How much should discs float ?

[utopia]

Ok, its not like I expect a definitive answer but .....
I've bought a pair of secondhand discs with the intention of converting them to fully floating.
These are they ....

http://i1326.photobucket.com/albums/u644/photopia132/Viper%20disc%201_zpsarmybhgz.jpg (http://s1326.photobucket.com/user/photopia132/media/Viper%20disc%201_zpsarmybhgz.jpg.html)

http://i1326.photobucket.com/albums/u644/photopia132/Viper%20disc%202_zpsnk8adp5c.jpg (http://s1326.photobucket.com/user/photopia132/media/Viper%20disc%202_zpsnk8adp5c.jpg.html)

Basically I will be replacing the wavy spring washers with plain shims, which I can machine to any thickness I like.
To cut a long story short, I'm wondering what axial clearance is normal between the buttons and the rotor/carrier.
Thus far I have failed in my attempts to obtain manufacturers specs so I was wondering if any of you folk with fully floating discs would be good enough to slip a feeler gauge in there and tell me what the clearance is.

Oh, and brakes are safety critical items and should not be messed with. ;)
But, for the record, I'm somewhat protected by an engineering background and a natural caution.

[slob]

never really thought about it much until you asked. My guess is: take your front wheel out, place discs in calipers, apply brakes & release. measure the gaps between pads, add together and subtract the thickness of both discs. I suspect about half the result is the right answer. I'll try and measure mine tomorrow.

[utopia]

Now, there you go !
That's why its always worth chucking an idea around.
I was looking at it more from the point of view of "what's the minimum float that will do the job".
Your approach looks more at "what's the maximum float that the system will tolerate" ie kinda from the opposite direction.
I think that makes a lot of sense.
Ta.

And I presume that you suggest applying the process to both calipers so as to then get an average.
And your aim is to determine the space available for the discs to "flap" in without excessive bouncing off the pads ...?

No hurry for the measurements ... but they would be reassuring.
Ta again.

[slob]

I guess you'd need to allow for thermal expansion of the materials and brake fluid. Of course that would allow GP commentators to speculate that 'the tank slapper out of the final corner had knocked the pads back making Stoner (et al) to run it deep into turn one.'

[Dirty]

When I tested my rear floating brake kit I was very disappointed and it took ages to get it out of the pond!

[slob]

different kind of float, the rear floats round the wheel spindle, front discs float side to side

[AndyC_772]

How much should they float? Much less than these...

http://www.scoobynet.com/non-scooby-related-4/1037318-just-rolled-into-the-workshop.html

[Dirty]

different kind of float, the rear floats round the wheel spindle, front discs float side to side

And there was me thinking it was to do with density

[BigOz]

I've got a spare set of buttons and rotors for mine I'll dig them out and check tonight.

[Dukedesmo]

I've got full-floating discs on both my Ducatis. I got an MOT advisory a couple of years back for the 916 (but not last year?) due to the amount of 'float'. These are Brembo HPK discs that are fully floating by design, the others (on the Monster) are standard Brembo discs converted to fully floating using STM buttons.

Funny how the original floaters float more (too much in the MOT tester's opinion) yet the converted don't, but it maybe due to them having alloy carriers rather than steel? Although I'm not sure if the wear is on the carriers or the buttons?

Anyway, not easy to measure but if you (Jeff) want to measure them/see for yourself, you know where I am. :thumbsup:

[slob]

...I'm not sure if the wear is on the carriers or the buttons...

That's a conversation I had with Jeff the other day, softer bobbins (ie Al not Ti) might save you having to replace the carriers

[BigOz]

It turns out I have spare washers and e-clips but not buttons.

I've got a spare set of buttons and rotors for mine I'll dig them out and check tonight.

[Flip]

I've got full-floating discs on both my Ducatis. I got an MOT advisory a couple of years back for the 916 (but not last year?) due to the amount of 'float'.

Funny how the original floaters float more (too much in the MOT tester's opinion) yet the converted don't, but it maybe due to them having alloy carriers rather than steel? Although I'm not sure if the wear is on the carriers or the buttons?

I had the same advisory a couple of times with a GSX-R I used to own a few years ago (same testing station but different tester)- the eventual wear on those brakes was on the alloy carrier.

[Dukedesmo]

I had the same advisory a couple of times with a GSX-R I used to own a few years ago (same testing station but different tester)- the eventual wear on those brakes was on the alloy carrier.

That's what I'm fearing and It would be a shame because the actual discs are hardly worn.

[utopia]

Yes, I'm wary of the mot situation but I've known my local tester for a long while and he's a sensible chap.
I may leave the old discs on 'til after mot time anyway.
I might even ask his opinion for future reference.

Oz .. thanks for looking anyway.
Tim .. I'll definitely pop over at some stage ... avec feelers.

On the bobbin material ...
I agree that it seems to make sense to have the bobbins wear in preference to the carriers.
But most of the aftermarket bobbins that I have seen in aluminium are hard-anodised 7075 alloy, same as Renthal sprockets, so those ones at least are not exactly soft.
To make my own out of such material would require subsequent treatment, ie hard anodising and maybe even some form of heat treatment, and all of that was just becoming too complicated.
On top of which, these discs came with stainless bobbins anyway, so titanium is no worse than that for being over-hard.
And .. I just happen to have some titanium in exactly the right size.

But the plot thickens, because .....
It turns out that the rotors on these discs are thinner than the carriers .. by about 0.018 ins.
I wasn't expecting that.
So I tried to imagine why.
I'm not sure that I have the answer but it soon dawned on me that the absolute minimum float that I can have on the rotors is 0.018".
This would require the buttons to be shimmed to zero clearance on the carriers, which is what I have done so far.
The rattle from the resulting 0.018" free float doesn't sound awfully loud when I shake the disc ... but it was loud enough for me to ask my original question about std bobbin clearance.

The best info that I have so far is that around 0.014" bobbin clearance is something like the norm (though this is not yet confirmed).
My gut feeling is that I would like to err on the low side of that if anything, ie maybe about 0.010" ideally, so 0.018 is getting a bit high ..... or is it ?
Because, with my bobbins shimmed close against the carrier, its only the rotor that moves, not the bobbins.
On the other hand, with "normal" discs where carrier and rotor are of equal thickness and the bobbins can move on the carrier, the lateral range of movement of the rotor is double the bobbin clearance. ....(think about it).
So maybe my 0.018" range of movement isn't far off my ideal of a tad less than normal.
Effectively its like having "normal" discs with 0.009" bobbin clearance.

Ok, if you're still up for this, there's one more thing.
Because the bobbins don't move (much) on the carriers, then maybe the wear is less than if the bobbins were loose in the normal fashion (I would say its highly likely).
And that is another reason why I might make my own in titanium.

Incidentally, these discs look like they've hardly been used and there is no discernible wear at the bobbin/carrier interface.
Indeed, the anodising on the mating surfaces looks pristine.

[Mr Gazza]

Looking askance, as LJK Setright might have said..(Indeed he did say that quite a lot!!)

I wonder if there is a relationship with the amount of desirable float, to the maximun allowed runout on a rigid disc...Whatever that might be?

[slob]

I don't think so, runout has to do with the resulting pulsing effect, which is one of the the areas where floating discs offer an advantage.

[Mr Gazza]

My thinking was that if there is a tolerable amount of runout that a solid disc will operate safely with...Albeit with a bit of associated pulsing. (and within MOT limits.)
Then that should be the maximum amount of float that should ever be needed.

That said, if a disc has a permanent runout of any sort it should be replaced. A floating disc is not a cure (or cover up) for that.

Perhaps a floating disc is more to cope with recoverable and temporary warping due to heat under extreme and prolonged braking. But I imagine the limits of any anticipated warping under such conditions would be in line with limits of permanent warps...or runout.

Just a though you see..since the brief was how much should the disc float...:D

[utopia]

Looking askance is always good, and it opens up the perspective.
Just ask the astronauts !

Dealing with runout and any consequent pulsing is not really my main area of interest in fully floating discs though.
Well, maybe that's not entirely true since I am hoping for an improvement in feel, albeit maybe only a small one.
But my interest in them was sparked when I realised just how much reduction in drag between disc and pads they offer.

My initial thoughts started from a similar area to Gazza's suggestion actually.
But as always, its a delightfully complicated conundrum with many contributing factors.
LJK would've loved it.
I would think that another major factor is the extent to which the elasticity of the seals pulls the pistons back when you release the lever pressure...as per Rob's earlier comments.
But I would guess that this is not at all precise or consistent in practice, as the seals deteriorate and dust accumulates etc.
And their capacity to do this is pretty limited in the first place.
Couple that with manufacturing/assembly tolerances, warpings, sticky bobbins et al and, one way or another, semi-floaters always seem to drag.

I'm also wary that there may perhaps be an issue if the float is too great, where the rattling discs could knock the pads back and leave a lag in braking response .. as happens in racing.

For this and other reasons it seems logical to aim for the minimum float that will achieve the objectives and 0.010 ins seems to me like a bare minimum, to cover any runout and reduce drag compared to a semi-floater.
Therefore if 0.020" does prove to be the norm, I'm happy enough with my 0.018".