by James Walker, Jr. of scR
motorsports Check out the site for more tech info!
Let's look at some common rotor `modification' and `performance'
upgrades that you may have been exposed to and try to separate the
marketing from the engineering...
Super Sizing
Bigger rotors will make your friends think you are cool, bigger rotors
look sexy, but bigger rotors do not stop the car. What a bigger rotor
will do is lower the overall operating temperature of the brakes -
which is a GREAT idea IF your temperatures are causing problems with
other parts of the braking system. Take for example a F500 racecar - a
small 800 pound single seat formula car. While the brakes are
certainly much smaller than those found on a 3,000 pound GT1 Camaro,
that does not necessarily mean that they need to be made larger. In
fact, swapping on a GT1 brake package would probably do more harm than
good - that's a lot of steel hanging on the wheel that needs to
accelerate each time the `go' pedal is pushed. So, the motto of this
story is bigger is better until your temperatures are under control.
After that point, you are doing more harm than good...unless you
really like the look (and hey - some of us do!).
Crossdrilling
Crossdrilling your rotors might look neat, but what is it really doing
for you? Well, unless your car is using brake pads from the 40's and
50's, not a whole lot. Rotors were first `drilled' because early brake
pad materials gave off gasses when heated to racing temperatures - a
process known as `gassing out'. These gasses then formed a thin layer
between the brake pad face and the rotor, acting as a lubricant
and effectively lowering the coefficient of friction. The holes were
implemented to give the gasses `somewhere to go'. It was an effective
solution, but today's friction materials do not exhibit the same
gassing out phenomenon as the early pads.
For this reason, the holes have carried over more as a design feature
than a performance feature. Contrary to popular belief they don't
lower temperatures (in fact, by removing weight from the rotor, the
temperatures can actually increase a little), they create stress
risers allowing the rotor to crack sooner, and make a mess of brake
pads - sort of like a cheese grater rubbing against them at every
stop. (Want more evidence? Look at NASCAR or F1. You would think that
if drilling holes in the rotor was the hot ticket, these teams would
be doing it.)
The one glaring exception here is in the rare situation where the
rotors are so oversized (look at any performance motorcycle or lighter
formula car) that the rotors are drilled like Swiss cheese. While the
issues of stress risers and brake pad wear are still present, drilling
is used to reduce the mass of the parts in spite of these concerns.
Remember - nothing comes for free. If these teams switched to
non-drilled rotors, they would see lower operating temperatures and
longer brake pad life - at the expense of higher weight. It's all
about trade-offs.
Slotting
Slotting rotors, on the other hand, might be a consideration if your
sanctioning body allows for it. Cutting thin slots across the face of
the rotor can actually help to clean the face of the brake pads over
time, helping to reduce the `glazing' often found during high-speed
use which can lower the coefficient of friction. While there may still
be a small concern over creating stress risers in the face of the
rotor, if the slots are shallow and cut properly, the trade-off
appears to be worth the risk. (Have you looked at a NASCAR rotor
lately?)
Too cool!
Last year we bought 4 rotors. Two were bone stock, and two
were subjected to a process know as Cryogenically Treating - one
of the high-tech buzzwords floating around the paddock. The
rotors were run back-to-back on the same track on the same car on
the same day with temperatures taken to make sure that they saw
the same level of heat. Following the track session, the parts
were removed and we had them literally dissected by a materials lab.
The testing conducted included surface hardness, grain structure
analysis, density, and surface scanning with an
electron microscope. Guess what - after seeing the heat of use, the
rotors looked identical in every regard. This is not to say that there
is not a benefit from treating other parts which see lower
temperatures and/or have different material properties, but treating
our rotors on our car showed no tangible benefits (note that it didn't
seem to hurt anything either). Come to your own conclusions, but in
our case, we'll pass.
Summary
So, what's the secret recipe? Again, there is no absolute right or
wrong answer, but like most modifications, there are those which
appear to be well-founded and those that `look cool.' If ultimate
thermal performance is your goal, look to what the top teams are
running (relatively large, slotted rotors). However, if `image' is
your thing, break out the drillpress - and be prepared to replace your
brake pads on a regular basis.
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