Matco Wheel Taper Shims
Hey guys, I thought you might benefit from this, since IIRC the RV-12 uses Matco wheels & brakes as well. I have a 2007 Flight Design CTSW that has had *terrible* tire wear issues. Here is a copy of an e-mail I posted to the CT forums about it:
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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 nuts, 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.
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Hopefully some of you RV-ers will find something useful in all that.