Brake Shoe Theory Question

[dcbullet]

Just wondering why the smaller of the brake shoes is put towards the front of the vehicle?

[68F250]

For drum brakes, the primary shoe is the rear one and usually the larger of the two due to more forward movement stopping. That's why it brakes better when going forward than backward.

The shoes are "pinned" at the top by the wheel cylinder but "float" at the bottom. When moving forward and the brakes are applied, the expanded shoes do a rotational move and wrap onto the drum. It's like a servo action and multiplies the braking force. The front shoe catches the drum and rotates the shoes into the drum. The rear shoe catches most of the braking action so it is made larger to take advantage of the leverage.

There's a name for this design but it escapes me right now. Not all drum brakes are like this, my VW had two wheel cylinders - one per shoe and no servo action, thus both shoes were the same size.

[dcbullet]

Thanks Barry. That's a great explanation.

I'm always trying to learn some theory rather than just remove and replace.

[flyboy2610]

They're called dou-servo brakes.
Your VW had leading-trailing brakes, which is what most drum brakes are.

[68F250]

Thanks flyboy!

[DuckRyder]

Actually, I belive the term is "Self energizing"

Its also why when ABS first came out you never saw it on anything but 4 wheel disc cars, it took a lot of redesigning to make the drums release fast enough for it to be effective.

[spartman]

They how did ford make it work on the trucks that only had rear wheel anti-locks.

Those were drum brake setups.

I know I had an 88 that was that way.

[DuckRyder]

They didn't work very well for starters, they sucked so bad they paid the bills for years.

Fortunately, I haven't had one apart for years and foget the details, at the training center whe had all kind of classes about how they made them suck bad enough to work semi-properly with the RWAL.

Edit: Sorry don't know how ford made them work, I was talking about the other guys...

[Wes]

They all used Rwal at some point. But Rwal is a simple system and weight sensitive. But there were plenty of 4wheel abs systems out there, hell Olds had ABS in 76 (cost to much to continue ). The new abs traction control systems need the release speed of a disc type break.
Duo-servo and self energizing are both correct depends on who made the drawings Bendix or Dana