Disc or drum

BillGrissom said
I never said or implied that the fastest way to stop was to skid the tires. Everyone knows you want to brake until just before the threshold of skidding, i.e. the old "pump the brakes" method and later ABS. But, if the brakes can skid the tires, they can also allow optimal stopping since that takes less force. That was my point, and it should be obvious to everyone.

No, it should not be obvious, because it is FALSE. Not true. Optimal stopping takes MORE force, not less.

The friction of the tire on the road when skidding is less, you know this because you know the fastest way to stop a car is not skidding the tires. That means it takes less brake force to hold that tire locked than it does to brake the rolling tire, which has more friction with the road. The flawed part of thinking here is that you have to "pass through" optimal stopping to lock the brakes. You do not.

The problem is that the static and kinetic coefficients of friction are not totally linear, and do not behave in a linear fashion. Without a better way to explain it, there's kind of an overlap between the two. A short impulse, ie, stabbing the brakes, can lock a tire with less force than it takes to threshold brake the same tire because you move straight to static friction, you don't have to go through kinetic friction first. The progression to locking the tire does not have to pass through the threshold point in a linear fashion if you just stab the brakes. The locking the tires up test is 100% useless with regard to determining your maximum braking power. Yes, if you can't skid the tires that's bad and your brakes are underperforming. But, just being able to lock up the tires does not mean you can't still improve your stopping distances. And non of that even addresses the fact that the locked brake isn't producing heat, because the rotor or drum isn't turning.

BillGrissom said
Braking is primarily kinetic energy into heat energy. When you stop very quickly, the heat goes mostly into temperature rise in the shoes and drum (or pads and rotor), since no time to dissipate the heat. That temperature is determined by the heat capacity, which is proportional to the mass of steel. If the drum and disk setups have the same total mass, those temperatures would be the same. 9" drums are probably lighter than the factory K-H disks. I don't know about Scarebird or Wilwood. Over a longer time, how fast the brakes dissipate the heat to the air is important, so the system is ready for the next braking cycle. Disks are much better at this, especially rotors with hollow inners with "fan blades" to move the air (not all), along with ducted air blowing on them (modern high-end sports cars). A different scenario is steady braking such as on downhill grades. In that case, at steady-state, the heat generation matches heat dissipation, so disks are much better. It is stupid to rely on brakes on grades but many drivers do so and the fed's mandated front disks for that reason (too many idiots running off cliffs). I think all truckers know better.

You obviously didn't read what I posted about truckers. Yes, it is stupid to rely on your brakes on grades, and that is a problem with drivers education. People don't get it. But, that driving habit by itself doesn't tell you which is a better brake. You can overheat disks too. In most cases (not all) it takes longer, but that depends on the size of the disk and the size of the old drum.

Regardless, you're arguing here that disks actually work better. They dissipate heat better, and withstand adverse conditions better. Being a better driver and being able to mitigate the disadvantages of having drum brakes does not make them better, it actually proves they're worse.

BillGrissom said
Obviously Mopar Muscle did not optimize both systems. If they had an adjustable proportioning valve and adjusted each system until the fronts skidded just before the rears, then both drums and disks should have applied the same force to the rear wheels. The only way that wouldn't be true is if the drums suffered fade and thus less braking at the end. I can't imagine the rear drums were designed to fade during a 60 mph stop. I didn't read they did anything like that, so it wasn't a fair comparison. I doubt their car is using the factory bias-ply tires, and any time you vary from that you need to adjust the F-R balance to get optimum performance. Their article is also about promoting a rear disk system. Buy it if you want, but understand that rear disks are mostly for bling, even in current models.

The article isn't a great one. They didn't really say what they did with the installation, whether they added an adjustable prop valve or not. They did mention that they just installed the Master Power kit, which doesn't include an adjustable prop valve, and they covered in detail the process for defeating the metering valve in the stock combination valve. They didn't say that's what they did, but I bet that's what they did. Stock combination valve with the metering valve removed, turning it into a distribution block.

But here's the thing. They started with a car that's pretty similar to most of the cars on this site. It didn't have bias plys, because almost no one runs those. And if you do you're right, no point in upgrading your brakes, your tires are the weakest link. No, they started with a factory braking system, disks and drums, with oversized radials on the front and rear, with larger rear tires than the front. You know, a set up similar to the majority of classic muscle cars. If anything, the front tires on their duster are wider than most folks run, which should minimize the effect of the rear brakes. And they just bolted on a rear disk kit. I realize the factory system wasn't optimized for the cars set up, but I seriously doubt they spent much time tuning the rear disks either. Especially with zero mentions of an adjustable prop valve being added. Maybe they added one, but I still doubt they perfectly optimized the brakes.

So what does that mean? I think they did what the majority of owners do. They upgraded the factory system, probably didn't fully optimize either system, and still ended up with a better stopping distance. Their gains were probably some percentage of luck, and with such a big improvement the factory system was underperforming. But there are other advantages to disks that may have played a role, namely, they're easier to threshold brake because they have better feel than drums. But that just means tha average drivers will have better braking with disk.

Here's the car in the article, if you didn't bother to look.

whitepunkonnitro said
There is more surface area with drums than discs... Which is exactly why it takes less line pressure to make them work.
On high performance B Bodies, they used 10 inch drums on the back when discs were ordered to maintain some balance. The normal 11 inch rear drums on those cars would overwhelm the disc brakes and cause the rears to lock.
From a historic perspective, NASCAR did not allow disc brakes on their Grand National cars until around 1975. Some people, like Richard Petty stuck with drums until the early 80s.
As someone who has been swapping disc brakes for drums on my own cars since the 70s, I can say first hand that I have never experienced any sort of braking issues from the "downgrade"... But then again, I don't hot lap my cars on autocross tracks.

Taking less line pressure doesn't make you stop faster.

NASCAR also banned Hemi's and wings because they worked too well. Not a sanctioning body I would quote when determining what system works best, unless it's to do the opposite of what they banned because they have a history of banning what works best. A rather long history of banning what works best, in fact.

rich006 said
Yes, their setup stopped the car shorter on dry pavement, but did they try it on wet pavement, maybe while turning? The rear wheels might lock up before the front wheels, making the car less safe to drive than with rear drums. The front/rear balance issue is one reason why I decided to stick with factory equipment when I switched from drum to disk brakes. (My '74 got the '73-'76 large bolt pattern brakes; on a '68 I might have gone with a pre-1973 factory disk setup.)

You run bias plys in the stock factory sizes? Because if you don't, the factory system is not optimized for your car anymore. Regardless, if anything the stock system was rear bias heavy. All of my cars with the factory brakes have locked the rears first with no tuning. Now, they were all running radials, so that might not reflect what happened with factory bias plys. But with the factory brakes and radials like most of us run the system is not optimized already.