Cracked rear rotor - suspicions confirmed

[dquarasr2]

OK, so I replaced the cracked rear rotor. (Reference my post from last week on this topic: http://www.nsxprime.com/forums/showthread.php?t=95581)

The link below has pictures of my new rotor after having been bedded in, and a picture of the cracked rear rotor.

My suspicions about the brake pads exerting uneven force on the rotor seems to be confirmed. View the picture of the new rotor. Its contact area is being only half used. The half of the contact area nearest the hub is getting all the contact. The area closest to the outer edge of the rotor is going virtually untouched. So, when using it on the track the uneven heating and then cooling is cracking the rotor, I suspect.

The other picture is the cracked rotor. It is likely not a coincidence that the two cracks are approximately the width of the caliper apart. Probably being caused by the rotor cooling, and the area under the caliper retaining heat while the rest of the rotor cools.

http://www.nsxprime.com/photopost/showphoto.php?photo=30395&cat=500&ppuser=13387

Here's the kicker: the pads (Cobalt GT Sport) are not wearing more near the hub, as you would think based on the wear on the rotors, but are actually thicker near the hub! Now that is really confusing me.

Questions:
1. Does the collective concur that the cracking is being caused by the uneven pad contact?
2. What could be causing the uneven pad contact?
3. These pads do not have return springs. Does the NSX not use return springs on the rear pads?!?
4. Why would the pads be wearing unevenly, but the opposite what I would be expecting (if the rotor is being touched only near the hub, shouldn't the pads be thinner near the hub)?

So many questions!

Thanks for all the help in this and the last post!

 

[BioBanker]

I cant imagine that anything other then uneven pad contact could cause this; what's causing it is the bigger question.

Who changed the pads last? Did you properly retreat the piston by turning it?

Could there be debris between the pad and the piston?

What does the backside of the rotor look like, same wear pattern?

I saw a similar wear pattern back a decade ago when I skipped putting shims in my old GSR brakes. I got wear right under where the piston was and nowhere else. Compare where the wear is to where the piston hits the pad. If there is not adequate distribution across the pad, maybe you need shims? Other side same issue?

 

[dquarasr2]

I cant imagine that anything other then uneven pad contact could cause this; what's causing it is the bigger question.

Who changed the pads last? Did you properly retreat the piston by turning it?

Could there be debris between the pad and the piston?

What does the backside of the rotor look like, same wear pattern?

I saw a similar wear pattern back a decade ago when I skipped putting shims in my old GSR brakes. I got wear right under where the piston was and nowhere else. Compare where the wear is to where the piston hits the pad. If there is not adequate distribution across the pad, maybe you need shims? Other side same issue?

Thanks for sharing your thoughts.

The pads were installed when the calipers were rebuilt by a local Acura/Honda tuner.

I retracted the piston by turning it about 1/2 turn, enough for me to comfortably re-assemble.

There didn't seem to be any debris between the pad and the piston. Remember I'm dealing with this problem with both driver's and passenger side.

Interesting question about the backside. The inboard side seems to have normal contact but the inboard pad is wearing more unevenly than the outboard pad, even if the rotor contact appears to be across the entire inboard side.

Shims? (Looks around the garage and scratches head . . . ) These pads, as installed, have nothing between the pad backing plate and the caliper, as just about every other car I've had has had. It's just the caliper touching the pad backing metal. Nearly every other car I've ever owned (likely all of them if I remember correctly) have some sort of anti-rattle or clips between the piston/caliper and the pad Are there %*$%$# parts missing?!?

 

[BioBanker]

I think some types of pads require shims but most do not. Ive never used them on any NSX pad Ive used, but Ive not used the Cobalts. They would have come with them if they were needed.

I would still try to compare where the piston is compared to the wear to determine if the pressure is not getting distributed. Something is causing the pad to inadequately contact the rotor. My bet is that it is parts related instead of failure related given that this is happening on both sides.

 

[dquarasr2]

Thanks, BioBanker, I concur. Trying to figure out what to do next.

Oh, BTW, for anyone reading this, I truly believe this has nothing to do with Cobalt. Just wanted to state that for the record.

 

[2slow2speed]

Not really sure if this will help, but a while back I noticed that the 2 pins that are used to hold the rear caliper in place are different (the pins where the caliper bolts are driven to hold the caliper, the pins that act like a piston to keep the floating caliper and bracket floating when the brake piston is not engaged).

This is wrong:
The top one is called out as PIN A with part # 45235-SL0-003 the bottom one is called out as PIN B with part # 45262-SL0-003.

It should be:
The top one is called out as PIN B with part # 45262-SL0-003 the bottom one is called out as PIN A with part # 45235-SL0-003 .

There might be a method to the madness after all, given that the rotor will turn a particular way 99% of the time (forward motion of the car) so the amount of force that might be needed to keep the caliper floating might not be even between the top pin and the bottom pin.

At a minimum you might want to check and see if they have been inserted in the right order and see if that might be the cause of the uneven pad contact/wear pattern that is leading to the rotors cracking.

 

[2slow2speed]

One more thing to keep in mind, with the OEM brakes the e-brake line goes to the rear calipers, make sure that when the e-brake is released that the rear brakes are not dragging/engaging at all, during street driving that might not make a difference, but perhaps at the track that might be enough to be causing the problems that you are seeing when the rear rotors get hot and expand.

After the cool down lap, do you keep driving your car for a while or do you park it right away?

Back when I had the OEM setup, after the cool down lap I had to keep driving the car for several minutes to allow the rotors to cool down, at a track like Laguna Seca I could get the Carbotech Panther+ to smoke

With the OEM brakes I used to see rotors temps that were about 75~100 degrees hotter in the rear of the car (no ducts or air deflectors in the rear)

And one more question that I forgot to ask before, are the cracks always on the outside face of the rotor where the wheels mount and never on the inside?

The reason why I am asking this is that the wheel will act like a heat sink helping to dissipate heat away from the rotor, so the outside face where the wheels mount should have in theory lower temps than the inside. (I'm assuming that there is a net dissipation of heat away from the rotor going to the wheel, that seemed to be the case when I measured the temperature of the rims and the rotors after sessions)

 

[Andrie Hartanto]

About the uneven pad wear, there is a possibility you install the pads wrong. The pads have knubbies that suppose to slot into the pistons. Often time, inexperience people forgot about this or install the outer pads on the inside.

 

[john@johnhudakracing]

+1 on the nubs. Andrie's right. If you didn't come across it before it is easy with the Cobalt's to miss the fact that you can swap the inner and outer pad. I made that mistake once too, and Eddie set me straight. This has the effect of cocking the pad so that it wears the pad at a clear angle or simply wearing it down pre-maturely. I would check that.

As to the cracking, several of us were cracking front stock S2000 rotors with the Cobalt's Sports or VR's pretty consistantly this year, particularly toward the end of the day. Usually off-track, while the car was cooling down despite pushing it to prevent hot spotting sometimes you'd hear it pop. I drove back from Portland to Seattle with cracked rotors several times, but it was never any biggie at $60 a set.

 

[dquarasr2]

Not really sure if this will help, but a while back I noticed that the 2 pins that are used to hold the rear caliper in place are different (the pins where the caliper bolts are driven to hold the caliper, the pins that act like a piston to keep the floating caliper and bracket floating when the brake piston is not engaged).

The top one is called out as PIN A with part # 45235-SL0-003 the bottom one is called out as PIN B with part # 45262-SL0-003.

There might be a method to the madness after all, given that the rotor will turn a particular way 99% of the time (forward motion of the car) so the amount of force that might be needed to keep the caliper floating might not be even between the top pin and the bottom pin.

At a minimum you might want to check and see if they have been inserted in the right order and see if that might be the cause of the uneven pad contact/wear pattern that is leading to the rotors cracking.

They *are* different! Where the shanks expand out near the mounting screw, the shapes are ever-so-slightly different. The curve of the metal is different.

Also, one has three evenly-spaced flat areas, and one does not. And finally, one is black and one is gold.

Now, which one goes where?!?

Anyone remember? I won't say where I found each (they were consistent driver's and passenger sides) so as not to taint your memories.:tongue:

Seeing as I don't seem to have anything to lose at this point, I'm going to reassemble with the position of the pins reversed, go do a test drive, and see what comes of it.

2slow2speed, I always try to do a cooldown lap where I touch the brakes only minimally (while trying to stay out of the way of the faster traffic) and when I come all the way off the track I'll idle around the paddock area 20-25 MPH (or whatever the speed limit for the paddock is) 5 minutes or so, then I'll park it and of course, never use the handbrake.

Got the nubbies right, thanks guys.

 

[nsxtasy]

It sounds like you may have identified the source of the problem.

Just to confirm what it's not - it's not the lack of shims. I've used lots of aftermarket pads, including the Cobalt GT Sport, without shims and without any problems as a result. And the GT Sports are not susceptible to any particular problems (I've gone through several sets on two different cars on both street and track).

 

[dquarasr2]

It sounds like you may have identified the source of the problem.

Just to confirm what it's not - it's not the lack of shims. I've used lots of aftermarket pads, including the Cobalt GT Sport, without shims and without any problems as a result. And the GT Sports are not susceptible to any particular problems (I've gone through several sets on two different cars on both street and track).

I wish I could say this was it. I swapped the pins top-to-bottom and there was no difference. In fact, now I get lockup of the left front with less pedal pressure than before, so I think I need to reverse them again.

BTW, the gold pin was on the bottom when I initially disassembled. I currently have it in the top, but it didn't seem to fix anything and in fact seems to have made it worse.:frown:

I've looked at this and looked at it and then looked at it some more. Can't figure out what's going on.

I think the only next step is to get a new, fresh set of pads (that don't have the angle bias in them), and have the calipers rebuilt again.

 

[2slow2speed]

I wish I could say this was it. I swapped the pins top-to-bottom and there was no difference. In fact, now I get lockup of the left front with less pedal pressure than before, so I think I need to reverse them again.

BTW, the gold pin was on the bottom when I initially disassembled. I currently have it in the top, but it didn't seem to fix anything and in fact seems to have made it worse.:frown:

I've looked at this and looked at it and then looked at it some more. Can't figure out what's going on.

I think the only next step is to get a new, fresh set of pads (that don't have the angle bias in them), and have the calipers rebuilt again.

BTW: In my earlier post I had the order of the pins wrong, the top pin is called out as PIN B and the bottom one is called out as PIN A, sorry for the brain fade :redface:

The way that I always had them installed on the caliper brackets was the gold one on top (PIN B) and the black one with the ribs on the bottom (PIN A).

I double checked the online parts manual at Niello and it seems to be correct, it's the same exact order for the front caliper bracket as well gold pin on top and the black pin on the bottom of the bracket.

You can check that yourself at Niello Acura's parts department.

http://acura.niello.com/jump_parts.html?RefShoppingType=P&Refid=DW&RefDealer=251357

Take a look at the chassis/brakes/rear brakes zoomed view and you will see the difference in the pins.

Not sure what is causing your lockup problem though, with ABS you should not be experiencing a lockup unless somehow you inadvertendly disconnected the sensor that is located near the hub assembly.

 

[dquarasr2]

BTW: In my earlier post I had the order of the pins wrong, the top pin is called out as PIN B and the bottom one is called out as PIN A, sorry for the brain fade :redface:

The way that I always had them installed on the caliper brackets was the gold one on top (PIN B) and the black one with the ribs on the bottom (PIN A).

I double checked the online parts manual at Niello and it seems to be correct, it's the same exact order for the front caliper bracket as well gold pin on top and the black pin on the bottom of the bracket.

You can check that yourself at Niello Acura's parts department.

http://acura.niello.com/jump_parts.html?RefShoppingType=P&Refid=DW&RefDealer=251357

Take a look at the chassis/brakes/rear brakes zoomed view and you will see the difference in the pins.

Not sure what is causing your lockup problem though, with ABS you should not be experiencing a lockup unless somehow you inadvertendly disconnected the sensor that is located near the hub assembly.

One more clarification, if you please. On my car the gold pin is the one with the ribs. In the picture on niello acura, it shows the ribbed one is on the bottom, which is the way it was when I first disassembled: gold, ribbed on the bottom, black plain on the top. Time to go back out to the garage and change it back. :redface:

When I mention lockup, it's invoking the ABS from what sounds like the front. It's locking up momentarily until the ABS takes care of it. Sorry if I wasn't clear about that.

Thanks so much for all your replies.

 

[titaniumdave]

I think the only next step is to get a new, fresh set of pads (that don't have the angle bias in them), and have the calipers rebuilt again.

Next step for me would be to put new rotors on, take the old pads to the belt sander and flatten them, put them all back together the correct way and drive it.

Caliper re-build is a waste of money if the dust boots are not melted, or seals leaking.