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Changing disc brakes - how can you tell if you need new rotors?


Joe789
07-07-2008, 10:04 AM
Changing disc brakes for the first time - 98 Olds Intrigue.

Brakes are a squeaking and it is time for their annual replacement.

Money is tight and I would rather not buy more than I need.

So, how can I tell if rotors will last?

Also, front brakes are squeaking, how can I tell if rears should also be replaced?

MagicRat
07-07-2008, 10:36 AM
New rotors are required when the old ones are too thin and/or rusty (inside where the vents are). A bit of rust is normal; heavy flaking means it's time for new ones).

You need a micrometer or decent machinist's calipers to accurately measure the rotor thickness. The minimum thickness rating is usually stamped on the rotor itself.

Every new set of pads MUST also have either new rotors or machined rotors. I would suggest removing the old rotors and taking them to a local mechanic and ask if they can be machined. They can measure them and tell you if they are too thin or rusty or if they can be machined. Around here the cost for machining is about $20 each.

You may find new rotors are so cheap that machining does not make much sense. For example, my '97 DeVille's rotors cost me $26 new. Therefore, it did not make sense to drive out of my way to get the old ones machined for $20. The six bucks was not worth the time and effort just to reuse old rotors.

As for the rear brakes, pull off the wheels and have a look at the rear pad thickness. If the lining is less than half the thickness of new brake lining, its a good idea to do them.

shorod
07-07-2008, 12:41 PM
The O'Reilly Auto Parts chain around here will machine rotors and drums. Last time I had it done it was $11 per rotor. But, yes, for very common cars, new rotors can cost just a little more than the price to machine them. You need to be careful with new rotors though. If they are not stocked properly, they can be warped out of the box.

-Rod

curtis73
07-07-2008, 12:53 PM
The machinist doing the work will measure them before cutting and advise you one if it can be done or if you need new ones.

I am not of the school of thought that new pads require fresh machining or new rotors. As long as you don't have a pulsation or other big problems, new pads cut rotors pretty hard, especially if you properly bed them in.

MagicRat
07-07-2008, 02:56 PM
I am not of the school of thought that new pads require fresh machining or new rotors. As long as you don't have a pulsation or other big problems, new pads cut rotors pretty hard, especially if you properly bed them in.
IMHO you can get away without machining but often the old rotor surface is not consistent from one rotor to the next. Machined or new rotors provide better balanced brake performance and longer lasting pads.
IMHO using the old rotor surface simply is a bad brake job. :)
I would only consider it in either an emergency or if the car I was repairing was so rare that new rotors were difficult to find (and I did not want to make the old rotors thinner by machining them.)

Scrapper
07-07-2008, 03:11 PM
squking pads might be loose in there or built up brake dust...and also like they say it's really cheaper to buy new rotors and i'd put O. E. pads on it they'll last longer...good luck...

Joe789
07-07-2008, 07:49 PM
Thank you all for responding.

Any recommendations on brands to use or those to stay away from?

Anybody know a good site for directions on how to change rotors and pads? :biggrin:

vgames33
07-07-2008, 10:11 PM
http://www.oplin.org/databases/proxy2/proxy.php?qurl=http%3a%2f%2farrc.epnet.com%2fautoa sp%2findex.asp%3fsid%3d151455211%26uid%3dcincy.mai n.autorefctr

The card number is 123456789 Go to the Cincinatti page. There should be an automotive section somewhere. It loads two different ways for me, depending on how it feels, so you may have to look around a bit to find it.

bobss396
07-08-2008, 08:33 AM
I used to believe in cutting rotors with every brake job. On today's cars, I sometimes don't bother since the new pad materials are so aggressive and don't need the break in period that the old pads required. Also anything you take off the rotor decreases the ability to absorb heat. Many rotors will withstand one cut and that's it, after that they're too thin to cut again.

if you want to renew the surface using a DIY method, sand each side of the rotors with an orbital sander and some 60 grit paper. Scuff it up evenly and don't let it dig in. I've done this many times and it never fails me.

For pads, especially on the GM FWD cars, use a good brand name pad and not the cheapest one the store carries. Carbon Friction Metallic is a good brand, Bendix also has a couple of good grades of pads.

Bob

curtis73
07-08-2008, 11:34 AM
This page seems to be hated by so many people, but I list it anyway. I had the opportunity to inspect several rotors with a scanning electron microscope when I had access to the college labs, and its probably the most accurate information I've found on the internet.

The old-schoolers HATE this site with a passion calling it new-age fluff, but I back it. Take it or leave it.

http://www.stoptech.com/whitepapers/warped_rotors_myth.htm (http://www.stoptech.com/whitepapers/warped_rotors_myth.htm)

MagicRat
07-08-2008, 12:43 PM
The old-schoolers HATE this site with a passion calling it new-age fluff, but I back it. Take it or leave it.


I think old schoolers hate the page because many of them (us??) have machined pulsing rotors and seen the warp. In my experience, as the rotor goes around and around in the lathe, severely warped rotors sometimes leave 'low' spots that show up due to the warp, or have an unusual pulsing sound as the cutting tips go through a high spot.

Now I am not certain if the 'warped' or distorted rotor actually causes a pulsing pedal, but sometimes, I have come across a pulse or vibration so severe, with such shaking of the brake pedal that I do not believe a change in friction coefficients is responsible. It must be a physical high spot on the rotor that is quickly forcing the caliper piston in and out of the bore to produce the vibration I feel occasionally through the pedal.

I and the old timers may be wrong, but to me it seems the most plausable explanmation for many of the pulsing brake problems. IMHO that site describes the cause many, but not all cases of pulsing brakes. :smile:

bobss396
07-09-2008, 10:06 AM
In one shop we had, part of our selling program was to SHOW the customers that their pulsating pedal was from badly warped rotors.

We had this clip on dial indicator that we could set up in seconds and show them if they were in doubt. It was something that Ammco (the brake lathe people) was giving away as a promotion.

Bob

curtis73
07-09-2008, 02:31 PM
I know they warp, I just don't believe that warped rotors cause the pulsation any more than paint color affects ignition timing.

Its easy to jump to the conclusion when you have a pulsating pedal and see runout, but to assume that its the warping that causes it is not a logical step given the knowledge of the metalurgy in the disc.

When you machine rotors, not only are you removing the runout, but you're also removing the inclusions in the iron that cause the pulsation; further reinforcing the mistaken idea that warped rotors caused the pulsation in the first place.

I've had rotors with no runout exhibit pulsation, and I've had rotors with significant runout that had no pulsation.

In all situations, resurfacing the rotors was the answer, but I strongly suggest finding someone with a blanchard grinder to put an OEM type finish on it. The lathe leaves pretty severe grooves like a record. The peaks get very hot under braking and can accellerate pad deposition, which means you'll be back again soon for more machining.

I also strongly suggest against any emery cloth or sandpaper. That just drives silicates into the porous casting and makes the problem more likely to come back.

MagicRat
07-09-2008, 10:32 PM
This page seems to be hated by so many people, but I list it anyway. I had the opportunity to inspect several rotors with a scanning electron microscope when I had access to the college labs, and its probably the most accurate information I've found on the internet.

The old-schoolers HATE this site with a passion calling it new-age fluff, but I back it. Take it or leave it.

http://www.stoptech.com/whitepapers/warped_rotors_myth.htm (http://www.stoptech.com/whitepapers/warped_rotors_myth.htm)
Awww come on Curtis....:smile:

By posting this you knew you would start a fight!! :evillol:

This is like a Monty Python sketch ("I came here for an argument!!")

Go ahead, say something nice about diesels...... I dare ya!! :lol2:

KiwiBacon
07-10-2008, 12:59 AM
Every new set of pads MUST also have either new rotors or machined rotors.

Why?
If my rotors don't pulse and haven't worn out, I reuse them. No problems yet.
I've never seen the point in machining a rotor, if they're chewed, pitted, warped or dished enough to need it then machining has the potential to create differing thickness which can pulse badly when warm.

If they need replaced, I replace them. If they don't then I just reuse them.

Joe789
07-10-2008, 10:11 PM
Thank you all again, especially vgames33 - EXCELLENT site!

Additional words of wisdom always welcome :)

curtis73
07-10-2008, 10:32 PM
Go ahead, say something nice about diesels...... I dare ya!! :lol2:

Diesels are an above average means of propulsion. There, I tempered it for you.

Joe789
07-18-2008, 09:51 AM
What is the opinion of Centric?

curtis73
07-24-2008, 10:41 PM
I've used their products many times. They are a box brand - meaning they purchase products from other rebuilders and suppliers and stuff them in a box labeled "centric", but I've not noticed any specific weak points or problems with their stuff.

odrazorbac1
11-05-2008, 03:17 PM
First time pad replacement I usually run my finger nail down the disk if no deep groves are detected or seen.
I just replace the pads with no bad effects.

My daughter drives fast & breaks hard & hers had a 1/2 nickle (5 cent) size hole in the edge of the disk so they needed replacement, no question about it.

Scrapper
11-06-2008, 04:35 PM
glad you caught that that could've been a wreck there.

vgames33
11-07-2008, 12:41 AM
We had a few rotors at school from the Mercedes dealership down the road. Some lady came in and said her brakes were grinding. The whole side of the rotor was gone. She was braking on the vents in the center of the disk.

Generally, if its my car, I'll let a few light grooves go. If its someone elses, I'll tell them to buy rotors before I touch the car. Better to charge them a few bucks than to be responsible when they wreck.

Scrapper
11-07-2008, 01:35 AM
you got that right i've done brakes and alignment for years well started doing that in 83. but yes there lives in your hands on those to parts. what if car dies or shuts down no big deal. i doubt they'll get killed but what were taliking about can kill them. glad you think the same..

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