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Hi Daniel,
A few things to check:
1. Nose wheel tire inflation. My Alon will shimmy if the tire inflation is way off spec, even though the scissors, etc, are in good shape. The nose gear is supposed to have 20 PSI, while the mains get 17 PSI if I remember correctly.
2. Tire balance - I've never had to balance a tire to get rid of shimmy, but they say that this can cause it too.
3. Worn out nose wheel bearings - you'd probably know about it if these were worn bad enough to cause shimmy.
4. Scissors worn - if your bushings are good, check the condition of the bolts themselves. A few thousandths wear on the bolts can result in a lot of slop. Even though the bushings are softer material than the bolts, the bolts do wear out. You can get some additional slop out by installing close tolerance bolts (they are not expensive in these sizes), which are a few thousandths larger in diameter than standard bolts. Check with your A&P to see if it's OK to do that. Also you can get wear on the surfaces of the scissors where the lugs on the scissors straddle the mating lug on the nose gear leg. This wear is not corrected by replacing bolts or bushings. If you have wear here, don't just tighten down on the nut to take out the clearance. This just causes the ears to bend inward, putting excessive stress on the part. The nuts should be slightly tighter than finger tight and no more. Use washers made from shim stock as required to take out the clearance. This should not be a problem unless your scissors has seen lots and lots of use.
5. Nose wheel steering rod connection to the steering arm on the nose gear strut - You should have a regular rod end on the end of the pushrod and a hefty bolt screwed into the steering arm on the strut. If you have the little stud with the ball and socket arrangement, you should replace that with the rod end and bolt. The stud and ball have been known to break off causing bad things to happen. If you already have the rod end and bolt, check it for wear. (Parts available from John at Skyport.)
6. Check the vertical clearance between the steering collar and the nose gear strut. You can check this by grabbing the steering collar casting and shoving it up and down vertically. There should be almost no clearance there (0.002 in if I remember correctly). If you have slop here, side to side movement of the tire/wheel assembly results in up and down movement of the steering collar, rather than a turning movement of the steering collar. There are peel off shims that you can get to take this play out, but you have to remove the nose strut to install the shims. This isn't really a hard job to do. (Shims available from Skyport.)
7. Rod end on aft end of steering pushrod worn. This rod end is back behind the rubber boot on the firewall and under the rudder pedals (if so equipped). Let me know if you figure out how to change this rod end!
Hope some of these ideas are of help. Let us know what you find.
Best Regards,
Wayne DelRossi
Alon N5618F
Hours logged since restoration: 182.0
Wayne DelRossi
Alon N5618F
Hours logged since restoration: 182.0
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