Can anyone explain Lockheed's 40's Ford step bore wheel cylinder design? These brakes were non-servo with bottom shoe anchors and used wheel cylinders that had a larger, front bore. This is counter-intuitive. With bottom anchors, due to "wedging action", the front shoe would produce more braking force per actuation force. So why would Lockheed reduce the wheel cylinder bore for the rear shoe? This would make the rear shoe almost useless. (If you wanted to utilize both shoes equally, the rear bore should be larger.) ***uming they wanted less rear shoe braking, this would also have been achieved with a straight bore wheel cylinder - which is exactly what was used on Studebaker's front disc brake rear drum cars: bottom shoe anchors and straight bore wheel cylinders. My only theory is that they wanted to have a maximum amount of safety margin remaining in the rear shoes if the fronts were totally faded. But back in the 40's, it wasn't known just how much margin would be optimal. So they went "all in"? I also believe upgrading to a straight bore wheel cylinder would improve braking significantly in these cars.
Been a LOOOONNNGG time since I worked on a set of those old Lockheed brakes, but what I do remember is that they weren't self energizing like the later Bendix brakes and I suspect this made it necessary to exert more pressure on the front or leading shoe by way of a larger cylinder bore. The floating anchors on the Bendix brakes help the front shoe apply pressure on the drum.
I believe the larger bore goes with the larger shoe to the front and the smalller bore to the smaller shoe to the rear, the thought being that each would produce the same amout of force, but yes the front shoe does most of the braking, I just went thru a 29 model A with these. Also its said that Henery didn't want to pay the royaltys to use the self energizers to Bendix
I guess I had it wrong. Couldn't help but do some searching and found this link https://www.speedwaymotors.com/the-toolbox/replacing-a-front-wheel-cylinder-on-an-early-ford/29865 Although it doesn't go into the why or theory maybe it will be helpful? Lynn
I would believe that Lockheed's engineers looked into all aspects of the braking system; stopping ability, brake balance, pedal effort, pedal travel, balanced shoe wear, time/mileage between needed adjustments, and who knows what else to design Ford's system.
The problem is Shoe Wear. If the front shoe wedges and brakes more than the rear, it will wear out sooner. And yet, they made the front wheel cylinder bore LARGER so it will Brake and Wear MORE than the rear shoe. Seems like the ONLY design consideration would be to NOT FADE the rear shoe so it becomes sort of an "emergency braking shoe." But I question if the design back then was that elegant?
According to a 1944 service manual the different bore sizes are for differences in pressure to the shoes. As far as the front shoes having larger lining (by 3 inches) that is because on a Lockheed two pivot brake setup the front shoes are the primary shoes and do more of the actual braking. Referring to Mr Creosote's post in post 6 yes the primary shoe always almost always wear faster than the secondary shoes on any brake system. With Bendix brakes with the rear shoes being the primary shoe the rear shoes always almost always wear faster than the fronts. That is just the way it works with drum brake shoes, the one doing the most work wears out faster than the other one.
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