Nose Wheel Shimmy

I have heard that one problem that can be identified during a high speed taxi is nose wheel shimmy. I plan a high speed taxi run just before first flight and I would like to know if others have had the problem and what they did to fix it? I have 35 lbs of air in the tires and have adjusted the nose wheel nut per Van?s procedure. I do not want to delay first flight so I want to be prepared to fix if shimmy is identified.

The plane is waiting DAR inspection this weekend, sitting all nice and pretty in the hangar with all the inspection plates removed. She is ready to go. Only 1.65 hours on the engine to verify systems and taxi. All I got left is to install the plates and wait until my test pilot is available, 12, 19 Aug? During first flight I plan to record all the data from the Dynon EMS and EFIS on my lap top, and I will have a cockpit camera on board looking at instrument panel and outside. Boy I would have loved to link to the ground. Live Web cast?

Larry
N72NJ
Fix Pitch Sensenich, Carburator, O-320, no paint, no pants, Light Speed and Slick ignitions,1047 lbs

Data point:
I run 25# air pressure in the nose wheel and
added a stiffener while my 7A was in construction.
No shimmy yet, after 7 hours in phase one.
Tom

Just make sure the breakout force is right

Make sure the breakout force is right and keep the nose gear off of the ground except for taxiing. I pull back on the stick at the time I push the throttle in and ease it forward as I feel the trim setting is holding the nose off then as soon as I'm airborne I crank out the UP trim as required to remove the stick pressure and sustain the climb. On landing crank in the UP trim but I keep it flying down to the runway and flare carefully at low altitude so as not to balloon but keep the nose gear from touching until the plane has slowed to taxi speed.

The breakout force will decay and the nut will have to be retightened after a very few flights and if you don't catch it before it goes too low (probable) the front end will shake violently and once started it won't stop until the plane is virtually stopped. This will happen again but the interval will get longer after a hundred hours or so it stays pretty much at the same level.

Tire pressure too low with close fitting wheel farings can be a problem when they come in contact and some damage can occur. Tire pressure too high removes the cushining effect and vibration will be intensified - even in the main gear. I fill mine at 35lbs and reinflate when they show slight deflation - usually by the time I catch it enough to go through the fairing removal reinflation process they are down to 22+ lbs.

Bob Axsom

Nose wheel shimmy

You should read Vans recommendations for nose wheel pressure. Van Knows

maybe he does but higher pressures have worked for me. fred

35 psi for me all round, never had any shimmy. no dampeners.

On my 7A I use 30# in the mains and 35# on the nose. I also keep my "break-out nut" on the tight side, insufficient break out will defininately cause shimmy.

I use 30 mains, 30-35 nose, for pressure. I have the main gear wood stiffeners, but none on the nose leg. Break out force set per Van's. I did add AN3 countersunk screws to stake the nosewheel bearing mushrooms. This prevents the mushrooms from turning and allows for less preload on the bearings, which allows the wheel to spin freely. No shimmy in any wheel.

Roberta

Walt said:

On my 7A I use 30# in the mains and 35# on the nose. I also keep my "break-out nut" on the tight side, insufficient break out will defininately cause shimmy.

Click to expand...

Same specs here. No shimmy. Initially check breakout force every few hours. After about 30 hours or so, I could cut this interval down to about every 25-30 flight hours.

Front wheel AN6-60A tightness

According to the book on the RV9A, the front wheel AN6-60A shaft is tightened to 7-10 ft lbs and relies on the fiber in the AN365-624 nut to retain torque.

Quite frankly, this scares the beejeebees out of this old man. Seems to me a cotter pin arrangement is in order like the main gear.

The book continues that after a few landings the rubber seals will break in and the tire will rotate more freely. But what that also implies is that the torque is less which means we're relying on the fiber in the AN365-624 nut to keep this deal in place.

Please share your experience. If you've seen no problem, please advise.

Gary Bricker

Several 6's and 7's in my neighbor hood are having problems with shimmy and it seems to be wheel bearings. This is a very poor boat trailer design and bearings and seal like the mains is the way to go. . Cleveland wheel is the answer.

One thought on staking the mushrooms.... The real drag from torqueing those special front wheel bearings is mostly the rubber seal.... if you were to remove the rubber seal (cut it off where it contacts the wheel) then the 7-10 ft-lbs applied to the AN-60A bolt would not produce a noticeable drag. What I fear is that if you stake the mushrooms and loosen the bolt until the wheel spins freely, your inner race must be spinning on the mushroom. Particularly just when the weight is removed and the tire is still spinning right after liftoff. I would check the mushrooms to see that this isn't happening.

If you imagine the forces involved when you have a 300+ lb vertical load on the nose gear/wheel combination, and up to an 1800 lb forward moving mass (the airplane) during takeoff/landing at 40-60 mph, then the tiny drag of the bearing seal would be insignificant.

I actually think the resistance to spinning produced by the 7-10 ft-lbs probably helps to eliminate shimmy as it provides a tiny additional drag to the front wheel under any rolling conditions.

Sometimes I see a foward rearward hoping motion of the nose wheel during roll out (Review of video from landing). This is not a left/right shimmy or a pittching hop, just motion foward and aft. I though this may be caused by the high rotating resistance of the nose wheel. I have only 10 hours on th e plane. Has anyone seen the same? What could cause this type of nose wheel action?

Balance

Did you balance the wheel?

That nose gear flexing in the fore/aft direction is the nsoe gear leg's reaction to surface irregularities. It has to flex in that direction as this is the only "suspension" the aircraft has with this style of gear. However, the friction contributed by the rubber seal in the wheel is insignificant. In all cases, the operation of any tricycle gear aircraft should be to keep the weight off of the nose gear under all circumstances. This primarily can be done by keeping the stick full aft during ALL ground operations. And particularly during high speed rolling before takeoff and after landing.

Chickenlips said:

This primarily can be done by keeping the stick full aft during ALL ground operations.

Click to expand...

Except during a very stiff tailwind. Lifting elevators then will put more weight on the nose.

Movie of nose gear shimmy

Larry said:

I have heard that one problem that can be identified during a high speed taxi is nose wheel shimmy. I plan a high speed taxi run just before first flight and I would like to know if others have had the problem and what they did to fix it? I have 35 lbs of air in the tires and have adjusted the nose wheel nut per Van?s procedure. I do not want to delay first flight so I want to be prepared to fix if shimmy is identified.

Click to expand...

Hi Larry,
My friend experienced nose wheel shimmy in his rv-6a. Here is a link to a movie where he mounted a camera in the cowl to film his nose gear while taxing. Quite eye opening. He ended up putting the wood dampener on his nose gear after trying everything else.

http://www.eaa538.org/azwing/pictures/nosegear.wmv

Thanks for posting

That video is one of the most informative posts I've yet seen on any nose wheel issue. Thanks so much for posting it. I hope Vans engineering gets to take a look at it.

What I find interesting is that this shimmy has the wheel moving fore and aft, not side to side (the way a Cessna shimmies).

I looked ahead on my plans and the nosegear leg doesn't call for a damper, but (spoken as an engineer) I think I'll be considering one. Or maybe another solution (like a tailwheel endorsement, just kidding)

I have witnessed shimmy on a number of "A" models, and every one of them was fore and aft shimmy like in the video earlier in this thread. I will say that most were not as severe as shown in the video.

There was a recent thread where people were modifying the nosewheel assembly by staking something <??> to allow reduced pressure on the bearings and hopefully letting the nosewheel turn more freely.

Can anyone provide an update on the effectiveness of this technique?

Where was the stick

Was the stick held FULL AFT in the video while taxiing? Or was the full weight of the plane on the nose wheel? I would be surprised to see this behavior if the stick was held full aft.

yup

hecilopter said:

Was the stick held FULL AFT in the video while taxiing? Or was the full weight of the plane on the nose wheel? I would be surprised to see this behavior if the stick was held full aft.

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Hi Rusty,
Yes that video was taken with aft stick.

I built and flew a 6A for about 300 hours. I experienced the side to side shimmy a few times. Re set the break out force and that went away. Eventually built a 7A that i now have about 150 hours on it. Never had the side to side shimmy, but have had the fore and aft shimmy. I flew both planes off of the same runways, so i don't believe the surface has a lot to do with shimmy. Upon taking the wheel and fork assembly apart, i noticed that the whole bearing assembly had been turning on the axle bolt. I believe this was due to the rolling resistance that the seal washer was imparting to the system. I cleaned everything up very carefully and used a torque wrench to carefully set the torque per Van's recomendations. I still, once in awhile have some shimmy, but not nearly as much as at first. I think that it just takes a long time for the washer to get wore in to where the wheel turns more easily. The 7A assembly will never turn as easily as the 6A did.

The 6A had a wood fairing that i glassed over. Looked real good and made the leg a lot stiffer. That's not always a good thing. The 7A is just the rod landing gear with the hollow fiberglass fairing.

Steve Ciha

Nose wheel shemmy

I think I posted this a year or so ago but I'm going to tell you my experience with the Nose wheel. The first plane I built was a 6A. At the first annual i tried to take the front wheel off. I didn't think I was going to get the axle bolt out. When i did manage to drive it out I found the aluminum spacers were turning on the axle bolt. The aluminum had built up on the axle bolt and that was what made it hard to drive out. i polished the bolt up and decided this was not good. Now this has been about 5 years ago. i called vans, I got the usual, never heard anything like that. Well they have heard about it now, many times. The first thing i did was drill through the fork into the aluminum spacer and taped the spacer 10/32. Then i took my center punch and hit the aluminum spacer 6 or 7 places where the bearing sets. That makes the bearing go on real tight. That is what I wanted. I had to drive the bearing on with a block of wood and a hammer. I didn't want anything turning but the wheel, I counter sunk the fork and installed the aluminum spacer with the 10/32 counter sunk screws. I'm not saying this is the thing to do, Its just what I did. Works for me. Have a good day.