Brake trueing
Page 38
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Stephen Gray investigates the whys and wherefores in brake lining and drum matching.
AT ONE TIME or another most a us have experienced the potentially dangerous consequences of unbalanced or uneven brakes. More often than not the cause of the problem can be traced back to inadequate or poor bedding-in Of the linings after a brake reline, or to distorted brake drums.
When a vehicle has been relined, there is a difference between the radius of the drums and that of the new . linings. Effectively this means that the area of contact between the two is greatly reduced, causing excessive heat to build up, which in turn can distort the drum or even the shoes themselves.
Beat spotting of the drum can also take place, and this leads to brake grab and premature lining wear. One way to prevent these situations from occurring is to grind the linings to suit each individual drum. Unfortunately, this is only half the story: the drum may be oval, heat crazed or distorted. To overcome this the drum has to be skimmed, and oversized linings ground to match Its new radius. Although the process has been common practice on the Continent and in North America for many years it is relatively unknown over here.
Trist-Draper of 804-818 Bath Road, Bristol, maker of TopDog brake linings and suppliers of flexible hoses, already operates Britain's first national brake trueing service from eight of its 20 depots, with plans to extend it to more in due course.
When an operator or garage is relining a vehicle, it brings the old shoes and drums, complete with hubs, to the nearest Trist-Draper depot. Each drum/hub assetnbly is then set up in turn on a Van Norman brake drum lathe. A rubber anti-vibration belt is tied round the outside of the drum and an initial cut of approximately 0.5mm (0.201n) is made to ascertain the extent of ovality and the depth of any heat 'crazing.
If the drum is too badly distorted or the heat crazing is too deep, the customer is told that a new drum is required, and linings only are supplied. If, however, the drum can be machined, a second fine cut is made and the machine's builtin grinder is used to follow the cut, about 6rrun (*in) behind.
The resulting surface finish is very good and, because the drum is machined on its own hub, there is no chance of eccentricity. Meanwhile, the chosen oversize linings are riveted on rap the shoes and set up as a pair on the PEL shoe• grinder. Setting up is simple. Anchor-type shoes are located at their anchor end by two studs, and a spacer of required size is fitted at the other end. A spring is used to keep a tension on the shoes.
Same radius
For non-anchor types, a circumferential ring of the correct size is fitted to the machine, on to which the shoes are damped to maintain their radius. The shoes are then ground to the same radius as the drum (a special measuring device ensures accuracy).
Now, both drum and shoes are marked to make sure that their perfect match is maintained. Trist-Draper says that the results are better than new and, in fact, one major vehicle manufacturer has a tacit agreement with T-D for it to perform its service pn vehicles which give brake problems during the warranty period. Net costs of the service depends on the size of the drum, ranging from £5.20 for under 76mm (ain) wide drums to £12.42 for 229mm (9in) wide drums, but this price includes re-radhising the shoes. Alternatively, shoes only can be re-radiused at 99p per shoe. The cost of the lining material is extra to these charges.
From the moment the skimming and grinding is done the brakes are effectively bedded in and a vehicle fitted with them will reach full brake efficiency alt once. The only problem that -Trist-Draper can foresee is that vehicle operators and repairers regard the service as a "once only" whereas. the company insists, it should be performed every time the vehicle brakes are relined.