The construction seems, at first glance, to be quite easy to comprehend. A disc which is linked with wheel is clamped, generally by two brake shoes. What can go seriously wrong there? Unfortunately, a great deal. On the previous pages, we've already dealt with the possible mal
function of the caliper mechanism, in particular that of the floating caliper. We'll assume that the brake is clamping faultlessly and, often enough even more important, that it releases properly.
The hydraulic system won't be our subject here either. There is no brake-fluid leakage, and nothing is scraping. Of course, we exclude the possibility of the discs bieng worn beyond the wear and tear limit. One checks them by using a special slide gauge and if necessary replaces them. This should also be done if the grooves are too deep. In this case, please check both the outside- and the inside surface.
You'll be asking yourself, if not this, what then. While we're having a look at the brake disc, perhaps checking it for cracks would be good idea. One should however, leave the allowed tolerances to the experts. Then, there is also the noise development when braking, whereby, at the moment we don't have any particular thoughts about the subject: maybe a paste between the piston and the pad or a suitable insert, cleaning of the pad-guides, bevelling the brake pads slightly and sensible running in of the brake pads.
So, we now finally come to the actual subject. Our brakes, when being used, bring a certain amount of disturbance into the chassis. This can effect the pedal-pressure, the steering or even the whole front-end of the vehicle. Of course such phenomena are also possible at the rear, but they are not that noticeable. That does not mean that one can carry on driving indefinitely. Such disturbances can be shown on the rolling tester if the gauge sways slightly back and forth.
Of course we must now refer to the preceding examination of the chassis and of course also the tyre pressure, however, the brake test does show the right direction, namely a non-uniformity of the brake disc. For this there are of course, apart from manufacturer errors, any number of other reasons, and they all have to do with the enormous heat build-up on the brake disc (up to approx 800°C and more). Added to this, is also the sudden cooling, e.g., from splash-water or the permanent distortion caused by not decreasing brake pressure after the actual braking.
It is vital, that this distortion is measured, in the above picture one can see roughly, how the instrument is applied. Attached quite far to the ouside of the disc, the gauge should vary no more than five-hundredths of a m/m, however, differing manufacturer defaults should be adhered to. Should the distortion be too great, the discs and the pads on both sides must be replaced. It is particularly important to meticulously clean the surface of the disc where it is mounted onto the flange of the wheel hub, because the price of slovenliness is very high.
What is to be done, if the disc, when mounted on the cleaned flange, still has too much distortion? There is also a solution for this, however, somewhat more complicated. In this case the complete wheel hub and the bearing must be dismantled. We're assuming a one-piece bearing, which is found in front-wheel drives, but sometimes also on the rear axle. Mostly there are spring-washers to be be loosened, after that, hydraulics are needed. Once all the new parts are mounted, there should be no more vibrations, either that or the new parts are outside the accepted tolerances. 08/10
|Is a disc braked too thin, there is a risk that it gets too hot (fading) and/or warping.|
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