Last post on May 03, 2013 at 4:53 PM
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#1988 of 2025 Re: Why is is...? [shipo]
Dec 04, 2010 (9:39 am)
So, if I machine the rotors on one of our CRV's leaving it well within specs and someone with an identical CRV REPLACES his rotors and we both make a panic stop from 60 MPH, are you telling me that there would be a measurable distance in stopping?
Of course, this assumes identical tires, same road conditions etc?
I'm not buying that. Sorry.
#1989 of 2025 Re: Why is is...? [isellhondas]
by MrShift@Edmunds HOST
Dec 04, 2010 (9:44 am)
I was thinking that there's a certain amount of glazing on pads and burning on old rotors, and some roughness, so all together yeah, you might get a few extra feet out of brand new rotors and pads vs. a set that's been working hard for 25,000 miles. Perhaps it's the overall combination of all the things about a "new brake job" that gives me that confidence.
#1990 of 2025 Re: Why is is...? [Mr_Shiftright]
Dec 04, 2010 (10:13 am)
Well, I was assuming both cars had new pads. One had NEW rotors and the other had freshly resurfaced in tolerance rotors.
Speaking of which, our 2003 CRV has 53,000 miles on the original brakes. 10,000 miles ago, they still had lots of pad left.
Time to take another look!
#1991 of 2025 Re: Why is is...? [isellhondas]
by MrShift@Edmunds HOST
Dec 04, 2010 (10:38 am)
I drive pretty darn hard, so I'm not comfortable with shaving any appreciable metal off a rotor. I'm okay with cleaning them up but I wouldn't run mine if they are close to minimum tolerance. I'm just too hard on brakes to take a chance.
#1992 of 2025 Re: Why is is...? [isellhondas]
Dec 05, 2010 (1:24 pm)
Lets say the stop is from 70 to zero; yes, there will be a measurable difference if said stop is at impending lock-up. Now lets say the rotors are already hot from heat yet to be dissipated from previous stops, then the difference stopping distance will be even more pronounced.
#1993 of 2025 Re: Why is is...? [shipo]
Dec 06, 2010 (8:22 am)
Is this fact or theory? Measurable difference? How much is total weight of an average rotor face? How much is machined off? How much heat would need to be dissipated before it would affect performance?
Sounds like something the auto parts industry would fund if putting on new rotors were noticeably more beneficial than machining. I mean why not put on a new set of tires after every 10,000 miles... same thing.
#1994 of 2025 Re: Why is is...? [jipster]
by MrShift@Edmunds HOST
Dec 06, 2010 (10:16 am)
Probably the only real advantage of not turning rotors is that, if you are a hard driver, a thinner rotor would probably warp more easily---let's say if you were coming down a long hill and then hit a puddle of water at the bottom.
It seems that many modern cars, especially entry-level types, are plagued with rotors that warp as easily as the tinfoil in an Italian TV dinner tray.
#1995 of 2025 Re: Why is is...? [jipster]
Dec 06, 2010 (10:25 am)
Sorry guys, the fact is, the thinner the rotor, the lower the thermal mass of said rotor; the lower the thermal mass, the higher the rotor will heat up with the application of any given amount of heat; the higher the heat the lesser the effective braking power. Is it significant? I believe it is and have read plenty of studies published in the SAE archives that back that up with hard numbers.
That said, you shouldn't believe some dumb schmuck on the internet; if you're interested in this kind of stuff you should do your own research. I've done mine and I change rotors at every brake job. That's my story and I'm sticking to it.
#1996 of 2025 Re: Why is is...? [shipo]
Dec 06, 2010 (3:03 pm)
Yes, the temperature rise is directly related to the mass of the rotor. The lighter the rotor, the more the temperature rise for the same amount of energy (heat) input.
That said, what kind of difference in temperature rise are we talking about for a machined vs a non-machined rotor? Well, we need an estimate of the before and after machining weight of the rotor.
Assume a stock rotor weighs 15 lbs. Further assume that after using up one set of brake pads (which wear the rotor) and then machining said rotor, it's weight it 14.5 lbs (0.5 lb metal loss seems high to me, but this will give us a worst case estimate or temperature rise).
Now, assume a braking scenario that raises the temperature of the stock 15.0 lb rotor 100 deg C. Then that same braking scenario, applied to the used, machined 14.5 lb rotor would raise its temperature 103.45 deg C, a 3.45 deg C increase.
So yes, the used, machined rotor does run hotter, but does the 3.45 deg C increase make that much difference?
If the rotor was half as heavy as the one I postulated (7.5 lbs from the factory), and the braking scenario twice as hard, the temperature difference between the two rotors would still only be 24.29 deg C.
I do not know if a 24.29 deg C difference in rotor temperature is significant or not. For the average driver who maybe needs to do 1 full blown panic stop from highway speeds a year, probably not. For those who like to push the envelope coming down a mountain, .
#1997 of 2025 All interesting...but
Dec 06, 2010 (6:50 pm)
The main reason for NOT turning modern rotors is because many are HARD SURFACED.... not because of loss of braking charictoristics. There is a min. number embossed on the casting to tell you what is safe. Turning them removes the all-important hardend surface whence the pads create friction.
Todays modern brake-rotor may be made up of several different materials.
- The hard-surfacing for the pads to bite into.
- softer material which conducts heat away.
- tougher material which resists warping near the bolt-holes
You may not be able to visually *see* these differences. Modern meterallgy tecniques like flame-hardening can make one cast peice have very different charactoristics.
Of course, you can always buy cheep rotors that will warp the 1st time you do a panic-stop.
Heck - the brake rotors on my motorcycle are aluminium inner section with fully-floating StainlessSteel outer area for the pads to bite into. (Just like race-cars use.)