Re: Ceramic Brake Pads - '99 T & C

Follow up to an earlier post.

Well I ordered some Akebono ProAct ceramic pads for my '99 and I installed them today. What a difference! The growling and minor pulsing has gone. the braking action is smooth and uniform at all speeds. I hope it also reduces the dusting that was occuring with those junk pads that were on there.

Frank

Reply to
Frank Boettcher
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I had to have the rotors and pads replaced at 27K miles. Is this the norm? They did pulsate but at this many miles? Is it because I didn't use the car for extended periods of time? I put 27K miles in 4 years.

Ken

Reply to
NJ Vike

Same need on a '98 Grand Caravan -- at five years and 38,000 miles, along with front and rear struts/shocks, starter and serpentine belt tensioner.

Not until all the OEM failures were replaced did the van work reliably.

NJ Vike wrote:

Reply to
News

That's sad. I'm starting to wonder about keeping my existing car and purchasing a new 300C. Perhaps it's time to look elsewhere?

Reply to
NJ Vike

One thing is that the lug nuts have to be torqued to spec, which is really tight- for mine, 100 ft/lb. The wheels clamp the rotors, and if the lug nuts aren't tight enough, the rotors warp.

This applies generally to vehicles with disc brakes, it's not make or model specific. I got this bit of wisdom from a pro.

-Paul

Reply to
carbide

Interesting. The conventional wisdom for years was just the opposite - that overtightening was the cause of rotor "warping." However, I've read an article or two more recently that suggests that the issue isn't warping of the rotor (which can easily be verified with a dial indicator), but rather uneven accumulation of pad material around the disk and/or rust patches. This causes the coefficient of friction to vary around the circumference of the disk causing the pulsing during brake application. I'm becoming more and more convinced that this is the real cause more often then not.

The reason I think this is that the cure that folks who claim this suggest seems to work and it certainly wouldn't fix a truly warped rotor. The cure is several successive hard stops as I recall, somewhat similar to breaking in new pads. This heats up the disk and redistributes the pad residue and cleans off any uneven rust. I've done this a couple of times on my minivans with good success.

Matt

Reply to
Matt Whiting

You don't hear the "undertightened" theory too much. I suppose you could argue that undertightening will mean less intimate contact between rotor and hub so that the rotor heat won't be able to get as much cooling by conduction into the hub - and that in fact could be a contributing factor. That's why there is a *correct* torque value - not just a max. or min.

I've come to that same conclusion and have posted about it several times here and elsewhere. See my second post from just two days ago in this thread:

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However, there are some pads that are so bad that the "several succesive hard stops" trick just causes thicker uneven layers to deposit - I had that happen last year with a set of very gummy ceramic pads.

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

Reply to
Bill Putney

Unless the lug nuts were loose enough to let the wheel wobble, I don't think heat transfer will be much affected. And I'm pretty sure that the fastener torque value has everything to do with ensuring the parts remain fastened and nothing to do with heat transfer. You want the studs stretched enough to keep the lug nuts from coming loose and to ensure that the wheel stays tight against the hub under expected loads, but not stretched so far that the normal loads will cause the stud to exceed is elastic limit.

Every rule has its exception! :-)

Matt

Reply to
Matt Whiting

Yes, I believe your analysis is correct. There's no rust or anything but shiny metal on the friction areas of my rotors, so foreign material on there is not the cause.

In my case, my brakes worked smoothly, then I removed my front wheels and just used my impact wrench to reinstall the lugnuts. I remember thinking "man, these lugnuts are tight" when I took them off. I had to use a breaker bar to get them loose. I torque things like head bolts, but have never bothered to torque lug nuts.

Soon after, my brake pedal started pulsating. Same rotors, same brake pads. So I'm convinced the pro who explained the cause to me was correct. There's a reason why that torque spec is so high.

So try it folks- torque the lug nuts to spec. It's the right thing to do, and it's an easy and cheap solution if it works. Then get back to us and tell us if it worked.

-Paul

Reply to
carbide

Umm - you used an impact wrench to tighten them, and then you had the pulsating? Your problem was *not* that they weren't tight enough - they were probably *too* tight (as further evidenced by your having to use a breaker bar to get them off the next time).

(Your post is a little confusing - if you're saying that the first time you took them off, they were extremely tight, then whoever had put your wheels on the previous time had overtightened them - probably with an impact - if it was a shop that did that, find another shop to work on your car.)

I'm confused. The pro (golfer? wrestler?) told you they were too loose, yet you had tightened them with an impact wrench, after which you had pulsation, and had to use a breaker bar to get them off?

I'll go along with that.

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

Reply to
Bill Putney

Yeah, my post was confusing. I'll try again. I went to change my struts, and the lug nuts were so tight I had to use a breaker bar to break them loose. My wimpy impact wrench wouldn't loosen them. I put the lug nuts back on with my wimpy impact wrench. I didn't measure the torque before or after, but I'm guessing they were to spec before, and probably half of spec after. Spec is 100 ft/lb, which is a lot.

After that, my brakes started pulsating. Prior to that, they had been silky smooth. A professional mechanic (pro) explained to me that this was a common problem with many makes and models. If the lug nuts are not tightened to spec the rotors warp. And I believe he meant undertorqued. It hadn't occurred to me that this could be the cause of my problem, but it made some sense to me.

That's kind of the lesson I took from it. Here's some more info from Wikipedia on rotors warping. This discussion prompted me to do a search. OK, that's my 2 cents worth. ;-)

-Paul

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Warping is primarily caused by excessive heat, which softens the metal and allows it to be reshaped. The main causes of overheating are: undersized/overmachined brake discs, excessive braking (racing, descending hills/mountains), "riding" the brakes, or a "stuck" brake pad (pad touches disc at all times).

Another cause of warping is when the disc is overheated and the vehicle is stopped. When keeping the brakes applied, the area where the pads contact the disc will cause uneven cooling and lead to warping.

Warping can also be caused by improperly torquing the lug nuts when putting on a wheel. When tightening the lugs nuts, it is best to follow the directions included in your manual. A general rule of thumb is to skip every other lug nut in a clockwise or counter-clockwise direction. Also, tightening to final torque in two or three stages helps to balance the stress and avoid warping the disc.

Warping will often lead to a thickness variation of the disc. If it has runout, a thin spot will develop by the repetitive contact of the pad against the high spot as the disc turns. When the thin section of the disc passes under the pads, the pads move together and the brake pedal will drop slightly. When the thicker section of the disc passes between the pads, the pads will move apart and the brake pedal will raise slightly. This change causes pedal pulsation. The thickness variation can be felt by the driver when it is approximately 0.007 inch or greater.

Not all pedal pulsation is due to warped discs, however. One reason is brake pad material operating outside of its designed temperature range and it has left a thicker than normal deposit in one area of the disc surface, creating a "sticky" spot that will grab with every revolution of the disc. Grease or other foreign materials (usually deposited on the disc during wheel maintenance) can likewise create a slippery spot on the disc, also creating the sensation of a grab or warped brake disc.

Reply to
carbide

Actually, 100 ft-lb isn't much at all. I can do this easily with a 1/2" drive ratchet. I use a torque wrench on my lug nuts as it is just too easy to overtighten them. Lug nuts that are properly torqued will generally be quite easy to remove and won't snap like they do when tightened to 250 lb-ft by the garage monkey with the 1/2" drive impact wrench.

I'll bet you that he was talking about overtightening, not undertightening.

Wikipedia is a very lousy source of information on many topics, and this is one of them. Most (except the very end) is posted below indeed was conventional wisdom for many years, but more recently a number of folks have measured "warped" rotors with a dial indicator and found that most pulsating rotors aren't warped at all.

Except that it is very hard to get the metal hot enough to be reshaped. The metal is well into the red hot range before this could happen, and I've never seen brakes glowing red hot on a passenger vehicle, only on race cars.

I believe this situation does cause pulsating brakes, but not for the reason given. I believe the real reason is that keeping pads in contact with a hot disk causes pad material to be essentially bonded to the rotor surface causing uneven friction. So, it is definitely a good idea to not hold your brakes firmly after coming to a hard stop. Let the car slowly inch ahead as you wait for the light.

I believe this is definitely a good practice, but I'm skeptical that it makes a huge difference unless you are using an impact wrench and torquing one nut at a time to twice the recommended torque value. I tighten my lug nuts using a 1/2" ratchet in the pattern recommended above and usually get to 80-90 lb-ft in two stages. I then use my torque wrench to add the remaining 5-25 ft-lbs to bring them to spec.

And very rarely do pulsating disks have runout or thickness variation even close to 7 thou. Ask your mechanic to put a dial indicator on your rotors next time. I'll bet you don't see more than a couple thou variation.

I believe this is the culprit 95% of the time. And a few really hard stops will correct this.

Matt

Reply to
Matt Whiting

But only if you keep your foot off the brakes until the rotors have cooled down a bit. If not you will transfer excess pad material onto the rotors in the spot where the pads were held down on the hot rotors.

Richard.

Reply to
Richard

Sure, the same holds true when breaking in new pads. Come to a hard stop or nearly stopped then let off the brakes right away and accelerate back to speed and repeat 5 or so times.

Matt

Reply to
Matt Whiting

Actually, it applies to cars where the rotor is retained by the lug nuts (like front-drive cars) much more than to cars where the lug studs are pressed into the rotor and the wheel bearing is integral to the rotor (front disks on rear-drive cars).

Reply to
Steve

What *I* want to know is if "deposits on the rotor" are to blame, why all this fooling around with disk brakes and pedal pulsation has *never* been and is STILL never a problem on my '60s and '70s rear-drive cars with big massive brake rotors, and is an ongoing problem with 90s' and

2000's front-drive cars with dinky little rotors.

I don't buy the uneven deposits theory too much. Its undersized rotors that get too hot. Now whether that causes true "warping" or somehow aggravates a mystery deposit process, I can't say for sure.

Reply to
Steve

I don't doubt that heat is a key ingredient in the process of transferring material from pad to disk. Also, the pad material may be different for the older model cars. They used to use asbestos, but I'm sure that isn't the case now, but it may be that the larger, older brakes can get away with a material that doesn't cause the problems that the material used in newer, smaller brakes uses. I don't know.

Matt

Reply to
Matt Whiting

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