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Nose gear shimmy

 
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AirMike



Joined: 27 Feb 2007
Posts: 512
Location: Nevada

PostPosted: Sat Sep 23, 2017 7:30 am    Post subject: Nose gear shimmy Reply with quote

I just started getting some severe nose gear shimmy after 400 hours of flying. I figured it best to deconstruct my nose gear to bird-dog the problem from the ground up. I removed the wheel and raised the nose gear off of the ground using an eye bolt in the floor of my hanger. First off, I am getting a new front tire that I will balance before I install it.
But as I moved up the gear leg, I noticed that there is about 1/16" of play in my gear leg in the horizontal axis. This translates to 1/4"of play at the nose wheel.
There were quite a few posts about under sized bushings in the gear leg attach point a few years ago, but this is NOT the problem. My gear leg bushings are nice and tight to the ears of the gear leg. The problem is that the "ears" are too far from the "hole" on the engine mount. Could this be the problem that is causing my shimmy? Is there a remedy or should I just forget it, and why did this not cause a problem before. Has anyone else encountered this issue and how did you resolve it?


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PostPosted: Sat Sep 23, 2017 7:50 am    Post subject: Nose gear shimmy Reply with quote

Standby for other responses, but probably just need to tighten the nose strut by adding a shim to the elastomers. That's about the time when they start to compress.

Marcus

Quote:
On Sep 23, 2017, at 9:30 AM, AirMike <Mikeabel(at)Pacbell.net> wrote:



I just started getting some severe nose gear shimmy after 400 hours of flying. I figured it best to deconstruct my nose gear to bird-dog the problem from the ground up. I removed the wheel and raised the nose gear off of the ground using an eye bolt in the floor of my hanger. First off, I am getting a new front tire that I will balance before I install it.
But as I moved up the gear leg, I noticed that there is about 1/16" of play in my gear leg in the horizontal axis. This translates to 1/4"of play at the nose wheel.
There were quite a few posts about under sized bushings in the gear leg attach point a few years ago, but this is NOT the problem. My gear leg bushings are nice and tight to the ears of the gear leg. The problem is that the "ears" are too far from the "hole" on the engine mount. Could this be the problem that is causing my shimmy? Is there a remedy or should I just forget it, and why did this not cause a problem before. Has anyone else encountered this issue and how did you resolve it?

--------
See you OSH '18
Q/B - flying 8 yrs.




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philperry9



Joined: 23 Nov 2011
Posts: 348

PostPosted: Sat Sep 23, 2017 8:55 am    Post subject: Nose gear shimmy Reply with quote

Interesting. I'm getting ready to dig into mine a bit more because I suspect a similar issue. Going over a bump or lowering the nose to the runway usually results in a pretty good thump and my ears tell me there's some slack somewhere in the system that I need to find.

Regarding the ears, is it possible to ship the gap with washers so the ears fit tight?

Haven't looked at mine yet.

Phil

Sent from my iPhone

Quote:
On Sep 23, 2017, at 10:30 AM, AirMike <Mikeabel(at)Pacbell.net> wrote:



I just started getting some severe nose gear shimmy after 400 hours of flying. I figured it best to deconstruct my nose gear to bird-dog the problem from the ground up. I removed the wheel and raised the nose gear off of the ground using an eye bolt in the floor of my hanger. First off, I am getting a new front tire that I will balance before I install it.
But as I moved up the gear leg, I noticed that there is about 1/16" of play in my gear leg in the horizontal axis. This translates to 1/4"of play at the nose wheel.
There were quite a few posts about under sized bushings in the gear leg attach point a few years ago, but this is NOT the problem. My gear leg bushings are nice and tight to the ears of the gear leg. The problem is that the "ears" are too far from the "hole" on the engine mount. Could this be the problem that is causing my shimmy? Is there a remedy or should I just forget it, and why did this not cause a problem before. Has anyone else encountered this issue and how did you resolve it?

--------
See you OSH '18
Q/B - flying 8 yrs.




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johngoodman



Joined: 18 Sep 2006
Posts: 528
Location: GA

PostPosted: Sun Sep 24, 2017 4:46 am    Post subject: Re: Nose gear shimmy Reply with quote

I never had any nose shimmy with my RV-10 for 5 years. Then, my new partners wanted to change from a
Corvette engine to a Lycoming 540. We used a new engine mount, and a new engine.
Everything firewall forward was new. The shimmy started on the first flight. I am skeptical of assumptions that it's something to do
with the wheel or the mount. I'm leaning toward the 6 cylinder engine, versus a 4 or 8 (like my old corvette LS1).
Six cylinder engines are naturally unbalanced. I know the jury is still out on this one, but it seems to be just an RV-10 problem.
John
PS: sorry for the multiple paragraphs, but big photos make looooong lines of text...


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bill.peyton



Joined: 19 Sep 2010
Posts: 187
Location: St. Louis, MO

PostPosted: Sun Sep 24, 2017 6:23 am    Post subject: Re: Nose gear shimmy Reply with quote

First, you need to size your photos smaller prior to posting.

Several things can cause the nose wheel shimmy. Usually the friction nut needs to be retightened to take up the slack in the belleville washers. It's tough to over tighten this nut.

The second is the axle assembly. If you are using the matco axle, confirm the the wheel is centered in the fork assembly. My original bushings were machines incorrectly and my wheel was not centered. If you are not using the matco axle, you need to order it.

Third, the tire provided with the kit is junk and should be replaced with quality tire such as a Goodyear flight custom or Michelin. My kit provided tire was way out of round.

The VA144 bushing that you are seeing play in should be installed in such a way that it protrudes from the edge of the gear leg assembly. When you tighten the nut you are tightening against the VA144 bushing. As I recall, washers can be used to take up any play as require. It's been a while, but I recall having to machine a new bushing to get rid of the slop in the gear leg. I'm not in front of the drawings at the moment, so excuse me for lack of assembly part numbers.

I have over 700 hours on my 10, and I still get some shimmy, or more of an up down motion, after it sits for a week. It will usually disappear after a long taxi. It occurs around 14 kts.


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Kellym



Joined: 10 Jan 2006
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PostPosted: Sun Sep 24, 2017 7:08 am    Post subject: Nose gear shimmy Reply with quote

Don't know where you got the idea that a 6 cylinder is unbalanced. Six
cylinder engines in the 3 most common configurations are very smooth.
That would be flat six, V6, and straight 6. I've driven all 3 in auto
applications---Ford straight six from the 50s, Corvair flat 6, and a GM
V-6 in a Jeep. (BMW straight and V-6s are some of the smoothest engines
around). Big 4 cyl car engines are either very rough (Pontiac Tempest
194) or have balance shafts.
Lycoming perhaps doesn't use a cam design to be as smooth at low speed
as it could be. Continental 6 cylinders are very smooth, if somewhat
heavy. Six cylinder engines fire every 120 degrees for a very smooth
output. Four cylinders fire either every 90 or 180 degrees and for all
but the under 2 liter sizes need a balance shaft to smooth them out.
V-8s fire every 90 degrees and have enough cylinders to create balance
the 4s do not have.
OTOH, Lycoming 4 cylinder engines are about as unbalanced as it gets.
Big six cylinder engines like the 540 and the big Continentals run
smoother with a 3 blade prop than 2 blade..but I am not suggesting a 3
blade would make any difference in shimmy. 3 blades are a PITA for
removing the lower cowling, and most cost 3-5 kts in cruise, while
climbing better than 2 blade.
There likely are some explanations in the Lycoming Dynafocal engine
mounts and how that interacts with the nose gear, but it isn't the
engine smoothness.

On 9/24/2017 5:46 AM, johngoodman wrote:
Quote:


I never had any nose shimmy with my RV-10 for 5 years. Then, my new partners wanted to change from a
Corvette engine to a Lycoming 540. We used a new engine mount, and a new engine.
Everything firewall forward was new. The shimmy started on the first flight. I am skeptical of assumptions that it's something to do
with the wheel or the mount. I'm leaning toward the 6 cylinder engine, versus a 4 or 8 (like my old corvette LS1).
Six cylinder engines are naturally unbalanced. I know the jury is still out on this one, but it seems to be just an RV-10 problem.
John
PS: sorry for the multiple paragraphs, but big photos make looooong lines of text...

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johngoodman



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PostPosted: Mon Sep 25, 2017 5:01 am    Post subject: Re: Nose gear shimmy Reply with quote

I think it has to do with the piston reversing directions without a power stroke, but I'm no expert. But, the "wet dog shimmy" is very noticeable.
John


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Kelly McMullen



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PostPosted: Mon Sep 25, 2017 5:45 am    Post subject: Nose gear shimmy Reply with quote

Every 4 cycle engine regardless of number of cylinders has a non-power stroke.How much heavier was that LS1 engine? Perhaps that has something to do with the shimmy or not.

-sent from the I-droid implanted in my forearm

On Mon, Sep 25, 2017 at 6:01 AM, johngoodman <johngoodman(at)earthlink.net (johngoodman(at)earthlink.net)> wrote:
Quote:
--> RV10-List message posted by: "johngoodman" <johngoodman(at)earthlink.net (johngoodman(at)earthlink.net)>

I think it has to do with the piston reversing directions without a power stroke, but I'm no expert. But, the "wet dog shimmy" is very noticeable.
John

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johngoodman



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PostPosted: Mon Sep 25, 2017 6:36 am    Post subject: Re: Nose gear shimmy Reply with quote

Quote:
Every 4 cycle engine regardless of number of cylinders has a non-power stroke.How much heavier was that LS1 engine? Perhaps that has something to do with the shimmy or not.


The LS1 was 15 pounds lighter than an IO-540 - it's Aluminum.
John


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Kellym



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PostPosted: Mon Sep 25, 2017 6:45 am    Post subject: Nose gear shimmy Reply with quote

Interesting...does that include weight of radiator and coolant?

On 9/25/2017 7:36 AM, johngoodman wrote:
Quote:



> Every 4 cycle engine regardless of number of cylinders has a non-power stroke.How much heavier was that LS1 engine? Perhaps that has something to do with the shimmy or not.
>


The LS1 was 15 pounds lighter than an IO-540 - it's Aluminum.
John

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johngoodman



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PostPosted: Mon Sep 25, 2017 7:08 am    Post subject: Re: Nose gear shimmy Reply with quote

It included the radiator, but not the coolant. It used a little less than 2 gallons of water. However, it only needed 5 quarts of oil, plus 2/10ths for the oil filter. The prop was 2.5 inches further forward, so I had to compensate with some ballast to bring the nose weight to normal.
You can't buy an LS1 anymore, so most folks are using the LS3. It's close, but lots more horsepower.
Frankly, the additional power is a mixed blessing, since you can't use it with the Vne limits we have.
John


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Strasnuts



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PostPosted: Fri Sep 29, 2017 6:32 pm    Post subject: Re: Nose gear shimmy Reply with quote

I agree with Bill, you need something like 20-25 lbs of breakout force on the nut and belleville washers. It states the correct number in the Vans' plans.

I also agree with Kelly, I haven't had any nose wheel shimmy with my IO-540. I'm thinking engine preference is not the issue.

You might stick a camera on your tie-down mounts and make sure its the nose and not the mains. If it is the nose it is either the belleville washers, the axle or the play in the bushings on the nose landing gear mount/engine mount. I had to make new bushings that had better tolerance than vans' bushings.


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Tim Olson



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PostPosted: Fri Sep 29, 2017 6:47 pm    Post subject: Nose gear shimmy Reply with quote

On the RV-14 (Same nose gear as the -10) I could swear that the original
plans
had it bumped up to 35lbs of breakout.  I just checked the plans now
though and they list 26lbs.  I would say stay on the high side of Sean's
numbers,
not the low.  By the time it gets down to 20 it's going to be pretty loose.
And, they wear in pretty quick, so after your first flight, check it before
50 hours, and then check it again at maybe 100.  After that it should be
more stable.
Tim

On 9/29/2017 9:32 PM, Strasnuts wrote:
Quote:


I agree with Bill, you need something like 20-25 lbs of breakout force on the nut and belleville washers. It states the correct number in the Vans' plans.

I also agree with Kelly, I haven't had any nose wheel shimmy with my IO-540. I'm thinking engine preference is not the issue.

You might stick a camera on your tie-down mounts and make sure its the nose and not the mains. If it is the nose it is either the belleville washers, the axle or the play in the bushings on the nose landing gear mount/engine mount. I had to make new bushings that had better tolerance than vans' bushings.

--------
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RV-10 SB N801VR Flying
780 Hours
SuperSTOL 60 hours


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AirMike



Joined: 27 Feb 2007
Posts: 512
Location: Nevada

PostPosted: Thu Oct 05, 2017 5:29 pm    Post subject: Nose gear shimmy - resolved Reply with quote

Soooooo, I am the original "poster" on this thread. Anyway, I installed (with Epoxy) an eye bolt in the rear concrete floor in my hanger. I raised the nose well off the floor. Then I completely disassembled my front gear and then put it back together. I bought an all new front tire (a Condor tire by Michelin) that my local FBO balanced for me. I did not know that Condor is Michelin's sub brand - might even be the same thing. Made in Thailand by Michelin.
It is important to align the little yellow dot with the valve stem to facilitate the balancing. It was a fun but time consuming project. I also did the installation of the AD/SB plate that I had not done (Vans S/B 14-08-29). The big shimmy is gone, but I did have a little bit at about 20-30mph on a rough runway today.
But as the shimmy is gone at high speeds, I consider the issue resolved. My vote is that if you have not had shimmy and it develops, consider a new tire and GET IT BALANCED CAREFULLY!


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