Disc runout ETZ251 ('89 original)

[Puffs]

Just a learning point for me that might help someone.

I had taken the brake disc off the hub for painting, and after reassembly I had quite some runout on the disk. I noticed this because the pads were grinding the disc at the high places, while staying well clear on the low spots. The rubbing caused the disk to warm up, also without braking, and of course robbed the bike of some speed.

Making sure the mating surfaces of disc & hub were properly clean did not help, so I measured the runout as shown:

Disc runout.jpg (27.58 KiB) Viewed 29 times

and found something like 0.5 - 0.6mm between the highest spot & the deepest one. So an axial runout causing typically 0.3mm of amplitude in the disc.

The disc can be placed in 6 different ways, so I figured: let's try them all. I had first chosen the worst, while the best had an amplitude of less than 0.05mm (0.09mm between min & max).

There is a number cast on the disc bracket between the mounting holes, quite possibly that should have been aligned with something on the hub, but I didn't do that. The learning is: make a note how the disk is orientated before you take it off.

[Blurredman]

Interesting.

And you say you cannot see any alignment marks?

[Puffs]

Well, I hadn't seen any alignment marks (other than the '1 2 3 4 5 6' cast between the holes on the disk bracket). If there were alignment marks, I hadn't looked carefully enough. At any rate, I had initially just put it back the first way it fell, thinking it wouldn't matter. But apparently here it does.

Apparently both mating surfaces have a small axial runout (meaning surfaces not exactly perpendicular to the axis), and while assembling it in the factory they must have picked the alignment giving least runout in the disc. Note that what I saw can not be explained by just 1 mating surface having a bit of axial runout (or for instance a bent disc), while the other surface would be completely square to the axis: in that case you would see the same runout, regardless of how you position the disc. It has to be a combination of 2 runouts, in some positions enhancing the amplitude, while in the optimal position counteracting each other.

[AlanJ]

Hi Puffs, I had a similar problem with my Saxon 301 when a Co powder coated my frnt wheel, they sprayed over the mating surface of the disc. I got it sorted by having the surface skimmed on a drop bed lathe, cost me a tenner. I did think of a solution before the skim but not before the paint people had a bloody go with a file which made it worse. Anyway have a look in the mechanic's posts under Disc Brakes in my name, it's all in there. An idea I thought of to get the paint off the mating surfaces is an electric soldering iron sharpened like a chisel or heat up an old chisel.
Wish you well Alan.

[Puffs]

Hi Alan, thanks, but here it was not caused by paint or lacquer on the mating surfaces. That is what I first thought too, but I cleaned the mating surfaces and that did not solve the problem. I cleaned the surfaces with a suitable knife to remove all paint, down to the bare metal, on both hub & disk bracket. I did so being careful not to remove any metal or deform the surfaces, and with a suitable knife that is easy & quick, and both surfaces looked flawless. In a case like this, you shouldn't use files or sandpaper to grind the surface, as that will inevitably remove some of the base material too. But if you do it the luthiers way, carefully with a knife, no re-work on a lathe is required; the process & result is similar as with your proposed heated chisel. It is important that the knife has a completely straight edge, is sharp, and that you control angle & pressure, so that you don't actually cut into the metal.

Just FYI, my take on the mechanism of how braking works. When you brake, the master brake cylinder pressures-up the brake fluid inside the calliper. As a result of this, the pistons do not normally slide outwards over their seal; they only do so the first time after installing new pads (and occasionally a minute bit, to compensate for wear of the pads). Instead what happens is that the rubber (Viton) seal deforms a little, thus allowing a little movement of the piston outwards, towards the disc. That deformation of the seal is elastic, and normally there is no movement between the surface of the piston & the surface of the seal.

Also interesting to realise is that the seal, being elastic, is at the same hydraulic pressure as the brake fluid, so it presses onto the piston (radially, inwards) with that same pressure, plus the little bit of mechanical pressure it has from the somewhat tight fit around the piston. But leaking is prevented by the hydraulic pressure inside the seal, and that same pressure prevent leaking between the seal and the groove it is in.

And then when you stop braking, the piston in the master cylinder is pushed back by a little spring. The hydraulic pressure in the system reduces to 0, and the elastic deformation in the piston seals vanishes: they spring back, and the pistons retract inside the calliper. Just a little bit, maybe 0.25mm, and as a consequence the disc should run free again (as long as the disc runout is less than the amount by which the pistons retract).

It happens rarely, but when the pistons have actually slid outward over the seals, the fluid volume inside the calliper, behind the pistons, has increased a little, and that volume of brake fluid is then sucked from the reservoir & comes ahead of the master piston. IMO that little spring inside the master cylinder is never strong enough to create sufficient suction to pull back the pistons in the calliper (furthermore, if that's how the pistons would return, then only the one that moves easiest would be pulled back), all it does is aspire a bit of fresh brake fluid.

When the seals get old, the material hardens. Then what happens is that your piston moves much less, and also retracts less, and you run the risk of a dragging brake.

[AlanJ]

Hi Puffs, Yes, you are right, it's the deflection of the seals with and without pressure that moves the piston and hence the pads off of the disc. Incidently the frnt wheel still had it's tyre on when I had the disc mounting skimmed. Still on the lathe, after the skim we bolted on the disc and dial checked the runout and it was 0.05. Everything is really great with that brake now. That is on the Saxon 301 but I wish that the frnt hub brake on my TS 250/1 was a lot better, infact it was one of the reasons why I bought the Saxon. Anyway Puffs wish you well with the disc. Alan.

[DAVID THOMPSON]

do mark the disk and hub on the 4cycle bikes i had this problem on my 2002 rt125
way too much wobble after a disk removal and reinstall
had to try it with a dial Gage several times before it seemed correct
had disk off to change tires and wheel bearings and had a mess getting it sorted
oem wheel bearings were really nasty at 10,000 miles which was part of the problem
DAVE