Pads backing off the rotors?

[Slowpoke]

Before we get into removing calipers and trying to fix this issue properly, I'd like to solicit other ideas and opinions.

'04 Subaru Impreza STi (~300hp, 3500lbs with driver)
OEM Brembo front and rear calipers (Aluminum, 4pot front, 2pot rear)
Technical details on brakes in this link. (http://ken-gilbert.com/wrx/STi_Manual/05.%20CHASSIS/09.%20BRAKE/01.%20General%20Description.pdf)
Appearance: OEM Brembo blanks. Even wear, no heat cracks.
Modifications: Small brake cooling ducts to front brakes. (Home made-minimal flow.) Removed front splash shields. Rear shields still in place.
Additional data: According to rotor temp paint. Front rotors are above 430C (806F) and below 610C (1140F). Rear rotors aren't reaching 430C.
RBF600 fluid, less than 6 months old, bled twice with no air coming out. Very firm pedal when cold.
Pedal is firm and holds pressure on long application.

Symptom:

Brakes have great engagement when cold, or when applying the brakes constantly. A period of cooling when the brakes are at track temperature (e.g. a 400m straight, or section with curves that doesn't require braking) and the next brake engagement has a "soft" feeling pedal... going deep with less pressure.

I'm currently left foot tapping the brake while on the throttle right before a braking zone to build brake pressure / move the pads closer to the rotors (?) as a work-around to this problem. This makes the following engagement very strong and no problem whatsoever. (Like my brakes used to be.) For that reason, I don't think it's really fade. If I don't tap the brakes before the zone, then I get a "soft" pedal that goes deep.

I use race compount Ferodo DS3000 (and never had this problem before) and have tried Gransport GS3 with the same result.

Although my "workaround" works, I have less confidence in the brakes. So, I have to brake earlier, hurting my lap times. And when I forget to tap the brakes and wait to my normal braking point... things can get a little scary.

Suggestions I've received:

"The heat from the rotors pushes the pads back. Can disassemble the calipers and add small ~5lb springs to keep the pads in constant contact with the rotors with minimal pressure to not cause wear or heat."

"Sounds like it could be worn wheel bearings or loose axle nuts. The rotor pushs the pads back into the caliper during hard cornering and then you get a low pedal as you push the pads back against the rotor."
- Checked for play in the suspension, wheels, caliper, etc. The OEM bushings are a little flexible, but seem to do the job as well as when I didn't have this problem. Did not pull axle apart or bearings out.

"On Brembo's, if you drive in salt, then after a few winters, the slider plates typically have to be removed, and the corrosion taken out with a wire brush/grinder, etc. Otherwise the pads will stick, and lead to random pedal heights for the same reasons Hanif mentioned. Heat the little hex bolts that hold the slider plates in place before trying to undo them, as there's pretty strong loctite on them, and it is easy to strip the hex heads (BTDT)."
- I think I have a different Brembo caliper than the Porsche ones that Craig is used to? Mine slide on pins and there are no moveable plates in the caliper, other than the pads themselves. I haven't removed the calipers for a really good grinding, but I have cleaned the pad contact surfaces with brake cleaner twice a year. The car is winter driven, so this is a possibility.


Before I get into pulling the calipers off and cleaning them on the bench per Craig's suggestion, are there any other things to check? What do you think of the spring idea? The car is driven on the street as well. Relatively few KM's... ~ 14K a year ... half of that is driving to/from track days and on the track.

[slai]

Here's a good write-up on pad knockback from Stoptech. I was experiencing this a fair bit last year in the G35.

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

[Slowpoke]

That sounds like a pretty good description, even though there wasn't much play when we checked it. Hmmmmmmm. Okay... 49,000km for front wheel bearings in a car that's thrashed as much as mine isn't a bad preventive maintenance interval, I suppose.

Any other things?

[Pete@Marcor]

That sounds like a pretty good description, even though there wasn't much play when we checked it. Hmmmmmmm. Okay... 49,000km for front wheel bearings in a car that's thrashed as much as mine isn't a bad preventive maintenance interval, I suppose.

Any other things?

Steve, does this occur only on the track? What about street conditions? Does the pedal feel soft, or just have long travel before any effectiveness starts? Have you pulled your pads out to look at them? Are they perfectly straight? When did this problem start to occur?

[Slowpoke]

Steve, does this occur only on the track? What about street conditions? Does the pedal feel soft, or just have long travel before any effectiveness starts? Have you pulled your pads out to look at them? Are they perfectly straight? When did this problem start to occur?

Hi Pete,

Yup, it happens to me only on the track, on about lap two or three, and then gets progressively worse if I push the brakes harder. Aside from the DS3000's looking a little like pure metal in patches, (glazed?) the DS3000 and GS3 pads are evenly worn. DS3000's have about 5mm left, GS3's have about 7mm left.

It's mainly long travel. Pedal is firm as can be hot on the track after a left foot brake tap.

With stock rotors (22lbs) I feel no vibration. When I ran my two-piece 16lb rotors I had bad pedal vibration starting on lap two or three. (Again, perfect when cold.) THAT vibration was harsh enough to shake the car out of 4th gear at Shannonville. Four times. I stopped using those rotors (dimpled and slotted) until I can resurface and check the runout properly. Maybe the lightweight aluminum hat bolted to the steel rotor combination is more susceptible to the same problem and shows vibration as a symptom?

I felt a hint of this problem last fall during my last track day on the DDT, but thought I must be getting fade. Didn't use the brakes hard enough over the winter to get them that hot. First track day of the spring (April, TRAC Racing, St. Eustache) I had the problem quite bad. Wasn't bad on Mosport Big Track the week after, but it was cold and rainy and I didn't really heat the brakes up those days. It doesn't happen at AutoX. (doesn't get hot enough, I assume)

I hadn't thought of wheel bearing because they're not making a peep under street loads, and I'm not feeling excessive heat in the wheel hub / wheel center. The car feels *slightly* loose when taking off-ramps at speed, but I figured it was the -3.2 degrees of camber and soft street tire pressures. Otherwise, feels firm.

[Pete@Marcor]

Hmm. Very mystifying. I have seen this kind of problem with a 1 or 2 piston calipers, but rarely in an opposed caliper design like yours. Tapered pad wear is not uncommon, but usually the problem will get better with use. First time the pedal is brutal, the second time it is way better.

I was once told by a pretty bright guy that rotors rarely warp. Usually it is an incorrect transfer layer of the pad material on the rotor. High and low spots cause the vibration. I cannot say I am 100% convinced, but it is a possibility.

This, and the possible rotor run-out would push the pads in, giving you a long travel on the first application. Is the pedal hard after you go through that first long motion?

[Slowpoke]

Yes, it is.

The two-piece rotors have some minor grooves and material transfer. I really think the DS3000's don't like this broken surface. (I bought them because they were discounted to 33% of their original cost but had only 0.02mm wear.)

I switched to some brand new Brembo blanks this spring, and there's no vibration, no material transfer issues. With street pads you can see your face in the rotor surface. With track pads, there's a perfectly even transfer of grey material to the surface.

If you read that article from Stop-Tech, apparently a floating caliper design is less prone to this problem. I've found it mystifying as well, but it isn't going away, and I'm not going to put up with it any longer. The Scooby can only take so much off-roading at that ride height.... ;)

Bearing work scheduled for next Monday. I'll remove and clean the calipers this Saturday and change the fluid to Castrol Super Blue.

Thanks for the help, folks!

[Pete@Marcor]

If you read that article from Stop-Tech, apparently a floating caliper design is less prone to this problem. I've found it mystifying as well, but it isn't going away, and I'm not going to put up with it any longer. The Scooby can only take so much off-roading at that ride height.... ;)

Bearing work scheduled for next Monday. I'll remove and clean the calipers this Saturday and change the fluid to Castrol Super Blue.

Thanks for the help, folks!

You have a floating caliper? I thought that your calipers were rigid mounted. Or, did you mean floating rotors? Still, I think you should have a non-floating rotor.

What have you done, and in what order to try to fix this issue? Did changing to new pads or rotors (hopefully one at a time) change or help the problem?

I think you mean ATE Blue. :D

[ADAM]

WOW.... cool article on knock back...

that really opens my eyes on this issue... I fought with the turbo car for 2 years with this exact same problem.... I never thought it could be bearing related....hmmm..runs off to check the cars front bearings

hmmmm very neat.....

i just ended up doing a 2 tap before every heavy breaking zone...

[Slowpoke]

You have a floating caliper? I thought that your calipers were rigid mounted. Or, did you mean floating rotors? Still, I think you should have a non-floating rotor.

What have you done, and in what order to try to fix this issue? Did changing to new pads or rotors (hopefully one at a time) change or help the problem?

I think you mean ATE Blue. :D
Yes, I have rigid calipers, but the article mentioned that floating calipers suffer less from the effects of this problem.

Attempted fixes:

Add Ducting, change and bleed RBF600 fluid meticulously: Does not happen on Mosport big track in the dry, but still have pad back-off on DDT. (DDT then Big Track, then DDT Fall 2005)

Try "directional" dimpled and slotted Project Mu two-piece rotors mounted "backwards" (slots in opposite direction): No vibration, but still have pad back off and car darts right-left under braking (TRAC, April 2006.)Switch to GS3's with vibrating 2-piece rotor: Worse with GS3 than with DS3000's. (SoloSprint 1&2, Summer 2006)

Use brand new Brembo Blank with both pads: No vibration, but pad still backs off. Equal problem on blanks with DS3000 and GS3's. (Solosprint 3&4, Summer 2006)

Adam: That's what I'm doing now, but I DON'T want to be doing this. I never had to, and I want this to stop. And I don't want to instruct guest drivers on it when they're trying to learn the dynamics of a Subaru that doesn't want to turn in and a short-ratio gear box.

[Pete@Marcor]

I would agree that floating calipers would be less susceptible to this. Assuming you are actually experiencing this. Have you put any brand new pads into your calipers, to see if the problem is cured?

Have you tried just cruising, but cornering hard, to see if the problem is still there? If it is pad knock-back, then it will not require the brakes to get hot for the problem to occur.

[Carguy]

I concur the article is a real eye opener. Just a thought... The last time I had to pump the pedal before a corner I had bad master cylinder seals. Maybe the master cylinder seals have deteriorated through heavy track and rally? usage (but certainly not due to using DOT 5 fluid since RBF600 is DOT 4).

The pad back off I get in my 240SX just lowers the pedal a bit but no need to pump the pedal - I'm using Ferodo DOT 5.1 with ss brake lines, 4-pot 300ZX brakes and Brembo blank rotors.

My 2 cents. :cool:

[neon boy]

Steve. From all the brake testing we do at Williams I have experienced several cars with knock back. Most of these are on cars with dual block bolted aluminum calipers such as yours.

They look Racy but do not perform well and yes I know they have the Brembo name on the product.

To check defelection you must mount a dial gauge from the caliper and let it rest on the COLD rotor. Let the car sit at least 3 hours prior to racing.

This will give you the amount of rotor and caliper defelection.

The second record you need is the travel length of the brake pedal. Mount a gauge between the floor and the brake pedal.

The third record you need is fluid temp and brake pad temp.

The reason you need all this info is to determine if you are experiencing deflection or a gas interlay between the pads and rotors.

E-mail me the info and we can tell you what is going on with your braking system.

William Dickson
www.wpfri.com

[Slowpoke]

Have you tried just cruising, but cornering hard, to see if the problem is still there? If it is pad knock-back, then it will not require the brakes to get hot for the problem to occur.
Yes, that's essentially my first lap on track, and highway off ramps and it doesn't happen under those conditions. And yes, I tried new pads... that's why I started using the GS3's when the DS3000's still had lots of life left.

[Slowpoke]

E-mail me the info and we can tell you what is going on with your braking system.
Thank you William. I am aware that the Brembo system on my car is not a true racing caliper... I'm only seeking the more than adequate performance and that I used to have. I did an almost two hour session on the DDT with no cool down in spring 2005 with no hint of fade or knockback, with the front splash shields still on and no ducting... Even nearly keeping up with James Mewett in the clockwise direction. The brakes used to exceed my ability... no longer the case. So I don't feel that this is a design issue.

I don't have a run-out gauge, so I'll be taking the slotted/dimpled Project Mu rotors to a shop for that measurement. The OEM Brembo rotors don't have any vibration, so I don't think there's a point to measuring it on those.

I will be painting the pads this time with temperature paint as well, but I don't have a thermocouple in the brake fluid line to check that temperature. And the temperature in the reservoir wouldn't be relevant, would it?

If this means anything, with Opal and I double-stinting the car, we managed to flake the paint off the back of the GS3 pads, even though the rotor didn't reach 610C. The DS3000 paint did not flake off.

Does anyone know someone who can resurface slotted / dimpled rotors? Most shops tell me they won't do it because it breaks their lathe tip.

[Pete@Marcor]

What has changed, causing your problems now? You used to have no issues, but now you do?

[P.Po]

Just a quick 2 cent addition...

I had the very same symptoms with a 944 Turbo I was using for lapping days.
I chased this problem for about three weekends until I changed a marginally loose wheel bearing at the rear of the car.

Hope this helps...

-Patrick

[jay fieger]

Hi
I don't know if it will be of help
a few questions....
how often do you bleed the brakes?
have you really gotten them hot?
what is the spec requirement for fluid type
have you ever mixed fluid types?

There are 2 areas to look at
in order of liklihood
if it feels like a soft pedal
the fluid may have been over heated or you have air or water in the fluid
the fix is to heavily bleed the brakes, be careful to use the right spec fluid essentially you are trying to change the brake fluid

secondly look for mechanical issues
if the pedal is firm but brakes don't seem to be working right
are the pads glazed at all?, hot/cold cycles tend to polish the pad surface
you can scuff the pads with sand paper or replace them
check the pistons for "sloppyness" they do wear and if you have play you might get one not applying pressure at different temps
check the calipers for leaks they can be inexpensively rebuilt, kits of seals and 0-rings etc should be available. if the pistons are showing wear, you should replace them, you can try polishing them with emery paper
does the master cylinder or the lines leak at all? check the presure by watching the flow rate as you bleed each corner
some master cylinders can be rebuilt but most OEM types do not have kits available
let me know how you make out
Jay

[Steve Moore]

In 1981, Bob Tullius cured the problem on the XJS Trans Am car by pressurizing the brake fluid system with 1-2 lbs of air.

[Carguy]

If this means anything, with Opal and I double-stinting the car, we managed to flake the paint off the back of the GS3 pads, even though the rotor didn't reach 610C.

I estimate this has very little meaning . The paint on my GS3 pads also flaked this weekend after first use, and my rotor paint showed no signs of overheating.

[Slowpoke]

What has changed, causing your problems now? You used to have no issues, but now you do?
A few things:

- 16,000km of wear and tear on the car including at least 12 track days and 12 AutoX'es.
- +30 hp, +30 ft-lbs torque in 2006 (faster speeds, higher braking demand)
- +75% higher spring rates over stock (was +25% over stock at last DDT lapping day when I didn't have the problem in early 2005, and later in 2005 when I started to get it. I assume this rules it out as the root cause, but the increased grip may be aggravating it this year.)
- Using 235/40/17 R-Comps instead of 225/45/17. (Transferring higher G-Loads)
- Added brake cooling and removed inner splash shields (Should only make the problem better, I would assume.)

In 1981, Bob Tullius cured the problem on the XJS Trans Am car by pressurizing the brake fluid system with 1-2 lbs of air.
Interesting!! Did they do this through the reservoir? Although I would probably use nitrogen, not air, this could be a cheaper way than disassembling the rotors to add springs... and you could vent the pressure for street driving, and pressurize it again for the track..... was this a typical reservoir, or did he have to specially seal it? I know that I'll have to add a valve to the system in the very least. Was it manual brakes or power brakes?

how often do you bleed the brakes?
have you really gotten them hot?
what is the spec requirement for fluid type
have you ever mixed fluid types?

There are 2 areas to look at
in order of liklihood
if it feels like a soft pedal
the fluid may have been over heated or you have air or water in the fluid
the fix is to heavily bleed the brakes, be careful to use the right spec fluid essentially you are trying to change the brake fluid

secondly look for mechanical issues
if the pedal is firm but brakes don't seem to be working right
are the pads glazed at all?, hot/cold cycles tend to polish the pad surface
you can scuff the pads with sand paper or replace them
check the pistons for "sloppyness" they do wear and if you have play you might get one not applying pressure at different temps
check the calipers for leaks they can be inexpensively rebuilt, kits of seals and 0-rings etc should be available. if the pistons are showing wear, you should replace them, you can try polishing them with emery paper
does the master cylinder or the lines leak at all? check the presure by watching the flow rate as you bleed each corner
some master cylinders can be rebuilt but most OEM types do not have kits available
let me know how you make out

A few of those answers were in my earlier messages...
- Have I gotten them hot? What do you consider hot? I've never had "fade" since buying my first set of track pads. Not over 610C or 1130F, but over 430C or 806F.

- I tried bleeding again after the first "knockback" occurence and it made no difference, and I bled them again in this spring before the first track sessions. I change fluid completely every six months. I've been using the same fluid up until now for the last few years... Motul RBF600. With the frequency of replacement and the thoroughness of bleeding with no effect, I don't think it's air in the system causing this issue.

- The GS3's are brand new and are not glazed. The DS3000 seem a little glazed, but still had good bite. At $250 US a set, I'm going to scuff them with sand paper instead of replacing them.

- I'll have the calipers off, so I'll wiggle the pistons for play. That's a good suggestion.

- No external leaks, I'll see if I can get behind the dust boot without damaging them. Fluid level doesn't drop at all, though. Pressing and holding my brake pedal holds the position. It doesn't slowly drop like it should if this was an issue. This would also rule out the master cylinder, no? It passes Pete Mills' tech inspections for AutoX. Hmm. Maybe I shouldn't have posted this where Pete can see it. I'm in for an EXTRA THOROUGH scrutineering next time I bet!

- Flow rate when bleeding is consistent.

[jay fieger]

I still favour the likelihood that its a problem with the fluid.
It sounds to me that you've got good components and everything is in decent shape
Brake fluid is a water misable composition
essentially it loves to absorb water, additionally a brake system is a hydraulic system that is open at one end this tends to promote air intake, particularly when under G loads. How full is your reservoir?
We run a GT 1 car, we bleed the brakes at least every other track session and change out the fluid completely 2x per year.
If we open a bottle of fluid we date mark it and don't use it if open more than 30 days old. This is somewhat dependent on how full it is.
with a 600 degree fluid you may well be boiling the stuff, especially if you're at the limit of the brake system capabilitites, if this happens, even once the fluid needs to be replaced
what diameter rotors? single or double plate rotors?
what does the car weigh?
I'm not familiar with the spec of your rotors
4 caliper? 6? what size pistons? do the calipers have cross tie pins?
what kind of track times do you turn? (mosport/Shannonville)
how much fluid in your system (educated guess)
are you doing the solo run at Mosprt this weekend?

Jay

[Slowpoke]

Thanks again for more suggestions, Jay.

I still favour the likelihood that its a problem with the fluid.

Okay, I feel that I've ruled that out by flushing and bleeding last fall after first experiencing the problem. Also, the pedal does not go deep if I just tap it before heavy braking. It's firm as granite which wouldn't be the case if there was air in the system and it needed to be bled.

How full is your reservoir?
Full enough that I can slowly push the pads back with a piston retractor at one corner without it spewing out the cap. My car triggers the ABS light if the level is too low in the reservoir. Only seen that once.

Your mechanical questions are covered in the first message of this thread. Look in the first 8 lines... there's a link there to the specifications and diagrams of my braking system. I have the STi with the 17" brakes. (Not the 15" or 16")

what kind of track times do you turn? (mosport/Shannonville)
My last timed lap around Mosport was a 1:42 in the fall.
Shannonville Nelson: 54.511 (cool track temps)
DDT CCW 1 Kink: 63.009
DDT CCW No kink: 58.623 (needed braking confidence here and didn't have it)

how much fluid in your system (educated guess)
are you doing the solo run at Mosprt this weekend?
The manual says that complete replacement of fluid is 500mL. No, I won't be doing the Solo run... I'll be throroughly cleaning the calipers of brake dust and flushing and filling the lines with fresh fluid this weekend.

[malcolm]

so have you checked the wheel bearings yet? takes two seconds to check... I assume you know how... (grab top and bottom of each wheel try to pull/push the top and bottom of the wheel and see how much play there is... it should be barely noticeable by feel)

edit: I see you checked in the first few posts... but it wasn't clear what exactly you checked.

[Slowpoke]

Just to clarify, I've been out of the country since getting back from the DDT, so I haven't had time to check any of this yet. I'll be starting on fluid flush and dust/corrosion cleanup this weekend. The car is scheduled in for bearing work on Tuesday of next week. I'm inclined to just change both front bearings with the mileage I have (no screwing around), and check the rears.

I had an inspection done in April this year of the bearings, and the play was not excessive. It may, however, have been marginal enough that it is causing knockback as per the Stoptech article under extreme cornering.

So, I'll be applying what I learn here this weekend/next Tuesday & Wednesday, then trying it on the track at Tremblant July 7-9. Wish me luck!

[P.Po]

I know you are going to get your fronts replaced and your rears checked, but my problem was with the rear bearing. When I had the symptoms you described, I couldn't really tell by hand (at the track) that the bearing was loose, only under track load it would vibrate and knock the caliper pistons.

I chased this problem for awhile until it got to the point that I had to bleed the brakes at the track. That day I noticed the fluid visibly burnt but only at one rear corner!!! (Cdn Tire Ford Fluid).

Changed just that bearing, problem solved. Wish I knew that beforespending money and time on brake ducting, pads, rotors, master cylinder, fluid etc.

My advice is to not change all the bearing unless you can afford it. But try your best to see what bearing has failed and then change the pair (front or back).

YMMV,
Patrick

[Slowpoke]

My advice is to not change all the bearing unless you can afford it. But try your best to see what bearing has failed and then change the pair (front or back).
Thanks Patrick.

Apparently it's usually the fronts on our cars... especially the ones that have been Autocrossed. BUT... based on your first message we're not going to ignore the rear. He's going to use a stethoscope while turning the rear. I told him to change it even if it's mildly questionable... especially if the fronts didn't show any wear when those are changed out. They have hubs on site for me, too.

Yeah... it won't be cheap... but brakes that work right are priceless.

[P.Po]

Whatever happens, please post the results!!

Good luck, I hope this works out for you very soon....

-Patrick

[Slowpoke]

BTW... I meant to mention that anytime I've flushed my RBF600 fluid I've never seen any signs of burning. No black carbon bits. No apparent discoloration to speak of.

I've also not seen any corrosion on my calipers... lots of brake dust build up yes, but not corrosion. Either way, I'll clean that out on Saturday.

I won't know for sure until after July 11th if it all really worked or not, but I'll definitely post results. Thanks for the suggestions, everyone! There's no BAD ideas during a brainstorming session.

[Logan]

Interesting how Ralf Schumacher withdrew from today's USGP due to "long pedal" which he described as a bad front wheel bearing.

[Slowpoke]

Brief update: Removed the calipers on the weekend for cleaning, as per Craig's suggestions. My slider inserts (ah! Now I know why you said plates) had some built up dust on them that took some fine emery cloth to remove... Pads dropped in like butter after that. Will it probably plug up again the moment the GS3's start dusting? Yup. But I'll make this an every spring routine in the future.

Still waiting for wheel bearing change...

[Tashko]

Yes, I'm bringing up an old thread, that's what the search button is for. :)

So, what was the root cause Stephen? You've left us hanging for three years!!

[Slowpoke]

It took a bearing upgrade to the 5x114.3 size bearing that Subaru went to on the 2005+ STi's. There's a company in the U.S. that retrofits 5x100 hub bolt spacing and '04 STi / almost all other 90+ Subaru strut hole spacing onto a set of '05 knuckles/hubs. You can even order extra camber if you want it. That avoids the need to buy new wheels and suspension.

And it costs about as much as one front STi bearing and hub change installed at each corner anyway.

I still have some issues with vibration with some pad compounds (e.g.: Hawk DTC 70) but it's not knockback related. Other compounds like PFC-97 don't have that issue. I just need some more cooling. And maybe some more aggressive Hawk pads.

[Tashko]

Ah, I thought yours was the good one to begin with. I'm having knock-back issues and figured I'd try do something about it over the winter.

Not many options I fear for my lowly Talon but maybe some good bearings are out there...

[Slowpoke]

If you have the same style bearings that we do (press on), clean out the factory grease then pack it with Amsoil Synthetic Racing Grease. With the better grease, you can torque them a bit higher (5% - 10%) than the factory spec. That seems to give a little more stability.

Next thing to do, I suppose, is keep your hub cool with ducting. There's some titanium spacers that are supposed to help:

http://forums.nasioc.com/forums/showthread.php?t=1856542