172 shimmy damper mounting play

The main gear might get involved too. Warped discs can cause the nose to

want to move left- right and back due to uneven braking.

Main gear dynamic unbalance can be apparent by watching if the wheel

shakes immediately after take- off. Braking or time will stop it.

Nosewheel harder observe. Some aircraft have “brakes” that stop

tire rotation at retraction.
 
Magman said:
Main gear dynamic unbalance can be apparent by watching if the wheel

shakes immediately after take- off.
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All it takes is static imbalance to make the mains hop after takeoff. The presence of the brake disc in the inboard side of the wheel prevents dynamic balancing. One needs to be able to attach weights in both sides of the wheel to get dynamic balance.

This video shows nosewheel shimmy. Speed is shown. Converting from the metric, we see that the shimmy starts at around 57 MPH and continues down to around 40 MPH. It gets pretty violent around the middle of that range. Resonance. Part of the problem is the pilot landing fast and therefore flat, and at that nosewheel RPM imbalance has a much bigger effect on wearing out all the linkages. One should touch down near minimum speed, as you do with a soft-field landing.
 
Wow, that video gives a good idea of the violence to the parts.
 
Yup....you can really see the natural frequencies. That tire is way outta balance.
;)
 
Resonance by itself is a major contributor, but the interface of a loaded non-uniform tire against a road (or runway) will cause a "squirm" or self aligning torque about a vertical axis, especially if the tire is worn or underinflated. The self-aligning torque provides the excitation, and the rake and trail geometry of the nosewheel system provides the rest.

I wonder what would happen if the rake and trail numbers were modified?

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They all (should) lock on extension in flight. The thing is how much tire vertical load after touch down before it unlocks and the dynamics of the system are revealed. I'm convinced that dynamic unbalance is the shimmy initiator and that static balance is only a secondary source of shimmy. The world needs a simple expeditious way to evaluate dynamic balance qualitatively.

Dan Thomas revealed in his earlier posts how he simply did it. Clever! - if maybe a little bit unnerving....! I'd like to try it, but don't have an airplane to fool with anymore.

There may well be a combination of more optimal rake and trail dimensions that were never seriously considered in the original design of those nose wheel steering systems.
 
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nrpetersen said:
There may well be a combination of more optimal rake and trail dimensions that were never seriously considered in the original design of those nose wheel steering systems.
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I suspect that the tire/imbalance is what initiates the shimmy, but believe that Cessna's nosewheel design coupled with neglect is a significant contributor to the chronic shimmy problems we hear of with the Cessnas. Other manufacturers seemed to come up with a design that keeps shimmy suppressed, even with neglect and while using the same or similar wheel and tire configurations (and no balancing) so I don't see it being solely a tire problem. Runway surface type may also contribute to an aircraft's tendency to start shimmying or not.

Another thought, the Cessna heavy duty nosewheel and fork might help suppress the shimmy problems. I don't think I've ever had one of those be problematic.
 
Many aircraft have different types of shimmy issues.

Mooneys, Cherokees, jet fighters and various various Tailwheel types are

subject to it. The worst shimmy I ever had was on a Clipper. IMHO Maule

tailwheels are more prone to it. A Scott ( AK Bushwheel) often cures it.

Somehow I acquired a device that tire shops used. It is a electric motor

driving a flywheel and is portable on 2 little wheels. The flywheel is put in

contact with the tire and rotates it on the aircraft. It does not provide

adjustment info like newer types but is helpful at times.


Tailwheel types have factors like rigging, springs, caster angle just

like nosedraggers.
 
mondtster said:
Runway surface type may also contribute to an aircraft's tendency to start shimmying or not.
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Agreed. And have noticed this on about three occasions over the last few years (all while taxiing no less) when having no shimmy issues otherwise. Seemed to be centerline cracks that triggered it.
 
Don't see if anyone actually answered the original question, is play in the mounting bracket something that needs attention? I don't have any nose wheel shimmering but definitely have plenty of play in the mounting bracket.
 
Larry Korona said:
Don't see if anyone actually answered the original question, is play in the mounting bracket something that needs attention? I don't have any nose wheel shimmering but definitely have plenty of play in the mounting bracket.
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These things are mostly old and beat up. The steel bushing in the aluminum steering collar on the oleo suffers some wear, but more commonly it wears the hole is a sideways oval shape due to the offset weight of the damper hanging on it. The damper then can flop up and down, but the shimmy damping relies on there being little or no slop fore and aft.

The steering collar rotates on the steel oleo cylinder. The steering rods from the rudder bars are on a fore/aft angle on it, and the up/down forces from them against the collar tries to cock it on the cylinder and wears the cylinder's lower flange and the shims that are there to close up clearances under the big lock ring on the cylinder. That rocking represents more slop. The torque links to the nosewheel fork also make that collar cock, and the cocking wears out the torque link bushings, spacers and other stuff. More slop.

So the shimmy damper slop is only one small problem among many.
 
Thanks Dan! I'll look into the repair kits with bushings.
 
moparrob66 said:
Thanks Dan! I'll look into the repair kits with bushings.
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That oleo needs to come out and everything on it gone over. You can replace bushing kits and still have slop due to a rocking steering collar, and you can't fix that rock unless the oleo is out and apart.

Besides that, the aluminum around the bushing and bolt holes gets wallowed out. What are you going to do about that?
 
Can it be reamed for an oversize bushing?
 
moparrob66 said:
Can it be reamed for an oversize bushing?
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Cessna makes no provisions for that. There isn't a lot of material there to allow for that anyway. What Cessna will do is sell you new parts.
 
That sounds really pricey. Are new parts still available for a 1961 model?
 
I’m wondering Cessna has established dimensions for wear items?

Older manuals usually do not provide that. Leaving it up to individual

folks to make a subjective call on this can be interesting. If someone were

to obtain an STC for bushing that is .005 o/s there would be people doing it

w/o the permission of the STC Holder. One possible route might be for

a Type Club to hire a DER to address feasibility. Selling bushings and renting

the reamer could fund the project.
 
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