[geeks] Drum versus disk brakes (Was RE: car question RESOLVED)

[Kurt Huhn]

> > Although to be fair, drums and shoes do cost more than pads and rotors.
> > Much more complicated metal bits in them (not just a flat sheet of metal =) )
> 
> Under what circumstances are drums better than rotors?  Performance cars use
> rotors all way round. Most braking is done with the front wheels, which are
> always rotors.  So why do car manufactures still use drums?  To subsidize
> people for being able to charge us more?
> 

Oh - you misunderstand.  More comlicated != better.  Drums are very poor
compared to disks these days.  My truck has disk front and drum rear -
I'm seriously thiniking about buying the disk conversion kit for the
rear so that I can stop from 35MPH in less than a nautical mile...

Drums, are cheap and easy to produce - there's tons of tooling that has
paid for itself many times over with regard to drum production.  I think
the theory is mostly pandering to the lowest common denominator.  Drums 
have less of a chance of locking up the wheel it's attached to - less
chance of skid - less chance of accident.

Think of it like this, I'll use my truck as an example:
The rear end of my truck weighs significantly less than the front. 
During hard braking, the front will dive, causing even more weight to
shift forward.  Since the front is doing most of the braking at this
point, it is equiped with disk brakes because the combination of
coefficiant of friction + weight equals forces that the front disks can
handle without locking up.  Now the rear end, on the other hand, doesn't
have nearly as much weight on it, thus the combo of coeficient of
friction + weight is much lower.  A disk brake setup on the rear would
*easily* lock the rears and cause a spectacular spinout - since dynamic
coefficient of friction is much loweer than static - this results in the
front end slowing down and the rear continuing on at-speed - creating a
spin.  This is especially dangerous in slippery road conditions.  Since
the rear has drums, it's more difficult for the drums to overpower
friction and stop the tires from spinning - resulting in a somewhat
safer panic-stop situation.

However, there are drawbacks.  The above relates to the necessity to
pander to people who have no idea how to properly modulate brake
pressure while manuvering.  To an experienced driver, disks are better
since they can modulate the brakes to keep all four tires unlocked and
stop in impressive times - but the average person can't/won't.  To make
it safer in panic situations, overall braking performance decreases - as
a result of having drums in the back.

My truck in particular has an odd bit of technology called RWOAL (Rear
Wheel Only Anti-Lock).  Essentially, when the sensor determines the rear
has locked up, it pulses the brakes (using excess pressure in the master
cylinder) in an attempt to unlock them.  If they're still locked after
the three pulses (all in one second), it relieves *ALL* line pressure
from the rear brakes and the rear tires spin freely.  in order to get
them to apply pressure on the drums again, you have to pump the brake. 
This is a serious piece of heartburn for people that know how to drive,
but saves the asses of the unknowing millions from dangerous spinouts.

RWOAL is *not* the same as true 4-wheel antilock brakes - which use a
batch of  electronic pumps to modulate brake line pressure to all four
wheels, keeping them unlocked, but still slowing the car.

Kurt
(car-geek at large...)