OK, so there are a number of things automotive that I have always done and just generally accepted them as the law of the land. Thats not to say that there are not some good logical reasons why we all do some of them. Well today I was doing some front end maintenance on the falcon, which included throwing a new set of shoes on. I, since a teenager was always taught that the "shorter" shoe with less material goes towards the front, with the "long shoe" going to the rear. Well today it got me thinking about it, and I could not really come up with a good reason why its not the other way around. Obviously there is a reason, or the shoes would be manufactured in such a way that there were equal friction material on each shoe. Can anyone shed some light on this for me? The wheel cylinders are designed in such a way that the bore is equal on each piston. Therefore, each shoe exerts the same amount of force outwards towards the drum. I'm sure that squirrel, or bruce lancaster will chime in here in a second with an answer that will be so simple that it will make me feel like an idiot for not knowing. Thanks in advance. Am I the only one here who lays awake at night staring at the ceiling wondering about little things like this?
the brakes are "self energizing", and if you understand about coefficients of friction, vectors, and stuff like that you'll see why the shoes are the lengths they are. Basically, the front shoe helps push the rear shoe into the drum, so the rear shoe has more force on it, and needs a longer lining so the front and rear linings will wear evenly.
SO is this due to the rotational force that is exerted upon the front shoe by the rotating drum? Therefore directing it away from the top guide pin, and into the rear shoe?
Which obviously means that the rear shoe is having force applied to it from both ends. Wow, I never thought about drum brakes like this before.
yeah, that's it. The early brakes had two anchors, at the bottom, and did not have the self energizing feature. mopars had one anchor at the top, one at the bottom, and two wheel cylinders...they worked better going forward, and terrible going backwards. Bendix is the company that made self energizing brakes, I think Lincoln was one of the first to use them, they were a big advance, the basic design is still being used, only real change was the addition of a self adjuster.
I respectfully disagree with Squirrel on one point in his explanantion. I DO AGRRE with the self energizing aspect of drum rotation on the leading shoe (short one), but do not agree the rear (trailing shoe) has more force acting on it. I believe it has LESS force acting on it and, therefore, requires more surface area (lining) to achieve the same braking effort or effect as does the self energized leading shoe. The trailing shoe is rotated into the wheel cylinder, opposing the wheel cylinder pressure, whereas the leading shoe's self energizing effect works WITH the wheel cylinder and in eeffect, increases it's effectiveness. I may be incorrect in these conclusions but it is my understanding of the principles at work in this mechanism.
I do believe that Squirrel is pretty much correct. and the secondary or back and longer shoe does do more of the braking than the front shoe. That can be seen when you pull a drum off most vehicles and see that the wear on the secondary shoes is usually noticeably more than the primary shoe. I can dig out one of my old auto mechanics textbooks when I get home and get the real scoop but the wear patterns alone should tell that the secondary shoes do most of the work. <input id="gwProxy" type="hidden"><!--Session data--><input onclick="jsCall();" id="jsProxy" type="hidden">
when in doubt, draw a free body diagram. I think you'll see that there's more force between the shoe and drum on the rear shoe, than on the front shoe. That is why the rear shoe needs a longer lining....since it has more force on the lining, the lining is longer to reduce the pressure (and wear) to the same level as the front shoe.
The rearward shoe is against the top fixed anchor, so it does not/cannot push on the rear link to the rear wheel cylinder piston. The forward piston applies the front shoe, which is then forced downward (by the rotation of the drum) against the lower adjuster link, and the link then against the rearward shoe. This is when stopping going forward. This is the "self energizing" or "servo" feature; the pedal force is multiplied by the turning drum, providing more stopping with less pedal effort. The rearward shoe is longer because it ends up doing the most work. When stopping while going in reverse, the above scenario is reversed.
One thing that has not been mentioned is the material that composes the lining. The short shoe is of a harder material, and the long shoe is a softer material. These are also factors of the self energizing action.
I taught brakes in Provincial Trade School for several years and have my USA (ASE Master tech) and Canadian (Ip Red seal)tickets. Some of you actually "know" what you are talking about. Good job. Good explantion. Have a Walnut! Don
Rather than comment on the individual errors, just read my article: http://victorylibrary.com/brit/2LS.htm
That clears it all up. (I did find two relevant paragraphs in there under "brake shoe types", thanks for the link)
I would like to know how come they don't mount the wheel cylinder at the bottom instead of up top.?. That way the fluid that leaks out would not ruin the shoes. HMMM.
Less ruined shoes = less shoes sold = lower corporate profits????? No wait the wheel cylinder at the top is an old idea, the above lesson in modern economics is a (relatively) new one - must be another reason....
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