. i'll tap the brakes but i am on the search for better bearings. maybe porsche motorsport if they fit my wheel carrier. are you going to LS end of 01/30 or 01/01-02?

M758

On my 944 I have experienced this. On the exit of one particular corner of one track. There are a set of gators on the track out point. Take the corner a bit wide and you can run on those gators, but the car vibrates like hell even with soft 350lbs/in spring rates and tallish 225/50 tires.

Anyway when you get to the brake zone you have zero brakes for 1 push. Then push agan and the brakes are back to normal. I can only guess that is pad knock back from the vibration of the gators. Not fun, but not big deal since I don't want to use the gators anyway. Spooked me like hell the first time though. For me that is the only place it happens. Of course it is the only place I have run on gators as well.

SundayDriver

Ah, my favorite subject. First time I experienced it was under braking for a hairpin with a concrete wall on the outside - kinda got my attention when the brake pedal went to the floor.

This can be a design problem or a mechanical issue. Design problems in regards to knock-back are usually found on lightweight cars that approach Formula Atlantic cornering forces but sometimes on production based cars.

Start with mechancial checks. It is almost certainly coming from the right side brakes, most likely the front but could be rear. Check the bearings and the rotor run out. My first time with this problem, it was really hard to find as it was a combination of a loose bearing and a slightly warped rotor. Neither was bad enough to be obvious but the combination caused the knock-back.

If it is design, then the solution is harder. I don't know which of these suggestions will not work with your car and ABS, but here is what I have done on the Stohr, which had pretty severe issues.
Check valve - Wilwood (I think) makes a check valave you can add to the brake line. Holds a couple psi pressure to keep the pat in contact with the rotor.
Springs - addin springs behind the pistons can keep contact.
Floating rotors - eliminates most of the flex but these are noisy on a street car.

Finally, a touch of the brake pedal with the left foot as you approach the braking zone will get the pads back where they belong.

Hope this helps.

JimB

I had this problem last year. It was an early warning of wheel bearing failure. I'd check that first. Jim

chris walrod

Possible culprits are caliper seals that are sticky and in need of replacement, too much rotor float (for floating rotors, obviously), worn wheel bearings, loose hub nuts, bent wheels, excessive rotor runout, brake balance bar improperly adjusted (for cars with dual master cylinders).

Other causes, like mentioned above, are running over curbs, big bumps, long long straights. Road America and Montreal usually produces knock back with the open wheelers due mostly to the long long straights.

These problems are sometimes tough to figure out.

JW in Texas

Agree with Chris & Jim. Glen & I experienced this in my car during the RA enduro. Ended up being a RF wheel bearing.

SundayDriver

That is one I haven't seen (at least that I know of). Does this contribute to knock-back (if so, I am curious if you can expand) or just a long pedal?

Thanks
(I still think you have the coolest job.)

Adam Richman

I don't know Stoptech brakes but could it just be the depth of the pads, have you tried new pads? If you take the pads out of the caliper, is there any taper (front to rear, top to bottom)?

I too would be on the lookout for worn wheel bearings or loose hub nuts.

wlittleman

I'm afflicted with this problem. I have the stoptech kit for a 350z, and I have pad knockback pretty bad. I installed the brake kit when the car had only 10,000 miles on the odo and two track days within those miles. I experienced knockback on my first track outing with the new brake kit. I had brand new pads and bearings that should be practically brand new.

With knockback this bad, I'm wondering how much worse it's going to get when I put on r-comps next month, and load up the hub even more. After much thought, discussion, and research, it have come to the conclusion that the 350z has weak hubs/bearings. There is no play in the wheel when I jack up the vehicle and do the shake test either, and the wheels seem to rotate well.

Many folks who DE their 350z's and use stoptech brakes are experiencing this, and now that I hear that karlooz has this problem, I am again wondering if this is a hub flex problem or a brake design problem. The Porsche brake design shouldn't be that far different from a stoptech kit should it? Do you guys experience this with your oem brakes?

karlooz, was this your first experience with knockback with this brake kit, or is it a brand new kit? If you've been using this kit for a while now, and just started noticing knockback, then I think you should get your hub/bearings serviced or replaced.