When the topic is drum brakes, there seems to be a lack of a consensus as to where the larger shoe should be positioned.
Here is my view:
The larger shoe should go forward, not rearward.
Remember: Bolted together as one cohesive part, are the rear-wheel, brake-drum and axle-shaft. When the brakes are applied it is that cohesive part that brutally grabs at the road - an action that forces this wheel/drum/axle-shaft assembly to violently leap relatively rearward - pressing the inner surface of the brake drum most-so upon the forward-facing shoe.
Ergo: The proper place for the larger shoe is forward.
Christopher Baer
Thats the right position on self energizing brakes according to my ken-Kevin
If thereās an anchor pin at the bottom where the two shoes come together, then it should make no difference where the larger shoe goes.
Assuming the bottom of the shoes can float (ie, no anchor pin to the backing plate), then Iāve always learned the larger shoe goes in the rear, not the front.
The reason is this: As the wheel cylinder pushes the shoes out to the rotating drum, the shoes want to turn the same way the drum is turning. This āturningā action puts additional forces on the rear shoe. So if the rear shoe is larger, you get better stopping power.
I grew up with this being the āaccepted practiceā everywhere I worked, and it always made sense to me. I never saw any design docs for brakes to know if they were designed that way.
For years I always put the large shoe on the front. Every car I worked on the large shoe was in the front. Then I changed the brakes on my 90 Pathfinderā¦and the large shoe was in the rearā¦So I guess it depends on the vehicleā¦or really doesnāt matter.
While doing dozens of brake jobs on US cars from '60s and '70s, the shoe with the shorter friction surface (I assume thatās what youād call the āsmallerā shoe) always went on the front. I donāt know if thatās changed.
BINGO - I found this on the net (and several other places, but this is a good quote), at least itās consistent with my memory:
āAutomotive brake shoes consist of a primary and secondary shoe. The primary brake shoe is the front shoe and normally has a slightly shorter lining than the secondary shoe. The secondary shoe is the rear shoe and has the largest lining surface area.ā
In all the drum brake jobs Iāve done the longer lining shoe faced the rear of the vehicle and shorter lining shoe faced the front. The front shoe which has the shorter lining is the self-energizing shoe.
Tester
They are called primary and secondary shoes. The shorter lining goes in front. The front shoe helps to servo the rear one. The lining is usually thicker on the front one too. It helps to keep the metal at the top from dragging on the drum. You just need training, thatās all. The primary shoe spring is supposed to be installed before the secondary spring. You will notice that the anchor pin is tapered in order to put more tension on the secondary spring.
This helps the brakes to engage when driving in forward gears. You may notice that the parking brake works better when the car is facing downhill too. Put them on wrong and the opposite will be true. If you have more questions, read the manual first. It isnāt a consensus, itās instructions.
The initial poster, right or wrong, forwards a good point: there is no solid, one-thought on the subject of which shoe goes forward. Clear evidence of that stark fact has been available to anyone who has removed hundreds of brake drums over the years. Some had the greater shoe forward while others did not.
The initial post seems to make good sense, yet you seem educated on the topic as well.
One thing is certain: long-held beliefs are often overturned.
Perhaps you were instructed in Drivers? Education that placing your hands at the 10/2 ?o clock position was sound thinking. Today that configuration is considered an unworkable concept. (save for NASCAR steering wheels which are low, and removable to allow for exit.) .
So, which position is correct for the shoe with the longer friction material? (I suppose we should start with the more basic question: ?Why does one shoe HAVE more material??, and go from there.)
Do we contact the engineers at Bendix (almighty) over this, can Click & Clack settle the hash, or what?
Well, several technical resources on the web agree with the above posts. Explanation for the opposite? The last guy did them wrong.
They for YEARS you have been putting them on backwardsā¦If the wheel cylinder has two pistons, well see belowā¦
Virtually ALL drum brakes today are āBendix Self-Actuating Duo-Servo Drum Brakesā. Bendix once had a patent on that design. Today, everyone uses it in one form or another. If one shoe has more lining material than the other, that shoe, the āsecondaryā, is ALWAYS mounted towards the REAR of the vehicle. Period.
Here is why. At rest, the shoes are centered by a pin at the TOP. When you step on the brakes, the wheel cylinder pushes BOTH shoes off the pin. The front (leading, primary) shoe is ādrawnā into the rotating drum, a wedging effect, BUT it is connected by the bottom self-adjuster link to the rear shoe. Remember, both shoes are trying to rotate with the spinning brake drumā¦The REAR SHOE is getting force applied from TWO DIRECTIONS, the hydraulic wheel cylinder AND THE BOTTOM LINK. This means the secondary (rear) shoe gets more overall force applied to it than the leading (primary) shoe. The big shoe goes on the back sideā¦Many backyard mechanics make the mistake of installing them backwards, and in another life when I was a State Inspector, I would flunk their cars for 'Incorrectly Installed Brakes". Usually, they will work okay, but they will wear unevenly and sometimes can get grabby as the self-actuation of the too big front shoe nears 100%ā¦
Interesting.
Yet are you failing to see something?
As the forward shoe is forced down, the back-shoe is forced upward, against itsā wheel cylinder?s piston, thereby adding more pressure to the net pressure within that cylinder. With that pressure having to go somewhere it must affect the not-immobile forward half of the piston, pushing it forward even more so.
Add to this the forward-most section of the brake drum?s inner surface is pressing hardest against the front shoe. There can be no doubt about that, right?
I?m confused, which shoe actually IS getting the most overall net force against it?
I appreciate that you have been educated on this overall topic, yet who was your teacher? Clearly you are merely repeating / arm-chairing what you?ve been told. I mean you?ve conducted no scientific testing / industrial assay on the system we?re discussing, correct?
It does appear at this point gentlemen that we have an open and unresolved issue.
??? if every technical source says āAā, why do you think āBā is better answer?
Every car I bought new had the shorter lining toward the front of the car. A lot of old Chrysler products that I owned, but didnāt buy new had equal length shoes and a wheel cylinder with one piston at both top and bottom facing in opposite directions so that both shoes were self energizing. This was a superior design but lost out to the cheaper to manufacture Bendix when power brakes were able to overcome the disadvantage of the Bendix design.
Both primary and secondary shoes are self energizing, the primary at the top and the secondary at the bottom. Itās interesting to note that a spare brake shoe set for my VW shows the same length and thickness primary and secondary linings and from visible appearance, the same material.
So far, these ātechnical sourcesā are merely regurgitating that which was once told to them (and they never stopped to question that information. An aside here can be, blindly following is not virtuous. After all, who invented the airplane, all the Ph.Ds in the world put together, or two guys in a barn?) Iām eager to learn which camp is correct on this issue, should any organization ever put the question to scientific scrutiny.
The short shoe belongs on the front. With the long lining at the front the brakes are prone to lock up.
The long shoe goes on the back because thatās where the engineers who designed these brakes many, many, years ago wanted itā¦Most of them are probably dead and now we have people wanting to debate their motives and engineering skills. And yes, some designs do indeed use two identical shoesā¦ But I have seen to two long ones on one side of the car and the two short ones on the otherā¦
Hmmmā¦I guess you want Mr. Bendix to show up and set your mind at easeā¦Iāve checked out dozens of web sites, but they may not satisfy youā¦but I could find NONE that conflict with the above-stated info. Anything you care to contribute?
The rear when you press the brakes while going forward, the top of the rear shoe stays firmly seated on the pin at the top of the brake backing plate. The rear-facing piston in the brake cylinder does not budge. The rear shoe does not add any pressure to the fluid in the brake cylinder.
While I would not rule out the possibility that somewhere there are brakes designed upside down relative to the ānormā (that is, they have the slave cylinders at the bottom and the adjuster at the top) every car I have ever worked on, foreign or domestic, needed to have the shoe with the longer friction surface mounted toward the rear of the car.