Uneven MATCO Brake Pad Wear

[Brett H]

I have an original RV-12 (i.e., undrilled gear legs) that is having uneven MATCO brake pad wear.

Three of the brake pads have even and equal wear. But, the right outboard stationary brake pad has 0.020” of tapered wear and at its thinnest spot is 0.025” thinner than the other brake pads.

MATCO technical support tells me that this is due the aluminum brake line acting as a big spring that prevents the brake caliper from being free floating. Namely, the brake line is pushing the caliper to one side rather than allowing it to stay centered on the brake disc. This causes uneven wear and is a known issue on RV’s.

Have others experienced this? If so, what have you done to resolve or minimize this?

Thanks.

Brett H
Columbus, IN

[lr172]

Originally Posted by Brett H

I have an original RV-12 (i.e., undrilled gear legs) that is having uneven MATCO brake pad wear.

Three of the brake pads have even and equal wear. But, the right outboard stationary brake pad has 0.020” of tapered wear and at its thinnest spot is 0.025” thinner than the other brake pads.

MATCO technical support tells me that this is due the aluminum brake line acting as a big spring that prevents the brake caliper from being free floating. Namely, the brake line is pushing the caliper to one side rather than allowing it to stay centered on the brake disc. This causes uneven wear and is a known issue on RV’s.

Have others experienced this? If so, what have you done to resolve or minimize this?

Thanks.

Brett H
Columbus, IN

Yes, this is an issue. It is a floating calipar arrangement and it moves on the pins with a good amount of clearance in the bushings. Because of this clearance, it can get cocked, which will create the tapered wear pattern and also prevent the calipar from floating, which causes the wear on just one pad. If the lines were poorly bent the last time the calipar was off, they will force the calipar into a position that encourages the cocking and sticking.

To address, remove the stationary pad and keep adjusting the aluminum lines untill the calipar rests with the main body situated such that the pad is touching the rotor, the pad is parallel with rotor AND the calipar can easilly jostle around on the pins with NO resistance.

Larry

[DHeal]

What Larry said above plus: Be sure to keep the two holes / pins that each caliper "floats" on clean and free sliding. MATCO advises not to lubricate these pins as it attracts unwanted dirt and grime. However, I usually sparingly apply a touch of dry-ish lubricant to these pins and have had no issues.

[Canadian_JOY]

This is another reason why I suggest using flex lines at the brake calipers. Matco's aren't the only brakes which exhibit this tendency. Hard lines may be just perfect and present even brake wear until somebody snags a line and bends it slightly, presenting the caliper with an uneven load. When this happens one suddenly starts to see uneven brake wear.

[seagull]

I had the same problem, sometimes I could hear a squeak in the brakes when pushing the plane with the engine off. I replaced the lines on the gear legs with ones from https://aircraftspecialty.com/rv-12.html, problem solved.

[lr172]

Originally Posted by Canadian_JOY

This is another reason why I suggest using flex lines at the brake calipers. Matco's aren't the only brakes which exhibit this tendency. Hard lines may be just perfect and present even brake wear until somebody snags a line and bends it slightly, presenting the caliper with an uneven load. When this happens one suddenly starts to see uneven brake wear.

These are not hard lines. The 3003 is VERY soft and quite easy to bend into a position that works well. It just requires some knowledge and care from the person doing the work.

[Canadian_JOY]

Originally Posted by lr172

These are not hard lines. The 3003 is VERY soft and quite easy to bend into a position that works well. It just requires some knowledge and care from the person doing the work.

There's quite a difference between 3003 and a braided line, something like at least an order of magnitude difference in flexibility. For this reason the solid wall 3003 tubing is referred to as a hard line as opposed to being a flex line.

Once bent, 3003 wants to stay where it's put. True flex line does not exhibit this property.

This lack of flexibility in 3003 lines is the primary contributor to fatigue failures of these lines at junctions with movable parts like brake calipers where the most common failure is a crack in the shoulder of the flare. This shoulder of the flare is where "the rubber meets the road" in terms of the mechanical interface between a movable and non-movable part. Many attribute these failures to poor flare fitting technique but the reality is fatigue happens when one part moves and the other remains stationary, or reasonably stationary. In the case of brake calipers, the caliper is being moved by a comparatively large force (wheel rotation) thus the 3003 line is, in relative terms, the immovable part. Differential movement between the two parts eventually causes the shoulder of the flare to crack.

[Brett H]

Thanks for the replies.

I have made the suggested tweaking of the aluminum brake line to better center the caliper.

It appears to me that servicing the inboard floating brake pad has the potential to undue the tweaking that was done to center the brake caliper.

So, switching to the braided brake line seems interesting. I was surprised to see on the Aircraft Speciality Flightlines website that braided brake line was zip tied to the landing gear leg in three places. I have attached the picture below. The lowest zip tie was located where landing gear bends downward to attach the axle. This short distance between the lowest zip tie and where the braided line attaches the caliper would seem to make the braided hose stiff. Can this last zip tie be located higher to minimize the load applied to the caliper?

Thanks.

Brett H
Columbus, IN

[Brett H]

Originally Posted by seagull

I had the same problem, sometimes I could hear a squeak in the brakes when pushing the plane with the engine off. I replaced the lines on the gear legs with ones from https://aircraftspecialty.com/rv-12.html, problem solved.

Walt:

Thanks for your response.

When using the AircraftSpecialty braided braided brake lines, how far down the landing gear leg is your last tie strap located and is the original hose fitting in the brake caliper used or a different fitting?

Thanks.

Brett H

[seagull]

I didn’t change any fittings, just the hose. I will post a pic of the tie strap placement later today when I get out to the airport.