Know-how 8—hidden cause of brake snatch
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AT some time in its life, almost every commercial vehicle has suffered front brake snatch, and as a rule the cause is not too difficult for the experienced motor fitter to find.
The obvious reasons for this defect follow a certain pattern. For example, a hurried brake reline and a rubbing shoe can overheat the drum and hub, and before the tight liner rubs free a small amount of grease reaches the drum, taking on dust from the liners and breaking liner to drum contact during light applications.
Another cause of snatch can spring from neglected adjustment, particularly with two leading shoes. However, this is no real problem for the skilled fitter. On the other
hand a heavy vehicle descending a long hill at speed under heavy braking can overheat and warp front brake drums. This distortion can still be in evidence after the drum has cooled, and the only answer here is to skim the drums if the variation is not excessive, or exchange them.
Adjustable steady pegs positioned in the back plate can also cause snatch and vibration if they have been set up incorrectly, and to set the pegs without the shoes in firm contact with the drum can mean that the shoes may be unable to meet the drum face squarely and will chatter heavily.
Another cause is the dust I mentioned earlier. Certain drums seem to hold this dust more than others, and after a wetting on some storm-swept road, front brake snatch can be in evidence for several miles, until the lumpy dust is again dried out and spread around.
These are well-known features to the practised transport fitter, although the younger mechanic may have yet to meet them all. However, there is always something new to keep the old hands on their toes as well, and brake snatch reared its head on several new vehicles defying all attempts to find a cure for several weeks.
All the normal ailments and remedies were considered or applied, and none brought more than _ temporary relief, the fault reappearing without warning in a few miles. Only when a front spring main plate failed
was it discovered that the rear-most top shackle pin was locked solid in the bushed bracket, and required some very solid thumping before it would move out. It was clear from the appearance of both pin and bush when finally withdrawn, that movement had only taken place under really severe spring-action, and very much to the detriment of pin and bush.
Slipping the nut from the opposite shackle pin disclosed the same situation; this pin was also an interference-fit in the bush. Withdrawing the lower pin and attempting to swing the shackle called for a 5 ft. bar and even then it moved very much under protest.
Light dawned at last. With full resistance at the swinging link, foot brake application called for the links to move in sympathy with spring deflection, but as the links resisted movement, so the stress built up, until the links finally moved with a catapult effect—not in unison.
This gave the effect of brake snatch, but was in fact direct interference with the steering geometry. It continued to affect steering control until the vehicle slowed down or the brakes were released.
Reaming out the bushes to .006 in. clearance cured the trouble completely, although how 16 new vehicles of the same model managed to leave the assembly lines with interference-fitted shackle pins is a question yet to be answered.