Re: Brake Noise & Fading

Posted by DavidPackard On 2022/11/17 23:20:44

Marty;

I think you’re on the right path of identifying the differences in the primary v. secondary springs, and the secondary leaving the anchor at the beginning of the process. I’m at a loss what would sustain that activity to produce a pulsation. If both wheels suffer the same condition, and become out of phase, I suspect some pedal pulsation would be felt. I’ve never had a set of drum brakes cause a pulsating pedal, but when a set of disk brake rotors warped the pulsation was quite severe. I would think putting the drums on a lathe would prove or disprove whether the drums are warped.

My vote is the primary spring is too stiff, or the initial preload is excessive. Perhaps the difference in the two springs is quite subtle, and if the wire diameter is the same, as you reported, the difference is embraced in number of active coils, coil diameter, or perhaps the length of the hook ends.

Without knowing the coefficient of friction, I don’t know what to say about the length of the primary shoe friction surface. You would think with 15% more surface area the initial spring force should be reduced. Once the available wire diameters are known, and the winding mandrels have been made, I would think one of the few variables left is the length of the primary shoe. That’s the last ‘adjusting screw’ that’s turned.

The two part numbers for the friction material may be nothing more that the geometry of the shoe (thickness, width, arc diameter, and location & diameter of rivet holes), and given the known difference in shoe length having two different part numbers makes sense.
The sensation of fading could be the loss of a portion of the ‘self-energizing’ character of the brake . . . they just won’t stop as well, and the pedal force is increased.

Warning: I’m going to go full geek on this part of the posting:

The equation for the spring constant (k - force per unit deflection) for a coil spring is given by the relationship
K = Gd**4 / (8nD**3) where d**4 = d to the fourth power and D**3 = D to the third power
G is known as the Modulus of Rigidity, which is a property of the material, steel in this case, which would be the same for both retraction springs in a drum brake
d - wire diameter
n - number of active coils.
D – mean coil diameter which would be the outer diameter of the coils minus the wire diameter
A weaker spring will have more coils, all else being equal
A weaker spring will have a smaller wire diameter, all else being equal
A weaker spring will have a large coil diameter, all else being equal.

The shape, length, and configuration of both ends of the spring will not affect the spring constant, but will affect the initial hydraulic force required to initiate motion. That alone can also affect the initial motion of the primary shoe. It is my understanding the design intent is for the hydraulic end of the primary shoe to move first, which infers the static force of the primary shoe is less than the secondary shoe (with single diameter wheel cylinders). The initial static force is the combination of the spring constant and how far the spring is stretched.

With the wire diameter being raised to the fourth power small changes can become significant. In this case a 10% change in wire diameter suggested by Ross’s estimate of primary/secondary spring wire diameter difference will result in approximately a 46% change in spring constant (all else being equal). With the mean diameter being raised to the third power small changes can become significant. If the same 46% change is used the outer diameter of the coil would need to grow by 13%.

All of the variables can be changed simultaneously, and I’ve intentionally not included initial coil binding, which is another contributor to initial spring force to be overcome by hydraulic force.

dp