The Mighty Arc

 

Many a woeful tale has been told about the brakes (or lack there of) on 1st generation Toronados (excluding disk brakes). Here is a simple hint to maximize your full breaking potential.

In order to fully explain how the arcing machine works, we have to brush upon our geometry and play Mr. Wizard! I designed a little experiment that will allow you to fully understand the nature of concentric circles in seconds! Take 2 paper plates. Take one paper plate and cut 2 inches or so off the circumference. Keep it as round as you can. Then, cut it exactly in half (two equal hemispheres).

Place the smaller paper plate hemispheres (as if they weren't cut) in the center of the large paper plate. Now you have concentric circles with different diameters.

The large plate edge represents the inside of the brake drum. The outside of the smaller circle (two hemispheres) represent the outside edge of the brake shoes.

Gently slide the hemispheres out to the respective edges of the large paper plate. This replicates the action of the brake shoe articulating against the inside of the drum.

Observe, you have a diminished contact patch. Only the high side of the shoe hits the inner diameter of the drum translating to diminished braking capacity.

Now, slide the hemispheres farther out to where the ends are in conjunction with the outside edge of the large plate diameter. Take a pencil and trace the excess or overage between the two points around the circumference. Now cut the newly defined arc. Wallah! You have just arced your brake shoes!

Now the outside diameter of the brake shoe equals the diameter of the inside of the brake drum. 100% of the shoe, is now in contact with the drum. The amount cut off is exaggerated here. Usually only a small amount of shoe stock is ground off...but enough to make a difference.

This MUST be done with each drum separately, as each drum won't have the same exact inside diameter. Especially after numerous cuttings.

 

Good brakes depend on proper installation and the experienced mechanic will make sure that there is uniform contact between drum and lining by arcing the new shoes to the same radius as the drums. Even new drums should be checked and ground true.

In Summation:

What the arcing machine does, is grinds off the high spots of the brake shoe to match them with the same exacting inside drum diameter as to ensure complete contact. I feel so strongly about this that we bought a used arching machine, as seen in the picture, for the shop as it is rarely done in brake shops anymore.

I hope this sheds a little light on the subject. It vastly improves breaking and greatly extends the life of the brake job, maybe by years. You also will not have to cut the drums as much as the wear will be more even across the width, assuming you don't go down to the rivets!

The next time your mechanic tells you that the new shoes are pre-arced, ask them how they knew what the inside diameter of your drums were. Yes, shoes do come pre-arced, but they are arced to the inner diameter of NEW brake drums, not your Toro that has 50K on the drums.

Happy Toroing (and stopping),

 

Greg Kalkhoff

TC # 563

 

11/25/96

Photo by: Bill Culp