AR7E - Alcester Racing 7's Equipe
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How the rear brakes work...OK, First of all, let's cover the basic design of the rear callipers.
Click Image For Full Size If you look at the above picture, you can see the outer pad (edged in light blue), the fixed calliper chassis (edged in yellow) and the sliding calliper body (red - with green arrows showing the direction of movement). The rear callipers have two identical pads (inner and outer), but only one piston (the front callipers have one piston dedicated to each pad). The piston is part of the sliding calliper body and acts directly on the inner pad only. So how does the outer pad do any work? Well, the sliding calliper body forms a sort of claw or hook reaching from one side of the brake disc to the other. When the piston actuates under braking, it first forces the inner pad against the brake disc. Once the inner pad is in contact with the disc, it cannot move any further - so additional movement in the piston actually starts to force the sliding calliper body away from the inner pad. Now, you will remember that the sliding calliper body hooks over the brake disc and behind the outer pad - so eventually the movement of the calliper body will pull the outer pad onto the disc as well - and that is how one piston can actuate two pads... Incidentally, the purpose of the fixed calliper chassis is just to hold the sliding calliper body in position and to provide a sort of guide rail for the pads to slide on.
Accessing the PadsThe brake pads themselves are not actually bolted to anything - they are just trapped between the calliper chassis and body. So, to access the pads, we simply detach the sliding calliper body from the fixed calliper chassis and then lift them out... Take a look at the picture below - it is the same as the coloured one at the top of the page, but has some of the key components labelled.
Click Image for Full Size. Key: A - 15mm Lock Nut B - 13mm retaining bolt C - Bleed Nipple D - Handbrake actuator spring To replace or inspect the pads, we only need to worry about the lock nut and retaining bolt in this case. Once the bolts have been removed, the sliding calliper body can be lifted away from the fixed chassis as in the picture below (noting that care must be taken not to damage the rigid brake lines...)
Click Image for Full Size. Key: A - Inner Pad B - Slide rails for outer pad (pad removed) Inspecting the PadsYou can now inspect the pads - and if you are happy with them, simply replace them on the sliding rails, replace the sliding calliper body and then retighten the retaining bolts against the lock-nuts.
Aside - You can identify which of the two lower pads was the inner and which was the outer by looking at the marks (highlighted in yellow) on the back of the pads. The one with the central, circular mark (made by the piston) is the inner pad - the one with the two hook/claw marks is the outer pad... On this car, however, we noticed some minor problems: i) Uneven Wear
First of all, by checking the amount of pad material left on the inner and outer pads, we could see that the inner pad was fractionally more worn than the outer pad (this was the same on both the rear brakes). This wasn't a significant issue - but it did suggest that the calliper body wasn't sliding cleanly - and so the brakes were applying pressure to the inner pad without the outer pad being called into play so strongly. This can be partly caused by the resistance to movement created by having fixed brake lines (as opposed to flexible hoses) -but to be on the safe side, we cleaned and regreased the sliding bolts (covered by the rubber gaitors in the pictures above) and also cleaned and copper-greased (lightly and carefully) the sliding rail surfaces that the pads sit on. The chipped edges, general glazing and reduced amount of pad material prompted us to fit a new set of pads (standard Lucas pads are approximately £20 for a complete rear axle set - so it was a sensible precaution). However - pad replacement (as opposed to simple investigation and refitting) raises the level of complexity of this maintenance procedure one notch... Pad Wear and Piston MovementYou might be surprised to find out that when you take your foot off the brake, the pistons and pads do not fully retract. If they did, then as the pads wear, the piston would have to move further and further before the pads actually come into contact with the disc - and so more and more brake pedal movement would be necessary to brake over time. In fact, the piston and pads only retract just enough to allow the wheel to rotate reasonably freely - the pads will actually continue to lightly skim the surface of the disc (unlike bicycle or car drum brakes where a spring forcibly parts the pads from the friction surfaces). Now, what all of this means is that when you come to replace old pads with new, the piston will have moved out of the calliper body to compensate for the pad wear - and the nice, juicy, fat, new pads won't fit. The piston will need to be forced back into the calliper body somehow... Piston WindbackIt takes quite a lot of effort to force the pistons back into the calliper body. A lot of effort. You can try levering the piston using a combination of spanners, wedges and blocks - but this still doesn't actually make the task easy - and if you slip whilst applying a large force, you could end up damaging the delicate fixed brake lines... As with most things in life, the cost and time eaten up by using inappropriate tools (and breaking things along the way) means that the proper tool is usually worth the outlay even if it is infrequently required. So, let us introduce the brake piston windback tool:
Key: A - Tommy bar (winding handle) B - Threaded driving shaft C - Main body D - Pressure plate E - Square drive F - Piston adapter plate G - Piston locating lug And here seen in situ...
Key: A - Main body B - Pressure Plate C - Threaded Driving Shaft The piston adapter fixes to the driving shaft via a square-drive socket and rotates with the shaft and handle. The locating pins fit into recesses on the piston face and force it to rotate too. As the shaft is wound in, the main body pushes against the pressure plate which in turn pushes against the sliding calliper body - and thus the piston is driven back into the calliper. When the tool is first placed in position, it is important to ensure that the piston adapter plate is properly located (via the two locating pins) onto the piston - otherwise the plate will not be able to rotate, and the tool can jam. It is also important to ensure that when the tool is removed*, the piston is left with its locating recesses arranged in a North, East, South and West arrangement so that the piston aligns with the locating pin on the back of the brake pad itself. *Tip - when the piston is fully wound back, do not try to release the tool by turning the handle anti-clockwise - it can jam... Instead, turn the barrel of the main body clockwise and it will release the tool more easily and reliably. That's about it, really - the windback tool can be helpful during the bleeding process as well - but more on that elsewhere...
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