Floating vs non-floating discs

[deviant]

One of those things I've never really got straight in my head, so it's time to pick the brains of PBland...

Q1 - What do floating discs do that non-floating discs don't?

Anyone who answers 'float' gets a virtual slap round the back of the head. Not least because they don't actually float unless they are worn out.

The fountain of all knowledge suggests that it is to do with allowing different amounts of thermal expansion between the rotor and the hub, thus avoiding warping. Which seems feasible.

I'd guess there's potential to reduce unsprung/gyroscopic mass with a two-part disc because the hub can be alloy, although it's a pretty trivial contribution compared to the wheel. Mixing materials would make it even more important to allow them to expand at different rates of course.

Anyone got any other reasons?

Q2 - Why do you almost invariably see non-floating discs combined with sliding calipers, and floating discs combined with opposed-piston calipers?

I think the answer to this one is just cost - non-floating discs are cheaper, sliding calipers are cheaper, so they tend to both appear on cheap (read: shite) brake setups.

Can anyone add any sort of engineering reason not to mix them in any other arrangement?

ta,

D

[chancho196]

A floating disc is able to move slightly, rather than warp, when the pistons don't move at the same rate when applying pressure.

A sliding caliper doesn't need this because it should always apply equal pressure or, in other words, move to suit the disc (as long as the sliding mechanism is free).

[Pete]

I was always under the impression the floaters were predominatley used for reducing the friction caused between the pads and discs during heavy use (ie racing conditions) and therefore lowering the rolling resistance of the bike.

Pete

[Liamo]

I was always under the impression the floaters were predominatley used for reducing the friction caused between the pads and discs during heavy use (ie racing conditions) and therefore lowering the rolling resistance of the bike.

Pete

er ... isnt friction the whole point of the exercise when it comes to braking?

[Jay]

Floating discs have free movement for heat expansion hence the bobbins, this means they dont warp as easy under extreme load.

[Pete]

er ... isnt friction the whole point of the exercise when it comes to braking?

Sorry, I didn't mean for it to read like that.

I meant that due to the increased heat of heavy braking the discs can expand and with the floaters having free play there's always a little "give" left, which means the wheel will spin more freely (when not braking) ie have less friction, than a fixed rotor due to them having no free play. It was Whitham that I heard this from, so have ago at him not me!!

Its probably all balls anyway

Pete

[deviant]

A floating disc is able to move slightly, rather than warp, when the pistons don't move at the same rate when applying pressure.

A sliding caliper doesn't need this because it should always apply equal pressure or, in other words, move to suit the disc (as long as the sliding mechanism is free).

They don't float side to side though.

[TriGGer]

They don't float side to side though.

But the rotors do flex on the bobbins, including side to side.

[Evilchicken0]

Yeah ... it's all about heat dissipation.

Iron discs (for exaple) scrub speed by converting the kinetic energy into heat through friction (I read that in the PBMag when they did the braking article) All that heat causes the wearing disc to expan a lot so to stop it warping it's discoupled from the carrier by the bobbins. There shouldn't be any forward / backward movement on the bobbins it's all left / right and the discs have spring washers on the inside so they can flex without rattling. The older spring washers get flattened a bit so the discs rattle but as long as the forward / back movement is withing manufacturers tollerances that's NOT AN MOT FAIL. You simply get the bobbins taken apart and the spring washers replaced ... bobins are a couple of quid each and spring washers are pennies.

Fully floating Brembo disks - see the new spring washers .... (cost £35 fitted and posted for both disks)

New disks .... because the garage decided the old ones (above) fail the MOT (cost £340)

You can buy the new disks (about 300miles) for £240 + PnP

[VeeTeeArr]

If you have a solid disc of large diameter and it gets hot the rate of expansion of the friction area will be much greater than the inner mounting area. The only way the outer friction area can expand (while being held solid by the inner part) is to distort (become wavy).

Mounting the disc on bobbins allows the hot friction area to expand away from the cooler mounting hub, reducing the amount of stress and distortion.

The larger the disc and the harder it is being worked the more need there is for it to be mounted on bobbins. Some disc's, even though they aren't floating, get around the expansion problem by having large cutout areas where the friction area meets the inner effectively mounting the discs on thin spokes cut at an angle. These 'spokes' flex to let the disc expand away.

Some axial float can be benificial to allow the disc to align itself between the pads, but too much axial float can cause more problems, especially on bikes that like to shake the front wheel, due to the gyroscopic effect of the disc pushing the pistons back into the calipers if the front wheel slaps about over bumps.

[porter_jamie]

my old mono used geniune tz250 floating disks. they didnt really float very much, and i replaced them for another geniune set that had a load of float, ie you could wiggle them from side to side a few mm. The difference in braking performance was massive (the 'floating' set felt way more powerful)

no science, but might help...

[lorenzo]

Have you ever used a bike with fixed disc and calipers? Did they rub like fuck and still have shit performance? That's because they're shit. If your discs seize on the bobbins and stop floating your brakes will suddenly turn shit and sticky. It is for this very reason, shitness, that they'll make either one or the other componant float. Floating calipers tend to have a fair bit of 'slop' in them, they move much more than a floating disc and hence lack a bit of feel. Also, have you ever seen a floating caliper that has opposed pistons, rather than just a pad held on a slider on one side and the pistons on the other? Once again, it's that shitness aspect coming into play.

[tonk]

Now I'm confused, I've got twin fixed Brembo 320mm discs with 4 piston, opposed Brembo calipers (not radial though). All on Brembo wheels.

Is this a load of kak then, by everybody's reckoning?

PS They're the best brakes I've ever used, so what would 'good' ones be like? Wow.

[Evilchicken0]

Now I'm confused, I've got twin fixed Brembo 320mm discs with 4 piston, opposed Brembo calipers (not radial though). All on Brembo wheels.

Is this a load of kak then, by everybody's reckoning?

PS They're the best brakes I've ever used, so what would 'good' ones be like? Wow.

Sounds like a Ducati setup - do they look a bit like this ...

If so they're goldline calipers which were available in a few different giuses and not a load of kak. They could be improved though, obviously new brakelines and a change of fluid will help, and then a change of pads and a lastly a radial master cylinder.

One other thing about improving the brakes are that you could be looking at the whole bike as a package so do the first 3 and get your suspension sorted before the radial master.

[VeeTeeArr]

Now I'm confused, I've got twin fixed Brembo 320mm discs with 4 piston, opposed Brembo calipers (not radial though). All on Brembo wheels.

Is this a load of kak then, by everybody's reckoning?

PS They're the best brakes I've ever used, so what would 'good' ones be like? Wow.

Don't you ride a BMW? What model is it, because I can't imagine the dics's aren't floating

[tonk]

Yep, its a new BMW K1200S, ABS, brembo wheels, brakes and discs. Discs are definitely fixed - no bobbins. Calipers are fixed too but not radial, they have 4 pistons, 2 pads ( I know cos I took them apart the other day to clean the salt off).

I'm guessing the discs have a lot of iron in them, they're usually orange the next day after being put away wet. The only odd thing is some unusual washers between the disc and the wheel, can't describe them but they might allow a teeny bit of movement, maybe a millimetre at the very most.

It's got braided lines too and I've fitted adjustable levers - the clutch is on the longest setting for fine clutch-control and the brake lever is on the shortest cos it will stop dead from any speed with one finger.

I've got no complaints at all but was puzzled by the comments on how 'shit' fixed discs were. I hope mine is an exception.

Never seen them on anything other than a beemer, guess they're just being weird for the sake of it. Aftermarket wheels convert them to floating discs (BST) but I can't afford £2,000 for carbon wheels. This month

[VeeTeeArr]

They are floating, the disc mounting holes are slightly elongated (radially out from the centre point), and are mounted on bobbins. This allows the disc's to expand when they get hot to prevent warping.

Because the holes are elongated, the bobbins will not allow the disc to have any movement forwards and backwards. Only up and down movement is allowed but you won't feel anything because only when one holes in aligned up & down the other four are trying to be moved side to side. It's only when the disc gets hot that all five holes move outwards along the bobbins.

Disc showing elongated mounting holes----------------Mounting bobbin

[gregtrx]

I was wondering about these when I test rode an f800s a few years back, the brakes seemed fine but I thought it was very odd to have fixed discs. VeeTeeArr explains it all!

[Evilchicken0]

They're fixed to the wheel aren't they (???) it saves the weight of the carrier and the brakes act directly on the wheel.

[morepower]

There are several things which can contribute to warping. But basically it all stems from heat in the disc. Solid discs when hot can put tension in the disc as the bolt area can be cool and the hot disc will expand and have no where for the expansion to allow movement in the disc so hot outer section will then warp. This can also happen with full floating or semi floating discs if the pad track (area where the pad actually touches the disc) is a lot wider than the actual area needed by the pads. CCM had huge discs with really big pad tracks but the pad only worked on about 50% of the pad track. These could warp really quickly as the outer would get hot enough to turn blue and the inner portion would still be quite cold.

Now semi floating discs which have the correct pad track width can warp if they get hot very quickly like in race use as the heat expands the rotor all over and it can actually expand and lock on the semi floating buttons and warp. But this really is when they are used hard and most OEM calipers are not really true full floating discs as they are held too tightly to float when they are cold.

Genuine full floating discs rattle and have a really good resistance to warping but they do have to be made so that any heat transfer would be dissipated quickly through the buttons. Some discs do not use any buttons and these can be made from Cast Iron which retain heat and can be quite good with sudden heat increases as the disc will not fully heat up quickly as they are poor conductors of heat but once hot they can then lock up in the carrier and as they take a long time to cool then this can lead to warping.

There is more to warping than just full or semi floating discs..Even pads can make a diffence as these even carry heat in different ways and the heat transfer is different from one pad to another. Carbon based pads do not retain heat very well so do not really transfer much heat into the discs, but some heavily sintered pads which have a lot of metal in them will heat up the discs and retain heat which can be a major factor in making a disc warp.

This is one reason I prefer SBS pads over EBC as they do have less metal in them and even the Carbon pads have stainless steel backs which dissipate heat very quickly to remove heat which also has the added benefit of less brake fade.