Drum brakes : Braking force multiplied?


#1

Proponents of drum brakes say one benefit that design provides is the braking force applied by the piston in the wheel cylinder is multiplied. What’s the mechanism for that?

Photo source: Mopar Magazine, Nov 2018


#2

Uh oh. Here we go again…the leading (front) shoe pushes out on the drum, and the drim’s rotation helps energize the back shoe, along with the brake cylinder. I wouldn’t call it “multiplied “.


#3

The flip side of this is that braking force is reduced when going backwards, when holding at a stop on a steep hill with manual brakes, it could take both feet on the pedal to keep from creeping backwards. Been there.


#4

Is this what you mean by “energize the back shoe”?

In the photo above the front of the vehicle (by way of the driveshaft orientation in the background) is toward the right. So the right shoe is the front shoe, right? The drum is rotating clockwise (meaning the car is going forward) and the brake pedal is applied. That forces the wheel cylinder to spread those little wings slotted to both shoes in opposite directions, moving both shoes outward. The rear shoe (left side) butts up against the post at the top, so it can’t push on the front shoe. But the front shoe (right) rotates slightly clockwise, so the bottom of the front shoe presses on the bottom of the rear shoes via the adjuster. So the rear shoe is pressed outward both at the top (by the cylinder) and at the bottom (by the front shoe). So the rear shoe grabs against the drum with more force than the front shoe. It is sort of harvesting the momentum of the vehicle and turning it into force applied to the rear shoe, via the rotation of the drum. Does that sound correct?


#5

I’m not seeing that. It seems like it would work in reverse the same way, only the “extra” force would be applied to the front shoe rather than the rear shoe. The reason it might not be as effective in reverse is just that you are going slower in reverse, or even stopped. The extra force comes from the movement of the car, and if the car is stopped there isn’t any extra force available.


#6

A drum brake has a much larger friction (aka swept) area so it takes less pressure to stop the wheel from turning. There may or may not be a multiplication factor depending on the piston area in the MC vs the total piston area in all the wheel cylinders, I’ll leave that to the physicists to discuss that part. It’s been a long time since I last studied physics.

The down side of drum brakes is that they retain both heat and water much more than disc brakes. Drum brakes can start to fade even in the first stop from 60 or higher and if you go through a puddle of water, you better not need to stop on the other side because chances are you won’t.


#7

The self energizing of the leading shoe was the reason Chrysler Corporation used double leading shoe brakes on the front wheels. This required a single wheel cylinder at the top and bottom of each wheel rather than the double wheel cylinder at the bottom we have on the drum brakes we still have on some car today.

This helped stopping power a lot, before cars had power brakes.

Chrysler made the first mass produced hydraulic car brakes in 1924, only Dusenberg was earlier in 1923 but there brakes did not work very well and the next year they changed to the Chrysler patent design.


#8

You could really tell by using the parking brake. In neutral a car would roll backward but never forward with the parking brake on. The lining on the front shoe is supposed to be shorter and thicker. The adjuster would angle so as to force the back shoe into harder contact with the drum. The brakes pictured are almost worn out because most of the front lining is gone. You can’t always picture what causes things to happen. Experience and experimentation can tell you that it does happen.


#9

Keep in mind, there are different designs of drum brakes. What you show is a duo-servo. Not all drum brakes us a duo servo. Here are 5 different configurations of drum brakes.

Note the drawing showing 5 different drum brake configurations.

The duo-simplex gives you two shoes that gain force but require two cylinders. Some early Chryslers had this, from what I’ve heard. One of the reasons you don’t see drum brakes on the front of cars anymore is that small errors in adjustment give large errors in brake torque when you design in a lot of gain. EU brake standards require very equal brake force side to side and drum brakes won’t pass anymore. Also, the picture shows pressure distribution across the shoe face - and it is not equal by any measure. Variations in drum inner diameter, shoe arc and wear patterns give large variations in brake torque with a given hydraulic pressure.


#10

Considering George_SJ’s response, we may both be correct about reverse braking experiences as different designs of drum brakes behave differently. Referring to Wikipedia’s drum brake entry, there is a graphic of 5 designs: Simplex, Duplex, Uni-servo, Duo-servo, and Duo-duplex. In both Servo versions the frictional forces of the “leading” or primary pad is transferred to the “trailing” or secondary pad increasing the latter’s pressure and effectiveness. The Uni-servo design will have weak braking in reverse where-as, as George points out, the Duo-servo version should work equally well in either direction (provided the design is geometrically symmetric). The image the OP presented appears to be Duo-servo, the servo action works in both directions. I don’t recall the specifics of my heavy '50 Pontiac’s manual drum brakes, but when loaded it was nearly impossible to hold when facing up a very steep hill, but no problems facing down the same hill.

Assuming frictional force is close to the Newtonian ideal, absolute speed shouldn’t affect braking effectiveness (until things get hot and materials change). Once any of the servo brake designs are mechanically loaded with all forces acting (some movement is required to take up slack), whether they are static or moving shouldn’t affect the servo action. In most materials systems static friction is greater than sliding friction.

George_San_Jose1

    December 3

ken2116:
The flip side of this is that braking force is reduced when going backwards, when holding at a stop on a steep hill with manual brakes, it could take both feet on the pedal to keep from creeping backwards.

I’m not seeing that. It seems like it would work in reverse the same way, only the “extra” force would be applied to the front shoe rather than the rear shoe. The reason it might not be as effective in reverse is just that you are going slower in reverse, or even stopped. The extra force comes from the movement of the car, and if the car is stopped there isn’t any extra force available.


#11

“Self Servo” drum brakes have the front and rear shoes connected and the front shoe’s leading edge is shorter than the rear shoe and reversing the position of the shoes will cause the brakes to lock up quickly when applied and when the drums are turned to the limit the shoes need to be “crown ground” to put the full length of the friction pad in contact to reduce grabbing despite having the front lining shorter. All the FWDs seem to have a fixed pivot point on the backing plate to eliminate “self servo”

And I saw a Dodge pickup at the dealership getting the entire rear end replaced and recognized that the shoes were reversed. When I asked what was wrong with the axle the service manager said the brakes were locking up and after replacing the master cylinder and booster and proportioning valve and repeatedly inspecting the rear brakes the factory sent a new axle. It was difficult to get out of there without laughing.


#12

Not long ago I had the pleasure of working on a Chevy with rear drum brakes. Customer had been to local chain tire/brake/alignment store and had the brakes replaced. The RR grabbed. They replaced the wheel cylinders. Still grabbed. Replaced the drum. Still grabbed. Replaced the master cylinder. Still grabbed. Charged him and sent him on his way. I found the RR shoes installed backwards. Fixed his brakes so they worked perfectly and charged him $105, he seemed taken aback at the price since the whole affair only took 20 minutes but eventually realized the value of a job done right the first time.

Unfortunately, we gained him as a customer since no one else around could or wanted to work on a feedback Varajet carburetor engine performance problem.

I’m only 49, why do I feel so old?


#13

Good mechanics can be very tough to find and when you do find one they are often booked up but often the McShops are ready to jump on your car quickly and after 3 or 4 returns they usually get the problem and your bank balance taken care of. I was disappointed when I visited a Toyota dealership and found they didn’t have a single ASE Master in the place. I must guess that they have someone certified for each of the various systems. Could the lack of mechanics be motivating the manufacturers to be leaning in on moving to EVs?


#14

Part of the equation is the leverage you get with pivoting brake shoes. Calipers just push the pads closed on disc brakes, but the piston in a drum brake pivots the shoes outward.

Today’s power brakes don’t need the advantage of leverage like cars did when brakes were manual.


#15

50 years ago I worked on medium duty trucks that had 2 master cylinders in series with the primary located where it is now the customary to put them and it was attached to a slave cylinder that actuated the vacuum boosted secondary master cylinder. It seems that with the need to push a lot of fluid down the line under a lot of pressure the geometry of piston diameters was used to operate the drum brakes satisfactorily. And I seem to recall that all the wheels had twin trailing wheel cylinders.


#16

??? So which shoe gets “self energized” in the photo above? The front shoe (right), or the rear shoe (left)? To me it looks like the rear shoe gets some extra force applied (beyond the wheel cylinder piston force at the top). The extra force comes from the front shoe as the drum turns clockwise and pushes the bottom of the front shoe into the bottom of the rear shoe via the adjuster.


#17

In the photo above the parking brake attaches to the lever behind the rear (left shoe). I think that’s the parking brake cable heading toward that lever, look through that hole in the hub at 6 pm, so the PB cable attaches at the bottom. . The pivot for the PB lever appears to be upper left, there’s a “C” shaped little clamp holding it on to the shoe. So when the parking brake is applied, the cable pulls the bottom of that lever to the right, which pushes the front shoe against the drum (via that bracket just under the wheel cylinder). If the car moves forward, a clockwise rotation of the drum, this would pull the front shoe into the rear shoe and so both shoes would be braking. If the car was tending to move backward, that would produce a counter-clockwise rotation of the drum, and the front shoe would still be forced against the drum by the lever/bracket assy, but it wouldn’t be pushing against the rear shoe. Instead it would be pushing against that post on the top. In this case (counter-clockwise rotation of the drum) the bottom of the rear shoe would be pushing against the bottom of the front shoe via the adjuster as before, but the rear shoe isn’t forced against the drum as much in this design by the PB. That might explain why the PB works better against a forward roll than a backward roll.


#18

??? So which shoe gets “self energized” in the photo above? The front shoe (right), or the rear shoe (left)? To me it looks like the rear shoe gets some extra force applied (beyond the wheel cylinder piston force) from the front shoe as the drum turns clockwise and pushes the bottom of the front shoe into the bottom of the rear shoe via the adjuster.


#19

[quote=“ken2116, post:10, topic:130091”]
Assuming frictional force is close to the Newtonian ideal, absolute speed shouldn’t affect braking effectiveness [/quote]

I think you are right about that. At least in the dynamic dry-friction case. Good explanation.


#20

I can’t see the photos you are referring to and I don’t have much luck posting links.
In the Chrysler design the shoes and linings were equal length on the front wheels and were designed with two wheel cylinders per wheel so that the shoes were anchored at one end and the wheel cylinders pushed the unanchored end into the rotation of the wheel. Of course, when you were backing up the rotating drum was trying to push the shoes away from the drum.

You don’t need as much braking force backing up because you cant go as fast.

This design is probably why the parking brake was a drum between the transmission and driveshaft.