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Discussion Starter #1
I'm sure that most of you that have replaced the shoes on the rear, or front, drum brakes, have noticed that the top part of the shoes are worn the most.

It makes sense as the wheel cyl. is located at the top of the brake assembly, which applies pressure to the shoes to slow the drum.

Looks like another wheel cyl under the cyl on top would apply braking force more evenly vs a single wheel cyl.

I can see why this was left out from the factory as it probably saved millions not to have an extra wheel cyl, when one would do the job.

I think I saw where a 55, or 57 Thunderbird called the "Battle Bird" had two WC's.

Anyone done/tried this?
 

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If your wearing out the top of the shoes your not adjusting the bottom out enough. When you adjust the shoes you hear a slight drag on the shoes and when the cyl. at the top applies pressure you should get a pretty even wear from top to bottom.
 

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I don't think you would want two wheel cylinders.... Your pressure on the pad wouldn't be balanced anymore, and your shoe would wear randomly.
 

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Back in the old days a separate wheel cylinder for each shoe was fairly common. It was a service headache to get both shoes adjusted. Some places had special fixtures or modified drums so feeler gages could be used for adjustment. The modern floating shoes coupeled at the bottom made adjustment a breeze. Soon the self adjusting almost eliminated service adjusting.

IMO a big part of the problem getting drum brakes working right is hardly any place grinds shoes to fit the drums. Back before dirt (when I was young) you'd take the drums to the parts store when you got new brake shoes. They would turn the drums and grind the shoes to fit the drum.
 

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duo servo drum brakes

Please do not try this at home, adding a second wheel cylinder is not a good idea. Google drum brake design, or duo servo drum brakes and educate yourself on how drum brakes work before you consider this any further.
 

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Imagine the way the drum turns (rotates). When you apply the brakes, the shoes move slightly in the direction that the drum rotates. Up until the top part of the shoe is resting against the anchor pin. The other shoe pulls away form the anchor. That is the fulcrum point in the system. Having said that, only the forward piston of the wheel cylinder is doing any of the work.
It don't matter, the pinch between a disc brake caliper works even better, so, don't waste your time, but the disk brake set-up on.
 

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Please do not try this at home, adding a second wheel cylinder is not a good idea. Google drum brake design, or duo servo drum brakes and educate yourself on how drum brakes work before you consider this any further.
As I said many cars/vehicles were designed and built back in the late 30s and 40s with 2 cylinders (one for each shoe). For many reasons the current design was adopted long, long ago! The single cylinder was a huge improvement in simplifying adjustment and need far less frequent service.
 

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Discussion Starter #8
Please do not try this at home, adding a second wheel cylinder is not a good idea. Google drum brake design, or duo servo drum brakes and educate yourself on how drum brakes work before you consider this any further.
Internet, knowitalls here at work.

First I never said I wanted to try it, although I have built my own 13" front disc conversion for one of my 65 Mustangs as well as fabbing up some chassis parts.

I just wanted to hear opinions about this as a way they might have ran race cars back in the day.

As some one already stated they did do this in cars, but it was too complicated to service.

Even if you do adjust your brakes out to where they just drag, {which I have been doing since I was 16 years old} it still don't look like there would be an even force generated when the brakes are applied to the full surface of the shoe with just one WC.

Maybe I'm just not looking at it the right way......lol.
 

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Have to agree with Roddster. By the time you engineer and fab a dual cyl system, you could have already converted to disc brakes, without even taking into account the inherent advantage of disc's. I like to design and fab too, but somethings aren't worth the effort.
 

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Even if you do adjust your brakes out to where they just drag, {which I have been doing since I was 16 years old} it still don't look like there would be an even force generated when the brakes are applied to the full surface of the shoe with just one WC.

Maybe I'm just not looking at it the right way......lol.
I think you are...using two wheel cylinders on one shoe, you'll never have an exactly equal force applied to the shoe by each wheel cylinder.

In fact, it'll effectively be less as the harder you push with one cylinder you'll have to counter that with the other cylinder. And since they have a fulcrum between them the problem is magnified.

There is a reason you only see one cylinder used in hydraulics. You see this principle applied everywhere...construction equipment, elevators, dump trucks, etc.
 

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Discussion Starter #11
I think you are...using two wheel cylinders on one shoe, you'll never have an exactly equal force applied to the shoe by each wheel cylinder.

In fact, it'll effectively be less as the harder you push with one cylinder you'll have to counter that with the other cylinder. And since they have a fulcrum between them the problem is magnified.

There is a reason you only see one cylinder used in hydraulics. You see this principle applied everywhere...construction equipment, elevators, dump trucks, etc.
Interesting.

I wonder why big trucks still use drum brakes on the rears?

Is it because of the fact that drum brakes are better for emergency brakes? {on a drum as the brakes cool the drum fits tighter on the brakes, where as on a disc as the rotor cools it shrinks away from the pads.
 

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Interesting.

I wonder why big trucks still use drum brakes on the rears?

Is it because of the fact that drum brakes are better for emergency brakes?

I grew up around trucks and have driven trucks for 25 years. I'm also a Federal Motor Carrier Safety Inspector. The reason that heavy trucks use drum brakes is due to way air brake systems are designed and work. They tried doing disc air brakes years ago, but had too many problems. Also I believe that the way truck drum brakes are designed (very deep drums) allows them to have more friction surface than a disc brake would. I'm not aware of any heavy truck (air brake system) out there that has disc brakes on the front. You might see a medium duty truck with a hydraulic brake system using discs.
 

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Part of the reason is that drums with leading shoes have a self-energizing effect when applied, thus requiring less hydraulic (or air) pressure to activate them. Disks have very little to no self-energizing effect. Leading shoes no longer self-energize when the vehicle is in reverse.
Trailing shoes are not self-energizing unless the vehicle is going backwards. Before disk brakes became common on off-road motorcycles commonly the front brake shoes were both leading. The rear brakes usually had one leading and one trailing. So if you grabbed a handful of front brakes you could about throw yourself over the bars. But stopping on a steep hill where you wanted to put both feet down was a challenge. Some front brakes wouldn't hold at all to keep you from rolling backwards. Made for some embarrassing moments. If you could spare a foot to hold the rear brakes it wasn't a problem.
 

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Discussion Starter #14
Good info, thanks guys.
 

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If you increase rear stopping power then check to make sure the fronts lock up before the rears. If the rears lock up first you will go into a skid.
 
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