Matco brake issue

[des4900]

I have a 2008 CTLS that was converted to MATCO main wheels and brakes a year ago.

I just recently started having a problem that first started with a strong shimmy in the right main landing gear.  After some hours spent with a flashlight and mirror, I determined that there was about 1/16" gap between the brake pad that was closest to the tire, and the brake disk.  An A & P examined the wheel, told me that both brake pads should ride on the disk, and then figured out that both left and right main wheels did not have the axle nuts properly tightened, since the wheels would spin rather freely when jacked off the garage floor.  He tightened them to specs, and that got rid of the shimmy.  However, there is still a much smaller but still noticeable vibration and a some apparent brake squeal at landing speeds, most of which goes away at taxi speeds.  The brakes otherwise work well, and don't pull to one side or the other at landing or taxi speeds.

I spent some more hours today using flashlight and mirror with that right main wheel jacked about 1/2" off the hangar floor, and eventually decided that the disk on that right wheel was rotating true with the wheel (there was no runout or vibration that I could see by spinning the wheel), but the brake cylinder itself was not square with the brake disk.  Looking closer at the brake disk, I observed that the surface of the disk that faced away from the tire was more heavily worn/polished on the outer part of its circumference, and the surface that faced the tire was more heavily worn/polished on the inner part of its circumference.  I did some measurements using a steel rule, and it appeared to me that the brake cylinder was about 1/32" out of parallel with the disk.

Could that be the source of the vibration?  Can it be fixed or adjusted, or do I need to replace some parts?  Should I be investigating the bolts that attach the wheel assembly to the landing gear?

Or could it be something else altogether that I am not thinking of?

[Tom Baker]

The brake caliper should run true to the disk and wheel. If someone shimmed in the wrong place that could cause it to be off, the other possibility is a bent axle. I have never seen that with the Matco wheels, but I have seen it with the Marc. Take the wheel pants off and look at it from the front to see if the wheel is canted in at the top.

Normally vibration either comes from a bent or warped disk or an bad or out of balance tire. I think you will need to take things further apart that just jacking the airplane and spinning the wheel. 

[Roger Lee]

The loose axle nut may have caused a wobble which worn the disc unevenly. I'm with Tom. Pull the wheel and check the bearings, the grease, the brake pads, the actual disc and make sure it isn't wrapped. Al you might need to do is install some new pads if the disc isn't warped or a bearing / race isn't messed up and grease the bearings too.

[des4900]

Thanks, Tom and Roger, for your expert advice.  I also got a callback late this afternoon from Matco, and they essentially said the same thing.  They told me to pull the wheel, and examine everything carefully.  I did that, and I didn't find a smoking gun.  The axle nut was tight enough to prevent the rubber washers from turning with the wheel.  The brake disk was dead flat, as measured with a steel rule every which way.  There were no lumps on the tire that might have been indicative of broken cords.  The top of the wheel was not canted towards the body of the airplane, indicative of a bent axle (which Matco said was unlikely with the Matco wheels).  The brake pads were new when the wheels and brakes were converted from the original Marc to Matco at the end of September, 2020.

However, I just re-read my squawk list to the service shop that installed the wheels and brakes,   I had written the following at the end of the work-order on September 30:

Quote

"I will also note that I have the wheel pants in my hangar.  I took them off to try to figure out why the wheel on the passenger side of the aircraft was grabbing and causing a "shimmy" when the brakes were applied, and I decided to leave them off until all the brake problems were resolved.  This effect was noticeable while taxiing, even at fairly low speeds, and particularly if you were braking hard.  This makes me wonder if the right (passenger side) axle was loose, or if the brake cylinder was a little out of alignment with the axle, or something like that... ???"

Now I am wondering if there is a different issue, possibly with the gear leg, because the wheels and brakes were brand new 6 months ago.