fixed vrs floating Calipers

[lftnwhls]

I am looking at putting Wilwood Dynapro rear brakes on my 69 Camaro to match the brakes up front.

They dynapro's are fixed. Is there a reason I should go with the floating D154 instead?

[alocker]

C-Clip axles can cause pad knockback issues with fixed mount calipers.

[Schwartz Performance]

What he said!

Throwing the car hard around corners can cause the axles/rotors/wheels to move slightly and push the pads back into the calipers so you have to pump the brakes to get all of your stopping power back. That's the only drawback with the fixed mount.

[lftnwhls]

Thanks guys!!!!! I do have Stock C Clip axles. I love Pro-touring.com tons of knowledgeable guys that aren’t afraid to share information.

[bovey]

Yes to floaters on a c-clip rear-end. I just had this problem on my 12 bolt in a C10 with the fixed Dynapros. The floater callipers are more than likely the way to go, BUT if your c-clip rear-end has a lot of play you may still have some knock back. The only way to know is to check your end play before ordering. Or put the brakes on and see what happens - there are loads of options for c-clip eliminator kits for your style rear-end. Or you could go all out and put a full floater rear-end in. Most builds get away with the floater callipers tho.

[79-TA]

As others have pointed out, floaters are less prone to knockback than their fixed counterparts. However, they are not a complete solution. I suggest considering the following remedies as well:

1. Address the actual problem.

The caliper is not the problem, component deflection is.

Minimize deflection. Get a rear axle setup with a proper bearing retainer. Make sure your wheel bearings and hub assemblies are relatively new and in good shape. The driveway "jiggle" test does not simulate the forces that your 3,000 - 4,000 pound vehicle is putting on that corner assembly at 0.7+g.



2. Rotor Diameter. A smaller rear rotor means less deflection for the caliper if all other things are equal. Figure out how big your rear discs really need to be for a balanced system with decent thermal capacity.



3. Clocking. Where the caliper sits affects how much it is affected by a given deflection. The closer it is to 3 or 9 o'clock, the less knockback it will experience.



4. Master cylinder bore size. Increasing the size of the master cylinder bore will reduce line pressure and hydraulic force multiplication. It will also mean that less pedal travel is eaten up by knockback when the pistons displace fluid. If your system is overboosted, this is something to consider.

[Quickboat]

Fixed or Floating

Callipers are either of “fixed” or “floating” design. Fixed callipers are mounted rigidly to the axle and employ one or more pistons on either side of the calliper to press the pads against the rotor. A floating calliper, common on production vehicles, has a piston or pistons only on the inboard side of the calliper. The floating calliper is mounted on pins or slides so that when the piston extends and presses the inboard pad against the rotor, the whole body of the calliper slides on its pins or guides in the opposite direction, bringing the outboard pad into contact with the rotor. Each design has its advantages and drawbacks as follows:

Floating callipers are smaller, lighter, and easier to package. They are also cheap, readily available, and easy to mount.

Fixed calliper is bigger and heavier than floating calliper. It will also overheat less readily and be more rigid than a floating calliper.

Floating callipers cool better as the fluid and piston are only on the inboard side of the rotor.

Floating callipers have fewer moving parts and seals to leak or wear out.

Fixed callipers tend to apply the pads more evenly, preventing uneven brake pad wear

Floating calliper design more easily incorporates a cable-operated parking brake.

Here is a must read for anyone modding a brake system...
http://www.pirate4x4.com/tech/billavista/Brakes/#MasterCylinders

[Skip Fix]

Also when looking at piston area a fixed only uses one side in the equation compared to a floating-something my brain can't figure as the pistons on both sides "squeeze".

[Ron Sutton]

As others have pointed out, floaters are less prone to knockback than their fixed counterparts. However, they are not a complete solution. I suggest considering the following remedies as well:

1. Address the actual problem.

The caliper is not the problem, component deflection is.

Minimize deflection. Get a rear axle setup with a proper bearing retainer. Make sure your wheel bearings and hub assemblies are relatively new and in good shape. The driveway "jiggle" test does not simulate the forces that your 3,000 - 4,000 pound vehicle is putting on that corner assembly at 0.7+g.



2. Rotor Diameter. A smaller rear rotor means less deflection for the caliper if all other things are equal. Figure out how big your rear discs really need to be for a balanced system with decent thermal capacity.



3. Clocking. Where the caliper sits affects how much it is affected by a given deflection. The closer it is to 3 or 9 o'clock, the less knockback it will experience.



4. Master cylinder bore size. Increasing the size of the master cylinder bore will reduce line pressure and hydraulic force multiplication. It will also mean that less pedal travel is eaten up by knockback when the pistons displace fluid. If your system is overboosted, this is something to consider.

Well put.

[Ron Sutton]

Fixed or Floating

Callipers are either of “fixed” or “floating” design. Fixed callipers are mounted rigidly to the axle and employ one or more pistons on either side of the calliper to press the pads against the rotor. A floating calliper, common on production vehicles, has a piston or pistons only on the inboard side of the calliper. The floating calliper is mounted on pins or slides so that when the piston extends and presses the inboard pad against the rotor, the whole body of the calliper slides on its pins or guides in the opposite direction, bringing the outboard pad into contact with the rotor. Each design has its advantages and drawbacks as follows:

Floating callipers are smaller, lighter, and easier to package. They are also cheap, readily available, and easy to mount.

Fixed calliper is bigger and heavier than floating calliper. It will also overheat less readily and be more rigid than a floating calliper.

Floating callipers cool better as the fluid and piston are only on the inboard side of the rotor.

Floating callipers have fewer moving parts and seals to leak or wear out.

Fixed callipers tend to apply the pads more evenly, preventing uneven brake pad wear

Floating calliper design more easily incorporates a cable-operated parking brake.

Here is a must read for anyone modding a brake system...
http://www.pirate4x4.com/tech/billavista/Brakes/#MasterCylinders


Good points.

[David Pozzi]

Also when looking at piston area a fixed only uses one side in the equation compared to a floating-something my brain can't figure as the pistons on both sides "squeeze".

When comparing caliper piston area (in square inches) between floating & fixed, only one side of the fixed caliper piston area is used.

If you have manual brakes, you will experience knock back problems with fixed calipers. The fix is to use corvette unit bearings in front, & full floater or Corvette bearings in the rear. Power brakes cover up the symptoms by 50%.

[Skip Fix]

So David and other brake engineers. Say you bolt half a fixed caliper in a bracket on a rotor. Will it not push against the rotor and exert force? Possibly causing some deflection since only on one side but it has to exert some force on the pads to the rotor. Now you add the other half and it give additional force and counter acts the deflection.

I could see the piston sq in not being identical to a floating by some weird formula but still seems it would exert some force on a fairly rigid rotor.

[79-TA]

So David and other brake engineers. Say you bolt half a fixed caliper in a bracket on a rotor. Will it not push against the rotor and exert force? Possibly causing some deflection since only on one side but it has to exert some force on the pads to the rotor. Now you add the other half and it give additional force and counter acts the deflection.

I could see the piston sq in not being identical to a floating by some weird formula but still seems it would exert some force on a fairly rigid rotor.



In your half a fixed caliper example, deflection would be a major issue as there would be no opposing force from anything other than the stiffness of the rotor itself.

Your intuition is partially correct. By losing one of two brake pads, you do have only half of the friction force resulting in your half caliper example.


First, remember that for something to be not moving, it must not be accelerating. To not be accelerating, the sum of the forces in the direction that it is not accelerating in must equal zero. 5 lbfs right plus 5lbfs left = net force of 0 on that axis. For the caliper to squeeze the rotor, we want each pad to exert the same force on the rotor. Otherwise, any imbalance in force will be taken up by the rotor.


For the following examples, imagine that you have a floating 2 piece rotor. For the sake of demonstration, pretend it can float very far and cannot resist any force from the pads. It can only be stopped by both pads applying the same force in opposing directions. This is what calipers are designed to do: squeeze evenly. This includes floating calipers with pistons on one side.

Fixed calipers are easy to look at. Each piston has an equal and opposite piston to work with. If the line pressure is 500 psi and the area of the pistons one one side is 2 square inches, it will exert 1000 pounds of force as will the opposite piston.

But how do we get equal and opposite force from a floater with that 2 square inches on only one side? To answer this, look at what's behind the piston. That fluid applies force in all directions. We can easily see that the fluid pressure pushes the piston out toward the rotor. It does the same thing to the outer pad, but in an indirect matter. That fluid pressure is also applied to the back of the piston bore. Instead of being resisted by caliper mounts, it is resisted by the caliper body and ultimately the opposite pad. To reiterate, the pressure on the back of the piston bore pulls the entire caliper and the opposite pad. The equal and opposite force is coming from opposite sides of the same fluid in the piston bore.

So going back to only half of the area being taken into account, you can see that whether fixed or floated, that force, pressure times piston area, is getting an equal and opposite reaction either way. The difference is how the reaction forces are distributed among the caliper and its mounts. Fixed calipers are much much stiffer and don't have the slop of the sliding mechanism, so fixed calipers have a quicker more direct response . . . no only to braking inputs but also component deflection.




So the next kind of strange (but not really once you make sense of it) part about braking force equations is that once you have the force applied to the rotor, you double it, as it is being applied on two separate brake pads. If you did have a braking system where it was up to the rotor's stiffness to resist force from one single pad, then yes, you would half the amount of braking force relative to an equal two sided system.


Tell me whether or not I explained that well . . . :hmm:

[79-TA]

So yes, the irony of the floating caliper is that it is its own (usually) undesirable component deflection helps isolate the piston from the deflection of other components.

Here's a diagram to help with the explanation of the fixed caliper. I could have used solidworks or probably have found an existing diagram . . . but ms paint is more fun.

82369

Red = caliper (the floating part)
Blue = brake fluid . . . we're cool and run the now illegal ATE super blue
Black = brake pad
Light grey = caliper piston
Dark grey = rotor

Not shown - caliper bridge or sliding pins



ok ok ok, here are a couple of results from a quick google search:

82370

82371

82372

[jlwdvm]

Not to beat a dead horse, but how much of the knock back issue applies to non c-clip rear ends? I'm looking at a Strange S60 with a TruTrack carrier, but haven't decided on rear brakes for my 69 firebird track/street car. I have Wilwood 6-piston 13" for the front and would like to make a decision on the rears. I could go with the 4-piston to match the fronts in appearance, or go with a d154?