Brake Knock Back Issue

[ADAM]

Hi Weasels,

so I am trying to correct the knock back issue on the v8 240sx...its just too hard trying to drive it having to pump the brakes before every corner...i have to start braking way sooner..and it stops me from being able to heel toe..and its scary :)

1= replaced the stock brake master with a larger 1 inch bore 300zx cylinder one..
2=replacing the front wheelbearings with new ones...

is there anything else I can do to stop this?

[philip_240sx]

I'm sure you remember these articles/posts:

http://www.casc.on.ca/forums/showthread.php?p=80843

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

I can't think of anything else.

[Slowpoke]

I'm learning that there's a lot of STi owners who struggle with knockback. The one I borrowed for TMP had it too.

In the springtimer, when doing an on-vehicle rotor run-out test, Fourstar found 0.003" runout from a bent RF hub. They replaced it, put in a new bearing and it was better in the spring.

But it started to get worse again in the summer... so KVR just rebuilt my front Brembos with 4lb knockback springs. Shakedown at the DDT will be Sept 3rd, with three days on the Big track Sept 7-9. Then Shannonville Pro on the 22-23, and big track again on Oct 6,7, DDT on Oct 21... and then it's ORRC time!

I'm hoping that I don't have to do knockback springs in the rear too, because it ain't cheap. The price was more than fair, but it ain't free.

[MazdaMatt]

This is probably a useless post...but I once saw a product in a magazine that was basically a low pressure check valve... it kept 2psi of pressure in your brake line at the caliper end. This resulted in the pads lightly being held against the rotor. The only issue with this would be if your brakes overheat from rubbing gently ALL the time.

[ADAM]

YES KNOCK BACK SUCKS....

its a bugger going 180 into the first corner at TMP, pumping the peddle (means having to brake way earlier) then trying to heel toe and turn...

sucks not knowing if you will have brakes the next stop or not...


hopefully the new master and bearings will solve my issues

[ADAM]

so KVR just rebuilt my front Brembos with 4lb knockback springs


how is this done? some small springs inserted in the pots?

[Slowpoke]

They also have to groove the back of the piston and the piston housing to make room for the spirng when it fully compresses. Other shops can do it as well, such as Fourstar and the like. Rallispec.com sells a Group-N 4-pot / 2-pot Subie brake conversion where they're pre-installed. Do the on-vehicle run-out test like we did to see if there's a bent hub. (Lanark Highlands was fun, but I hit stuff.)

If I were you, I'd pick up the left foot tap technique while going down the straight. Full on the throttle with your right foot, you shouldn't feel a physical slowing of the car when you tap with the left, just a slight depression of the pedal. If you don't put any extra steering input in, the pads will stay in contact with the rotor and be there when you need to brake for the corner. That's all you need to do for big courses without many turns (like Mosport), but the infield of TMP is pretty busy and it takes some effort to get the physical dexterity right.

Fourstar's Group-B Audi (not used for competition) has a hydraulically activated clutch because Frank left foot brakes in every corner to keep the turbo spooled. So with right foot on gas, left foot on brake, he presses a small switch on the shifter knob and presto... clutch drops to floor. Shift and release the little switch for the clutch to come back up. Free mod under Solosprint rules as far as I can tell.

[Chris91GT]

Adam, have you checked your diff for too much play?

[Pete@Marcor]

Adam, have you checked your diff for too much play?

Chris, you do realize that he probably has an IRS car, and it does not directly affect the axle run out? ;)

[shawn cormier]

Adam, have you checked your diff for too much play?

I was thinking the same but I didnt know how IRS axels are placed my car had the same problem and it was worn c-clips.There must be some kind of play in IRS for suspension travel.:confused:

[ScotcH]

I was thinking the same but I didnt know how IRS axels are placed my car had the same problem and it was worn c-clips.There must be some kind of play in IRS for suspension travel.:confused:


Nah, the CV joints take up all the slack. The bearings are pretty much fixed in the arms, and unless the wheel bearing is bad, it won't affect the rotors at all. Basically, I can move my axles back and forth about 1/2" by hand.

[Chris91GT]

Chris, you do realize that he probably has an IRS car, and it does not directly affect the axle run out? ;)


LOL! [smack]

Yes, yes... I forgot... he's been domesticated, but not relegated to 1969. :D

[Pete@Marcor]

LOL! [smack]

Yes, yes... I forgot... he's been domesticated, but not relegated to 1969. :D

Heh. Yes, the ENGINE is a domestic, but the rest of the car is still mainly an import. :-)

Adam, how warped (if any) are your rotors? Do these low pedal situation occur at a particular place on a track? Meaning after the slow stuff, after the high speed stuff, or after corners?

[philip_240sx]

Nah, the CV joints take up all the slack. The bearings are pretty much fixed in the arms, and unless the wheel bearing is bad, it won't affect the rotors at all. Basically, I can move my axles back and forth about 1/2" by hand.

Yes, Adam's 240 is still IRS. The rear wheel bearings and hubs are bolted to upright/knuckle. The half shafts also have CV joints and are free to move independently. That being said, the rears bearings are probably just as prone to wear.

FYI: Adam is running the stock 240 floating calipers (single piston) in the rear. The fronts are '90-96 300ZX (fixed caliper) aluminum 4 pots.

[ADAM]

it happens almost every time in need to brake...especially after hard corners

[Chris91GT]

You sure you didn't go for the full domestication and drop a solid lump of iron in the back?? :D