Posted by Gordon Maney on Monday, June 12, 2006 at 9:10PM :
In Reply to: wheel cylinder question posted by g tauer on Monday, June 12, 2006 at 3:14PM :
The front shoe is the primary shoe. The front shoe is typically the leading shoe. [Primary and leading are different brake terms. Primary is positional, leading is circumstantial.] If the truck is going forward, the shoe closer to the radiator is the leading shoe. Wheel cylinder orientation is chosen for most effective brake application going forward.
The primary shoe is self-energizing. That means that friction between the shoe and the drum tends to cause the shoe to move further outward, creating more friction and greater braking power. The secondary shoe is not self energizing, so it needs more force applied.
Place the end of an eight foot 2X4 against the middle of your chest, place the other end on the ground, and then try to push it around your yard. The end will tend to dig in. You will not enjoy the experience.
If you tied the end of the 2X4 to the middle of your back and pulled it around the yard, it would be much easier and it would not dig in, though there would be friction between the 2X4 and the yard. A crude analogy, perhaps, but I suspect you get the idea.
The large bore goes toward the rear. That shoe needs more help, as it is not self-energizing. The large piston applies more force [pressure X area] than the rear.
There have been fixed anchor brakes [that is what the Dodge has....] that had wheel cylinders top and bottom. Those tend to be poor brakes going backwards, but better brakes going forwards.
I have taken a number of Dodges apart that had the wheel cylinders in backwards.
In a servo type brake [the kind that has what you call a star wheel adjuster at the bottom] both shoes self-energize in both directions. That is a far better brake. We just did not happen to get them on our 1-ton Power Wagons, M-37's, and WWII Dodges. Oh well....