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Nose wheel break away nut moving

seagull

Well Known Member
I am trying to track down a slight shimmy when I land and apply the brakes. I have gone through the mains, warped rotors, wheel bearings, tire roundness, it all checks good so I am focusing on the nose wheel.

I have rechecked the break-away torque but notice the castle nut moves within the cotter key when the fork is moved side to side. I don’t recall noticing this before and would think it would eventually wear the threads.

The shimmy started shortly after the new nose leg and fork was installed.

Advice?
 
I am trying to track down a slight shimmy when I land and apply the brakes. I have gone through the mains, warped rotors, wheel bearings, tire roundness, it all checks good so I am focusing on the nose wheel.

I have rechecked the break-away torque but notice the castle nut moves within the cotter key when the fork is moved side to side. I don’t recall noticing this before and would think it would eventually wear the threads.

The shimmy started shortly after the new nose leg and fork was installed.

Advice?

I think this is to be expected, as the friction radius of the nut is smaller than the friction radius of the belleville washers. I saw that as I was building in the late 90's, and found a split collar with the 1.25-16 threads. I had to machine a chamfer on it to clear the wheel fairing. I have looked on and off for a supplier of this collar, but have never found it again. Bottom line - it is normal. Probably good to replace the cotter pin from time to time.
 
You checked the mains and the nosewheel. If the shimmy is present on takeoff roll or landing rollout with the nosewheel off the pavement it’s not the nosewheel.. if it’s a main wheel it should be apparent from the vibration source.

I had a Yak-52 with a castoring nosewheel that would shimmy on landing. It sounded like a buzzer. My Cherokee shimmies sometimes and the feedback through the rudder pedals makes it obvious that it’s the nosewheel. I’ve never experienced any shimmy on my RV-12.
 
I don’t notice it on takeoff or landing until the roll out and I use the brakes, at that point the nose wheel comes down so it is hard to be sure of the source.
 
If your runway is long enough try using aero braking until the nosewheel comes down on the pavement without mechanical braking. If it still shimmies it’s not the mains.
 
If your runway is long enough try using aero braking until the nosewheel comes down on the pavement without mechanical braking. If it still shimmies it’s not the mains.

I have done that, let is slow down without brakes, all 3 wheels firmly on the ground.
The combination of the nose wheel down and semi hard braking will cause it. It is not severe like a bad oleo on a C172, but it is noticeable, and new.
 
I think this is to be expected, as the friction radius of the nut is smaller than the friction radius of the belleville washers. I saw that as I was building in the late 90's, and found a split collar with the 1.25-16 threads. I had to machine a chamfer on it to clear the wheel fairing. I have looked on and off for a supplier of this collar, but have never found it again. Bottom line - it is normal. Probably good to replace the cotter pin from time to time.

I found this play around the cotter pin to create a 5 lb difference in force left vs right. I didn’t like it, so i used a #6 flat head screw/nut instead of the cotter pin. Keeps the nut from loosening in one direction and tightening in the other.

I have had my break out force drop to 10 lbs early in phase I and no shimmy. Just moved the ball off the center in flight. I would be surprised if the shimmy was due to break out force.
 
I have done that, let is slow down without brakes, all 3 wheels firmly on the ground.
The combination of the nose wheel down and semi hard braking will cause it. It is not severe like a bad oleo on a C172, but it is noticeable, and new.

I've seen hard braking cause this on 3 different RV-12s at the airport I am at. Usually a problem if we land short and try to get off at taxiway C instead of D or E, on a 4000 ft runway.

More a problem if we land clean, flaps up. Less of a problem if we land slower with full 2 notches of flaps added once the runway is made and 50-52 kts flaring just past the numbers, with full ground effect in force.

YMMV, these are just my observations. I suspect that hard braking causes splay and alignment issues, and the shimmy is the tires and mains springing back into alignment, as hard braking causes some sort of torsional flex in the mains, along with flex in the tires, and alignment causing an oscillation in the tires, wheels and main landing gear. Can't prove it, just know hard braking is a consistent factor in it occurring.
 
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Shimmy

I had this problem and finally installed a GoPro under the wing to look at it after trying everything from tire pressure to balance, etc on nosewheel. Turned out to be left main gear, not the nosewheel. Once something like brakes starts the movement of the main to the rear, it then wants to return to the front, then the oscillation begins until a lower speed is reached. The natural resonance of the gear leg and the mass of the wheel, tire, pants, etc. causes this once excited. Tire pressure a bit lower than recommended will dampen it. I run about 30 on my RV9A
 
I am not saying that stopping the castle nut movement is necessary but this is my fix.
I machined the "crowns" down .080", still room for the cotter key. I completely removed the crowns on another nut to use as a jam nut. I had these nuts laying around so I re-purposing them rather than buy new ones specific for the job.

The cotter key still does its job, and the jam nut keeps it all stationary.

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