Gee, where to begin?

On every disc brake system I have worked on, DIY only, there had always been an accordian type rubber seal/boot around the brake piston and secured to the caliper body. If you remove this seal/boot you can discover that it strongly "dislikes" being extended and will always return to the collapsed position due to the internal springs.

"in the caliper" Never said so.

"There is no "wobble" in the rotor either"

Even the most perfectly machined rotor will have enough "wobble" to force the brake pads back away from the majority of the rotor surface. Now, also, jack up any wheel and grasp the top if the tire and push/pull to check for bearing looseness, always just a little bit there, right...??

And finally..

In case you haven't noticed, and obviously you haven't.

There is an ad running currently that touts the braking advantage for a new system that lightly applies brake pressure to the brakes if the windshield rain sensor indicates rain, wet climate. What would be the need in that if the pads were already always in contact with the rotor...??

 

I can't say discs don't retract at all... but they don't retract MUCH. Whenever I've done a disc break job I've had a HECK of a time getting the darn caliper to slip off the disc because the pads were trapped by the groove they wore in the rotor. Same goes for drums. In the case of drums (I've only done that once) there was some way of using a tool (screwdriver?) to retract them I think.

In any case... I always thought discs "touched" the rotors at all times, but I can't verify that.

 

The main reason, as much as the marketing folks deny this, is cost. Drums are cheaper and do the job at hand. Are they the best option? No. Disc offers superior stopping power but in a vehicle that wasn't designed with performance in mind, is it really needed? No, but from a safety point of view, disc are the better option. Also, the drag that we keep hearing about is negligible at best, the weight difference of disc (lighter) vs. drums will probably negate this issue.

 

"in a vehicle that wasn't designed with performance in mind..."

More of a matter of the fact that the rear braking capability isn't normally a significant portion of the overall "braking" equation.

But again, BEANCOUNTERS...!!

 

Willard,

I think we can agree to disagree on this one (well at least I can).

If you are proposing that the expandable dust boot in some way retracts the piston (the same piston that you either have to spin or use a c-clamp to retract when replacing pads), I would have to say you are incorrect.

The accordian style boot does nothing more than keep dust and moisture out while allowing for significant piston travel as the pads wear down and the piston must move further out during the pad service life....

You are right that no rotor is perfect, but the runout you speek of is measured in thousandths. If it were enough to push in the piston, you would feel that when applying the brakes...

Two examples prove that your pads are always in contact...

Ever notice after your car sits for a few days, the rotors get a little rusty... notice when you first drive you can feel and HEAR the scraping sound for the first few miles?
This is the pads rubbing on the rusty surface (even when the brakes aren't applied).

Also,

Next time it rains out and you arrive at your destination... check your rotors, they will be dry.

Immediately after driving through a large puddle, you might feel less braking, but by applying the brakes even slightly harder, this lack of braking can be reduced even more.. (that is the marketing ploy that you referred to...)

Another Idea,,
If you are still doing brake jobs, as soon as you get the wheel off the ground give it a spin with the caliper still on....
Now take the caliper off and give it a spin again..

There absolutley WILL be a difference, every time, every car...


Respectfully,

Sean

 

"Marketing ploy..."

Sounds as if you have never had the "joy" of ending up out in an intersection since the brake rotors had a thin film of water on them when you started seriously braking.

No, not a marketing ploy, sound, VERY sound, engineering design.


Respectfully,

Willard West

 

All you guys ( Except Willard ) have missed the obvious.

When you start a DIY brake job or even as Mark has done in a professional shop, how did you get the car/truck to the driveway, garage, or lift???

Did you let it coast there from a high speed? Or did you use the brake pedal to stop the car JUST BEFORE taking the wheel off?

Willard touched on this and I agree.
During DRIVING the rotors will push the pads 100% off the rotor.
There is zero continuous drag.

At high speeds, this "pushback" is nearly instantanous.
On a lift, from a dead stop, it may take a few tire revolutions, or, by hand, you may not have enough muscle to push them back at all. This may have given some of you an illusion that the pads always gently rub. They for sure DO NOT!

You only need 0.001" of clearance, and that's not a lot, a thickness of a sheet of paper will be more than enough, but it is enough to avoid wear, and it is enough to prevent heat build up.

There is no "suction" on the hydraulic lines, and there is no spring that I am aware of.
Neither is needed in reality. If your discs drag all the time, there is something mechanically wrong with them.

-John

 

So when you flip the caliper up, there is never a drag from the pads?

When I shift to drive from park, I apply the brake just like you.... but then I RELEASE the pedal to begin moving forward,,, applying the brake when you are stopped (to shift to D) doesn't remove any rust....

Your cockiness irritates and it's unfortunate that you must refer to adding it to the debate.

For that reason, I will let the others here continue with the discussion...

,

08FEH

 

Good-o.

 

As a trained auto mechanic who used to make my living fixing cars, I was going to weigh in on this thread with what I was taught in vocational auto classes and technical school. But, I see it's becoming a fight and I'm tempted to STFU. I expect I'll be told to do just that shortly.

Couple of points:
Fluids are not compressible to any real extent, if they were then hydraulic systems would not work. They will expand and contract a bit with heating and cooling. This is how a thermometer works. Pressure placed on a fluid in one location is immediately transferred to everything that fluid touches.

Disc brakes are far superior to drum brakes as far as stopping capability. You would know this if you had ever ridden an old motorcycle with drum brakes. They provide far better stopping power with far less heat fade than drum brakes ever could. They are also simpler and easier to repair, for the most part.

No disc system I have ever seen has retraction springs. They are not needed. The disc system is designed to move the pads away from the rotors by the SLIGHT runout that all rotors and bearings have. This is not nearly enough "wobble" to be felt or cause other problems. Pulsation felt in the pedal is usually a rotor that is warped well beyond spec.

The dust boot has no effect on retraction of a disc caliper although the piston seals have some slight return effect. The bulk of the return force is from rotor/bearing runout.

When a wheel has spun at road speed, the discs have retracted from runout so there is very little drag. We used to do on-car wheel balancing that used a pair of driven rollers to spin the wheel up to highway speed to find imbalance in the entire wheel, tire and rotor (or drum) assembly. When that was done to a wheel, on the car, there is no discernible drag and the wheel takes quite a while to spin down and stop. If it doesn't, it's an indication that something is wrong in the braking system or wheel bearings. This is freewheeling, the balancer device was rolled under a wheel while the car was raised on the lift and the wheel could spin freely.

The first full rotation of a wet rotor that has been clamped by the squeezing of the brake caliper is enough to wipe the water from the disc. The brakes may feel like they grabbed when that water is gone as the pads are now contacting the rotors.

There is no pressure in a disc brake system when at rest. Excess fluid can return to the master cylinder through a hole that is only open when the master cylinder piston is at rest position.

When you unbolt a wheel caliper and try to slide it up off the rotor, you are dragging it across a ridge at the edge where the pads never sweep. Rotors are only (relatively) flat all across their surface when new or freshly turned.

Drum brake systems rely on compound braking action where the leading shoe is forced into the drum, then the rotational force helps put additional pressure on the trailing shoe through the link at the bottom where the star adjuster is. Springs are used to retract the shoes and force brake fluid back up into the master cylinder. Drum systems are much simpler to apply a hand brake to, you just run a cable to a lever that forces the shoes out against the drum.

The lion's share of your braking is done by the front wheels due to weight transfer. When you hit the brakes, most of the weight shifts forward to bear down on the front wheels. A vehicle the size of the FEH will do fine with rear drum brakes. In fact, I traded in a 2003 Mazda Tribute 4WD V6 for my 2006 FEH 4WD. The Tribute had front discs and rear drums. I drive down a winding canyon road to work every day. The Tribute was just as good at handling that road as my FEH. It just got 18 MPG doing it.

Does that answer anybody's questions?