How much advantage in adding rear disk brakes?

[RustyRatRod]

Can't speak for where you are but around here the only dirt track guys using drums are in entry level classes where the rules require it.

Not so here. Lots of the pros use drum brakes. They mod the CRAP out of them. Drill cooling holes in drums, cut the shoe material into several small sections, all kinda tricks.

 

[92b]

Not so here. Lots of the pros use drum brakes. They mod the CRAP out of them. Drill cooling holes in drums, cut the shoe material into several small sections, all kinda tricks.

Interesting. I guess the guys around here need to " get with the times".

 

[72bluNblu]

I converted my Duster from 11x2.5" rear drums to 11.7" rear disks from @DoctorDiff ( I run 13" front disks from doctordiff as well). The most noticeable thing for me was that I have better braking control, I can brake harder without locking up the rears and the pedal feel is a lot more accurate than compared to drums. I couldn't say if the car stops significantly shorter, but I know that hard braking events feel a lot more controlled.

And yeah, the drum auto-adjuster thing sucks. They only adjust when you're backing up, and if you always back while turning the same direction (like if you back out of your driveway or into your garage the same way) then you can have one drum tighter than the other. The way I used to park my Challenger and my Duster at my house meant I was frequently turning the same way, and on more than one occasion during hard braking the rear wheel that would be on the outside of that turn would lock up first because it would adjust more frequently than the other side. Disks are always "adjusted" the same.

And DoctorDiff's 10.7" and 11.7" kits work with the OE style tapered adjusters.

Mopar muscle did a rear disk conversion on a '73 Dart Sport and checked the stopping distances between the rear drums and rear disks from 60-0. Their result was that from 60 mph factory disks up front and factory drums in the back the car took 133 feet, 6 inches to stop. After the rear disk conversion, the stoping distance improved to 122 feet 4 inches. It wasn't a super scientific way to test it, but it was better than most of the documentation out there. And the car involved was set up the same way as a lot of these cars, much larger rear wheels than fronts. The 80/20 thing is an estimate, it changes a lot based on how the car is set up- it's actual weight balance, the front to rear rake, the size of the rear tires. If you run much larger wheels out back than up front your balance may be a lot closer to 70/30.

The online article is a total mess now, probably something with being converted over or moved to Hot Rod when MM was bought out. The final distance used to be a caption on the second to last picture, but I don't see the captions popping up anymore. The hardcopy article is easier to follow, but that's the way it goes.

Rear Disc Brakes - All Bound Up - Mopar Muscle Magazine

 

[RustyRatRod]

Interesting. I guess the guys around here need to " get with the times".

I think it entirely depends on the track length. Most of the tracks around here are short tracks with relatively slow speeds compared to long tracks, so the brakes don't get as hot. On the longer tracks, I think disc brakes have an advantage because they don't fade with the heat.

 

[92b]

I think it entirely depends on the track length. Most of the tracks around here are short tracks with relatively slow speeds compared to long tracks, so the brakes don't get as hot. On the longer tracks, I think disc brakes have an advantage because they don't fade with the heat.

I see. So drums are probably an advantage on tracks where you don't use your brakes as much.

 

[DionR]

I remember @AndyF saying that The Red Brick used to brake fine from 160 with rear drums.

I put disks on my 8.25, partly because I wanted to. I do think they have better control at the edge of traction and are more consistent due to the lack of self-energizing. But I didn't figure I was going to see any real performance advantage.

 

[DionR]

Is that an 8.25 rear?? How did you do the e brake setup?

I was thinking about doing this to my '42 Dodge pickup because it has an 8.25 and I have the Jeep brake setup laying here.

I have the e-brake working well on mine. Couldn't see doing without an e-brake due to the 4 speed. I used the GC cables by cutting off the end of the factory cable and then "un-swedging" the jacket end. Then I cut the jacket down and put the original jacket end back on (didn't swedge them though) and just stuck that in the OEM e-brake mount on the frame. Then I tried to get the cable cut to the right length and get new ends swedge on. I say "tried" because they were still like 3" too long and required a loop in the original cable to get everything working. Need to go back and fix that. But it has been working well, holds the car on a good hill.

Be aware that the cable is plastic coated, so make sure to strip that off before getting new ends swedge on. I didn't and pulled the end off one cable on the first go round. Luckily it was when I applied the brake and I was still sitting in the car so I could shut the car off and put it in gear. Hate have seen the results if I had been away from the car and it started rolling.

 

[AJ/FormS]

My car does not have rear discs. She is a streeter.
She has 235/60-14s up front and 295/50-15s out back.
She has the KH 4-piston calipers up front and 10X2 drums in the back.
I gutted my P-valve, and adjusted the rear "proportioning", by changing the w/c's to 7/8s, then to 15/16.
She stops real nice with the front-end only diving a little (1.03 bars and HD Munroes)
The whole car just digs, how I imagine throwing out a parachute would feel like.
But; I wear out rear shoes about two to three times more often than the fronts, which is by design, cuz I have plenty of shoes and only one spare set of new pads left.
But I also have an 11/1 360 with a 4-speed so, you-know, plenty of compression braking.

BTW; the rear self adjusters, in the factory system, should not be self adjusting under light braking. The P-valve is actually a hold-off valve, so it takes a good amount of pedal to get the rear shoes
moving, and even more to get a click of adjustment. I run NO Proportioning at all and have never had an issue, and I back up every time I back out of my carport, ending in a 90* turn. I never give the brakes a second thought.
IMO, warning; OPINION coming;
To you guys with adjuster issues;
I'd be willing to bet that you guys are running automatics, and probably "built" ones and IMO, probably need to idle your engines down so they don't bang into gear and/or pull so hard. And before you say that your big cam monster street engine won't idle any slower; if it's smaller than [email protected]; that's malarkey. My [email protected] PurpleCam would idle down to 550 still pulling itself in First gear with an A833 and 3.55s.
Some of you just insist on running mountains of timing, because you can get away with it. Well here is just one example of why not to. I mean why do you feel the need to run 18 to 25 degrees of more of IdleTiming, with a 2400 or more stall, hmmmmm? Do you think the springs might not be able to keep up, lol. I don't get it, and I don't tune that way. Furthermore;
Ma Mopar put a wavy-spring in the Hi-drum for a reason. And a spring on the L/R servo for another reason. But you know, your "built trans" guy, probably left the one on the build-table and probably never gave the other any thought.
Anyway, this ain't helping the OP, sorry for the sidebar.

 

[72bluNblu]

My car does not have rear discs. She is a streeter.
She has 235/60-14s up front and 295/50-15s out back.
She has the KH 4-piston calipers up front and 10X2 drums in the back.
I gutted my P-valve, and adjusted the rear "proportioning", by changing the w/c's to 7/8s, then to 15/16.
She stops real nice with the front-end only diving a little (1.03 bars and HD Munroes)
The whole car just digs, how I imagine throwing out a parachute would feel like.
But; I wear out rear shoes about two to three times more often than the fronts, which is by design, cuz I have plenty of shoes and only one spare set of new pads left.
But I also have an 11/1 360 with a 4-speed so, you-know, plenty of compression braking.

BTW; the rear self adjusters, in the factory system, should not be self adjusting under light braking. The P-valve is actually a hold-off valve, so it takes a good amount of pedal to get the rear shoes
moving, and even more to get a click of adjustment. I run NO Proportioning at all and have never had an issue, and I back up every time I back out of my carport, ending in a 90* turn. I never give the brakes a second thought.
IMO, warning; OPINION coming;
To you guys with adjuster issues;
I'd be willing to bet that you guys are running automatics, and probably "built" ones and IMO, probably need to idle your engines down so they don't bang into gear and/or pull so hard. And before you say that your big cam monster street engine won't idle any slower; if it's smaller than [email protected]; that's malarkey. My [email protected] PurpleCam would idle down to 550 still pulling itself in First gear with an A833 and 3.55s.
Some of you just insist on running mountains of timing, because you can get away with it. Well here is just one example of why not to. I mean why do you feel the need to run 18 to 25 degrees of more of IdleTiming, with a 2400 or more stall, hmmmmm? Do you think the springs might not be able to keep up, lol. I don't get it, and I don't tune that way. Furthermore;
Ma Mopar put a wavy-spring in the Hi-drum for a reason. And a spring on the L/R servo for another reason. But you know, your "built trans" guy, probably left the one on the build-table and probably never gave the other any thought.
Anyway, this ain't helping the OP, sorry for the sidebar.

Nope.

Manual transmission in my Duster, no converter at all. Drums were fully rebuilt and adjusting exactly as they should with nice new springs and the adjusters spinning nice and loose before I swapped them out for disks. Challenger was an auto but it had a basically stock 318 with a stock transmission and no crazy idle or anything like that. Drums fully rebuilt and adjusting as they should. Timing shouldn't change a thing unless it's altered your idle speed, and even that's a stretch. So your argument is we're riding the brakes too hard in reverse and adjusting more than is necessary? Except the brakes can't adjust unless there's space for them to do that. Otherwise you'd always have the rear brakes dragging, which should be really obvious for anyone.

Simply put, if you always turn the same way when you're in reverse, no matter what your engine or brakes are doing, you can bet your adjuster on the outside of that turn will click over first. Will the brakes always be unbalanced? Of course not, the other side should follow soon enough. But if you drive enough you will end up making a hard stop and find you locked up one rear drum because it wasn't balanced on the adjustment. Been there, done that. Adjusted the brakes after and guess what? One extra click needed on the "inside turn" wheel.

As far as brake wear, I have driven old Brit sports cars, old American trucks, new vehicles with ABS both trucks and cars, and old stuff with manual transmissions and 10:1 compression like my Duster where I use a lot of compression braking, all front disk/rear drum. I've never experienced anything more than about a 2:1 wear ratio for the front pads vs the rear shoes (2 sets of front pads for every one set of shoes). Hell I've gotten 3:1 on some of the trucks. If you're changing more shoes than pads, well man, that just goes against physics. By the CG you should be doing about 70% of the braking with the front brakes, even with the big rear tires. So, 30% with the rear. The math is easy, 70% is a little more than twice as much right? So 2 sets of pads for 1 set of shoes. Even 1:1 wouldn't be right, to have the rears doing as much as the fronts is *** backward. To have them doing 2 to 3 times as much? You're saying you do 70%+ of your braking with the rear brakes. Despite the physics that says 70% or more should be up front.

 

[Ironracer]

Not close to the same improvement you made from drum front to disc fronts.

All I know is, on a scooter, 85% of Your Stopping power is up Front

 

[MoparLeo]

IMO save your money for something else that your car needs. The rear brakes are just fine.
If your car is being used for a very special use, I am sure there are also a lot of other things that are also modified.

 

[Car Nut]

Are we talking sandbagging ?

Sandbagging?

 

[YY1]

My 2000 Dakota R/T didn't need rear shoes until almost 100,000 miles!
Many of those miles were either "spirited" or hauling or towing something.
During that time I think I put on three sets of front pads and 2 sets of rotors.

I'm glad AJFormS mentioned wheel cylinder sizes as I'm entirely convinced that's how the factory achieved "proportioning" rather than with a valve like you read about so often.

I've seen a LOT of people not touch the adjusters when doing rear shoes.
I always inspect them, lube them inside and out, replace them if they bind, and manually activate them by pulling the cable to verify operation. Routing that cable correctly is crucial as well.

 

[Oldiron440]

I’ve got 11” drums on the rears and have no problem knocking off 20 miles an hour in the traps when needed.