Basically it's the proper method to ensure proper shoe/drum contact and ensure good smooth effective braking and operation of them. Yes, the roller tracks sometimes need to be tweaked to get things centered up. Not in and out... but up or down.
Here's my centering advice/process.
Centering the shoes ensures that the contact of the top and bottom of the shoe are contacting the drum equally between the two shoes with relation to the contact surface of the drum. A perfectly round drum is best ( recently trued/turned) .
The best and most thorough approach is:
The first thing which is probably best is to see how the shoes fit the drum when they are off the backing plate. pull shoe to the drum and use a feeler gauge to see how the surface contact is. The ends of the shoes should be chamfered to the 1st rivet on the top and bottom of the shoe, otherwise high spots will skew and unchamfered they tend to squeal. If the shoes rock they curve ( size) of the drum is more than the arc of the shoe, if the shoe ends touch first, then you know that there is high spots in the lining. Either way the shoes should be arced to fit the drums. Experience comes to play here with analysis and corrective action.
Repeat for other shoes/drums
once installed on the backing plate a "centering gauge" either KRW or current available from brattons/snyders can be used on the spindle to check the relation of the shoe to spindle which should correlate to the drum center. You basically want equal distance/contact on the shoes to ensure full contact of the shoes to the drum. Roller track adjustment, brake shoe pins/rollers/wedges, etc all have an effect on shoe alignment. Yes, there is a certain amount of wear in... but there is no substitute for proper procedure here. These are just the processes required for proper restoration/setup of the brakes.
One thing for sure... if you don't do it... no telling what you have and you can count on less than optimum brakes...
Larry Shepard
