Massive crunching / knocking when hard braking

[Kekotic]

Hey guys.

I noticed this issue a while ago but brushed it aside as I have never had to really actually brake super hard before, but I decided tonight to test it again and the problem is still there.

Basically I was doing 60km/h to dead stops and every time I slam on the brakes, a big knocking and crunching feel goes through the pedal and I can also hear it too with the window down.

I have the PHAT GT dump pipe for post facelift GT, and also the cusco brake stopper (not done up tight) Everything else that might relate to the front end in that regards is stock.

Any ideas on what might be causing it?

brakes still work correctly and I can stop very fast, but something is definitely knocking against something else when I'm doing it, and although I'm not exactly familar with what ABS feels like (have felt it once in my dads clubsport when he was driving), I know how it's supposed to work and this does not feel like that at all. This is just crunching sound and the brake pedal knocks around a bit. With the window down I can definitely hear something crunching and smashing against something else.

I'll undo the Cusco brake stopper tomorrow to double check it's not that, but the sound seems to come from the front below the engine on the right hand side, which is why I think it has something to do with the DP?

Thanks.

[jimbo]

I'd say it's your ABS it feels and sounds very wrong in my liberty, very much what you are describing

[Kekotic]

It shouldn't feel like this though?

It feels/sounds like someone has thrown a bunch of pebbles in between the brakes. Just this horrid crunching sound. Something is definitely hitting another thing.
I appreciate your post but, surely this cannot be right? It's horrible.

Edit: Imagine standing on a pebble surface and rubbing your foot across the ground. That's what it feels like when hard braking. A lot of small pebbley crunchings. hard to explain I guess.
I'll show TeK at the next meetup and see what he says.

[Std08GTB]

Pebbles is probably the description I would use when describing ABS. ABS isn't some magical thing that happens behind the scenes, you will certainly hear and feel it as you have described. Try it on a gravel road if you want to easily replicate it rather than chew out your tires on bitumen. I suspect that the ABS pulses much faster on a Subaru versus a Holden, hence the difference. You must be hitting them pretty hard to experience this feel.

[cruisn]

My Gen2 GT had '08 spec B brakes on it with all new suspension.

I hit the brakes hard once and it was exactly like you described it. Its not a nice feeling and it made me get under the car and check everything out the next day.

I then went and did advanced driver training and was able to replicate it.

Sounds pretty normal to me.

[Nottage969]

I had this feeling half way through the SA cruise not so long ago and although I had felt the feeling in my old car, never in the suby! It scared the shit out of me but the guys assured me it was my ABS kicking in once we stopped to regroup. I tried it later that week on dirt roads and sure enough that's what I can put it down to. Makes me stop like there is no tomorrow, but definitely feels like there are pebbles in between the pads and the discs! Good description of the feeling! Mind you, I have never heard clunking or crunching sounds!..

[Dropz]

The crunching type noise is the brake system, every component in it acting at its full potential, caliper pistons shooting in and out, pads grabbing the discs and letting them go, tyres screeching intermittently on the road, the piston in the master cylinder moving in and out, and the pump operating, etc.

From the description given, yes it is definitely your ABS operating. The reason the pedal vibrates is because the pump forces more fluid into the calipers until the wheels lock, then the fluid is released back into the master cylinder and this forces the brake pedal back up, then when the wheel spins freely again the pump operates and locks the wheel again, this is when the pedal goes back down. It all happens VERY fast and it can lock and release the wheel multiple times each second, ALOT faster than a human could do pumping the pedal with their foot.