Drum brake width?

Ok guys

This will be some of the first posts i have done here so here it comes.

I have a 1956 F100 with rear stock drum brakes, 11" in dia and 1 3/4" wide.

I am planing to switch to a 9" rear with 11" dia and 2 1/2" wide drum brakes.

Everyone says that the 2 1/2" drum brakes is a HUGE improvement, BUT:

Would that effect the brake distribution? Brake torque front vs Brake torque rear? Given I Keep the stock front brakes.

According to my schoolbook the formulas, for calculating the brake force (friction force) between drum and shoe, does not include the width of the shoe/drum interface as a parameter.

However i understand that the size (width) would increase the effect that the drums can handle without fading (i.e temperature rise) as the brake effect (energy over time) would be dstributed over a larger surface.

The larger surface would also increase cooling.

Sooooooo:

Will the width effect the brake distribution? Brake torque front vs Brake torque rear?

I am hoping for a good discussion as other forums has turned up with "Zero" replies.

Thanks

 

Assuming the master cylinder and the rear brake drum's slave cylinders are the same size as the 56 F100 OEM brakes, the pressure applied to each brake shoe will remain the same - given the shoes' fulcrum height and lever length inside the 9 in drum is not much different than the OEM units... However, you will get more work out of the 9" brake shoes because they are wider (they have more surface area). Therefore, the 9" brakes will be more effective. The larger brakes should run cooler than the narrower units (less likely to fade), because the amount of force applied to each shoe will be spread across a larger surface area. You can always play with your front to rear proportioning valve(s) to adjust your front to rear braking bias. Ideally you want something like a 60/40 bias, front to rear.

 

Like jr9162 stated.
If all else is similar(Bendix/Wagner; Delco-Moraine; Dual Servo) increasing the width of the drum/shoe(s)(with-in normal confines) will not change bias or effective brake torque. You will have a more noticeable variable on how stiff the brake components are, but usually it is the applied pressure and the radius of the drum that are the two main factors. Not so much the shoe width.

 

To my simple minded reasoning, the wider rear shoes will produce more braking action for a given amount of pedal pressure. If the wider shoes had no significant effect, there would be no need to increase the width in some applications. All rear shoes could be the same width for a given diameter.

Given that; a) the front brakes do most of the braking, due to typical forward static weight bias, especially on an unloaded pickup truck...and b) the forward weight shift when stopping....and c) rear wheel lockup, prior to front wheel lockup, usually causes a sideways skid........I think you will need to pay close attention to the results and either
add a proportioning valve, as mentioned above, or possibly use smaller bore wheel cylinders on the rear brakes to regain the balance need for safe operation in heavy braking situations.

 

Seems as said above a proportioning (adjustable) would address any issues ... but it seems your desire to go with the nine inch rear is for other reasons than improved stopping power, which I tend to disagree with. One would think this would also be to optimum time to upgrade the front binders, maybe going to disc brake (front) or, at least have future plans to do so.

 

Why would anyone want to up grade 1/2 a brake system then restrict it to work with the other end, front in your case? All 4 corners should be done as a system or it's just a waste of time and $$$$$.
The Wizzard

 

Most your braking comes from the front as already mentioned. As the weight shifts forward under hard braking, the rear wheels/tires get "light" and are more likely to skid prematurely when you improve the capability of the rear brakes. This doesn't accomplish anything. If you want to really make a difference, improve your front brakes

 

1-if the improvement's huge, it'll just skid the rears under light pedal pressure

2-the rears don't generate much heat to begin with in normal driving

3-see #1

 

But then they want to pass the fronts

 

As usual some stupid snipes mixed in with good points..1) If you have the whole 9" rear, use it and just put in an adjustable proportioning valve and follow the directions, thats what they are designed for; 2) IF you carry something heavy you will want to increase rear brakes force ; 3) IF you have some wide tires in the rear you will want to increase brake force...4) as one suggested you could use smaller wheel cylinders but some what of a guessing game and a pain to change back and forth..5) PLEASE PUT the proportioning valve where you can reach it from the drivers seat, putting it under the hood like so many Magazine feature cars is stupid; might as well put the head light switch there too..

 

Think of a rifle cartridge. During the firing process the bullet (piston) is moved forward to engage the rifling lands under initial pressure increase (above atmospheric). Because of the interference fit between the bullet and the rifling the bullet momentarily stops, pressure continues to build inside the brass cartridge case since the smokeless powder burns at a scheduled rate - it doesn't explode. The building pressure expands the malleable brass cartridge case until it firmly grips the rifle's chamber and is fully back thrust against the face of the rifle bolt (breech block). The powder continues to burn and as the pressure reaches 38,000 to 55,000 lbs per square inch (http://kwk.us/pressures.html ) the bullet is forced down the rifle barrel, spinning at a rate of twist anywhere from 1 turn in 9 inches to 1 turn in 40 inches depending upon the caliber of the bullet. As the bullet exits the muzzle the pressure begins to decrease and eventually reach atmospheric. Bear in mind, all these events occur in milliseconds. The malleable brass shrinks to a size smaller than the chamber, but somewhat larger than it was originally. The rifleman may then eject a spent cartridge case and chamber a new round.

All the above info leads up to my original statement regarding pressure (force) versus surface area (piston) and work performed. If one changes the bore diameter of the cartridge above from say .308" to .338", leaving all other factors similar (same case volume), same pressure applied by the burning powder (increasing fluid pressure due to rapidly expanding gases), the larger bullet (brake shoe) due to it's increased surface area (piston), performs more work. In this case the .338's velocity will be higher than the smaller .308.

However, the force applied by the larger brake shoe against the brake drum is applied over a larger surface area. This diminishes heat and makes the brakes more effective due to less fade. From 1938 to the late 1950's Buick increased the shoe width on their heavier Roadmaster models while keeping the single reservoir master cylinder bore size, and wheel slave cylinders bore size constant at 1", and the brake drum diameter constant at 12". I'm not talking about a vacuum assisted or power boosted system. All Buick did was increase the width of the brake shoes on their heavier cars. Why? Wider brake shoes stopped the heavier cars with the same system pressure applied (brake pedal feel), with less brake heat and fade. Using narrower 1952 Special brake shoes and drums on a 1952 Roadmaster will burn up the brakes muy pronto.

In 1956 Buick began placing fins on their front brake drums, and offered power brakes as a option. In 1957 or 1958 Buick began using the 45 finned aluminum brake drums (lined with cast iron), in the 1960's Buick increased the aluminum finnage from 45 to 90 (double). All this effort to dissipate heat induced by the brake shoes (shoe width did not increase on the Roadmasters from 1952 forward). Aluminum wasn't used to decrease brake (unsprung) weight it simply dissipated the heat energy more efficiently.

Yes, front brakes provide the majority of the braking effort on a 4 wheeled vehicle. The rears engage first and then the fronts (ever ride a motorcycle?). If the front brakes are engaged first, the rear of the vehicle is still moving at the same speed, the front of the vehicle is moving slower, and inertia will case the rear of the vehicle to want to pass its front. For this reason, pressure differential and proportioning valves are utilized to control brake bias during the stopping cycle.

I agree, use the larger 9" Ford drum brakes in the rear and install a Scarebird or similar disc brake conversion up front. Utilize a dual reservoir master cylinder. It's more important to decrease your vehicle's stopping distance than it is to increase it's ability to accelerate and maintain a higher speed. You'll enjoy the ride longer...

 

The thing that is a little puzzling is that what the OP is saying is true ....the friction force that is acting on the drum from the shoe is the Normal Force ( the force normal or perpendicular to the surface) multiplied by the coefficient of friction of the brake shoe material on cast steel. There's no area in the equations. The friction force x the moment arm ( 1/2 brake drum diameter ) is the braking torque at the drum. I have some old Machine Design Books from the 1930's I'll check and see what I find.