R Haug wrote:
>
> List,
> I am considering replacing my standard rotors with the drilled and
> slotted rotors for my 67.5.
> Has anyone used these on the street?
> If so, is there much difference in braking with these as compared to to
> standard rotors?
> Is there any increased or decreased noise?
> I would think there may be an improvement in fade resistance.
> Maybe a slight decrease in unsprung weight.
> Anything else?
> Bob
Drilled rotors and slotted rotors are both addressing the same problem, brake
fade due to a buildup of some sort of material between the pad and the rotor,
such as a buildup of gasses due to outgassing of the pad, or a buildup of water.
Like the tread in a tire, the holes and slots give the material somewhere to go.
As the material builds up between the pad and rotor, it acts as a lubricant,
reducing the brakes' efficiency. Its just like running slicks in the rain vs
running grooved tires in the rain. But as, someone else has mentioned, pads do
not outgas as badly as they used to, and water deep enough to cause a problem is
rare. Many people think that the drilling is to improve cooling and while it can
appear that it does help cooling if you have pads that tend to outgas, by
reducing brake fade due to outgassing, the fact is it does little to lower rotor
or pad temps. Drilling might help with cooling if you could direct enough air
across the rotor and through the holes. But normal airflow patterns of a
spinning
cross drilled rotor are such that little or no air passes through the holes.
There is a slight increase in surface area from drilling but the stagnant air in
and around each hole acts as an insulator making this extra surface area nearly
useless for heat transfer. OTOH by drilling you are reducing the total mass of
the rotor which reduces its transient heat load capabilities. During a stop, the
rotor's cooling abilities, that is its ability to shed heat, can not possibly
keep up with the heat load being pumped into the rotor.
During braking the heat is being generated much faster than it can be shed.
The excess heat is stored in the rotor to be shed later, over a longer period of
time. The rotor acts as a battery of sorts, storing the heat energy as it is
rapidly pumped into the rotor during a stop and then "slowly" releasing it to
the environment after the braking is over. As with any battery, its ability to
store energy is limited by its mass. The more massive the rotor, the more energy
it can store between cooling cycles. But, a rotor's primary duty is not energy
storage but energy conversion (brakes convert kinetic energy to thermal energy)
and its ability to convert energy effectively is limited by its maximum
operating
temperature. The more energy it can store without exceeding its upper operating
temperature limit, the more energy it can convert (in other words the better it
will stop a car). If you reduce its mass, you reduce its ability to store
energy
without exceeding the operating temp limits. This will not affect the braking
until you brake long or hard enough to cause its temperature to rise beyond its
maximum operating temp limit. Then you will lose your brakes.
Another way to think of it is this; The rotor acts as a thermal shock absorber.
The thermal loads in the braking system come hard and fast during braking, and
the rotor damps them out by absorbing them and dissipating them slowly, just the
way a shock absorber in the suspension damps kinetic loads.
In fact that analogy is even more accurate than it seems at first. Both shocks
and rotors are subjected to sudden and large inputs of kinetic energy which they
convert to heat and transfer to the surrounding environment over time. However
in the case of the shock, the road spring also helps, acting as the primary
energy storage unit. With the brakes the rotor serves both functions.
Anyway, the point is that during braking, the only thing that prevents
heat-induced brake fade is the rotor's ability to store energy without
overheating, because *no* rotor can shed heat fast enough to make a difference
*during* braking. And the rotor's ability to store energy without overheating is
directly related to its mass. A rotor's ability to shed heat comes into play
after the braking is over (ie between braking events). During this period there
is no more heat being pumped into the rotor and it sheds heat as fast as it can.
If the time between braking events is long enough, or the rotor's ability to
shed heat (its cooling ability) is great enough, then the rotor will be
completely cooled by the time the next braking event occurs and the rotor will
have full storage capability. OTOH if the rotor cools too slowly or the next
braking event occurs too soon, the rotor will start at an already elevated
temperature and will overheat more quickly. In this scenario you will eventually
reach a situation where the rotor overheats during braking. But this will happen
only when the car is driven hard with repeated and quick-succession braking.
This is the situatio where a rotor with an imporved cooling capabilty will help
prevent or delay heat-induced brake fade. If a rotor overheats in one or two
hard stops, the likelyhood is that the rotor does not have enough mass. If it
takes repeated hard stops and happens only when the brakes are used frequently,
then they problem may lie with either the rotor's mass and/or its ability to
shed heat.
Crossdrilling does almost nothing to improve a rotor's ability to shed heat and
does reduce its mass (ie its ability to store heat) and so it should be used
only when pad outgassing-induced brake fade is causing problems. And even then,
a change in pads will often do more to address that problem than crossdrilling
the rotors will. Some cars respond fairly well to crossdrilling, and I used to
use this technique for street driven MGBs, but for the most part it is a
band-aid solution that is not really addressing the true problem. If you feel
you have an outgassing based problem, I recommend slotting instead. It will
address the problem without reducing the rotor's mass as much. It also avoids
another big problem with cross drilled rotors which is crakcing. At each hole
stress risers are created when the hole is drilled. These will generate cracks
as the rotor heat cycles. You can address this by chamfering the holes and
shotpeening the rotors prior to surfacing, but this adds to the costs and most
manufacturers do not do this. So you will still have to go through this even if
you buy predrilled rotors. Porsche "crossdrilled" rotors are not really cross
drilled for just this reason. Instead the holes are cast into the rotor during
manufacture.
BTW, from what I can see, the design of the Roadster rotor, with it's large
integrated hat section with holes, is a fairly effective design. The hat section
acts as a radiator to help shed heat and the holes in the hat section improve
the airflow through and around the hat section. If the rotor were more like a Z
with little or no hat, it would probably have much more serious fade problems.
If the stock setup isn't enough, look to a true vented rotor instead. That will
not only increase the rotor's mass, but also its ability to shed heat. The 280ZX
rotor, while woefully inadequate for a car as big and heavy (and fast in the
case of the turbos:-) as the 280ZX, is great for cars in the weight range of the
510, even in hard racing use. I would suspect that it would also work very well
on a Roadster. And it is small in diameter, allowing the use of smaller wheels
(most vented rotor conversions using race parts require the use of larger
wheels, the ZX rotor was designed for and fits in 14" wheels, and will even fit
in some 13" wheels).
--
Marc Sayer
82 280ZXT
71 FJ510
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