I apologize if this issue has been discussed here before but a search did not answer any of my question.

I have just recently returned from a day at Infenion raceway participating in the NASA time trials. i had major brake issues all day long to the extent that i missed 2.5 of my 4 sessions that day. the issue started out when after bedding and bleeding the brakes i went out for the first session. the brake pedal would fall halfway and then abruptly brake and ABS would come in immediately. this caused me to lose a lot of confidence in my braking system which caused me to end my session halfway through. after returning to the pits i bled the system one more time and took it out for a test run same thing. frustrated i again bleed the system. still no resolve. so i start asking around for people with similar issues. and, am surprised to hear 3 different people who have stoptechs tell me that they are notorious for pad knock back problems. myself never hearing of this issue, asked for potential fixes. the nice people at the track recommended i pump the brakes before i hit the braking zone. so for my final session i did as recommended and the brakes worked fine after being pumped before every braking zone. now i also asked for any potential fixes for this issue and one person recommended me to install a 4 lb spring inside the caliper to keep the piston from falling back into the caliper. can anyone who has performed this fix give their impressions? i really don't want to have to pump my brakes for them to work as my OEM brakes did. I talked to a stoptech rep and he told me it could be a wheel baring issue. i don't think the problem lies in the wheels bearings as my OEM brakes never gave me this problem. any help on the matter would be greatly appreciated.


my car is setup in the following manner. i have basic bolt on's. stoptech 14 inch kit with ST40 calipers upfront and 13 inch rear kit with ST22 calipers out back. I have ran both street tires and Hoosiers at this track event. I am using Hawk HT14 pads upfront and HT 10's in the rear.

Hammad

 

I have the same stoptech kit as you and have knockback. Read this article, it covers it well:

http://www.stoptech.com/tech_info/wp_knockback.shtml

I haven't installed knockback springs yet, I just tap the brakes before the braking zone (or just after coming out of the previous turn, with my left foot - this takes practice to just tap it barely).

Zeckhausen racing (and other Stoptech vendors, I'm sure) sell the knockback springs.

Also, if it's really bad, maybe your wheel hub & bearing assembly is worn.

There are other discussions of this on this forum. Try searching for knockback or something. I know the search on this forum sucks... but it's out there somewhere.

 

Here is the thread you are looking for...
https://my350z.com/forum/autocross-road/134966-hub-flex.html

 

Hi Hammond, it was nice meeting you at the track and it was nice to see some other Z's out there competing. Yup that thread mentioned above is the one I started a couple years back. Knockback springs will cure all of your problems. Your stock calipers didn't have this issue because they are floating. Stoptech's are fixed mount. The PFC springs that I mentioned are the cheapest and work great.

 

would pad knockback springs have any detrimental effects when bedding in new pads? i know they are only a small force (in-lbs), but would that be enough to actually enable the pad to transfer some of its material over to the rotor when stopped?

 

Hey Dave. Do you have a link to the PFC springs for the ST BBK? Thanks.

 

http://www.hrpworld.com/index.cfm?fo...action=product

 

Mahalo!

 

another question - how much would R-compounds add to the knockback effect? since the tires are stickier, wouldn't they cause the hub to flex even more under load?

does no one have any opinions on the detrimental effects of knockback springs when bedding (see post #5)

 

Yes

No effect on bedding.

 

^Knock back is supposed to increase with R-comps since the hub has more flex. That being said, it didn't get much worse on my car, and to be honest I'm not in need of knockback springs yet.

From my understanding the springs won't really affect bed-in since they're only 4-6lbs springs. I would' just cool them down a few more minutes before coming to a complete stop. I haven't heard about it being an issue.

edit: daveh beat me to it. Got most of the info from him anyway

 

-Never tried knockback springs on a ST brake system...

But it is a common problem with the wheel hub flex and caliper position. It has nothing to do with your fluid so flushing the fluid will not make it any better. Whenever you come up to a brake zone, just tap the brakes to seat the pads against the rotor, then when you hit the brakes, the pads will already be against the rotor, and you'll have a stiff and consistent pedal. You just need to get into the habit of tapping your brakes before applying them though.

I agree the problem becomes worse with stickier tires. The knockback wasn't terribly bad with BFG KD tires, but going from the KD to the R-compound BFG R1s, (285 all the way around), the added grip and hub deflection made the knockback significant. Now we are running on Brembos with a relocated caliper -which solved our problems.

Again, never tried the knockback springs, but will look into them.

0.02

 

thanks for the input guys. i tap my brakes in every brake zone...it just gets distracting sometimes. maybe the next time i'm up for a flush, i'll go ahead and rebuild the caliper and add some knockback springs in. i'll follow the advice of mistico (daveh) and allow them to cool off longer before coming to a stop.

stuntmant, where did you relocate the caliper to solve your problem?

 

Tapping the brakes before the brake zone can be distracting, heck, I dont like it. But a lot of cars I drive have that problem so it's something you need to practice and have become second nature. Its probably cheaper to try the knockback springs to see if that helps. If you're going to R-comps (or have them) the problem will be a little worse than street tires.

We rotated the caliper on the Brembo setup to reduce the knockback effects. You might be able to do it with the ST calipers, but that will require new caliper brackets. We went to Brembo F50 calipers and developed a position that worked for us.