"Disc Pad Shift Knock" ?

[Losiho]

Whenever I reverse my Outback, then select a forward gear, my front dics pads make a knocking noise. For instance, I'll select reverse, slowly reverse out of a car parking space, then stop. I then select drive, drive for a short distance, and as soon as I hit the brakes with the car travelling forwards, the pads will knock (a clunking noise from the front end).

 

I complained to my service department, who said it was "normal" and anti squeal applied to the pads is the only solution, but I have never had a car that's had the same "problem" (if you could call it that - more of an annoyance). The only thing is the effect of the anti squeal they applied to the pads at the last service only helped for about 100km before the knock came back.

 

Do all Outbacks suffer the same problem ?

[blitz]

It's normal. Actually all disk brakes do it, but it seems like Subaru lets the pads flop around more than most, possibly to help thwart caliper binding due to mud and rust from off-roading excursions, etc.

[Nuwan]

I don't think that really should be happening, it is obviously a problem with the pads the dealer is using. The backing plate stamping must be slightly undersized from what the specs should be at , allowing it to move around more than it should.

 

I'd suggest going to aftermarket pads than the dealership stuff. Cheaper and better performance too.

[99obw]

No, that sounds like an unusual problem.

 

It sounds to me like the stainless steel pad guides or shims might be missing. That might allow the pads to move around enough to make an audible noise.

 

I have a 99 outback and the pads fit very tightly in the pad guide bracket thingy (technical terminology) with the stainless guides and shims installed. The car emits no audible brake noise, period.

 

If someone at some point used non OEM parts then the stainless guides or shims might be missing, as they don't come with most if not all aftermarket brakes.

 

OEM is the only way to go IMHO.

[BoxersOnly]

I had the exact same thing on my '99 RS.

Never seemed to cause any problems though,

except for annoying the hell out of me.

 

Rob.

[outback_97]

I'll copy my response to Losiho's post on Nasioc:

 

Losiho:

 

I think I have had a somewhat similar problem (annoyance is probably more accurate) with mine. At slower speeds (5 to 25 mph or so) the front left wheel would make a light repeated knocking noise, related to speed. Light pressure on the brake pedal would silence it. It was somewhat intermittent but happened a lot.

 

I took it to a brake shop and they fixed it with some sort of shim behind the pad. It was only $15 and it was quiet for over a year, but now it's starting to do it again. Guess I'll have to have them do it again, but they said it shouldn't affect brake operation, so it's not really a safety concern.

 

Steve

[Losiho]

Thanks guys.

 

My service dealer (Subaru) seems to think it's normal, but I might contact them again to make sure.

 

As I said over at the Nasioc Forum, I want to try Bendix Ultimate pads (I think they're called Axis Metal Masters in the USA ?) with DBA slotted rotors all round - the braking performance should be much better, but this combo definitely isn't dustless.

[tcspeer]

I agree with 99 O.B.W. I think the pad guides are gone. I dont see how the shims on back of pads would have anything to do with the sound you are hearing. I think you need to clean and grease caliper silde pins and replace the rails the pads slide on or in. It may also be the ends on the pads are not close enough to correct size. Clean, grease pins, replace rails and put on different brand of pads should take care of it.

[1qtlow]

I don't know your car specifically, but the disc brakes I've worked on all have something like an anti-rattle spring to reduce this problem. It is sometimes a thin multi-bend steel spring wire, or a springy thing made from a narrow steel band. Sometimes they break and don't work any more. If the installer of new pads didn't have a new one handy, they might have just slapped in the pads without it.

 

Many disc brake replacement instructions call for a thin coat of brake grease on the edges of the caliper opening wheer the pads almost touch. That grease may muffle the click.

 

The goo that's put on the metal sides of the pads may or may not do anything for this. It's meant to eliminate vibration and squealing, not the click of pads hitting the caliper opening.

 

I agree it's probably not anything but an annoyance. Things like this are one reason many people do their own brake work - you know what's been done and whom to blame!

[RallyKeith]

How sure are you that this is the brake pads??????

 

Subaru's have a lot of driveline to move and with each piece of driveline you have certian amounts of what I'll call slack. When you back up, you take the slack out in one direction, then procede to go forward and now you take up the full amount of slack in the other direction.

 

Here's the thing. I own an STi and have lots of friends that own WRXs and newer outbacks. All of our cars make the same clunking like you described. The key here is that the STi has Brembo Brake calipers on it which are nothing like factory subaru calipers.

 

Also, has the car always done this or is it a new thing? My STi has made the noises since the day I picked it up. No shims worn out, or cheap pads installed with mine.

 

Also, have you checked the lug nut torque? make sure they are tightened to factiry specs and no less. The brake rotors on subaru's are held in place by the wheel and the lug nuts. The rotor has holes for each stud to go through and they are usually a little bigger than the stud. If you actually go and remove a tire and then grab the rotor, you can spin it back and forth a bit without the hub spining, each time with a little clunk as it hits the studs.

 

So you car start to see how there are a lot of things on our Subarus that could add up to a bit of a clunk when you do something like reverse and pull forward. This can very well include to some extent the brake pads, but as said before there should be metal spring clips in the caliper that prevent the pads from slapping around.

 

Keith

[Losiho]

It's got nothing to do with the AWD system or tyre pressures - the clunk happens as soon as you touch the brake pedal. There is no noise at all when shifting from forward to reverse until the brakes are touched.

[blitz]

The pads are simply changing position, fore or aft.

 

Disk brakes are simple mechanisms. During brake application the hydraulic pressure pushing on the piston, deliberately distorts (stretches outward) the neoprene piston seal. When the hydraulic pressure is removed, the seal resumes it's shape (retracts) which in turn retracts the piston off of the rotor face.

 

Because the stretched seal is the only thing pulling the piston back off the rotor, it's important that both halves of the caliper (and pads) remain completely free of any friction or bind. Mud or rust scale WILL cause the caliper, slides, and/or pads to bind, which in turn will cause accelerated pad/rotor wear.

 

This pad thunk thing is a long-standing and well documented sube phemomenon. All Subaru would have to do the eliminate the noise would be to fit all the parts together really tight and my guess it that they don't want to.

 

I'll second the suggestion to keep the slides lightly lubed with silicone brake grease at all times to keep things moving freely and to retard rust formation. After I lubed my brakes, the thunk did lessen a bit. They ship 'em completey dry from the factory.