The reason for the poor m.p.g. on my PT

Untill you get into a vehicle such as the Caravans or Town and Country vehicles with ABS Brakes. Crack a line and get air into the Hydraulic unit and now you need a scan tool to perform a bleed test of the Hydraulic unit

Me personnally the Japanese Metal is junk.

Keep in mind that the piston does not slide in the bore of the caliper, it actually slides on the seal ring inside the caliper. rust pitts wont hurt the caliper unless it is at the seal. Also you have to be careful pumping the brake pedal past its normal travel when trying to bleed the brakes, thats a good way to tear or damage the seals inside the master cylinder. If the piston extends past the normal travel in the bore the corosion/buildup will damage it

See below re: United Brake Parts rotors.

Nope - Hate to break it to you, but United Brake Parts is NAPA's in-house brand - they are private-labeled for NAPA by - wait for it - RAYBESTOS (parent company: Dana) - made in Canada IIRC. You did find those in a NAPA store, right? 'Cause that's the only place they are sold in a United Brake Parts box.

Mmmm - cheaper I would agree with. Way cheaper? That's arguable. It's common to pay $60 per axle ($30/wheel) for an upper-mid grade of pads.

Rock Auto Parts

Drilled down for calipers for my '99 Concorde. If I counted right, 28 permutations (left/right, front/rear, various manufacturers, different grades, some with pads included) of part numbers.

Examples in their list (all prices are netted out after core returned and core price refunded): Raybestos right front - $32 Raybestos right front "Professional Grade" *loaded* *with* *pads* - $52 Raybestos right front "Professional Grade" *loaded* *with* "Super Premium" ceramic pads - $62

Similar results on

BTW - I just checked calipers for your Corsica at napaonline.com and Rock Auto: Prices ranged from $17 to $37 w/o pads on both sites. Rock Auto - Raybestos Prfessional Grade *with* *pads*: $38!

Next question! :)

Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my address with the letter 'x')

True, however if you maintain your brakes, change the fluid, don't let them get ground down to the steel, why then would this happen? Actually both my T&C's are ABS vehicles and it is true about that, but if you don't do something stupid and the bottom of the vehicle isn't all corroded out due to salt on the road or some such, then you won't get air into the lines.

And, even if you do - well then, do all the parts replacements, fluid flushing and such, and take it to a dealer to get it bled. You still will save a bundle.

Yup.

Well, once again we are talking maintainence here. If you change the brake fluid every once in a while, water isn't going to corrode up the inside of the MC. And if the MC is corroded to the point that it will only hold a seal during a very small amount of travel, well what happens during that panic stop when you jam down on the brakes, and push the cylinder into the corroded zone?

If the MC cannot survive a regular bleed cycle, it's dangerous to have on the vehicle IMHO. Better to find that out when the car is in the garage, I think.

One other thing too I'll say about bleeding, if you instruct your assistant to push gently on the pedal, you can control how far down the pedal travels by how much you open the bleed screw, and it doesen't take much pressure at all to bleed them. I've personally tried a number of the brake bleeding tools, like the one way valves, the vacuum bell jars and such, and nothing works as fast or as well as an assistant on the brake pedal. That's where it helps to have kids - wives are rather unpredictable doing this job, they tend to get impatient and take it out on the brake pedal.

Ted

You mean the one where you stomp on the pedal 50 times in a row or some such? I think that only worked on the Bendix 10 systems that were pieces of crap. The bendix 4 systems you have to use a scantool, and the later teeves ones, I don't remember.

Ted

The factory Service manual does state that the DRB has to be used to bleed the brakes with that set up. You are right as far as the Bendix 10 system, since there is no program to bleed them with a scan tool use a scan tool to actuate the hydraulic valves as you are pushing on the pedal and it makes it alot easier

Glenn

Actually I put that in there to yank your chain, Bill. I guessed you might read that. :-)

What was that you were saying last year about supercooling in liquid nitrogen?

Most of the auto rotors I've seen are machined castings.. I very much doubt that the machining on the casting is any worse than the wear the rotor is going to get in actual operation so I'd dismiss that. It's more likely you could have casting problems, such as voids in the material, molding problems, or the metal may not be of the same composition throughout the casting itself or might not have been cooled evenly. But, the question really is how much accuracy is really needed here? As soon as you use the rotor your going to be fatieguing it.

That is a good point.

Ah, Bill, but your missing some things. When braking a vehicle not all of the energy is being absorbed by the friction components. Some of it is going into the frame of the vehicle and stresses on the suspension and tires.

A panic stop is going to put a very high amount of kinetic energy into the suspension and frame and tires. The frame, in fact, will compress much like a spring, as the energy is dumped into it, as will the suspension.

You can see this very easily by doing a very hard stop at a stoplight, you will feel the car bounce back after it stops, as the springiness discharges the stored kinetic energy to move the car backwards. An oscillation will occur here, which the frame and suspension is designed to very quickly damp out, of course. But, the point is that this energy will not be discharged into the friction components.

By contrast, a long slow stop won't slam the car frame back and forth, because much more of the kinetic energy is being discharged into the friction components. So the brakes will get hotter.

Now, think about the difference between a hard and a soft pad. A hard pad when pressed against the rotor is not going to deform on a microscopic level as much as a soft pad, with the same amount of braking force. So there will be less friction between the hard pad and the soft pad for a given braking force. Less friction means that the rotor is going to spin more during the braking decelleration process, and the frame and suspension will not be called on to absorb as much braking energy. Thus the longer spinning means that the pad is going to get hotter, since it's absorbing more braking force.

But there is the rub, braking isn't just the pads and rotors. You can get the same decelleration with 2 vehicles at a different rate. One drive may brake harder, earlier, then let off later on. Another may brake lightly, but constantly. The decelleration, measured over the same stopping distance, is the same, but the brakes are taking up more kinetic energy with the second driver, the frame is taking up more kineic energy with the first driver.

And, even there I also suspect your assumption about given braking force is wrong too. I think people tend to only want to press on the brakes at a certain foot pressure, and they learn with bigger vehicles to start the braking earlier, and with lighter vehicles to brake later on.

Did you ever see the Lucille Ball/Desi Arnaz movie "The long long trailer" If you never have, watch it sometime it is a scream. There's a scene in it when they just got the trailer that Desi brakes at a traffic light - and ends up in the middle of the intersection with the trailer perfectly at the stop line, and everyone looking at him like what an idiot.

It doesen't have to be a big difference between the soft pad and the hard pad. I think we all can agree that material of both the rotor and pad will wear off faster, the hotter the materials are, since the molecular bonds are weaker the hotter the material gets.

That's why if I descend a long hill here in my 68 Torino, if I let it coast then press hard on the brakes to slow it down, periodically on the way down, rather than just gently riding the brakes all the way down like the rest of the idiots on the road do, that at the bottom of the hill my brakes haven't faded yet. Before I learned to drive I did ride the brakes all the way down once or twice and with a 4 wheel drum system, when it fades, it's just like trying to brake on solid ice, and it definitely faded!

If the hard pads only get 10 degrees hotter then the soft pads, I think that's enough of a difference to affect wear rates.

If a DIY'ers time is that valuable, it would be cheaper for them to take the vehicle to the garage and pay whatever they ask to fix it. Otherwise I think the DIYer is putting on airs.

Ted

I suspected as much. Good thing I didn't over-react. (you bastard!) :)

It changes the molecular structure - has to do with grain structure, precipitation of carbide particles. At the end of the process, the metal structure is extrremely stable - no distortion due to heat because of the extreme consistency throughout the part. The process is not just a mere brief temperature change - it involves soaking at the super-cooled temperatures for many, many hours - the change in the grain structure doesn't happen immediately. It's grain growth - but in reverse. Take a course in materials science, specifically metals and alloys, at your local university.

That's not an accurate use of the word "fatique" when talking about materials. The point is that the best, most stable and warp-resistant rotor will be the one with the most uniform and stable composition and structure throughout, all stresses relieved. You allude to much of that in that previous paragraph, yet you seem not to grasp their importance. How much accuracy is really needed here you ask? How much runout is needed to cause pulsation - maybe 8 or 10 thousandths. Whatever that number is, the amount of thickness variation that it takes for the same affect is about 1/5 or less of that number - so we're down to 2 thousandths of an inch or less of thickness variation.

It almost sounds like you're now trying to say that warped rotors don't cause brake puslation.

But, for a given deceleration curve of a given vehilce, those factors are equal. That allows the discussion to focus on what happens at the friction components themselves, which is what you started out doing.

True. But you're just muddying the waters with that. Let's keep those equal and say that the driver is going to apply whatever pressure it takes for a given deceleration "schedule" if you will - I think that is a valid assumption once a driver adjusts to a new feel. For example - after the initial adjustment, the guy's wife is going to perceive the same driving cahracteristics of her husband after the more aggressive brake pad is installed - if she thought he was a smooth, slow driver before, she will continue to think that - if she thought he was rough, she will continue to perceive it exactly the same way - he will change how he uses the pedal to give the same effect. It's not like he's going to be throwing her thru the windshield and telling her that he can't help it - these new brakes are more powerful. No. He's going to adjust his pedal modulation to give the same effect as with the old crappy brakes.

Moot point for the experiment as I would set it up as explained above. And I think my assumptions are valid.

Unrelated to the discussion.

Back to my premise that under a given driving condition/slowing/stopping situation with the same driver, same car, the driver will have adjusted his use of the brakes to give the same effect. So - given that - the rotor will not spin any more.

We were previously talking about changing the pad characteristics - not changing the agressiveness of the driver in how quickly he stops the same car in a given situation.

Except we're not talking about variations due to the driver - we're talking about given pad/rotor combinations.

You're screwing with me I know.

yadda yadda yadda

We're talking about the same vehicle - pick one and stick with it for the discussion of comparing pads and rotors.

Yes - we're both old. :)

OK - no problem with that. *BUT* I disagree that there is more total heat generated in the brake components under the same deceleration schedule on the same vehicle with different pads - the law of conservation of energy says I'm right. I'm willing to give you that, though the total heat generated in the pad/rotor combination is the same (for a given amount of vehicle energy burned off at the same rate), the splitting of that quantity of heat between pad and rotor will be different. You've helped me see that as a possibility.

You're right about that. I understand that they now teach students to ride the brakes on a descent rather than pump them. I guess it's the results of having to teach them to hold the pedal down in a panic stop with ABS brakes, so they misapply and extend that to speed control on a descent.

I don't understand your conclusion that the harder pads will get hotter under the same slowing or stopping conditions. On the heat split between pad and rotor that I mentioned above, for the same total heat in the pad/rotor combination (according to the law of the conservation of energy), I would think the rotor would get hotter and the pad cooler with the harder pad, but that's a little bit of guessing on my part. But the total heat shared between pad and rotor is, by definition, the same.

Well - you know yourself that people have different availability of time and money at differetn times in their lives, or even from one week to the next. Yeah - if money is really realy tight, and I have free time on my hands, I would do the rebuild - been many years since that was the situation. But if I have even a little money to throw at it, and a new caliper costs $35, and I'm moderatly but not extremely busy, then I replace the caliper. If I have money but absolutely no spare time - yeah - then it goes to the shop - except not if the caliper is $250!

So there's no right answer. The variables that go into the decision are availability of time, availability of money, price of parts, paranoia about shops working on one's vehicle, desire to DIY for relaxation even if it costs more (then you call it a hobby and your wife has to accept it).

Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my address with the letter 'x')

Well, there are 2 problems with that argument:

1. The difference in amount of energy stored in the frame is insignificant when braking from a reasonable speed. How much energy is absorbed by the frame? If one releases the brakes at the optimum time, on some cars, one can get the car to reverse after stopping. But how fast? Maybe 5mph. Some energy is absorbed by the shock absorbers, but again, I think this is minimal.

We should remember that kinetic energy is calculated as 1/2 M V^2. In other words the total energy to be absorbed during braking is highly dependent upon the starting speed. So, when braking from 10mph, the amount stored in the frame may be significant, but when braking from 60 mph it will be insignificant. I believe the amount of energy absorbed by the frame is limited, irrespective of the starting speed (unless the starting speed is very low).

1. When braking at a higher rate, the amount of time to come to a halt is less. This means that the amount of time for the rotors, pads, etc. to transfer the heat generated when braking into the atmosphere is less. Hence the rotors will get hotter.

And item 3:

When stopping gradually, normal drag (tires, engine, wind, etc.) all help to reduce kinetic energy (and help to reduce heating of the rotors and pads).

All true, except, again, they all get negated when the driver, after a short adjustment period, modifies his use of pedal pressure to give essentially the same deceleration curve in a given situation to compensate for differences in pad characteristics. By "negate", I do not mean the effects of the things mentioned are not there - I mean they are essentially identical for the different pad scenarios and can be ignored for comparison of heat, wear, whatever effects on the pads and rotors.

Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my address with the letter 'x')

Bill,

I don't think we are in disagreement here. To brake from any speed to another speed requires the same amount of kinetic energy to be converted into other forms of energy. If the decelleration profile is the same, then the amount of energy that is converted into heat must be the same, irrespective of the pad/rotor combination.

I also agree with you that dirvers will change the amount of pressure applied to the pedal to get a similar decelleration profile if pads/rotors are changed. In fact, I just put on new Akebono pads and Brembo rotors and found a much reduced pedal pressure is required -- does this mean that I should expect to replace the pads sooner?

If you get air into the Hydraulic assembly you do need to use a scan tool to perform the ABS brake bleed. Im not talking about cracking a bleeding on a caliper.

I'm not sure if higher coefficient of friction inherently means faster pad wear.

Are these the Akebono ProACT's? Several people tried them and liked them on the 300M Club, but mine developed serious pulsation - I suspect a filming issue rather than rotor warp, but never did investigate that. I'd be interested to see how yours do over the long haul.

Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my address with the letter 'x')

They are the ProACTs. Filming has been an issue with just about every set of pads on this vehicle, although I did not notice it much with the last set. The original, factory-fitted pads had an uneven filming problem after about 25-30k miles. I have rented a similar model and it also had the same problem. It's a '97 Grand Voyager. I assume that it is a filming problem because the problem comes and goes -- and I doubt that rotors would un-warp themselves.

Well, there is one advantage these pads have over others: since the pad material is bonded, not riveted, there is slightly more material to use.

Would the factory pads have been ceramic?

Exactly. Finally someone gets it! (although it may be that some rotors could warp when hot and unwarp when they cool down due to uneven stresses - not sure if that's possible or likely, but it's the only way it could go thru cycles of warp and unwarp)

I think that's pretty much true on most vehicles these days - at least the ones I've seen in my DIY'ing.

Not sure - I don't own a Cruiser.

If you have filming problems, see if Performance Friction "Z-Rated" pads are available for the Voyager - regional chains like Advance carry PF brand, but generally not the "Z-Rate" - try

I have been using those for over a year now on my Concorde, and the brakes are as smooth with those for longer than they have been with any other pads I tried. I also got good service out of Hawk HPS pads - they would be my next choice if the Perf. Frictions weren't available. Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my address with the letter 'x')

Oops - meant Voyager (got thrown by original topic). But it's true - I don't own a Cruiser either. :)

Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my address with the letter 'x')