Custom Matco Axle Shims

[FlyingMonkey]

As some of you know, I have been struggling with excessive tire wear since I bought my CTSW three years ago.  The wear has always been greatest on the outside of the tire tread on both wheels, the left slightly worse than the right.  The problem is bad enough that I go through 2-3 sets of main tires annually.  In contrast my nose wheel tire is the original factory tire and looks fine with 430 hours on it.

 

I upgraded to Matco wheels and brakes to try to solve this problem.  However, experimentation and measurement (at least to the best of my ability) show that I have both excessive negative camber (tops of wheels farther out than the bottom of the wheels) AND excessive toe-in, which combined explain the outside wear pattern.  Whoever set my gear up originally must have done it on a Monday morning following a "fun" weekend...       

 

The Matco standard shimming procedure using washers won't work for my airplane, because I can't shim both directions to the degree I want to within the Matco limit of two washers per bolt station.  I tried adjusting only camber to the limits, and that helped some.  The camber in that config was just about neutral but the tire wear is still way too high.  I needed way to make large adjustments in *both* camber and toe-in.  

 

I sent an e-mail to Matco to ask about this, and they immediately replied with "you need taper shim plates".  A little back and forth with Matco showed that if you use a plate that fills the gap between the axle halves, there is no limit on how much you can shim, because there is no "air gap" as with washers, and the axle halves are fully supported.  They said you can put a taper shim of the correct angle in there for each axis, stacked together, and that should do it.

 

I talked to my A&P, who does a lot of Diamond maintenance for the local flight school, and he had shims for the Diamond wheels.  They were a perfect fit for the Matco wheels, with the exception that they use AN4 bolts instead of AN3 bolts like the CT so the holes were bigger.  The other problem is they Diamond shims come in 1° and 2° adjustments.  My measurements showed that I need 4° of camber adjustment and 3° of toe out.  That would be a LOT of stacked shim plates...and they cost $65 a pair.  Hmmm...

 

Long story short (too late, I know), a friend of a friend is a pilot and an engineer with access to commercial CNC milling and grinding equipment.  He offered to make me some shims if I gave him specs.  I asked him if he could makes shims tapered in two axes to minimize the thickness needed, and he said sure and sent me some CAD drawings.  It looked great, so he went ahead and made them, tapered 4° in one axis and 3° in the other:

 

Amazing work, he even engraved the taper on each axis!  Yesterday I installed them, they fit great.  The only issue is that the holes in the axles are at their very limits of play, and should probably be enlarged, but I only wanted to do that if absolutely needed.  So I assembled the shims to the wheels, tightened the bolts, then took it back apart and checked the bolts.  They were not bent or deformed by the side loads on them, so I think they are okay.  My plan is to taxi and flight test them, then change to fresh tires.  When I do that I will pull and check the hardware again, and if any stress is evident I'll ream the holes a little to relieve the shear force on them.  Here they are all assembled:

 

 

 

 

Fingers crossed that this solves my issue.  I fully expect that there will be some tweaking required, and possibly either new shims made or a slight grind to change the angle on the existing shim plates.  Just eyeballing everything, the right wheel looks great in both camber and to-in.  The left looks like it still has a *very* slight negative camber, and it might have transitioned to a very slight toe-out.  But I won't know for sure until I taxi it and observe the wear pattern and how the airplane tracks.  I'll use some chalk on the tread during taxi testing to see how and where it wears.

  

[Roger Lee]

Nice work

[Anticept]

I have a left wheel shim plate myself. Flight design usa installed it.

[FlyingMonkey]

I cross posted this to the RV-12 forum too.  IIRC they use Matcos too and they have outside tire wear issues, though not as bad as mine.

[John Vance]

You might want to consider spot-facing the inboard piece of the axle assembly so that the surface the nut sees is not angled.  That will raise stresses in the threaded section of the bolt and add to fatigue, especially on the bottom two.  I have no idea what the margins of safety are so maybe it's just fine. 

 

John

[Runtoeat]

Nice Andy.  The two axis machining is neat and really reduces the thickness of the shimming.  If you need to tweak, you can go with the washers for the 2nd go-round since these should be within Matco's 2 washer limit.  Or, if your friend is agreeable, have a new set machined.  John's suggestion to spot face the axle face is a good one, especially if you add more angle.  With the amount of shimming here, there is a bending load on the bolts.   Along with the spot facing, you may need to consider opening up the thru holes so there is a straight path for the bolt and there isn't a spot bending load put into the bolts. 

[Roger Lee]

Andy,

 

Those are compound angle shims. How did you work up a compound angle shim solution?

[Anticept]

Probably just approximated with washers. That's how I would do it!

[FlyingMonkey]

Andy,

 

Those are compound angle shims. How did you work up a compound angle shim solution?

I did a lot of measuring and some geometry calcs to get the approximate angles. I had the camber where I wanted it with the washers, so I measured that distance on the axles and did the math to get an angle. For toe-in I drew a line between the wheels, measured a 90 degree line by each wheel and measured from the front and back of each rim to the line. That made determining an angle possible.

 

Granted, the wheel angles are referenced to each other and not the aircraft centerline. But I know of no way to get a true centerline, and this should be pretty close. It's a first swag and will probably require tweaking one or both shims.

[Craig]

Nice looking shims!  I predict such an improvement that your wheel aligning days will be over.

[FlyingMonkey]

Nice looking shims!  I predict such an improvement that your wheel aligning days will be over.

Your lips to God's ears!

[Anticept]

Morden: if you draw a line along the centerline (Y axis), be it with laser or just yarn; then you can then make another perpendicular line (X axis) in front of the wheels. From there, you have a point of reference, just aim any lasers or straightedges off the X axis line, perpendicular. That's for a bit more precision.

 

Usually though, just throwing a bar down in front of the wheels and using a protractor off that is more than enough.

[FlyingMonkey]

Morden: if you draw a line along the centerline (Y axis), be it with laser or just yarn; then you can then make another perpendicular line (X axis) in front of the wheels. From there, you have a point of reference, just aim any lasers or straightedges off the X axis line, perpendicular. That's for a bit more precision.

 

Usually though, just throwing a bar down in front of the wheels and using a protractor off that is more than enough.

 

Similar to what I did.  I laid a piece of aluminum angle across the back of the wheels, measured to the center of that angle between the wheels, and drew a line at 90° out from that center point to some arbitrary distance.  Then measured from that point to the center of the front center of each tire.  If the numbers were the same or very close (they were), then you know that the reference line (the metal angle across the back of the wheels in this case) is perpendicular, and the wheels are not canted (one farther forward than the other) to the fuselage.

 

Yay geometry.  

[FlyingMonkey]

If this doesn't work, I will probably end up building a frighteningly complex wheel jig.

[Anticept]

Oh also, you're supposed to put the airplane on grease plates when you do alignment. It lets the plane rest in the most natural position.

[Runtoeat]

Someone mentioned laser?  Here's my alignment setup:

 

[FlyingMonkey]

Dick, I know you have posted before about this, but could you post (or link to an already posted) detailed explanation of this?  I can't envision it from the three pics here.  

 

My hope is that I can get this problem solved without resorting to very complicated jigs, laser alignment tools, etc.  But if I can't get very close with these custom shims (either this set or another set once we do some testing), then going full bore complex/precision will be the next step.

 

Also Dick, would you rent out your setup if I needed it?  I would pay shipping both ways plus rental charge.  

[FlyingMonkey]

Here's a pic of my alignment measuring setup.  The box tube is against the rear of each wheel and provides the reference line.  The drywall square makes sure everything is squared up, then the adjustable ruler square is clamped to it and run level across the center of the wheel.  The difference in the distance of the ruler square from one edge of the rim to the other edge combined with the distance (IIRC it's ~7 inches across the rim) makes a right triangle you can solve for the angle.  That angle will be the same as the angle of the wheel off the reference line, and will give you the angle of toe-in.

 

[Runtoeat]

Andy, your system might be more accurate than mine! If you need anything from me, you're welcome to use it. I basically established a center line on the hanger door and floor and used my axle laser jigs to figure out how far and at what angle I was at to the center line

[FlyingMonkey]

Andy, your system might be more accurate than mine! If you need anything from me, you're welcome to use it. I basically established a center line on the hanger door and floor and used my axle laser jigs to figure out how far and at what angle I was at to the center line

 

Once you determined how much your alignment was off, how did you determine how much and where to shim?

[ct9000]

I would'nt spend too much time worring about the camber because it changes so much under load anyway, the toe in is your problem with tire wear and it looks to me that tou have done a brilliant job fixing that.

[FlyingMonkey]

I see what you are saying, but my camber seemed extreme, pretty much all the weight on the outside of the tires. Wear definitely improved when I took the camber out. Though I agree toe is probably the bigger component.

[Anticept]

I see what you are saying, but my camber seemed extreme, pretty much all the weight on the outside of the tires. Wear definitely improved when I took the camber out. Though I agree toe is probably the bigger component.

Just do a few hard landings and make adjustments from there!

[FlyingMonkey]

Just do a few hard landings and make adjustments from there!

 

Way ahead of you there!  

[Runtoeat]

Once you determined how much your alignment was off, how did you determine how much and where to shim?

For the toe-in, I used the laser's distance from airplane center line projected on the hangar door.  Then used trial and error with washers and some trig. to come up with a small toe-in.  Used the laser's bubble to set up camber.  As others have observed, your setup should result in good numbers when your done.  Much more exact than mine.