172 shimmy damper mounting play

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moparrob66

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Moparrob
Changed a nosewheel tire today and added some balance beads to see if it helps the intermittent shimmy and I noticed that the shimmy damper seemed loose. The attach point to the steel collar (the end of the piston) seems fine, but the body of the cylinder is very loose where it attaches to the main strut body, allowing about 1/2" of up and down wobble. It looks like it would be easy to put a washer on top and below to solve this, but I thought if its too tight, it could break or bind.

Anybody know if its meant to be loose or should it fit more tightly?
 
moparrob66 said:
Changed a nosewheel tire today and added some balance beads to see if it helps the intermittent shimmy and I noticed that the shimmy damper seemed loose. The attach point to the steel collar (the end of the piston) seems fine, but the body of the cylinder is very loose where it attaches to the main strut body, allowing about 1/2" of up and down wobble. It looks like it would be easy to put a washer on top and below to solve this, but I thought if its too tight, it could break or bind.

Anybody know if its meant to be loose or should it fit more tightly?
Click to expand...
Vibration from taxiing over rough ground does that, along with nosewheel shimmy. There isn't anything you can do about it other than replace the steel bushing in the steering collar.

Those beads will make no difference to the shimmy. Shimmy is a dynamic imbalance issue, and beads are to fix static imbalance. You are using aspirin to cure cancer.
 
The strut scissor link is the most important.....if that's loose then it will shimmy. If the strut has play....it will shimmy.
 

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Scissor link seems tight as does everything else but its disconcerting that the body of the shimmy damper can flop around so much. I'll try to find some washers and a new bolt to add to the "sandwich" and see if that helps.
 
Checkout_my_Six said:
The strut scissor link is the most important.....if that's loose then it will shimmy. If the strut has play....it will shimmy.
Click to expand...
The root cause is always dynamic imbalance. Always. I have dynamically balanced shimmying nosewheels without touching the worn stuff you mention, and the shimmy stops dead. It's the shimmy that causes looseness in the torque links and strut. Tighten all that up and the shimmy will just wear it out again.

How many motorcycles have shimmy dampers? How many cars or trucks? None that I know of. They all (except bicycles) get dynamically balanced when tire are installed.

Tailwheels also shimmy, and it's due to TWO factors: dynamic imbalance, and steering pivot axis being bent forward at the top by tired tailsprings. Tailwheels very often have "shimmy dampers" that are nothing but adjustable brakes that offer friction to discourage shimmy, but they will still shimmy if the balance is bad or tailspring bent. Castering nosewheels are the same. And in either case it just makes steering difficult.
 
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I should also mention: while the wheel needs dynamic balancing that it probably won't get, that shimmy damper has to be full of oil. FULL. No air at all. To do that, it has to come off and the clevis end clamped carefully in a vise, with the slot at 90° to the jaw faces. Pull the cylinder up all the way so that the rod end disappears into the cylinder. Clean the outside of the cylinder with brake cleaner, and use duct tape to make a dame around the top of the cylinder. Fill that dam with oil, and work the cylinder up and down all the way to get all the air out of the cylinder. You have to loosen the vise so you can tilt the cylinder just a bit in all directions to get the last air bubbles out. The cylinder has a flat face inside that does not guide the bubbles to the rod opening, so it needs tilting. After the air is all out, including any foam generated while working that cylinder, push it down to get the rod into its hole, sealing off the inside, dump the oil out of the dam, and remove the tape and clean everything up.

Chances are that the O-rings in that damper are all shot anyway, and air will soon be back in there.
 
Show me a dynamic nose wheel balancer.....and Dan I'll give you a gold star. ;)
 
What should be added to the above are the details of Dan's dynamic balance scheme;

1) Remove the grease from the nosewheel bearings lube same with light oil or WD40,
2) Reassemble/slip together
3) Spin the nosewheel using a wire wheel on the tire OD while holding the axle with your hands.
4) Simply rely on your hands (or eyeballs) to evaluate the balance while placing weights.
5) Probably easiest to approximately radially balance things before trying to do correct dynamic imbalance.
6) Wipe off the lite oil. Repack w grease. Reassemble into nose fork.

(D - did I get it right?)
 
Checkout_my_Six said:
Show me a dynamic nose wheel balancer.....and Dan I'll give you a gold star. ;)
Click to expand...
I built this one:

1733172396836.jpeg

It has no fancy electronic sensors in it. It uses the same principles as this antique dynamic balancer:

1733172723803.png

I had earlier converted one of those ancient mechanical dynamic balancers to do aircraft nosewheels. Worked well. Before that, I did them by hand as Mr. Petersen has noted. That takes much longer than with the machine, but still worth it to stop that shimmy.

Some motorcycle shops have dynamic balancers for small wheels. Some POAers have had their nosewheels balanced. An example:

1733173802497.png

I have not seen a dynamic balancer aimed at only the GA market. Too small, I suppose, and you can't dynamically balance the mains anyway, with the brake disc in the way on the inboard side. You need to be able to attach weights to both sides of the wheel. But mains don't shimmy anyhow; static balancing is enough for most of them. Gear legs tend to be fairly rigid.

You can get a small-wheel bench-top balancer for about $4000:

1733174434202.png
Since most mechanics seem to know little or nothing about the differences between static and dynamic balancing, it's not likely a shop is going to buy one of these until it has been proven to the owner that they work at stopping shimmy. Most mechanics just say, "Oh, that's what Cessnas do" when confronted with nosewheel shimmy.

Automobile balancers are too big and their mounting shafts generally won't go through the bearing bore of the wheel. Motorbike balancer use smaller shafts and are more sensitive to imbalances at small radii.

Gold star time?
 
Dan Thomas said:
How many motorcycles have shimmy dampers? How many cars or trucks? None that I know of. They all (except bicycles) get dynamically balanced when tire are installed.
Click to expand...

Here's a motorcycle steering damper:

www.revzilla.com

Ohlins Steering Dampers - RevZilla

Ohlins Steering Dampers have been an important ingredient in almost every successful motorcycle race team throughout the world. Each Ohlins Steering D…
www.revzilla.com www.revzilla.com

Here's a truck steering stabilizer:

www.synergymfg.com

Synergy Fox Performance Series 2.0 IFP Steering Stabilizers

Synergy Fox Performance Series 2.0 IFP Steering Stabilizers
www.synergymfg.com www.synergymfg.com

Despite balancing their tires, both still occasionally get shimmy for one reason or another. A tank slapper on a bike can get ugly in a hurry. And yes, some of these are installed as OE.
 
I had a steering damper on a drag bike and one on a 87 ramcharger for the front end that had 33" tires in the past.
 
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Checkout_my_Six said:
You didn't have the tires dynamically balanced? :eek:
Click to expand...
Not on the drag bike, it only went 140 mph.

But i did on the ramcharger, remember lifetime tire balance?
I took them off myself and sent them back to the tire shop many times for a balance check. Towards the end I removed the weights so I knew they checked them. I kept my tire receipt on the glove box to prove it each time. They marked the tire serial numbers on it one visit.
Years later I bought a used tire balancer that I hardly use anymore...

I also use dynamic balancer on large trucks tires, strobe balancer. It has 5 hp spinner and a strobe light to determine where the weights go. It balances the whole wheel including the drum or disc brake.

For trucks I also have a tire lathe, after trueing a tire it rarely needs any kind of balance.
 
Gary Ward said:
I had a steering damper on a drag bike and one on a 87 ramcharger for the front end that had 33" tires in the past.
Click to expand...
I've seen them on big-tired pickups. I think that mud stuck inside the wheel might be the reason for those. That can throw the balance off quite a bit. Even snow wedged and frozen into the backside of a car's wheel will make shake and shimmy.
Gary Ward said:
For trucks I also have a tire lathe, after trueing a tire it rarely needs any kind of balance.
Click to expand...
McFarlane Aviation has a video of truing a nosewheel, on the airplane, using a belt sander. But that doesn't address dynamic imbalance.

Most aircraft tires aren't as accurately made as even cheaper auto tires are. Runout is very common. They are also bias-ply, and I think old manufacturing methods are still in use 50 years after the auto industry went to radials. Goodyear often uses heavy balance patches inside their tires, but those tires cost considerably more than the economy stuff. Tubes are also still in use, and they aren't that accurately molded, either.

My evidence is anecdotal. I was the director of maintenance in a flight school, looking after 7 airplanes and sometimes 8. The Cessna singles were bad for nosewheel shimmy and I used to replace torque link bushings and spacers and bolts. I took the whole oleo apart to rebuild and re-shim the steering collar to remove the rocking that results from shimmy wear and introduces slop. I re-rigged the nosewheel/rudder control system exactly as per the manual. I rebuilt the shimmy dampeners repeatedly, with new seals and whatever else needed to reduce any slop. It was all to no avail. It wasn't until I dynamically balanced those nosewheels that the shimmy was finally conquered. Many times the balance was all I did, and it stopped the shimmy.

That shimmy is destructive. It not only beats up the steering stuff, it can loosen the oleo mountings on the firewall, it wears the rudder bars and pedals and transmits violent vibrations down the cables to the rudder, it damages gyros and radios, and so on. Tolerating it is very short-sighted.

Tailwheel shimmy can break that airplane. This is the tail of one of our Citabrias:

1733240802482.png


See the wrinkling of the fabric just above the tailwheel spring mounting on the fuselage? Note, too, the angle of the tailspring, and the torquing of the lower fuselage members as shimmy racks that tailwheel from side to side. This is what happens:

1733240981180.png

Broken fuselage tailpost, just above the welded fitting that clamps the tailspring to the aft fuselage. This gets expensive, and it's because students and instructors wouldn't let me know it was shimmying. The shimmy was easily corrected with dynamic balance and a new tailspring to restore the correct tailwheel steering axis alignment.
 
Here is a video on a Cessna damper rebuild for anyone interested. I think it relates to @Dan Thomas' post #8 about getting the fluid in:
The Parker/Lord damper is fluid free, but I don't know if that's good or bad. More experienced folk can chime in. https://www.parker.com/us/en/divisi...olutions/zero-maintenance-shimmy-dampers.html

I'm in discussion with one of the local motorcycle shops about getting set up for GA dynamic balancing. I think some owners, and maybe even some of the local A&P's/shops would send them business, if it's not too much of a hassle or expensive to get set up with adapters. They just bought the machine, and want the business....if it shakes out. ;)
 
Dan Thomas said:
I've seen them on big-tired pickups. I think that mud stuck inside the wheel might be the reason for those. That can throw the balance off quite a bit. Even snow wedged and frozen into the backside of a car's wheel will make shake and shimmy.
Click to expand...

Volkswagen used them too, but they didn’t have big tires.

1960-1977 Bug steering damper

Fits 1960 (1960 Bug, starting at chassis # 2921552) -1977 std. Beetle.
www.wolfsburgwest.com www.wolfsburgwest.com

While I agree with your assertion that tire balance is a contributor, I don’t believe that it is the sole contributor. If it was, then all the OE bike and auto manufacturers wouldn’t spend the money to add the dampers. Steering geometry and configuration along with shock selection are significant contributors to the problem, and the problem can be magnified with wear and system modification. That’s why you see a lot of truck guys adding dampers - it is a means to mask undesired steering traits that they don’t know how to or can’t fix.

On bikes rake and trail will influence the way the bikes handle. Most of the bikes that have OE dampers have minimal rake and are more prone to tank slappers. The typical SE Cessna doesn’t have much rake/caster either, and I suspect that is a contributor to the tendency for them to shimmy, along with component wear and tire balance.
 
How many of the dynamic-balance skeptics here have done a dynamic balance on a nosewheel to see if it stopped the shimmy?
 
Regarding Dan’s pix and comments in #17:

1. Tried to post a vid of sheared rivets for 172 nose strut mount but
it was too big. You could see the entire fitting moving away from the Firewall. A prop strike would have been in the future.

2. I’ve had a similar event to pic take place with the tail of a PA-16.

3. A 152 was found with a broken lower tube on nose gear mount.

4. My belief is lots of radio and instrument problems develop from shimmy.

Bottom line = FIX it!
 
Dan Thomas said:
How many of the dynamic-balance skeptics here have done a dynamic balance on a nosewheel to see if it stopped the shimmy?
Click to expand...
I have replaced a nosewheel tire that solved the shimmy with loose linkage for about 3 years. Now the tire is getting worn again the shimmy is coming back.
Haven't tried the local motorcycle shop to see if they can spin balance.
If I find it out of round I will replace the tire again.
Also plan to replace the bushings and hardware in the strut linkage this time around as they are loose more than they were 3 years ago.
When I fly by myself it is not a problem, when I have a passenger and more weight it "rears it's ugly head".
 
And I have used a belt sander on the front tire before after I saw the McFarlane video with OK results. It's not near as good as using a lathe but is better than not trying if it is out of round.
 
I’m in the habit of yoke all the way back all the time while taxiing (like soft field) and on roll-out. That can help or mask shimmy.

I got a new tire at annual last year, didn’t even static balance it. About 1 out of 10 flights I now get a slight shimmy at the end of the takeoff roll or start of a fast landing.

Called my favorite go kart race shop and they don’t do dynamic balance. I’ll static balance it and report back next week. Otherwise will call around for motorcycle shops next.
 
In the pre-Internet days I developed a pamphlet called the “ Shimmy

Sheet”. This was advertised in T.A.P. with a guarantee to stop shimmy or

you get your money back. It was interesting to see destinations throughout

the world. No one ever asked for a refund.


Folks seem to focus on one magic cure to solve the issue. Often that is not

the case. Accurate diagnosis is essential. Some shimmy’s have the MAIN

gear as the problem. Pulling the yoke back on a Cessna takes weight off the

Nose wheel but can also lock the upper torque link and isolate the “ Upper”

components in the system.


I’m not a fan of modifications to a damper to alleviate the issue. Adding

internal resistance beyond the factory norm can result in excessive wear

of Major components.

FIX. It!
 
Mine just started recently as well as the tire has become worn.
Temporarily...one thing I've found to help avoid the shimmy from starting, is to perform loooong slow S style turns while slowing down. Basically moving from the center of one side of the runway to the center of the other.
Seems to keep some of the forces to one side or the other and helps avoid it from starting.
I'm sure it adds a little stress to things, but less stress than the violent shaking that occurs and the ensuing braking which puts even more stress on a convulsing nose wheel.
 
The more I read about balance beads, the more excited i get to try them. It seems to me like it does more for dynamic balance than static balance. I was able to get about half an ounce in, but it was so foggy out that the humidity seemed to make getting them in the valve stem especially difficult. Same fog kept me from doing any flying, as I had hoped to warm up the engine and change the oil too, but that will have to wait. I'll also take the shimmy damper home and make sure its full of 5606, then reinstall with some extra washers to reduce the flopperation it currently has. Will also check the new tire for eccentricity and belt sand until true!
 
moparrob66 said:
The more I read about balance beads, the more excited i get to try them. It seems to me like it does more for dynamic balance than static balance. I was able to get about half an ounce in, but it was so foggy out that the humidity seemed to make getting them in the valve stem especially difficult. Same fog kept me from doing any flying, as I had hoped to warm up the engine and change the oil too, but that will have to wait. I'll also take the shimmy damper home and make sure its full of 5606, then reinstall with some extra washers to reduce the flopperation it currently has. Will also check the new tire for eccentricity and belt sand until true!
Click to expand...
How are those beads going to migrate to the sidewalls to counter the dynamic imbalance? Centrifugal forces at typical wheel RPMs are not small.

Anyway, I am done discussing shimmy. Seems to me I already swore off it a couple of years ago after numerous posts over the years showing the phenomenon and what had to be done about it. One gets tired of arguing it with people who have NEVER done a dynamic balance of anything.
 
Moparrob

There is a steel bushing that often falls out when the damper is removed.

Assure it’s there.

Note that binding = buy new shaft.

You may want to try turning the nose left-right just a little with

weight on the wheel. Put your finger on torque link joints and see if

ANY movement. Might be easier with 2 people.


You may want to familiarize yourself with how the Rudder/ nosewheel

system is rigged. It’s common for the lower Rudder Hinge to wear due to

constant Pilot foot pressure and the cables may stretch slightly. This

upsets the action of the steering rod bungees and adjustment is needed.

Not hard but you have to make a gage to position the pedals.

You are dealing with a Flight Control so it MUST be done correctly.


The Shimmy Damper is the LAST line against shimmy. It does not cause

shimmy.
 
Checkout_my_Six said:
Yup....they spin at such high speeds. ;)
Click to expand...
My calculations indicate the 5.00/5 nose wheel on a 172 has a 14" diameter and 44" circumference*, and is turning 1,193 RPM at 50 MPH, and 1,438 RPM at 60 MPH. That's not insignificant.

*Numbers slightly rounded for convenience
 
My guestimation is that the oscillations are increasing from the time they start and by the time youve slowed down, theyre at their worst. Once an oscillation starts, it will increase in amplitude (?) unless something arrests it. Out of balance tire sets the shimmy in motion, sloppy linkage does nothing to arrest it, and by the time you exit the runway or stop, youve got all the involved parts shaking pretty good AND the caster angle of the nosewheel decreases since the brakes are on and more weight is on the nose.
 
Often at higher speeds the strut is fully extended.

Due to the centering stop on the upper link the lower portion

of the system is locked and isolated. Less weight on the tire means

less deformation of the footprint as well.

Application of brakes can exacerbate the shake. IMHO This type seems to

be particularly violent.


One Cessna had a persistent shimmy that was never really addressed.

A vid revealed how bad it was. When it was addressed the inner sides

of the fork ( where the lower link attaches ) was badly worn. Standard

shims would not fix the problem.


Deferring = higher cost
 
My intermittent shimmy shows up at higher speeds. If I delay rotation or land fast.

I think I’ve posted here before, I had bad enough shimmy just prior to rotation on takeoff that it stalled the engine on a rental 152. I thought it threw a rod, was thinking I oiled the runway like one of @Gary Ward ’s dragsters…turns out it was just tire shake :) felt like Warren Johnson - my eyes blurred for a second, no lie, couldn’t see the airspeed indicator clearly. I could understand how avionics would be damaged.
 
Checkout_my_Six said:
Why does the shaking get worse as it slows down?
Click to expand...
There is a resonant RPM, and it depends on the weight of the wheel and all the moving stuff in the linkages. We use (or used to use) pendulums to keep time. A given weight at a given distance will oscillate at a given frequency. That nosewheel will oscillate at a given frequency, and that frequency will be at some speed where the RPM resonates with the mass of the wheel. The weight and flexibility of the nosegear leg itself also feeds into it as it flexes from side to side trying to follow the wheel's track as it shimmies. When you land, the wheel's RPM will probably be too high to resonate with the natural frequency of the wheel, but as you slow, and as Magman also points out, as the weight comes on the wheel and the centering cam unlocks, resonance happens. A heavier, good-quality tire with new tread will shimmy at a lower speed than a cheap, worn-out tire.

And what starts it all is dynamic imbalance.
 
455 Bravo Uniform said:
I had bad enough shimmy just prior to rotation on takeoff that it stalled the engine on a rental 152
Click to expand...
Does it on 150s, too. It shakes that engine so hard that the carb sloshes fuel out of its bowl vent and into the carb throat, flooding the engine. That vent is hidden behind the venturi where the air is still and therefore at ambient pressure, and where fuel sloshed out doesn't splash on hot stuff and cause a fire.

On the 150 that carb's intake spider is mounted with Lock-O-Seal washers to let the carb move a little, to absorb some of that shake caused by the engine firing. Performance problems such as serious roughness or hesitation is often due to those washers being left out during engine or carb installation. The carb is sloshing bowl fuel into the throat.
 
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