Floating disks - bikes vs. cars

[dbrick]

The common wisdom is that brake disks "float" to allow movement both between the brake disks and their carriers (which are made of different metal, and thereby move differentially when the disks are heated by use), as well as movement within the disk itself, as it heats and cools. That makes sense.

But...why are the rear disks on bikes usually non-floating? And how about cars' brake disks? There's a *lot* of heat generated there. Why are they mounted rigidly?

[Terry]

Excellent question.

I'm guessing here but the fact that floating discs are fitted when you have two on the same wheel. If they didn't float, even the slightest misalignment would make them work against each other - you could at worst have only one effective friction surface per disc. It must also help with with wheel balance, surely more critical on a bike.

On performance cars at least, although the discs do not float, they are more ventilated than simple drilling, they are a sandwich of two discs with a distinct air gap between them - I suppose it's because size/weight are less of an issue and cooling is more critical as they are hidden under the wing.

Some European cars float the caliper itself - the trouble is, when the sliging pins stick, you wear one pad considerably more than the other ( it's always the one with the wear indicator wire that doesn't get worn !!!)

I am intrigued to hear the answers from contributors more knowledgeable than I am.

[MikeH]

I think that it's also due to the difference in design and construction between a car and a bike.

The discs on a bike are much thinner to keep the weight down, so will tend to distort more when hot. They are also bolted to the wheel so any manufacturing tolerance and play in the bearings will show up as misalignement at the caliper.

In contrast, a car's discs are much thicker and therefore much more stable as regards heat distortion. The disc is also part of the wheel hub assembly, as is the caliper itself, and alignement can be achieved to much closer tolerances. Any discrepancy or run-out in the wheel has no effect on brake disc alignement.

I think that the floating calipers seen on some cars is just a low cost option. Cheap and nasty - they are single piston design on only one side of the caliper, which therefore has to float in order to apply an equal braking force to each brake pad (or at least that's what the designers calcs showed!).

Looking at another option, there have been some design concepts recently on super sports and race bikes to have a much larger diameter brake disc attached directly to the wheel rim, with the caliper located higher up the forks. I imagine that advantages for this setup would be less heat buildup as it's over a bigger surface area, and lighter wheel construction as the braking force is transfered directly to the weel rim and not through the spokes. I can't see this being used on anything other than exotic machines though due to cost.

[Terry]

I can't see this being used on anything other than exotic machines though due to cost.
Bizarrely, the Sym Joyride scooter has a rim mounted disc.:dunno:

[MikeH]

Bizarrely, the Sym Joyride scooter has a rim mounted disc.:dunno:Ooops! I got that wrong. Was it a racing scooter? :D

[dbrick]

I believe that disks "float" for heat-related reasons - at least that's what Iv'e heard - and my original question was why car systems didn't float theirs. The best reasons I've heard are based in thinness of the disks and the primitiveness of their provisions for the circulation of cooling air, and the disks' concomitant proclivity to warp.

Terry and Mike both observe that floating disks are used to allow for manufacturing tolerance stacking and to allow for differential wear of brake pads. I disagree. In most motorcycle disk brake systems, brake pad wear is handled by movement of the pistons in the slave cylinders; as the pads wear, the pistons come to a rest position closer to the disk. By keeping the working face of the pads a constant distance from the disk, the brakes are self-adjusting; free play at the lever remains constant. The movement of the pistons is why changing pads always involves pushing the pistons back into their bores so there's clearance for the new, thicker pads. Diferential wear, manufacturing misalignment, and small alignment differences is compensated-for by ever-so-slight "cocking" of the piston and pad.

A two-piston caliper (one piston on each side of the disk) can therefore be ridgidly mounted to the forks. This is what we saw on Honda CB750 brakes, and others, and on BMW systems during the /7 and early K days.

If the bike's caliper has only one cylinder - like the ATE brakes on the /6 bikes - then the caliper must be able to move laterally to take up the gap which develops by pad wear between the fixed pad (the one which isn't mounted on the piston) and the disk. The piston's coming to rest in a different place will take care of the disk-pad gap on the piston side. Moveable caliper design is also seen in car systems.

I don't think that floating disks are there so that they can move laterally to compensate for disk pad wear and alignment issues...at least on BMWs.

[Terry]

Let's not start on aligning b%$^&*£)$("*£$ ATE calipers :bang:

[MikeH]

Dbrick,

If you read my original post I don't mention wear as a factor anywhere. :confused:

I think that the floating discs on a bike are there for 2 reasons.
a) to account for any misalignement or manufacturing tolerance between the disc and the caliper, i.e. runout.
b) to minimise any effects of distortion of the thin discs due to heat build-up .

[Ed Miller]

It really has nothing to do with the rotor thickness or heat or anything of that sort. It is simply a matter of design, cost, and weight. A typical automotive brake caliper is an inexpensive (and heavy) single piston unit, thus it needs to float in order to be able to apply pressure to both pads. If it was fixed it would only apply pressure on the piston side. As the brake pads wear, the caliper must move on the slides to take up the slack. Having the disc float is not an good option as it would have to move too far over the lifespan of the pads, plus it would be much more costly to build. Your motorcycle caliper is a multi piston unit, it has pistons pushing in on both sides equally, so there is no need for caliper movement in the design. However, even though the caliper is mounted solidly, there are other factors at work such as fork flex, runout and such that nessecitate having some allowance for the movement. If they were to build the entire system to be ridgid and not requiring that free movement, it would be either very expensive or very heavy or both. A floating disc is one answer to the situation, some bikes do use a floating caliper with a fixed disc, it just does not have the range of travel needed in an automotive single piston caliper setup.

One additional note on the heat issue. The rotor should not distort under heat, if it did, you would have serious issues with the braking. You would feel a pulsing sensation in the lever with even a minute thickness variation in the disk. If the disc was to develop too much runout, you would have excessive lever travel at the master cylinder. The rotor will expand radially with heat, and that is accounted for in the rptor design. The main difference with a motorcycle disc versus an automotive disk is the material. An automotive disc can have lots of mass, so using cheap cast iron rotors is ok. They are cheap to build, and the addition mass required to maintain stability under heating is not as big a deal. Nobody wants a twenty pound hunk of cast iron bolted to thier motorcycle wheel (well perhaps a Harley rider would tolerate it:D ), so a higher quality (expensive) material that will maintain stability under heating with much less mass is required. Thats why your bikes disc cost $200 - $300 each while a cars disc is often less than $25.

[dbrick]

Dbrick,

If you read my original post I don't mention wear as a factor anywhere. :confused:
Right you are, Mike. I responded to both you and Terry in one post, and wasn't clear about which of you made which points.


I think that the floating discs on a bike are there for 2 reasons.
a) to account for any misalignement or manufacturing tolerance between the disc and the caliper, i.e. runout.
I understood this, and disagreed with it.
(Jeez, if we all agreed on everything, we'd be riding H**** step-throughs.)


b) to minimise any effects of distortion of the thin discs due to heat build-up.
This one I agreed with...though Ed thinks we're both wrong.

[Paul]

Heat rejection is indeed part of the design concept, by using a floating disc the rotor and carrier can be built with different materials optimized for the application, allowing the development of larger diameter discs. Early Japanese discs were stamped out of stainless steel and bolted up solid to the wheel hub; they lasted forever but were very heavy and prone to warping under extreme usage (racing). Italian bikes used cast iron rotors which gave great feel and performance but were prone to rusting. With the advent of multiple piston calipers it was possible to instantly generate tremendous amounts of heat - remember, a brake works by converting kinetic energy into heat.
There is no need for a floating disc in back, because of weight transfer up to 75% of your braking power is up front, a small disc is easily locked up at that point and you are just not going to generate enough heat for it to be an issue.
The new EVO brakes are a very clever design in eliminating the carrier and using a floating attach point on the wheel spoke, gets rid of a lot of unsprung weight. And look at Buell's rim mounted disc, also very clever.
Cars don't need a floating disc, even in high performance applications. There is enough mass to remain dimensionally stable.

[MikeH]

What ever the design issues, modern disc brakes are fantastic and light years ahead of the old drum brakes we had to contend with a few years (Err . . decades ago). In those days you had to plan stopping 10 minutes ahead of time :) We didn't need ABS though - there was never any chance of locking the wheels up! :D