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

[geeks at sunhelp.org]

Debates of cost/effectiveness aside, having driven many vehicles with front
drum brakes (incl trucks), I can say that the best feature of disks is
cooling. Drum brakes hold heat and the shoes have no where to disapate heat
to. Nothing compares to coming down a long hill and experiencing brake fade
(been there). I've had drums fade on a pickup truck so bad that I could
stand on the power brake pedal and it didn't give me all the stopage I
wanted.

Disk pads can eliminate heat into the airflow around the wheel, the large
thermal mass of the caliper and into the disk itself, since it never looses
complete contact with the disk. Also, the disk can shed heat into the
airflow too.

Drum shoes are enclosed in the drum, so no airflow. The cylinder is enclosed
in that space too. Shoes don't touch much metal, so there is not much heat
transfer by contact. There is airflow around the drum, but not as much as a
disk (drums nearly fill the space inside the rim). There is also much less
fluid in a drum cylinder, so it heats up quicker.

James Fogg, Network Engineer
Vicinity Corporation - New Hampshire
(603) 442-1751

~ -----Original Message-----
~ From: Kurt Huhn [mailto:kurt at k-huhn.com]
~ Sent: Tuesday, March 12, 2002 12:38 PM
~ To: geeks at sunhelp.org
~ Subject: [geeks] Drum versus disk brakes (Was RE: car 
~ question RESOLVED)
~ 
~ 
~ > > 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...)
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