In the November issue of Chevy High Performance president of SSBC Michael Jonas claimed that a car with rear disc brakes will stop in 25 to 35 percent shorter stopping distance than an equivalent car with rear drum brakes. Am I the only one who thinks that statement is off the wall? Of course disc brakes are better, but you'd practically have to disconnect the rear drum brakes completely in order for the rear disc car to stop 35 percent shorter. Where am I going wrong?:confused:

might have something to do with being able to better modulate the rear wheel lock up with discs compared to drums.
drums are "self actuating"- which means that once they grab, they kind of wedge tighter and cause the wheels to lockup, which is bad for optimum stopping. you might not even notice it and think things are working properly, but are leaving a lot of stopping power on the table.

Yes, rear discs are more stable with less lockup problems, so you can dial in more rear brake bias and get a little more rear brake.

That said, there IS a guy that won 10 national autocross championships in C Prepared using drum brakes on the rear.

But 35% improvement? I doubt it.
David

I did a rear disc swap recently on my 68 and would not agree that it's stopping in 35% shorter distance. No way!!! My drum brakes were in good shape and worked fine though and I think they may (no - of course they are) comparing worse case scenario to best case scenario and then stretching it a little. At first I actually thought it stopped worse but realized it was due to the "grabbing effect" no longer being there that the above person described. At first they feel less sensitve but that's actually a good thing as described above.

I'm nopt buying it either. The rear brakes aren't even "in charge of" 35% of the cars stopping power!

It was my understanding for normal street driving drums in the rear combined with front disc's will be very close to the same as disc's in the back with the same fronts. The advantage to the disc's is they won't fade as much as drums under hard usage...

In a full panic stop with correctly biased braking front to rear and yes there is a difference in the amount of braking duty given to the rear of drum car versus a disc car. If done correctly the master should be changed (larger piston utilized in 4 wheel disc cars vs 4 wheel drum, the only reason I mention this is that a great deal of front conversions get away with out changing master therefore when rear is converted = wrong master) there would be a noticable difference in stopping distance. If this did not hold true rear discs would be an option on most performance oriented cars.

In a full panic stop with correctly biased braking front to rear and yes there is a difference in the amount of braking duty given to the rear of drum car versus a disc car. If done correctly the master should be changed (larger piston utilized in 4 wheel disc cars vs 4 wheel drum, the only reason I mention this is that a great deal of front conversions get away with out changing master therefore when rear is converted = wrong master) there would be a noticable difference in stopping distance. If this did not hold true rear discs would be an option on most performance oriented cars.

I'm very interested in what you said above regarding the master needing to be changed when going to a 4-wheel disc setup. When I asked Stainless Steel Brakes tech guy (very rude and impatient dude to be working in customer service by the way) that question when I bought their single-piston rear disc conversion they told me that I didn't need to change the master (which came with their front single-piston power disc conversion I bought about 8 years back). Are you saying that maybe they misinformed me and I should be using a different master now that I have also made the conversion to rear discs??

I'm very interested in what you said above regarding the master needing to be changed when going to a 4-wheel disc setup. When I asked Stainless Steel Brakes tech guy (very rude and impatient dude to be working in customer service by the way) that question when I bought their single-piston rear disc conversion they told me that I didn't need to change the master (which came with their front single-piston power disc conversion I bought about 8 years back). Are you saying that maybe they misinformed me and I should be using a different master now that I have also made the conversion to rear discs??


Bill they probably sold you a disc/disc MC when you got their front kit...

In the November issue of Chevy High Performance president of SSBC Michael Jonas claimed that a car with rear disc brakes will stop in 25 to 35 percent shorter stopping distance than an equivalent car with rear drum brakes. Am I the only one who thinks that statement is off the wall? Of course disc brakes are better, but you'd practically have to disconnect the rear drum brakes completely in order for the rear disc car to stop 35 percent shorter. Where am I going wrong?:confused:

Well, if you skewed the data, I'm pretty confident you could get 25%=35% shorter stopping distance, but in a single stop on dry pavement with the system dialed in for those conditions, I'd bet there is almost no difference (but the discs would be easier to modulate). Now if you wanted to stack the deck, you could set up both cars for the dry, then do a threshold brake maneuver in low friction (wet, snow, etc.) conditions. In this case, the discs should win by a large amount (fronts will lock up too easy with the drums)

Problem with drums is that they are self-energizing (stupid duo-servo brakes!). As a result, brake torque increases with increasing pressure, whereas with discs, brake torque is (roughly) linear with pressure. As a result, it's nearly impossible to get the F/R balance right for all braking conditions (light, moderate, and heavy braking). Tendency is to make the car "safe" by making sure the fronts lock up first under all conditions. This results in greatly reduced rear pressure, which works fine under heavy braking, but gives up a bunch of rear brake potential under light/moderate braking. As a result, under light/moderate braking the rear drum system that is optimized for the dry gives up some rear brake force... Of course, you could dial that back in with an adjustable prop valve, but it'd be spooky under heavy braking in the dry...

Of course, you could also just keep repeating stops until the drums are TOAST and the discs would kcik the drums but there too...

The first brake!!

I figure an all 11" drum set-up on our cars will stop ONCE as good, if not better than an 11" all disc conversion!!

But figuring out how to install 11" drums on our cars EXCEPT for the, what is it??, 201, '65, Z-16 Chevelle front's, no way!!

pdq67

Power brake master cylinder sizes

First gen MC bore drums" 1 1/8"
First gen MC bore disc: 1 1/8"

Second gen disc: 1 1/8"

Most GM cars and pickups that used the 2 15/16" bore front calipers used the 1 1/8" bore up thru the late 70's, also Corvette.

Disc brake master cyls do have larger reservoirs and disc brake systems don't use residual pressure valves, where many drum systems do.
David