Why turn rotors when doing a brake job?

[bull]

I was just thinking about this the other day. Why do people have their rotors turned when doing a brake job if the rotors are being reused? The only thing that comes to mind is perhaps the mating of one flat surface to another (pads to rotors) but are you really losing that much braking force by mating these two uneven surfaces until the pads wear into the rotor grooves? And once that takes place wouldn't you have more braking surface than if the pads and rotors stayed flat for a long period of time?

Just thinking out loud here so don't get too jacked up. That said, does anyone know how long it would take, generally, for new pads to mate to a used rotor?

[A383Wing]

rotors are turned for 2 reasons....to take out the warpage, and to give the new pads a surface to blend into. New pads on old un-turned rotors will not stop as good because the old pad is embedded into the rotor. It usually takes about 200 miles for new pads and turned rotors to be at optimum.

I have in the past put new pads on un-turned rotors...but I usually end up with a squeak or some noise. New pads on glazed rotors don't stop very well

I'm sure others may disagree with me...but I have been doing brakes for over 35 years....

[bull]

rotors are turned for 2 reasons....to take out the warpage, and to give the new pads a surface to blend into. New pads on old un-turned rotors will not stop as good because the old pad is embedded into the rotor. It usually takes about 200 miles for new pads and turned rotors to be at optimum.

I have in the past put new pads on un-turned rotors...but I usually end up with a squeak or some noise. New pads on glazed rotors don't stop very well

I'm sure others may disagree with me...but I have been doing brakes for over 35 years....

In my case turning the rotors caused them to warp; a common issue on the Durangos apparently...

[A383Wing]

they were probably too thin to start with.....and Durangos, Jeeps, and other vehicles like those have undersized brakes....factory only puts on what the minimum it would take to stop the vehicle.

Plus...if you are running oversize wheels & tires...that puts an extra load on the brakes and makes them hotter

[bull]

they were probably too thin to start with.....and Durangos, Jeeps, and other vehicles like those have undersized brakes....factory only puts on what the minimum it would take to stop the vehicle.

Plus...if you are running oversize wheels & tires...that puts an extra load on the brakes and makes them hotter

Well, to remedy my current issue I was hoping to just buy a new pair of drilled/slotted rotors and swap them out yet keep the pads, which is pretty much the opposite of what we're talking about here. But since I already 23k miles on my current brake job I should trade the pads out too. Mainly this is a "what if" question for next time. Do people usually turn drilled and slotted rotors or just scrap them?

[A383Wing]

there is a minimum thickness that they are allowed to be turned too...if they meet the specs, then we turn the rotors.

We also turn drilled & slotted ones as well......they are no different than regular rotors, except that they are not supposed to warp as stock ones do....but they will warp....depends on how hot they get and if the calipers are releasing all the way

if yer gonna swap out the rotors...put new pads on...

[Mike DC]

     
Compared to older cars the modern ones have plenty of brakes. 


If the rotors are warping just from normal turning procedures, then my guess is probably because the chinese rotor supplier got too cheap on the metal quality. 

   

[bull]

How much heat dissipation efficiency do you lose on the slots when you turn the drilled/slotted rotors?

Mike, I'm not sure what the deal was on those Durango/Dakota rotors but most people I know who own them were fine until that first brake job if the OE rotors were turned and reused. That's when the warping started and the vibration, steering wheel shimmy, etc. I assume, like you say, they were probably weak Chinese junk to begin with.

[elacruze]

I haven't turned rotors on anything smaller than an F-350 in many years. I have two J-body GM cars, and I got so tired of warped rotors that I spent the cheese to buy Powerslot rotors for both of them. I've put about 70-80k miles on each car with the powerslots, neither is warped and neither has had the rotors turned. They've both had 2-3 sets of pads on them too.

The quality of rotors these days is totally crap. Most new cars are good for exactly one panic stop, then the rotors are shot. Once they've warped, there is no way to bring them back. If they weren't up to the task new, they aren't up to the task after you shave material off of them.

I bought Powerslot (or equivalent) rotors for the Charger, and had them cryogenically treated to boot-about $300 just for rotors. I think it's money well spent. There's a reason a Brembo setup costs $3000.

[TylerCharger69]

   Turning the rotors is pretty much in my opinion is necessary....kinda like...one would not change the oil without changing the filter.   Yes it provides a better mating surface and faster break-in...not to mention new pads on warped rotors will not last very long,  and you may feel some vibration during braking if this isnt done.   Same goes with clutches.....I would never change a clutch without resurfacing or replacing the flywheel.    Also.....the "Durango"  model was brought up.....I believe these are a composite material  (correct me if im wrong)  and resurfacing is more involved which is why a lot of people just replace them  since they are relatively inexpensive.

[b5blue]

On this subject, is anyone using "non directional" turning of rotors? Apparently it uses a spinning round cutting face as the rotor is turned. My opinion is it is done (any turning) to give all 4 faces of the 2 rotors an equilateral (like that word?  ) grabbing surface area, so all have even grip with no pulling left or right or a differential inside and outside of the same rotor.   

[A383Wing]

we just use a standard brake lathe and after the final cut, we use some sandpaper to the surface before mounting to hub

[TylerCharger69]

Thanks b5blue...that is also true...a non-directional surfacing after the rotor is cut is very important....i forgot to mention that

[jeryst]

Rotors warp, causing pulsation and poor braking performance. The rotors get wear grooves that also diminish braking performance.
Rotors that are not smooth will tear up your brake pads, diminish their performance and reduce their life.

Older vehicles did not stop well when the braking systems were in top notch shape, so you are risking a lot by not doing everything you can to get that 5000 pound fire breathing missile stopped in the shortest distance possible. You know the old saying: "Pay me now or pay me later"!

I think you answered your own question when you mentioned that your rotors had grooves in them.
Even buying replacement rotors is not very expensive, so I wouldnt risk it.

[elacruze]

I was just thinking about this the other day. Why do people have their rotors turned when doing a brake job if the rotors are being reused? The only thing that comes to mind is perhaps the mating of one flat surface to another (pads to rotors) but are you really losing that much braking force by mating these two uneven surfaces until the pads wear into the rotor grooves? And once that takes place wouldn't you have more braking surface than if the pads and rotors stayed flat for a long period of time?

Just thinking out loud here so don't get too jacked up. That said, does anyone know how long it would take, generally, for new pads to mate to a used rotor?

To answer your questions specifically I'll add this.
I've padded shot rotors plenty of times in my broke youth days, and never had a problem.
That said; How long the pads take to bed in depends entirely on the pad composition and how bad the surface of the rotor is. It also depends on whether the rotors are glazed or not from overheating. How much flat surface you have left determines how much stopping power you have at first. a rotor that is severely grooved will have nearly no power at first but will quickly bed in and stop well. A rotor with 1/2 the friction surface good and 1/2 bad will have about 1/2 normal stopping power in the normal break-in period, but will take a very long time to improve any. The key is that the pad must wear away at all the high spots before it can touch the lower ones.
Clear?
A rotor that is worn but not damaged, lets say that it's cupped smoothly, may take 1/2 the pad life to completely bed in. Again it depends on how far from flat it is. Once it does though, it will have capacity equal to the flat rotor as long as the rotor isn't glazed or hardened by heat.

[jeryst]

If your rotor has grooves in it, the brake pads will eventually conform to the shape of the grooves, because the grooves are linear in respect to the motion of the wheel. Once the pads conform to the shape of the grooves, you should have full braking power. But remember that if you have some high grooves, those areas will become the thinnest areas on the brake pads, which means that once those areas wear through, they will be the first place that the metal backing of the brake pad contacts the rotor. In the case of very deep grooves, this can happen in a very short time. Also, once they wear through, the brake pads may crack, come apart, etc, causing unpredictable results (loss of braking, locked brake, etc).

If your rotor is warped, the pads can never conform to the shape of the rotor because the warping is perpendicular to the motion of the wheel, so you will never achieve full braking power. The pads float on the rotor, and when the rotor is warped, it has a series of high and low spots that occur one right after another. The brake pads simply hit the high spots, and dont make contact with the low spots. When you feel pulsation in the brake pedal, that is the high spots of the rotor banging against the brake pads. In that case, your braking power is diminished, and the life of your brake pads is shortened.

[b5blue]

Unless all 4 surfaces are flat the variable of the groves will introduce imbalance in gripping to some degree. With one side grabbing more the car must pull harder to that wheel, very dangerous on a wet road as that tire will then slip. On a beater it's dangerous, on a Charger it's also very expensive if you hit something. It's all about the degree of the wear on the surface of the rotors or drums, an adjuster finds excessive wear and you will pay dearly. Even my beaters have 100% braking systems. (I do totally get what you guys are saying and have sanded rotors and drums with minor blemishes or glazing)