Re: [DML] Re: Get performance!!!

[William T Wilson <fluffy@xxxx>]

On Wed, 5 Apr 2000, Lee wrote:

> rotors are a direct result of the video industry's need to see some
> kind of rotational movement when doing close up's of tire changes in

I guess it's time to clarify my opposition to drilled rotors. :}

Drilled rotors are quite common on race cars, but of course, race cars are
not street cars and the requirements are different. They do have a few
legitimate advantages which, in my opinion, do not justify their drawbacks
for a street car.

The first advantage is their weight. This little bit of weight
(especially since it is rotating mass on the wheels) can make a noticeable
difference in suspension performance. But it doesn't matter on a street
car, because the weight saved would be a very small proportion of total
wheel mass (but it does make a difference on motorcycles, which is why so
many sport bikes have drilled rotors even from the factory).

The second advantage is cooling. The little holes give the rotor more
surface area to shed heat with. This doesn't matter very much on a street
car either, because the rotors just don't get hot enough to cause
problems.

The third advantage has to do with 'outgassing'. Hot brake pads emit gas,
and the gas can act as a lubricant, reducing the effectiveness of the
brakes. It's much less of a problem with modern brake pads and isn't a
problem at all at the low temperatures encountered on the street. The
holes provide a place for the gases to escape to.

On the other hand, there are two big problems with drilled rotors - they
are not very sturdy and they crack. It's not a matter of if they crack,
but a matter of when. The jostling that street cars issue to their
suspension is very severe compared to what most (paved surface) race cars
encounter. This can stress the rotors and break them. Solid rotors
rarely crack because of stress, but drilled ones crack frequently. 

Sometimes, street rotors get heated unevenly - suppose you are
driving along the highway at 70 mph and suddenly have to make a panic stop
- say because of an accident or other traffic congestion ahead. This can
put quite a bit of heat into your brake pads, and then you sit in traffic
with your foot on the brake - wheels not turning. All the heat gets put
into just one area of the rotor, and the uneven expansion stresses it in a
way that a race car would never have. Even the regular heating-cooling
cycles that all street car components get are hard on these rotors. Race
cars usually wear their rotors out before they have a chance to fail due
to heat stress.