Why you should give your rear disc a quick look...

The other week I happened to look at the rear disc on one of our bikes and saw the above crack. Not pleasant.

On taking the rear of the bike apart and looking at the caliper I found this:



As you might be able to see, the piston has been replace by (what I suspect) is a non CB500 piston - it has remnants of pointy bits on the end (in the way that I suspect some pistons do). It was obvious that the previous bodger had attempted to flatten off the end of the piston but hadn't done a very good job, so the pad wasn't exactly meeting the disc square on. In addition to all that the pad spring was missing in action and there was no R-clip on the pin!

I 'finished off' the bodge that the previous owner had started by filing down the piston so it was approximately flat and roughly square with the bore - hopefully this will mean it pushes the pad less wonkily!

Thankfully I could 'borrow' a somewhat rusty pad spring and an R-clip out of the other bike (having access to another bike the same is really great). I'd bought a 'nearly-new' disc off ebay for £25, so I got that fitted.

I then managed to use a valve spring compressor as a make shift G-clamp (it *is* just a big G-clamp, isn't it?) to push the piston back in which allowed me to fit the new pads I had bought especially for this kind of occasion (the old pads were not worth refitting).

After a good bit of slider-pin cleaning and re-greasing it went back together and after a bit of riding around to re-seat the pads and test things out it all seemed to work OK.

That was all last weekend, during the week these arrived from Fowlers (that is a new pad spring and a new R-clip):



I then spent a bit of time on Saturday refitting these to the bike that was 'owed' new ones - who thought that having to take the rear wheel off to disassemble the caliper/slider mechanism was a good idea?

On Sunday the weather was nice and we went for a ride:

All I can say to that disc setup is eeeeek! Previous owners have a lot to answer for.

Funnily enough my rear disc had a problem caused by a chimp with a toolbox which failed me an MOT.

Scenario: Rear disc warped, guy bodged it by selectively loosening mounting bolts, filling the gap left with fiberglass, time passes, fiberglass deteriorates, flappy rear disc......

Now, although the gunk had crumbled, still had to get the wheel off, clamp the axle length ways in a workmate, set up a sliding tool post with a lathe tool in it and accurately plane the stuff off back to the alloy boss.

chromedome wrote:All I can say to that disc setup is eeeeek! Previous owners have a lot to answer for.

I can't quite blame the previous owner for the state of the disc - the corrosion is almost certainly our fault and/or the 'fault' of the weather. What was going on inside the caliper - that was certainly their fault!

Funnily enough my rear disc had a problem caused by a chimp with a toolbox which failed me an MOT.

Scenario: Rear disc warped, guy bodged it by selectively loosening mounting bolts, filling the gap left with fiberglass, time passes, fiberglass deteriorates, flappy rear disc......

Now, although the gunk had crumbled, still had to get the wheel off, clamp the axle length ways in a workmate, set up a sliding tool post with a lathe tool in it and accurately plane the stuff off back to the alloy boss.

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That's not the way to fix that problem!

Fibreglass resin can be quite hard to remove, so I assume that was what you might have had to plane off to get back to the wheel.

Yes, the resin was a swine to get off. Quite a lot had found it's way into the mounting bolt holes too.

If you want improvised thread lock, fiberglass is your chappy every time.

Anyway, after removing it, replacing the fruit bowl which the old rear disc had become with a new one, the run out  and shimmy was 0.0008" according to the dial gauge measurements.

Do you want to leave a wife and children in uncertainty or a girlfriend in the hands of a psychologist? change disc and pads and breathe deeply anchorite

Yes, I didn't fancy anyone riding the bike with that crack in the disc - it probably won't go wrong, but I wouldn't want to bet on it...