Fully floating discs

[AndyRob]

A couple of months ago I looked at an 1100s from around 1999 that had floating front discs, ie they rattled and moved a little on their fixing screws. I think this is called 'fully floating'. I didn't look to see if the back disc was the same.

But I've just realised that mine has 'fixed' discs - not rattling/floating/moving.

What is the time-line for the different methods used and the best practice for discs?

TIA, Andy

[Corvus]

It may be that your front discs aren't fixed. Not truly fully floating either. Bmw use a system where the "holes" in the disc are actually short radial slots. The disc looks, at first glance, to be held firm by the screws, but a closer look will probably reveal that it is held against the turret faces by preloaded wave washers. So the screws don't contact the disc face directly, but tighten against a stepped bush, or "bobbin".

It looks to me as though bmw are allowing the disc to expand freely, whilst holding it in place against the turrets. The wave washers would also allow some temporary warp as well, although I suspect warp is intended to be averted due to the freedom the disc has to expand in a radial sense.

If the wave washers are left out the thing still functions, but rattles. Also I've read reports on here that the turret faces are prone to wear, which would also allow more disc movement. Holding the disc, off the bike, I noticed how very easily it distorted just by light hand force.

I've seen wheels powder coated and the turret faces coated too. This strikes me as risky. Surely the paint on the turret faces will get sticky with the high disc temperatures and may grab, preventing the disc expanding freely? Also may hinder heat transfer into the wheel? This is purely my theory. I have no evidence for certain.

[RiceBurner]

As said - BMW discs are 'floating' and the main reason they don't rattle is the wave-washers.

[AndyRob]

Thanks for the replies so far.

Bruno might be on here soon so I won't steal his thunder but the first R11 I looked at, a good few weeks ago now, had disks that clonked noticeably, even at standstill, when you applied the brake and depressed the front forks.

They moved freely in the fingers and just didn't strike me as being right. They must have been noisy on the road.

[AndyRob]

Bruno - whose R11s I bought - wrote direct to me in case I felt I had a problem with it.

This is an abridged version of the very helpful advice he gave.

"The discs have elongated mounting slots. The arrangement consists of the mounting bolt, a top-hat bobbin, a sprung wave washer, the disc itself and a thin plain washer between the disc and the cast wheel mounting pillars.

The bobbins tighten against the plain washers and the the slotted disc mounting holes are supposed to allow a small amount of movement, the "float". Any rattle is supposed to be controlled by the wave washers.
Not truly floating, but allegedly better than solidly mounted.

Problems arose in early bikes because they didn't have the plain washer, and the aluminium mounting pillars suffered from wear.

That was reduced by the addition of the washer, but the top-hat bobbins still wear. They serve as a sacrificial component that prevent the wheel casting from being damaged, developing flats where they rub against the disc. They can become very thin in places - worth checking.

Some have suggested that its possible to slacken the bolts and rotate the bobbins to a position where an unworn portion of the bobbin bears against the disc. I preferred to replace the bobbins, wave washers and the plain washer as a set, though the torx bolts, bobbins and washers are quite expensive as a full set.

I once slipped up when I fitted new discs. I noticed that my (10k miles) bobbins were showing signs of wear and decided to replace them. I used a very small amount of VHT (very high temp) graphite anti-seize compound but, even though I used it very sparingly, it still managed to work its way out onto the friction surface of the discs.

I replaced the pads and cleaned the discs, but the graphite seemed to have become ingrained into the disc surface, making the braking feel ' grabby'. Replacing the discs solved that problem.

So, check the mounts every 5k and possibly rotate them before they show too much wear. The discs shouldn't really rattle, but they should move just a little under pressure. and don't use any lube on the mounts.

NOTE: The bolts are supposed to be thread-locked into the wheels but I've heard tales of stripped threads on removal. I use a hot air gun beforehand to soften the loctite, or whatever."


The grease issue, above, reminds me of my very early driving days - don't try this - I know there are a dozen reasons not to.

Drum-braked Morris Minors etc often leaked brake fluid onto the brake shoes. It was quick, cheap and easy to replace the seals in the cylinders but the shoes were relatively expensive so we'd sit them in a metal tray, pour petrol into it and 'fire' the shoes so that the fluid boiled out of them. It worked insofar as the brakes worked fine and, we argued, the linings had become 'hardened' in the process which saved us buying the heavy duty, anti-fade shoes that all 17 year olds thought were better.

The good old days!

Andy

[dysondiver]

ah ,,, and putting a coin ontop of the slave cylinder piston , which got the last bit of use from a thin drum ,,, oh the good old days , to be fair , 1300cc cars didnt do a hundred miles an hour then ,,, no power steering , no servo on brakes ,,, valves a must at 40k , and pistons and rings at 80 ...
oil pressure lights on at tick over , and setting the points with a bit of a fag packet.

[Corvus]

ah ,,, and putting a coin ontop of the slave cylinder piston , which got the last bit of use from a thin drum ,,, oh the good old days , to be fair , 1300cc cars didnt do a hundred miles an hour then ,,, no power steering , no servo on brakes ,,, valves a must at 40k , and pistons and rings at 80 ...
oil pressure lights on at tick over , and setting the points with a bit of a fag packet.

I was posh. I used to set the points on my 400 four ( and later 550) with a little bulb.

Later, when I was married, we had a fiat panda with the first "fire" engine. It was so sensitive to pinking that I finally splashed out and bought a dwell meter and strobe light. Has it down to a tee. Used to keep points accurate by resetting every 3000 miles.

[dysondiver]

and go to the scrap yard on a saturday morning to get some brand new .... old bits .. to fit that afternoon , usually a 28/32 webber ,,, or the likes , to get you out on a saturday night ,,,, lol

[Tapio]

They moved freely in the fingers and just didn't strike me as being right. They must have been noisy on the road.

For what it's worth: the front rotors on my K 100 RS don't have any wave washers.
When tapped lightly, they rattle. A lot.
In fact: a mariachi band could take the front wheel off of a '90 K 100, and use it as maracas.
Do i hear rattling while riding it? No. Absolutely no.

[nab 301]

Most of it is covered here



http://www.boxertrix.com/phpBB2/viewtop ... ting+posts

http://www.boxertrix.com/phpBB2/viewtop ... 7&start=15

[AndyRob]

There's nothing new under the sun.

I come here as a new owner but it appears this rattling discs/worn bobbins/washers/wheels (disc posts) issue has been on the go for years.

I have to say that these posts, and those from quite a long time ago, have been illuminating for several reasons.

When buying a 'pre-owned' bike (or anything really, especially a house) I've always argued that it's the previous owner you should evaluate as much as the item being purchased. That's why buying a bike privately, rather than from a dealer, can be advantageous.

In these brake posts, written over the years, I've read maybe a few things that have worried me - quite a lot in fact. To think that I may have bought a bike which has been serviced/maintained by previous owners who don't seem to know what they are doing is a concern I hadn't thought of before. The discussion on the use of SS bolts in 'mission critical' areas is a good example.

I should say here/now that what Bruno wrote, above, about the maintenance of his old bike which is now mine, gives me complete confidence. For example, his willingness to spend a lot of money on new, genuine BMW, parts when he felt it was needed.

So maybe, like me, you're back into biking after a long time away. Riding bikes that are much faster and more capable than anything we had back in the 60s/70s. And you're buying a classic, or modern classic, that is perhaps 50k miles/15 years or more old.

Trust no-one! Do a thorough end-to-end check of your bike - like the 50-odd point checks that BMW do on the used cars they sell and probably bikes as well? Checks which are quite separate from the Inspection 1/11/111 checks which is worth doing as well.

Keep safe!

[dysondiver]

theres a saying in the watch world ,,,
''buy the seller'' ,,,,,, it really does cover most things.