MZ/MuZ Riders

One of the nice things of these older MZ two strokes is that you can still buy just about any spare part. While New Old Stock has by now been pretty much depleted, because of the huge number of bikes MZ (and Jawa & CZ) built in the past, there is still a very significant number of them around. And that makes it commercially viable to re-produce certain parts.

Over the years, the front disk rotor of my '89 ETZ 251 has become significantly below spec:And that was a while back already, but the problem was: the stock of new, or even decent, original disk rotors was pretty much depleted. This was one of the items that was no longer available. Of course there was an alternative: you could fit a Grimeca disk, but then you also required a new calliper, and wheel hub. So in fact pretty much an entirely new front wheel. And that I thought was a bit intense. And looking at the disk & pads I thought I could carry on a bit more, and indeed I could.

There are of course significant risks associated with running a rotor that is too thin, particularly if your pads are also thin, but for now, let's just say the manufacturers always post a minimum thickness on their disks for a reason.

Now one of the things that happened during my absence from this forum was that new brake disks became available, with specs equal to the original disks. That means you can keep your hub & calliper, just take off the old one & fit the new one. So I ordered one of those disks (from Guesi, but several traders can supply them).Being a floating disk it looks a lot more modern. Also see the minimum thickness is specified at 5.0mm, while the thickness new = 5.54mm:Easily fitted, I could not help measuring runout:A video of that can be seen here: https://streamable.com/gvjgqc, but only for a few days after posting. It wiggles 0.02mm or less, which is fine.

So, there we go:
Of course it works fine. Soft pads may be best, as it can have a tendency to squeak a bit. And also, softer pads may cause less disk wear (though I'm not sure of that).

I'm curious of two things:

Why is the new disc minimum thickness 5mm instead of 4.5mm like the old disc?

And, with it being floating- is there enough to remove possible minimal runout that might occur?

I do not know why they prescribe min TH=5.0mm, and yes, on that the two are different. In fact, the NG disk is just a bit thicker than the original, and that makes fitting it with new pads a bit more of a hassle, but it can be done.

I do not know, but I can speculate. If you apply the brake, the disk gets a force on only 1 side. While in the rotor itself, this force is quite symmetric, so if that were the only force in play, the chance on that force causing the rotor to buckle would be quite small. So then you could possibly go to a much lower thickness. But this is not the only force in play: the rotor itself is held by a construction offsetting the rotor from the hub, and that construction is of course not infinitely rigid. When you bake hard it will flex a bit. And that causes other forces on the rotor disk, and for that it may need more strength --> higher min. thickness. Now on the original rotor, this fixation to the hub is rigid and all around the inside of the rotor disk, while on the new one, it is via a number of these big pins, floating as it's called.

On that floating: that's what it's called, but in reality this is actually pretty stiff (similar to what it is on top quality Brembo or Nissin stuff). But yes, it's possible that it sets & aligns the rotor disk under braking force, I guess that's the idea of these floating disks. For the record: a human hair typically ranges from 0.04 mm to 0.12 mm in thickness.
At any rate, the thing works fine and without drag, I've done several thousand miles with it since I fitted it in 2023.