WHY NOT Keep Drums?

[ultraranger]

Changes in the vehicles height, and especially the addition of weight (bed loaded or pulling a trailer or both) have a dramatic impact to the characteristics of how the brakes and vehicle will react --particularly under heavy braking. This is all additional kinetic energy added that the brakes will have to absorb (convert the kinetic energy of the total moving mass into friction applied to the brakes and convert that to heat energy, by the brake components, which has to be dissipated into the air). Greater loads and/or speed will generate higher temperatures on the brakes when the vehicle is decelerating.

A duo - servo (drum) brake system gives the lowest pedal effort for a given rate of deceleration, for all practical friction coefficients, compared to discs. However, you don't get something for nothing. If the drums get too hot, friction of the shoes to the drums decreases as the brake component temperature increases. --coefficient of friction on discs decreases too, just not the extent of drums if the mass of the drums and discs are the same.

Let's assume you have a vehicle and you're applying 50 lbs of force on the brake pedal (that's not much force but this is just hypothetical). Assume at normal brake temperature, the friction coefficient of the brake shoes and drums is 0.5. Now, assume you've made a hard stop with the vehicle. The friction induced into the brakes heats up the drums and brake linings and results in a drop of the friction coefficient from 0.5 to 0.4. This one-tenth drop in friction coefficient will result in a 115.2% increase of leg force (from 50 lb to 107.6 lb) being required to actuate the brakes to the magnitude of the first hard application just to get the same amount of brake force.

Given the same scenario above, (but for a disc brake system), will require 62.5 lb of force (from 50 lb to 62.5 lb) on the brake pedal or, only a 25% increase in effort on the drivers part to achieve the same results.

If you had a 3500 lb. vehicle and each drum weighed 5 lbs. each (20 lbs total) and were decelerating from 60 MPH to zero, the brake temperature after that one brake application would be 270 degrees (F). If you had a track car of the same weight and brake size and were decelerating from 120 MPH to 60 MPH, the brake temperature would be 813F. Brake requirements don't double with speed. They are exponential. It would take 4 times as much braking effort to decelerate from 60 MPH to zero as it would to go from 30 MPH to zero.

Unless there's some special (vintage) class requirement to run drums, almost all road track race cars have all wheel discs and they're usually very large. A large rotor has a better mechanical advantage since the caliper is further away from the spindles center line. A 13" rotor would have more stopping advantage (leverage) over an 11" rotor, just as a 4' pry bar would have more leverage than a 3' bar.

The heat induced into a brake drum has to travel across the entire drum before that heat can be dissipated into the air to cool the drum off. This is not the case for discs brakes. It's friction surfaces are exposed to the air and cooling to it is immediate. That's not to say discs can't suffer from brake fade but, a disc brake rotor (of the same mass as a drum brake) will resist brake fade for much longer than the drum.

As a drum brake heats up, the drum casting expands. Unfortunately, it's in the direction away from the brake shoes. This will increase the distance the pedal has to travel to apply the brakes. The expansion of the brake components is actually an advantage in disc brakes because it moves the friction surfaces of the discs towards the brake pads so, a little less brake pedal travel would be required.

On a brake drum, the open edge of the drum design is very critical to its performance. The stiffness of the lip will help keep the drum from going out of round (egg-shaped) or bell-mouthing (diameter increases at open end) when the shoes press into the drum.

I saw the photo of the drums being drilled for the purposes of cooling. I had never seen that before on a drum. I've seen holes added to the backing plate but not in the actual drum itself. I suspect the drilled drums were for a race-only vehicle that would be changed out after only a few races. In a street application, the brakes modified like this would be susceptible to grit, dirt or water easily getting inside. The added holes would probably result in a weakening of the drum and stress cracks running from one hole to another. If a drum is being used in a performance application, it's best to use as large a (finned) drum that can be fitted inside the wheel.

But, then again, if you stuffed as large a disc brake on the same vehicle, you would get more braking advantage out of it.

[BrockGrimes]

The drilled drum was an old racer trip from back in the day.
But people did run them on the street, like most things use your better judgement.

[motzingg]

Let's assume you have a vehicle and you're applying 50 lbs of force on the brake pedal (that's not much force but this is just hypothetical). Assume at normal brake temperature, the friction coefficient of the brake shoes and drums is 0.5. Now, assume you've made a hard stop with the vehicle. The friction induced into the brakes heats up the drums and brake linings and results in a drop of the friction coefficient from 0.5 to 0.4. This one-tenth drop in friction coefficient will result in a 115.2% increase of leg force (from 50 lb to 107.6 lb) being required to actuate the brakes to the magnitude of the first hard application just to get the same amount of brake force.

Given the same scenario above, (but for a disc brake system), will require 62.5 lb of force (from 50 lb to 62.5 lb) on the brake pedal or, only a 25% increase in effort on the drivers part to achieve the same results.

could you post your math on this? I don't disagree with the concept but i just don't understand how the math is any different for the two different geometries, seems like it should be proportional in either case. I don't see where the 115.2% is coming from.

i have yet to encounter a brake fade situation in my truck, granted I haven't hauled a heavy load down a mountain yet, but in daily driving, highway speeds and heavy traffic ( i commute on a road that is 65-0-65 with many stoplights) the manual drum brakes have done the job just fine. Even for a week when i was hauling a spare 800 lbs of engine around in my bed.

I've had brake fade happen on many disc vehicles, including a ranger i used to use for lots of heavy towing (cars, boats, tractors) and my escort which i drove like a jackass and warped a set of rotors every now and then.

i guess its like they say, nobody changes unless they are faced with a crisis... i haven't had a problem with my drums yet, but i am going to put a booster on them because holding down the pedal at stoplights is hard on my 'bum knee'. haha.