re: cutting rotors
I stayed out of this, but feel the need to throw in my 2 cents
worth. I think that whether, and how you cut rotors, probably
does not have an easy answer.
I have a '90 Chev Beretta with a 3.1L V6. It actually has been a
very good car, but the front rotors really are too small for this
model with a V6.
The rotors run very hot here in our Los Angeles traffic, and as a
result, front pads only last 12-15K miles. I drive it about 10K
miles a year (seven miles to work), so it is not terrible, but I
have done brakes on this car about eight times. I do all my own
mechanical work (other than machining), so I bought a spare set
of front rotors which I have turned and sitting on the shelf
waiting for the yearly pad change.
My neighbor is a aerospace machinist (contracts work for the
airlines, has CNC equipment, has been doing this for 20+ years),
is a car guy, too, and knows his stuff. He offered to turn/cut
the rotors on his equipment, which I gladly agreed to.
When I installed the rotors, they had a small pulse which
increased over a few weeks until I took the spare rotors to the a
local auto parts shop which is privately owned by a "car guy" who
does the machining.
I described the situation regarding the rotors that were turned
on a lathe, and he explained that the problem with cutting rotors
on a normal lathe is that if you do not cut both sides at the
same time, and if there are high spots, the cutter can deflect
the rotor, leaving a high spot.
Watching the knowledgeable local auto parts machinist cut the
rotors, he only skimmed enough to true them, and he did it in
multiple passes, taking about 15 minutes per rotor (he has an
older machine with a tired motor).
After reinstalling the rotors, the brakes had never been better
-- I mean they felt smoother and more positive than they have in
five years.
I guess what I am trying to say here, is that there is little
reason not to turn the rotors if the person doing the job knows
what he is doing. You only need to skim off whatever amount is
required to true them. If the rotors are in good shape, then
almost nothing will be removed. If they are warped, then, yes,
more will be taken off. I have had the two pairs of rotors cut
now a total of eight times, and they all still have enough "meat"
left to be turned more times.
I also would suggest, based upon my experience, that there
potentially are problems trying to turn rotors on normal lathes
if there are high spots.
On a car like my Beretta which is "pad hungry" and "rotor
deprived," having the rotors as "right" as possible makes a big
difference with pad life and brake feel. I generally will turn
the rotors each time I replace the pads. I have tried replacing
the pads without turning the rotors, and the brake feel and pad
life suffered.
I think the spare rotors cost around $40 for the pair, and buying
them was one of the smarter things I have done. You just jack the
car, yank the pads, yank the rotors, replace the rotors, replace
the pads, all in less time than if you did not change the rotors,
as the new rotors do not have any ridges, rust, etc., to make
mounting the pads more difficult. Because you have the fresh
rotors ready, there is no hauling your rotors down to the shop
while your car is up on jack stands (potentially interesting
situation), and then wait for them to be turned.
On the other hand, if you take your rotors to the chain auto
stores, and an 18-year-old minimum wage worker just chucks them
in, and blindly cuts them down, then no, this probably is not a
good idea, either.
For a vehicle that is "over-braked" like my old '75 Ford van (an
E250), with huge rotors, I did not turn them when I changed pads,
and it was just fine. The rotors are very thick and very
resistant to warping, and the new pads simply bed into the
surface.
Like many situations in life, "to cut, or not to cut" probably
depends on several factors, and a single answer probably is
inadequate for each individual situation.
doug
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