Recreation of the automatic brake system failure due to uneven braking effect

Hi,
The trailer brake on my caravan showed an uneven effect during the TÜV (German technical inspection) test. The friendly inspector adjusted a tooth on the weaker brake, and then the brakes were within the acceptable range.
The brake cable adjustment (travel) was set to the manufacturer's specifications on both the left and right sides. The brake, which had only 4000 km on it (so it was practically new), had been cleaned and lubricated according to the specifications beforehand. The adjustment reduced the clearance gap on one side by approximately 0.5 mm.
I'm missing a clear explanation of how this correction could have had any impact.
The balancing cam ensures a symmetrical distribution of the pulling force of the lifting device. The brakes will initially engage differently depending on the amount of play, but they should eventually equalize as the pulling force increases. The different tension of the return springs seems...
to be of little importance to me. I also don't believe in a significantly different self-reinforcing effect. But something must have caused a different clamping pressure on the pads, and therefore a different braking effect.
hg
Herbert.

Hello,

in times when I didn't have any brake cleaner at home: did I reach into the drum with greasy/oily fingers?

It sounds trivial, but I experienced it in at least one of my cars, which made me pay more attention to it in the future. Since then, I've always used brake cleaner for the final cleaning.

Best regards, Rainer.

Certainly not. Then the reenactment wouldn't have worked out so well either.
hg
Herbert.

Herbert wrote:

I'm missing a clear explanation of how this correction could have had any impact.

Hi Herbert,

As far as my brain can function after the heat today, I can imagine that, depending on the design, the metal casing of the Bowden cable on one side might be damaged.
and initially slides a short distance into each other before the force is properly transferred.
One side of the cable pull is more resistant, meaning the transmitted force reaches the pad "later" due to the equalizer bracket.

Best regards, Richard.

Hi Richard,
The entire vehicle has only 4000 km on the odometer. For that reason alone, I would rule out a faulty or stiff control cable. A damaged outer sheath would not re-expand even when the brake is released. Furthermore, none of the cables are stiff. And as I mentioned, adjusting the timing by just one tooth significantly increased the braking effect.
hg
Herbert.

Hi Herbert,

Herbert wrote:

...the brake, which has been used for 4000 km (so it's almost new)

I just took my foot off the brake...

Herbert wrote:

The entire vehicle has driven 4000 km. That's precisely why I would rule out a faulty or stiff control cable

.
I agree with you on that .

Perhaps it's a combination of component tolerances and brake pads that haven't worn down evenly, and the correction by one tooth has compensated for it...

Best regards, Richard.

Hi,
I think I've made some progress.
In a drum brake, such as the ones used by VW, the brake shoes are pulled upwards by the return springs against the pressure rod at the top and against the backing plate at the bottom. Once centered, the brake pads have a nearly defined resting position, and when the brake is applied, they are only evenly spread apart.
In the AL-KO trailer brake system, the brake linings are almost floatingly mounted against the expanding wedge, while the retraction spring only pulls the linings towards the expanding wedge. Only during braking does the trailing cheek rest against the adjustment unit - the toothed wheel.
If the brake is simply adjusted to a specific clearance as recommended by ALKO, it is not guaranteed that at least one brake pad will make full contact upon the initial resistance. Depending on the orientation of the brake pads with the locking mechanism, only one end may make contact, and further pulling force and rotation are required to align the brake pads for full effectiveness. It's like extra playtime.
That could at least explain the different reactions.
Now, the pivot point of the rotating jaw itself is also dependent on the clearance in the adjacent unit; the more clearance there is, the higher the jaw must be positioned within the drum. This may change the clamping force, and consequently, the braking effect.
hg
Herbert.

Hi,
Here's another thought: When braking while moving forward, as described above, the two brake shoes rotate slightly with the locking mechanism due to friction against the brake drum. The pivot point is approximately the axle. During this rotation, the attachment point of the brake cable moves, which creates a counter-tension on the brake cable, opposing the tension exerted by the parking mechanism. The clamping force is decreasing.
The more play there is, the more rotation occurs, which reduces the pulling force of the AE (Auxiliary Engine) and decreases the braking effect. When two brakes have different responsiveness, the braking force will always be uneven.
The ALKO service manual for wheel brakes only specifies an even adjustment of the clearance, without checking the braking performance. In my opinion, this is insufficient. The braking performance must be checked and adjusted for evenness. This can only be fully achieved on a test bench. The required braking force of 400 daN per wheel corresponds to a torque of approximately 1300 Nm. This means that using standard workshop equipment (torque measurement), I can check and adjust the first 10% of the braking force and its responsiveness.
hg
Herbert.