[hijack] So, while all the brake experts are in here, I have a rotor question. On my '72 C10 the front rotors each have a v-groove cut in each side of the rotor surface. It is a deep groove, about 1/8", and it was intentionally put there because it is cut in a perfect V and is in the same location on all 4 sufaces. So, what's it for? [/hijack]
CK5
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Why turn shiny brake rotors??
- Thread starter mofugly13
- Start date
If it were a racecar, I'd suggest it's there to clear the gasses from high speed braking..... in the case of the early trucks, I think it's a more humble functionality. Probably to help clear water from the center of the rotor when driving in the rain....???
They are "factory" grooves by the way.
They are "factory" grooves by the way.
Everyone's different...
The shops around here like Midas,etc,always turn the rotors--if for no other good reason to increase profit..most places just scrap them now,they are so cheap to buy new--
I used to turn my rotors and drums at the parts store for free--but I think many times it wasnt really nessasary,all I did was cut them down,and the thinner they are,the easier they warp...now I only turn them if they are pulsating and still thick enough to turn them---but now that you can get brand new ones for 25 bucks or less for most trucks,its not worth the aggravation-
-they get 10-15 bucks each to turn them at the local parts stores,you do the math--(and the boobs they have running the lathe at many stores ruin more rotors than they save in many cases--not many know the machine like they should..or can stand there and watch it while they are being turned..
.most of the time in my area,by the time the rotors need turning,they are all rotted away in the middle where the cooling fins are,and are dangerous--so they get chucked in the scrap pile...I have seen a few brand new ones need turning right out of the box,but not as often as some people lead you to beleive....nowadays I just buy new ones,and call it done--easier,safer,and less aggravation...
The shops around here like Midas,etc,always turn the rotors--if for no other good reason to increase profit..most places just scrap them now,they are so cheap to buy new--
I used to turn my rotors and drums at the parts store for free--but I think many times it wasnt really nessasary,all I did was cut them down,and the thinner they are,the easier they warp...now I only turn them if they are pulsating and still thick enough to turn them---but now that you can get brand new ones for 25 bucks or less for most trucks,its not worth the aggravation-
-they get 10-15 bucks each to turn them at the local parts stores,you do the math--(and the boobs they have running the lathe at many stores ruin more rotors than they save in many cases--not many know the machine like they should..or can stand there and watch it while they are being turned..
.most of the time in my area,by the time the rotors need turning,they are all rotted away in the middle where the cooling fins are,and are dangerous--so they get chucked in the scrap pile...I have seen a few brand new ones need turning right out of the box,but not as often as some people lead you to beleive....nowadays I just buy new ones,and call it done--easier,safer,and less aggravation...
Greg gets points for his answer.
You have come a long way grasshopper.
beater_k20
Banned
for those of you arguing that the brakes will work better on a glazed rotor, think of it this way. take a brake pad, and run it across a piece of glass. then run that same pad across a piece of sand paper. which one will be easier to push across the surface? the glass will. which one will tend to grab the pad more? teh sandpaper. there's your true answer. if what you guys are saying is true, then the next time i buy a set of tires for a stuck that will be used offroad, i'm buying slicks. because they have more of a contact patch, which means they'll get better traction.
Fierospeeder
Banned
beater_k20 said:
why would drag cars run slick? dont they want knobby tires instead?
A glazed rotor isn't perfectly smoothed. There is a certain microfinish required for brakes to work the best. Take a microscope and look at the layer of a rotor. You wont see it being smooth at all. And not only is there coefficient of friction we are dealing with, there is also something called a "clamping force" being applied to the rotor. Thats how hydraulics work.
4X4HIGH said:Greg gets points for his answer.
You have come a long way grasshopper.
At this rate, I may need to buy a bigger thimble to hold all my knowledge....!
beater_k20
Banned
not even going to waste my time responding to this.Fierospeeder said:
very good. at least you're not totally off base. nothing is perfectly smooth, some are just smoother than others. i've explained why you want a "rougher" surface like a freshly turned rotor has, rather than a worn out glazed rotor.
Fierospeeder
Banned
the example of tires in mud is a great example of how the physical aspects of hydraulic brakes work is great 
85jimmy3/4ton
Registered Member
The desired finish on both the pad and the rotor is 80 microns for the best friction (stopping power) some people don't turn rotors some do, I always use a single pass on the vehicle brake lathe when I cut mine, this compensates for any runout in the front end (to a certain point) and also saves the time of removing the rotor off the vehicle. whenever I do a brake job I use the lathe to at least take the glaze off.
Fierospeeder
Banned
beater_k20 said:
heres another website to explain common sense
http://www.babcox.com/editorial/tr/tr20244.htm
after you run your so called brake pad(friction material) over glass 1000 times, will it still be smooth? It will leave a finish that has the same ridges as the pad. And when a rotor has been removed from a vehicle. Will the rotor have "ridges" on the surface from the brake pad grooving it into the surface? And after a few miles, the brake pad surface matches the rotor in a perfect fit. But your "theory" is that a rougher surface is better. This will cause the brake pad not to "fit" into the rotors surfaces.
To make it easier to comprehand. If you were qualified, you setup the brake lathe to cut the rotor. It leaves 6 large points on the surface. Then you setup the lathe to cut finer(slower speed). This leaves 12 large points on the surface. Your theory, it is better to have the rougher surface (6 points) on the rotor because it "grips" more. In the real world, all the cuts are in one direction, so there is no better grip, because it was machined on a lathe. Because it has a "rough" finish. The brake pad will be pressing against 12 points on the rotor. Now what if we put a better finish on the rotor. Say we use 150 grit sandpaper on a lathe. This causes the points on the rotor to go up to 24. Now there are 24 points that the brake pad has to contact. Now the vehicle can stop in less distance because it is like having larger brake pads.
You can also use railroad track as an example too. the rails act as grooves like the rotor. The more rails, the greater the surface.
BTW, knobby tires have more surface area then slicks. So i dont know why you would want slicks for off roading. ??
Fierospeeder said:the example of tires in mud is a great example of how the physical aspects of hydraulic brakes work is great
I think you need to go back through all your posts and read what you have put out and try to do so through the way others see you and your responses. I just have because of the amount of complaints you get.
You seem quite knowledgeable but you come off harsh. Understand that there is a point where a debate becomes argument and it is no longer a productive.
Its one thing to debate a point with facts its another to belittle people openly by being sarcastic when they don't see it your way. You have quite often made remarks about small unimportant items, in a derogatory manner as you did above, to people trying to help others instead of just helping with a productive response to the original question or problem that was posed in the thread.
I will debate with you all day long about the pros and cons of something like turning rotors but I will not stand for you to become hostile or derogatory to others. That is 100% the opposite way the the owner and members of this board wish to happen. This forum is designed to be a resource to help people out and be an open friendly place. Please remember that.
Fierospeeder said:
Brake are about getting rid of heat. Think back to physics class. Your turning motion into heat through friction.
The point of turning is to provide as much uniform contact area as possible from the very start. Doing so also lessens heat concentration and helps prevent pad glazing. Pad glazing is a direct result of heat. If you put a flat pad against a ridged rotor at the peak of each ridge will be a hot spot that will glaze. That glaze will then harden the surrounding ares so that once it does conform to the ridges it is already glazed as well. Even though there is still a fine ridge after turning its many small ridges spread across a much greater surface area. This dissipates the heat better and lessens the glazing effect.
The second effect to a fresh turned rotor is you want material to come off the pad. Once glazed that is harder to do.
When a rotor is machined it is machined in a spiral so again that helps grind away the surface of the pad to help prevent thickening of glazed surface. A worn grooved rotor runs the same ridges full circumference.
Your reference to train track forgot to mention that trains drop sand on the rails to help provide more friction to over come the glaze on the rails. The sand cuts the glaze on both the wheel and the track. If the wheel were allowed the slip it would tool harden from heat and loose traction.
Fierospeeder
Banned
Grim-Reaper said:
i dont have time to get into this much right now. im eating a gyro.
actually, brakes are used to stop a vehicle. Heat is a byproduct of friction.
Yes, a lathe is used to make a rotor "flat". But you still need to provide a finish to give it more of a "surface". This works both in heat transfer and the brake pad having more area to clamp onto.
Theres no real issue with my rail road example. Because the brake pad has a different friction rating then the railing which is an example of the grooves on the rotor.
But the main point of the topic is the rotors dont need to be machined if they are within specs. If this was untrue, then GM engineers and the automotive field wouldn't point it out.
beater_k20
Banned
Fierospeeder said:
i dont know where you get that idea. there's a reason they call the area between tread blocks voids. i'd prefer a fresh, non glazed, and true surface for something that's quite possibly going to save my ass from time to time, rather than something grooved, hot spotted, and glazed. no point in arguing with you, as you MUST be right. i'm done.
After my truck has sat for several weeks the rotors (front and rear) get rusty, which is not shiny, smooth, or flat and it stops SUPER good. After the rust is cleaned off then they stop as usual, not as quick as when they were rusty. I just wish mine were rusty all the time instaed of smooth, flat, and shiny... 
Fierospeeder said:
Yeah I will concede the " within Spec" but its rarely "within spec". Most people bring in a car after the pads are worn and there is already peddle throb indicating the rotor is not true or when a run out check is made it is found. Part of in spec is the rotor not grooved over so many thousands. So when you get down to it it is very rare for it to be "within spec" with any kind of miles on them especially on a heady vehicle.
From my FACTORY 88 Service manual.
Thats both pads total variance together cannot exceed that so we are talking a max of .0025 per pad. Thats barely felt running you thumb nail across the rotor.
I have been told by a friend that is a ASE mechanic and use to be an instructor for SAAB that for you to feel peddle throb that run out is pretty excessive and will require a pretty excessive amount of material to be removed to correct.
Now while the grove depth seems excessive if you were to do the pad test that they first mention there is NO WAY that you will ever come near that .005 motioned as the very first check before you even get to checking run out and groove depth. You would never pass the very first test the factory manual says to perform.
Now to put some perspective on this 24lb copy/printer paper is s .0048 inches. So if run out or pad face variating is off more then the thickness of a sheet of paper the rotor is NOT "within Spec". I would say that I am fairly accurate to assume that most vehicles with over 10k of service on the brakes since the last turning or rotor replacement would fail to meet one of those specs.
So yes you are correct that turning is not required when it is "within Spec". Unforunatly thats not going to be very often on a vehcile that is used as a Daily driver.
Got rust??
If you want rusty rotors,just move to new england,especially massachussetts,where they dump tons of salt on the roads--my rotors are rusty every morning,and the rear drums get real "grabby" and make the rear wheels skid at the slightest touch of the brake pedal upon the first stop after sitting overnight--once the rust wears off after the first stop or two,they work normal!.--after 2-3 years,the rotors often split in half due to the rust eating the "fins" out of the center.....
If you want rusty rotors,just move to new england,especially massachussetts,where they dump tons of salt on the roads--my rotors are rusty every morning,and the rear drums get real "grabby" and make the rear wheels skid at the slightest touch of the brake pedal upon the first stop after sitting overnight--once the rust wears off after the first stop or two,they work normal!.--after 2-3 years,the rotors often split in half due to the rust eating the "fins" out of the center.....
Fierospeeder
Banned
diesel4me said:If you want rusty rotors,just move to new england,especially massachussetts,where they dump tons of salt on the roads--my rotors are rusty every morning,and the rear drums get real "grabby" and make the rear wheels skid at the slightest touch of the brake pedal upon the first stop after sitting overnight--once the rust wears off after the first stop or two,they work normal!.--after 2-3 years,the rotors often split in half due to the rust eating the "fins" out of the center.....
thats because the manufac are eliminating a metal in the rotors, which use to prevent them from rusting overnight. I know GM is trying to find a "cheaper" metal to use in their rotors to prevent the overnight rust.
to the mod
Sure there are people that dont care about their car and the ones that they do. I personally, deal with people that have newer cars. Late 90s and 00s. So when i see brakes they are in great condition. The calipers are not frozen, the bleeders dont break off etc...
Then you have people who run the pad until it is just the steel backing and that rubs against the rotor until there is no rotor left.
And if you look at my first post. If it is within spec, you dont need to cut the rotors. I do run a dial indicator, and have found rotors that dont need to be cut. If you find someone that drives like an idiot or doesn't care about their car, sure you will find a beat up braking system.
majority of small cars run non vented rotors. by the time they need a brake job, the rotors are too thin to even cut or reuse and new rotors need to be installed. And many new cars have issues with pedal pulsation. Because they use such a thin rotor, it gets warped from the excessive heat that it cant "transfer"
In conclusion, a brake doesn't need to be turned if it is within specs. Just like my original post.
