Shimmy Shimmy Shake

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4RNB

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4RNB
Background: C172M, purchased December, hundreds of landings with the front nose pant off. Had some work done on the plane, IA put the front pant back on. Did my first soft field takeoff leaving his shop. Landed back home, good slow energy landing, gentle brake application led to brief shimmy, alleviated by brake release. Same thing happened again.

Might putting the pant on have caused this?
Might something have happened during either mechanic work or soft filed unevenness?
What would you do at this point?
Mechanic said to use more yoke on landing, think I had plenty in use at the time.

My thoughts are to
1. Fly some more and observe.
2. Consider taking the pant off?
3. Have mechanic look at it.

What are your thoughts re cause and cure?
Thank you.
 
4RNB said:
Background: C172M, purchased December, hundreds of landings with the front nose pant off. Had some work done on the plane, IA put the front pant back on. Did my first soft field takeoff leaving his shop. Landed back home, good slow energy landing, gentle brake application led to brief shimmy, alleviated by brake release. Same thing happened again.

Might putting the pant on have caused this?
Might something have happened during either mechanic work or soft filed unevenness?
What would you do at this point?
Mechanic said to use more yoke on landing, think I had plenty in use at the time.

My thoughts are to
1. Fly some more and observe.
2. Consider taking the pant off?
3. Have mechanic look at it.

What are your thoughts re cause and cure?
Thank you.
Click to expand...
This has been discussed ad nauseum here. Do a search on this site for "nosewheel shimmy."
 
Dan Thomas said:
This has been discussed ad nauseum here. Do a search on this site for "nosewheel shimmy."
Click to expand...

Thanks. Do I use a "+" symbol when I type in "after maintenance" or "after wheel pant installed"?
 
So yesterday I did another flight, it did shimmy again. Another mechanic came and looked at it, thought it looked overinflated. I texted the guy that did the work, he thought he inflated to 42 or 49 (talk to text made it awkward to read). POH says 26. We let some air out though could not get our gauge in that small hole to test. I just did a couple landings and no shimmy!

Things I learned"
1. Don't ignore the obvious, in the case recent mx contributed.
2. Lots about the front wheel!
3 If I ever need to replace the damper I will likely go with McFarlane.
 
4RNB said:
So yesterday I did another flight, it did shimmy again. Another mechanic came and looked at it, thought it looked overinflated. I texted the guy that did the work, he thought he inflated to 42 or 49 (talk to text made it awkward to read). POH says 26. We let some air out though could not get our gauge in that small hole to test. I just did a couple landings and no shimmy!

Things I learned"
1. Don't ignore the obvious, in the case recent mx contributed.
2. Lots about the front wheel!
3 If I ever need to replace the damper I will likely go with McFarlane.
Click to expand...

Yup. POH or maintenance manuals have a lot of instructions that work when not much else does. One thing they don't talk about is dynamic balancing of the nosewheel, probably because most shops would never have one and because the aircraft manufacturers never dreamed that these things would still be flying 60 years later, some with tens of thousands of hours on them, with worn-out stuff and poorly-manufactured tires made in countries with the cheapest labor.

Just remember that replacing a lot of expensive parts to try to stop the shimmy is treating symptoms, not the root cause. It's like taking Aspirin, hoping it will cure your cancer. It just reduces the pain for a little while.
 
The truth regarding nose wheel shimmy... It usually related to multiple highly worn parts. The whole nose gear, tire, steering linkage, wheel pant, etc is a system.

Sometimes the shimmy dampener is the problem or a contributing problem. Everything must be checked if there is any unwanted to unanticipated movement with tail down/nose wheel up for testing.
 
NordicDave said:
The truth regarding nose wheel shimmy... It usually related to multiple highly worn parts. The whole nose gear, tire, steering linkage, wheel pant, etc is a system.
Click to expand...

No. In my experience, the worn stuff is the result of dynamic imbalance, and it that isn't corrected you are wasting time and money replacing "multiple highly-worn parts." I stopped shimmy on airplanes with worn nosegear stuff just by dynamically balancing that nosewheel.
 
I’ve had both C172 and C150 that shimmied.
NordicDave is correct, it’s a system and everything needs to be in spec.

with both, higher time airframes, they needed the nose strut shimmed in the motor mount tube. My most recent mechanic is a wizard at resolving those issues as he’s worked on small GA aircraft his entire career.
He started with shimming the strut, then replaced strut packing AND bearings and sleeves. Replaced wheel bearings, AND balanced tire/wheel. Replaced hardware and re-bushed and new seals on shimmy dampener.

Yes, it was a day’s labor and parts, but shimmy was cured. Removing the front wheel pant is just a poor boys “fix” for nose shimmy.
Any time I see a Cessna w/o a nose gear wheel pant I assume it’s due to shimmy...
 
AeroLudite said:
Any time I see a Cessna w/o a nose gear wheel pant I assume it’s due to shimmy...
Click to expand...
Or a 150 that flies from a grass field. There are those too. Wheel pants plus wet grass (or mud or snow) plus the extra weight in summer equals no bueno.

Not that my 150 is totally cured of shimmy issues. We too have the strut shim on the short list as well as somehow rebushing the dampener bolt attachment point that is worn out.
 
Suggest you enlist a KNOWLEDGEABLE CESSNA mechanic to go through the

entire ritual. It will be money well spent.

Hoping the shimmy goes away can cost big bucks.

The shaking can damage instruments, avionics and aircraft structure.

Ultimately; you may find a circular groove has been worn in the Fork

at the Lower Torque Link attach point.

That alone can set you back a lot more AMUs than the Tech would charge you.
 
Follow up number2:

After fixing the shimmy by removing excess tire pressure we had a nose wheel flat, two days in a row. It turned out that the tube had inside wall dry rot. We ended up being very grateful this happened at home whilst parked, not away from home on a landing. While getting that changed we went ahead and installed the MacFarlane shimmy damper. I had thought to install the Lord when I first read about it, but the MacFarlane information suggests it is a superior product.

Now the only alignment or shimmy issue is my need to keep the plane better aligned during IFR training...guess that will come with more time and increased awareness.
 
Dan Thomas said:
No. In my experience, the worn stuff is the result of dynamic imbalance, and it that isn't corrected you are wasting time and money replacing "multiple highly-worn parts." I stopped shimmy on airplanes with worn nosegear stuff just by dynamically balancing that nosewheel.
Click to expand...

I agree with this^^^^^^^^^^^

This has been my experience. If the tire is true and in balance it can tolerate some wear in the parts. Even if you eliminate all the wear, if the tire is out of balance then it is still going to shimmy.
 
A static 'bubble level' will suffice.

Also, you do know what the yellow and red dots on the sidewall mean?
 
pfarber said:
A static 'bubble level' will suffice.

Also, you do know what the yellow and red dots on the sidewall mean?
Click to expand...
Yellow spot is lightest area of the tire and where the valve stem should be located. Red spot is high spot for tire and where tread depth should be measured.
 
pfarber said:
A static 'bubble level' will suffice.
Click to expand...
Once again, the biggest problem here is misinformation. Do you have ANY idea what driving was like when all we had were static balancers for cars? It was awful. Constant shimmy and shake.

I've lost count of the times I've pointed out that a static balance WILL NOT suffice, and I've proved it many times over many years on many airplanes. How many shimmy problems have you fixed?
 
upload_2021-8-8_11-42-24.png

Now. Which one can be fixed with a simple bubble balancer? And which one will cause shimmy? The dynamic imbalance is often statically balanced but will still shimmy.

If we had the steering wheel starting to shake in our 40,000 mile cars, would an honest mechanic look at it and say you needed new ball joints and tie rod ends and all the other front-end stuff? Or would he just put the wheels on a dynamic balancer and get them right? Here on POA we have a constant stream of advice telling owners to spend money on a whole bunch of new bushings and shimmy damper stuff that will only mask the problem for awhile. Then it will need to be done again. And again. Meanwhile, the whole nosegear assembly is suffering wear: torque link and shimmy damper bolt holes wallowed out, oleo mountings working loose, and so on. All because so few understand the difference between static and dynamic imbalances.

Motorcycle shops use these. They don't use bubble balancers. They can balance nosewheels. The wheel MUST be spun to find the imbalance and calculate how much it is and where it is.

upload_2021-8-8_11-52-3.png
 
Gary Ward said:
Yellow spot is lightest area of the tire and where the valve stem should be located. Red spot is high spot for tire and where tread depth should be measured.
Click to expand...

I think you may be referring to automotive tires. Valve stem should be aligned with the red dot on an aviation tire since the red dot indicates the lightest part of the tire. I haven’t seen all aviation tires but the ones I have seen only have a red dot. I have yet to see a brand with a yellow dot
 
Timbeck2 said:
I think you may be referring to automotive tires. Valve stem should be aligned with the red dot on an aviation tire since the red dot indicates the lightest part of the tire. I haven’t seen all aviation tires but the ones I have seen only have a red dot. I have yet to see a brand with a yellow dot
Click to expand...
From https://aircraft.michelin.com/wp-content/uploads/sites/15/2018/01/technical_pockettireguide.pdf

upload_2021-8-8_20-55-34.png

The yellow dot shows up when the tube's heavy spot isn't at the stem.

But aligning the stem or dot with the red dot doesn't guarantee balance. It just gets the balance as close as possible without adding balance weights. Most light aircraft tires and wheels are poorly balanced as they come from the factories. I'm sure you've all felt the shake as the main wheels spool down after takeoff. I encountered one wheel so badly out of whack that it shook the whole airplane (C180) while still in the takeoff roll. It took a massive amount of weight to stop that. Aircraft quality, indeed.

And mains can't be dynamically balanced. They have to make do with static balance. Dynamic balancing requires weights on both side of the wheel, but the brake disc is in the way on the inside of the main wheels. A dynamically imbalanced main wheel doesn't cause 2% of the grief that a dynamically imbalanced nosewheel does. The main's axle is way too stiff to allow it to shimmy.
 
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