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Nose Gear Shimmy

rcwagner55

I'm New Here
I am new to RV's and just bought an RV7A. I consistently have a pretty severe nose wheel shimmy upon landing. I am sure it is the nose wheel by the fact that it does start until I set the nose wheel down. I have tightened the pivot nut to where the force at the axle to swivel the nose wheel is about 28 lbs. I have increase tire pressure to 50 psi. It seems the tire pressure increase helped a little, but still have a shimmy. Can anyone give me some advise on how to cure this issue?
 
shimmyshimmyshimmy

Hey Bob, where do you fly out of in beautiful Twisp? ;-)

I have sold my -9a, but had the occasional shimmy.
I think you are going to find a lot of info and opinions, so what works for you may not be someone else's magic bullet.
the higher pressure and tighter breakout are typical gremlins to go after.
what STARTS the shimmy can be hard to quantify, so if I had it start, touching the brakes seemed to tuck it into line, and it stopped.
whether the change of load, angle, g-force, it seemed to help, and then you'd passed thru that speed range, or cleared the rough asphalt, or whatever triggered it!

proponents of the AntiSplat bearing mod often say they've seen an improvement with the lack of friction; I can't verify that, altho I did add the axle spacer to make it easier to set the bearing crush, the drag hopefully more consistent then. good luck!

http://www.matcomfg.com/AXLEASSEMBLYA24125INCH-idv-3657-1.html
 
I'd make sure the nose wheel is balanced as well as possible. My -9A had no shimmy at all until about the 150-hour mark, but tire wear resulted in a minor imbalance. I bought a $30 motorcycle wheel balancer off eBay, and after adding about 2 oz. of weight, the shimmy was greatly reduced.

On the balancer itself, I sprayed its bearings with copious amounts of WD-40, then re-oiled them with a super-light machine oil to reduce the friction as much as possible to get the best result.

It sounds like you're good on the break-out force, which of course is critical. Make absolutely sure there's no slop in the wheel bearings. I'm using Matco's axle (like Perry's using in the above post) that preloads the bearings before they're put into the nose fork. Others swear by the sealed bearing mod from Anti-Splat Aero.

50 psi sounds a little high...I'm running 35-40 psi.
 
I've got a 9A and had no shimmy until I bought one of Desser's "real aircraft" nose wheel tires. It came with no dot on the tire. I thought great it's perfectly balanced, well that was the wrong assumption. Had a wicked shimmy at just the perfect taxi speed. I first tried "balance beads" based on success reported by other VAFers. It changed the speed at which the shimmy occurred but didn't eliminate it. Finally shook out the beads and somewhat crudely balanced the wheel. Shimmy is gone but it took about 12 oz of wheel weights. In retrospect the balance beads might have worked if I had put more in. I also have a Matco axle and have 27lbs breakout force and run 35psi. So a lot of words to say try balancing the wheel. If that doesn't help you can't go wrong with the anti splat option and it seems to be a sure fix.
 
I've got a 9A and had no shimmy until I bought one of Desser's "real aircraft" nose wheel tires. It came with no dot on the tire. I thought great it's perfectly balanced, well that was the wrong assumption. Had a wicked shimmy at just the perfect taxi speed. I first tried "balance beads" based on success reported by other VAFers. It changed the speed at which the shimmy occurred but didn't eliminate it. Finally shook out the beads and somewhat crudely balanced the wheel. Shimmy is gone but it took about 12 oz of wheel weights. In retrospect the balance beads might have worked if I had put more in. I also have a Matco axle and have 27lbs breakout force and run 35psi. So a lot of words to say try balancing the wheel. If that doesn't help you can't go wrong with the anti splat option and it seems to be a sure fix.

....Don't forget to check wheel for being out of round! If it is, you will never cure the shake with that tire. You can send your tire and wheel in to us for service, and we will get rid of the issues. We have done hundreds of these with a 100% success rate. Please have a look at our website and see all nose wheel related services and products. Thanks, Allan...:D
 
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Shimmy control

I had terrible shimmy problems until I followed Vans former employee Ken's advice and reduced the tire pressure to 25 lbs. The tire acts as a shimmy dampener at low pressure .Check the pressure frequently.
 
Check for chevron wear

I had a 56 C-172 that would have a vicious shimmy once in a while. Fixed replaced tightened all the usual suspects and still got it. Finally found a tech advisory on the CPA website (I think) that said to check the tire for faint chevron cuts in the tread face which came from the original shimmy. If you have them they will excite a shimmy even if everything else is fixed and the tire is balanced and n round. The fix is to sand the tread face until the cuts are gone. Sounds crazy but it worked. Tie the tail down and use a disc sander letting the tire spin as you work back and forth across the tread face. Black rubber powder will go everywhere. I'll try to find the PDF of the article
 
I had a 56 C-172 that would have a vicious shimmy once in a while. Fixed replaced tightened all the usual suspects and still got it. Finally found a tech advisory on the CPA website (I think) that said to check the tire for faint chevron cuts in the tread face which came from the original shimmy. If you have them they will excite a shimmy even if everything else is fixed and the tire is balanced and n round. The fix is to sand the tread face until the cuts are gone. Sounds crazy but it worked. Tie the tail down and use a disc sander letting the tire spin as you work back and forth across the tread face. Black rubber powder will go everywhere. I'll try to find the PDF of the article

If the tire IS out of round, this can fix it -- but get somebody who's done it to show you how. [And that likely won't be anybody younger than 50 or so...] First time, it can be perilously easy to make it worser and worser and
OUCHthathurts!
 
If the tire IS out of round, this can fix it -- but get somebody who's done it to show you how. [And that likely won't be anybody younger than 50 or so...] First time, it can be perilously easy to make it worser and worser and
OUCHthathurts!

excellent point. I was a virgin when I did it but my old friend, A&P watched me on Facetime.

I found the tech note. We don't have nose wheel steering or shimmy dampeners on RVs (at least my 6 doesn't) so part of it is N/A but the rest is good data


.... Since the oscillations are equal in divergence angle and time duration, the rubber on your tire is being scuffed in a uniform and distinct pattern that repeats itself each revolution of the tire. This wear pattern shape is directly related to the tire shape created by the amount of weight on the nose tire, the tire pressure, and the speed of the aircraft. The frequency of the shimmy is a derivative of these factors.

You might have noticed a braking feel to the airplane when severe shimmying is happening. The braking is from the nose tire skidding sideways during the more extreme angle divergent portion of the shimmy cycle.

Since shimmy generally takes place for a short time, the early stages of this wear pattern are microscopic and hard to detect visually or by feeling the tire tread by hand. After the first shimmy, the then-created wear pattern tends to start the oscillating action when the airplane speed and nose gear weight matches the speed and weight that the airplane was traveling when the shimmy wear pattern was created.

You might have noticed that shimmy starts at about the same landing or taxiing speed each time. The results are that the shimmy gets worse every time. It happens even if the mechanical issues that let it start shimmying the first time have been corrected and the shimmy dampener is working and trying to do its job. The shimmy dampener simply is not strong enough to prevent shimmy when a nose tire has an established shimmy wear pattern in the tread.

The key to shimmy problems is to prevent shimmy from starting in the first place. You have to start with the routine stuff that is in the service manuals. The Cessna Pilots Association has a very good article on fixing the mechanical issues associated with nose shimmy on Cessna airplanes.

Tech Note No. 001, Revision 004 dated April 15, 2010, does a good job of describing and illustrating the system and directing corrective repairs. This tech note seems to parallel the Cessna Service Information Letter SE84-21 on the same subject.

The first step in preventing the problem is to look for any un-dampened nose gear movement. This is motion of the nose tire without the shimmy dampener moving. Looseness in the nose gear system cannot be detected with the nose wheel off the ground unless the pressure is released from the nose strut. When you move the nose wheel right and then left, the shimmy dampener should also move.

If there is any un-dampened motion, tighten or replace the worn components, such as the torque link bushings and spacers, the steering collar, and shimmy damper attachments. Remove the shimmy dampener attachments. Check the shimmy dampener for proper fluid and proper operation. Check the dampener for seal condition and excessive wear in the piston and damper bore. The dampener shaft must have considerable resistance to motion when moved quickly but move easily when moved slowly.

Nose gear rigging is important to prevent shimmy. If the steering rods or bungees are biased, damaged, or holding improper tension, shimmy can be started. The aircraft service manuals do a good job of describing proper nose gear rigging procedures.

One of the hardest parts of proper rigging is determining where the nose tire is straight ahead. The method we use is to create an airplane center line by dropping a plumb bob from the center of the firewall to the ground (center can be determined from the rivet pattern or measuring from the motor mount attachments) and again dropping the plumb bob from the center of the tail tie down hook. Mark both of the plumb bob points on the shop floor and create a chalk line mark between the points. This is your airplane center line.

Extend the center line forward as close to the nose tire as possible. Place a straight 2?4 stud or a piece of straight angle iron against the side of the nose tire. Adjust the nose wheel and tire until the 2?4 is parallel with the airplane center line. Check your results by placing the 2?4 on the other side of the tire. The 2?4 acts as a tire angle multiplier, giving you measurable results. Parallelism can be checked by simply measuring the distance between the 2?4 and the chalk line in two places. Be sure not to move the airplane while you make your nose gear alignment adjustments.
Wheel bearings must be in good condition and properly adjusted. Bad bearings or adjustments can allow un-dampened tire movement. Tire balance is also critical for preventing shimmy as an out of balance tire puts cyclic centrifugal loads on the tire tread. Out of round tires will do the same thing.

One of the objectives of preventing shimmy is to not have any type of cyclic loads going into the tire system. McFarlane makes an inexpensive suspension tire balancer (P/N TOOL108) that works well for this.
Check the tire itself for casing shift or other damage as follows: Take the weight off the nose tire for a period of time to let the tire take its proper shape. Assure that the tire is inflated to the proper pressure for the aircraft. Spin the tire by hand and look for any significant lateral divergence (tire wobble) or vertical divergence (out of round). The tire must rotate true, but a little out of round is normal.

If tire casing shape problems are detected, let the tire stabilize longer without weight. If that does not correct the problem, the only fix is to replace the tire. "If the tire casing seems to run out true and the tire is determined to be airworthy in all aspects, remove the shimmy wear pattern in the tire tread.

How do you remove rubber on a good tire to get rid of this mysterious and evil tread wear pattern that nobody can see or feel? We use an electric disc grinder that is used in the weld shop for grinding welds and smoothing structural steel. Any large sanding disk power tool with a course grit disc or a belt sander would also work.

There will be some rubber flying around the shop, so this is a good job to do outside. Get someone else to do it if you have allergic reactions to latex or rubber products.

Block the nose gear off the ground and give the tire time to stabilize its shape without weight. Again assure that the tire has the correct inflation pressure. Touch the grinder to the tire at an angle that rotates the tire and removes rubber. With a little practice you will be able to control the tire rotational speed with small grinder angle adjustments.

If you allow the tire to rotate too fast, very little rubber will be removed. If you allow the tire to rotate too slow, it is hard to remove the rubber evenly. Taxi speed tire rotation seems to work best. You can actually remove small ?out of round? tire conditions by being steady with the grinder and allowing the grinder to work harder on the tire high spots. The grinder must be worked across the tire tread as evenly as you can. Never grind into the sidewall of the tire.

You can feel advanced shimmy wear patterns before you start and they will take more work to remove than the patterns you cannot feel. The tire must feel smooth and even when you are done.

Only experience will tell you how much rubber to remove. Be sure that the tire has good tread depth when you are finished, and verify that there is not any inadvertent damage to the tire.

Clean up the rubber grindings and high speed taxi test the airplane. You will probably be smiling with the results. It is a good idea to re-balance the tire after grinding the tread and before returning the aircraft to service. If the test does fail, repeat the process.

Yes, with a little patience, this shimmy beast can be tamed"



the tire is really tough and a disc sander with a 60 grit disc removed surprising little rubber at a time so this wasn't as intimidating as I thought.

good luck

Chevy
 
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