Brakes question

I was hoping someone could help me understand the brake calipers/system. I am trying to understand what keeps the caliper centered around the rotor. The whole caliper floats on the 2 dowels of sorts but what keeps the pads from dragging constantly on the disk or from contacting the disk at an angle?

I read in one of the threads that you want to be sure not to have any "preload" of sorts on the caliper from the brake lines. So how do you know if there is any odd pressure on the caliper when it floats relative to the disk.

Thanks! Andy

All disc brakes work this way. Even the ones on your car. The pads do "drag" on the disc but with no pressure, it's not a problem.
The dowel pins keep the caliper in alignment.

Mel said:

All disc brakes work this way. Even the ones on your car. The pads do "drag" on the disc but with no pressure, it's not a problem.
The dowel pins keep the caliper in alignment.

Click to expand...

That makes sense. So is there any way to cause a contact or dragging problem if your brake lines to the caliper put a little torgue on the caliper itself in one direction or another? From memory, I would guess not because when I pulled the caliper off to remove the pads, the inboard pad and backing plate were very difficult to remove due to the fact that they would bind if I had any non-uniform pressure on them as I tried to slide them off the dowls.

Is there a flex loop in the line before it is fastened to the caliper? the line should be pre bent in a way that it should be in close proxemity to where it goes into the caliper.

bret said:

Is there a flex loop in the line before it is fastened to the caliper? the line should be pre bent in a way that it should be in close proxemity to where it goes into the caliper.

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Yes, it was constructed with the spiral around the gear leg/axle. I feel good about the job I did recently changing the tubes and tires, and more recently, the brake pads. I just want to be sure I am not overlooking anything that I could screw up putting the calipers back on the disk with the new pad in them. The whole brake system was a bit of a hazy mystery to me in terms of the questions I asked above. Mel cleared up the haze for me but I just wanted to double check that there were no other important issues I might overlook when I put things back together.

After pressure on the brakes is relieved, the o-rings rotate slightly and retract the piston, at least that is the automotive theory. Automotive uses square cross section rings, that do this to a greater degree.

Additionally, any tiny wobble due to deflection of wheels, axial runout and any bearing play will tend to push the pistons back. If that happens, you will notice a slight pump will give a raised pedal position.

The drag is very slight to non existent.

OK, I appreciated the help. I am looking forward to buttoning everything back up with confidence now. Andy

Thanks for the help!!

Everything worked great! The shoes are anti-seized, torqued, and safety tied. This is the first time since I bought the plane that the right brake is quiet. Not a peep (or groan or squeal!).

I was able to hold run-up power (1800 rpm) right out of the hangar and cold. I did a couple or three quick taxis and firm stops. Actually, I never let the plane stop until I was at the FBO. After the brakes cooled I had even better power. No leaks, no sounds, no problems. Very happy!

Old pads


New pads

Automotive brakes are far superior

Mel said:

All disc brakes work this way. Even the ones on your car. The pads do "drag" on the disc but with no pressure, it's not a problem.
The dowel pins keep the caliper in alignment.

Click to expand...

I am referring to pin slider automotive brakes (NOT rail slider designs like Bendix -ugh!)

The best automotive designs use square section O-rings. The o-ring groove is machined into the caliper housing with a form tool (as opposed to a single point cutting tool). This is so the pad side circumference of the groove is smaller than the caliper bore side. When the brake is applied the o-ring compresses. When the brake is released, the square cut o-ring wants to slide toward the back of the groove, thus retracting the pads from the rotor and eliminating any brake drag.

This design feature, called the "high rollback caliper" is a primary reason current cars coast so well, aerodynamic improvements being the other.

GA brake designs have the o-ring groove turned on the brake piston and use circular section O-rings. Indeed the brakes drag slightly while taxiing. However, much cheaper to make in GA low volumes. Plus hopefully one's taxi time is small compared to flying time!

More than you ever wanted to know from a former Sr. Brake Engineer @ Kelsey-Hayes Corp, a Tier One automotive brake supplier.

Larry Tompkins
544WB -6A
W52 Battle Ground, WA