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Will Rear Disc's Stop Better Than Rear Drums??

54K views 90 replies 45 participants last post by  Irelands child 
#1 ·
I did a search on this but didn't get a definitive answer. Is it worth the time & money to convert my rear drum brakes over to disc? My rod has 11" front disc's, with a Ford 9" rear end & drums, power booster, combination valve and 205/65-15 tires in front, 235/70-15 rears.
 
#27 · (Edited)
No disrespect but, where are you getting your info. on braking? A 60/40 braking bias is a recipe for disaster! The rear brakes do aid in braking but, ad more to controling the direction to which the car in steering.

The normal braking bias is 70% and 30% max this is greatly in due to the dive designed into most vehicles.

I highly recommend visiting http://www.stoptech.com to better understand the complexity of the braking systems.

Brake bias.

http://www.stoptech.com/tech_info/wp_brakebiasandperformance.shtml

Some stockcars can achieve almost even brake bias front and aft due in part to anti-dive designed into the car.


Deuce said:
...........
It is generally accepted that the front brakes does 60 % of the braking and the rear does the rest ( 40 % ) .....on most vehicles.
 
#28 ·
Bang the drum

I had a 66 gmc 20 years ago that had manual drums. The stopping performance was so bad, I swapped in late model C10 power disk brakes, and as you would guess, braking improved dramatically. Fast forward to my current project, a 65 gmc. It has the same drum brakes, but it has a factory power booster. It stops better than the first truck did after modification.
If I were looking to improve braking on a rig, before I spent big money on a conversion, i'd Increase the pedal leverage, or increase the vacuum booster and see what happened. I hated drum brakes when I was younger, but I'm leaving mine in place.
 
#29 ·
A lot of cars and trucks have 60/40 displacement. Of these, many also have the same size rotors, and same type pads, front and rear, but most are also set up to fade rear first. I dont think it is a recipe for disaster unless it does not match the car. Most FWDs are 80/20.

I'm not pretending that displacement is the same as braking torque, but it does play a major role, especially when setting up your own ride. A heavy car or truck can easily have 60/40 displacement. Proportioning valves are your friend, but they should be used to optimize braking, not fix an 'oops' in the setup.

The displacement info can be determined through mathematics, or looking up the specifications in either the service manual, if listed, or the AMA specs.
 
#31 ·
Disk or Drum

Well as for me I have had both on my coupe ,and I can say the disk brake set up is much better , it stops better and less work to maintain and I think they look better . But they both worked . So it's up to what you like and can afford .

FAT40 :thumbup:
 
#32 ·
Triaged said:
I take it you are talking about anti-dive in the front end?...or are you talking about anti-lift at the back end?

Care to explain any of this to me? Why would the nose diving cause the rear tires to slide? How much anti-dive are we talking about?

I apologize for the delay in answering. I will try to keep it short and simple.

BOTH,

Think how it would be if there were no springs but solid bars. No lift, no dive, no squat. But we must compromise.

If you ever have the opportunity to see pictures of a Fox or SN-95 Mustang doing a maximum effort stop (in lots of magazines) and then the new BMW video ad showing the Beemer stopping..... you will see that the Mustang pitches nose down 3-4 inches and the rear rises 3-4 inches and the Beemer stays almost level to maybe an inch.

Front and rear geometry are fixed by design, and by nature are a compromise of the lift and dive (squat) by their design. What works very well for drag launches is poor for stopping ability, etc.

When a front end nose dives the weight of the vehicle is being removed from the wheels on each end. Actually if the front end did not dive, the center of gravity would stay higher and there would be more downforce on the front tires. If during a stop the front springs were suddenly solid... there would be more downforce on the tires.

When the rear squats on launch, the weight bias is being removed from the rear wheels. Either way a stiffer spring or better geometry would improve the traction.

Suprisingly shorter ladder bars are are good for the rear. They lift the rear on launch, planting the rear tires, and when stopping, they are short enough to try to "pull" the rear of the vehicle downward when braking. But ladder bars are not the best for either situation and have other handling deficiencies.

When the front end drops and the rear rises during a stop, the rear tires are especially unloaded and traction suffers. Even with sticky rubber there is not enough weight on the contact patch to maintain reasonable traction.

Most stopping ability is determined by the tires themselves, by contact patch size and rubber compound. Skinny front runners are poor stoppers. If you have ever driven a drag race car on the street you could appreciate the deficiencies of the suspension settings.

Me? I drive a 5 series Beemer every day.
 
#33 · (Edited)
ramblin65 said:
Xntrik,

Who makes the best shoes in your opinion? Has anyone tried these folks? http://www.praisedynobrake.com/index.htm

I've got 4 wheel non-power drums on my 65 Rambler Classic, and just want to find the best shoes for it.


I have had very good service with Wagner products.
If available I prefer the ThermoQuiet series.

Auto Zone sells a high performance brand that seems to work well for many people. Be advised that rotor material varies and so does the pad material... Not every pad is compatible with every rotor. I have seen new rotors installed with new pads and in 8,000 easy miles both were totally ruined.

So many people buy the cheapest pads they can get, insisting that lifetime guarantee is the important thing.. Cheapo NAPA or AZ pads are false economy. Buy good stuff that will STOP.

You can't imagine how many people I have seen convert to disk brakes and then buy $ 8 pads.... and later complain about increased pedal pressure and limited stopping ability.....they run around in circles chasing master cylinders, rubber brake lines, bleeding brakes. mmmmmm.
 
#35 ·
Hi. Lots of opinions on this topic. Me, I would go to a deserted stretch of road, go about 30 mph, slam on the brakes and see if I could lock up all 4 tires. If all 4 tires don't lockup you don't have enough brakes. IMHO brakes and tires go hand in hand. Put some skinny bias plys on and see how easy the brakes lockup compared to a set of 60 series radials. It's my understanding that you get the shortest stopping distance by keeping the tires on the verge of lockup. Sooo if you can't lockup the tires then you don't know what the tires are capable of in terms of stopping power. If you can lockup all 4 tires then ( in my mind anyway) you've got enough brakes unless you upgrade the tires to the point where they overcome the brakes.

I've been trying to get the rear drum brakes ( 85 Monte Carlo rearend ) to lockup on my 38 Ford PU. The truck stops pretty good but I know it can be made better so that's something to work on this winter.

I have 255/15 70r tires on the back of the truck. My question is, does the large size make it harder to lockup the rear tire? It would seem like it would because of the slower drum rpm ( large dia tire ) which exposes less drum/shoe contact surface per road foot than a smaller dia tire. The tire also has more rotating mass. Also the contact patch of the bigger tire is naturally greater which makes it harder for the brakes to overcome the tire.

Well that's enough rambling for now.


Moon
 
#36 ·
xntrik said:
.

If you ever have the opportunity to see pictures of a Fox or SN-95 Mustang doing a maximum effort stop (in lots of magazines) and then the new BMW video ad showing the Beemer stopping..... you will see that the Mustang pitches nose down 3-4 inches and the rear rises 3-4 inches and the Beemer stays almost level to maybe an inch.

Front and rear geometry are fixed by design, and by nature are a compromise of the lift and dive (squat) by their design. What works very well for drag launches is poor for stopping ability, etc.

When a front end nose dives the weight of the vehicle is being removed from the wheels on each end. Actually if the front end did not dive, the center of gravity would stay higher and there would be more downforce on the front tires. If during a stop the front springs were suddenly solid... there would be more downforce on the tires.
I find this quote misleading. It would seem that you are saying that weight transfer under braking is caused by chassis dive, which is incorrect. At the point that the chassis begins to pitch forward, weight transfer has/is already taking place. The only portion of weight transfer that can be assigned to chassis movement would be that caused by the vertical movement of the center of gravity (CG) under dive (or squat under acceleration). IIRC, in his book Tune To Win, Carroll Smith attributes something like 5% of weight transfer to movement of the CG. Additionally, from the formula HERE , weight transfer is purely a function of vehicle weight, CG height, wheelbase, and brake pad coefficient of friction.

Ramblin65,

There are several people on another board I frequent (Moparts), including a couple of autocrossers, who have used and recommend the Praise Dyno brake shoes. You might do a search over there for more info.
 
#37 ·
Moon said:
...If you can lockup all 4 tires then ( in my mind anyway) you've got enough brakes unless you upgrade the tires to the point where they overcome the brakes...

I've been trying to get the rear drum brakes ( 85 Monte Carlo rearend ) to lockup on my 38 Ford PU. The truck stops pretty good but I know it can be made better so that's something to work on this winter.
While I agree that if you can't lock up all 4 that you have serious problems, just being able to lock them up doesn't make for good (or even sufficient) brakes. Try stopping from 80 as fast as you can without locking up the brakes. Then do it again and again 'till the brakes fade and you can't even stop the vehicle. If you can't get at lease a few stops in without the brakes fading than you need more brakes.

As for your problem I think I have a solution. Try using rear wheel cylinders from a manual brake S10 of the same year. They came with larger wheel cylinders and will give you some more rear brakes.
 
#38 ·
Rear Brake Lockup

Moon[/QUOTE]
I've been trying to get the rear drum brakes ( 85 Monte Carlo rearend ) to lockup on my 38 Ford PU. The truck stops pretty good but I know it can be made better so that's something to work on this winter.
Moon[/QUOTE]

Unless you like driving backwards at high speed, I would recommend abandoning this pursuit....Particularly since A/G GM rear brakes get really grabby in the wet....
 
#39 ·
QUOTE--"Unless you like driving backwards at high speed, I would recommend abandoning this pursuit....Particularly since A/G GM rear brakes get really grabby in the wet...."

I'd never try to lock them up going more than 30 mph. But I can't even lock them up going 5 mph on the grass in my backyard. That's not right. I've got the proportioning valve ( Speedway.. on the rear brake line ) wide open. My goal is to be able to lock the rears up at 30 mph and then I can start closing the proportioning valve down until they barely squeal. This should give me about the shortest stopping distance with the setup I have.

One possible problem I might have is the master cyl is a disc/disc and I have a disc/drum. I don't know if the internals are different than a disc/drum. Guess I'll have to call one of the brake vendors and ask the tech dept

Moon
 
#40 ·
The 85 Monte rear cylinders are 3/4" bore. They might be too small for you. If you want more rear brake, try swapping in some 7/8" cylinders listed for a manual brake 84 S10. They are the same clip-in style, and should give about 25% more braking back there.

That is, if you haven't already done this :embarrass
 
#42 ·
IIRC the larger (7/8") wheel cylinders were about $5 each for new ones. When you have it apart you might as well spend another $12 and put new shoes on it.

I used them to balance out the 12" discs I put in up front and they worked great even with the stock prop valve.

As for master cylinders there is no difference between disc/drum and disc/disc except maybe bore and the size of the reservoirs.
 
#43 ·
Master Cylinder

Moon said:
One possible problem I might have is the master cyl is a disc/disc and I have a disc/drum. I don't know if the internals are different than a disc/drum. Guess I'll have to call one of the brake vendors and ask the tech dept

Moon
What you're missing is the residual pressure check valve....I don't know if you can put one in a disc/disc master cylinder....I've removed them from drum/drum master cylinders to make front discs work better....I suggest you get an A/G body master cylinder and hook it up just like the service manual for a Monte Carlo or Malibu tells you....Then you'll have a built-in residual pressure check valve that you need for your rear brakes....
 
#44 ·
xautoracer said:
What you're missing is the residual pressure check valve....I don't know if you can put one in a disc/disc master cylinder....I've removed them from drum/drum master cylinders to make front discs work better....I suggest you get an A/G body master cylinder and hook it up just like the service manual for a Monte Carlo or Malibu tells you....Then you'll have a built-in residual pressure check valve that you need for your rear brakes....
I don't think GM has put the residual pressure valve in the master cylinder for a very long time (if ever). It will be in the combination valve (along with the proportioning valve, hold off valve, and idiot light switch).
 
#45 · (Edited)
I've got the proportioning valve ( Speedway.. on the rear brake line ) wide open. My goal is to be able to lock the rears up at 30 mph and then I can start closing the proportioning valve down until they barely squeal.
Hello Moon,

I picked up on your statement above and I want to make sure I understand it correctly, I was struggling with my rear brakes and the proportioning valve until I discovered that the adjusting knob should be turned the opposite of what I thought. It should be screwed in (clockwise) all the way to give you maximum pressure to the rear lines and brakes. It should be backed out all the way open to REDUCE the rear brake pressure. You stated that you have the valve "wide open". If it is the same type of valve I have then this would give you minimal brakes in the rear. We talked about this at this link too with a picture of the valve I have:

http://www.hotrodders.com/forum/proportioning-valve-70163.html?highlight=proportioning+valve

Dan B.
 
#46 ·
Hey Dan... guess which way my proportioning valve is turned. Bingo, all the way out. I was an industrial plumber before I retired and used to work with flow controls once in awhile. When those valves are turned all the way out your at full flow. I ASSUMED the prop valve was the same. Don't think I read the instructions either. I sure would be interested to see an internal flow diagram for one of these valves. Sometimes I have a hard time pictureing something until I see the print. I'll try cranking the valve all the way in and see what happens. Nice catch Dan, thanks.

Moon
 
#48 ·
Back to the topic...

The Question was " will discs stop better thabn Drums".In my experience.Yes!,I have a several old mercedes here,the newest car i drive is a 1974 280,with 4 wheel discs,Stopping is so easy in traffic you'd think it had ABS ,which was an option on the '78 and later big mercedes models.One of my other projects has vented rear rotors( a '69 300SEL 6.3) ,somethinmg you DO NOT get today,and does it stop?,does a fish swim?,
Mercedes started using 4 wheel discs in 1959 on the 300SE cars,after they got their *** kicked at Lemans by Jaguar,which had discs and mercedes still persisted with drums.
And why is this????
When you apply the brakes,you are creating heat energy...with drums they hold all heat in the drum,the heat doesn't dissipate very quickly and you get overheating and in the old days you lost your brakes. :(
Discs on the other hand are right out in the breeze,so the heat energy being generated by the braking action is being dissipated into the air and the more you apply the brakes ,the more energy is being absorbed and lost in the air...Better brakes.!!!!Thats why disc pads are so small,they don't need that big area that brake shoes require to lose that heat energy.
As for Drums being better in a damp climate,that sounds a bit strange to me,I spent years clearing mud out of Drum brakes on landrovers etc because once it's in there,it cannot escape,unlike the self cleaning action of the discs.

It's the 21 st century now,get with the program and STOP! :welcome:
 
#49 ·
I havent read all four pages but why bother, i'm sure they all say DO IT! after having a buick regal with 4WDB i just had to have them on my 55 truck and it stops on a dime, for a ton of bricks like it to stop that fast is absolutely amazing.

here is a link to a guy i just found out is near me, that is WAY LESS than anybody i've heard of: www.scarebird.com i plan on putting 4WDB on my Tempest before i drop the 455 into it. cheap cheap cheap! (i have not used any of these products... yet) but rear brackets for under a Buck is way better than paying $500 like i did for brackets and factory calipers. and btw! at the Truck dealer they have all these Firestone airbags and compressors out there, the counterman says if you want to do all the cutting on the control arms or wherever, and he says he has scores of kids coming in there to buy these things for their cars, (honda's i would assume) you can get a complete airbag suspension for half of what you would pay AirRide Technologies. the airbags should be online somewhere as they are made by firestone, and the truck dealer has all the hoses and fittings as well
 
#51 ·
black1970 said:
rear disc are hard to get to work right and they are expensive. got to be just right. take it from one who knows. disc/drum are the simplist and best.
I have to disagree,I may also venture an opinion here,you may think drums are simpler because they are already on the diffs you are using.
Have you seriously considered the benefits of NOT having to adjust brakes shoes all the time?
Or how about "brake FADE"?
If your brakes have self adjustors ,do they still work after 20 or so years?
What is simple about Ford or Bendix brake systems,?
Can you really say that Mopar rear drum brtakes are simple?
Can you change both sets of rear shoes in under 1/2 an hour ?
and honestly say you enjoy removing stuck drums full of deadly asbestos then have to fight with at least 4 springs ,retainers,leaky wheel cylinders,hard to find bonded or riveted linings,scored drums ..... :drunk:
How about rusted or seized ajustors or seized articulating pins?
You see discs don't have ANY of these problems because the bits aint there.
...But have you ever considered why Disc brakes were utilised on cars in the first place?.
It was to get better brakes with the utmost simplicity and effectiveness...if drums were so good they would still be used on PERFORMANCE cars..I admit that even Toyota use rear drums on the Cheapo hacks still.
And HEY! we're Building High Performance vehicles here!, It's not good enough to build Big power and then use 19th Century technology to try and stop the thing ,Is It? :)
 
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