REFLECTOR: Shimmy and Nose Gear Pivot Bushings

Scott Baker sbakr at comcast.net
Sat Nov 19 15:09:32 CST 2005


Hello Bob,
Welcome to the Reflector!
The compressed spring washers (Belleville washers) under/over the nose gear fork assembly act as a shimmy dampener.  The design of the shimmy dampener assy has not changed in some time - and the factory has no immediate plans to change the design.  We are concerned about nose wheel shimmy because when it happens, it is violent affair - and usually causes damage to the nose gear system.  A few builders have experienced nose wheel shimmy - and are rightfully concerned to find a fix.  Most aircraft have no problems at all.  The factory demo XL and the V173FG training aircraft have logged more take offs and landings per flight hour than any individually owned Velocity.  I would guess N94VA has experienced over 2000 takeoffs and landings; and N271TC, close to 600.  Neither has had any problems with nose wheel shimmy.
Some helpful maintenance tips to avoid nose wheel shimmy are:
1)    Balance the front tire and make sure it is inflated to the recommended pressure.
2)    Check the nose gear system for slop.  Are the axle spacers in place?  Are the spacer washers in the nose gear pivot bolt assembly in place (RG models)?  Are the nose gear bolt attachment bushings solidly installed into the keel?  Is the nose gear retaining clamp (strap) tight in FG models?  Is the king post snug inside the Silicone-Brass bushing?  Are the metal nose gear strut gussets free of cracks and broken welds?
3)    Clean and lubricate the Belleville washers.
4)    !!! Tension the nyloc king nut, compress the Belleville washers so that "it is almost difficult to steer the aircraft during taxi" !!! (This is a real scientific explanation, I know).
If you have any questions about how to do any of this, pls call the factory for details.
5)    Always check the tension on the nose wheel fork assembly before every flight.  This involves lifting the nose of the aircraft and pivoting the nose wheel left<>right to confirm the nut is tight and the Belleville washers are providing friction.

A year ago the factory tried a locking nose wheel assembly - which would lock in the center position during takeoff and landing; and be unlocked during taxi.  The system had little to no tension on the Belleville washers, making the aircraft very easy to steer.  Ken Mishler installed the system on his aircraft - and he likes it very much.  The nose wheel, while locked, still has enough "warp" in the gear leg to make subtle directional changes during takeoff and landing.  We see talk with Ken on a regular basis and continue to monitor how the system is working.  The next step is to evaluate and perfect the system.  You (Bob Buls) will have a chance to meet Ken when you come down to the Service Center.

Best regards,
Scott B.



----- Original Message ----- 
  From: robertbuls at aol.com 
  To: reflector at tvbf.org 
  Sent: Saturday, November 19, 2005 2:04 PM
  Subject: Re: REFLECTOR: Shimmy and Nose Gear Pivot Bushings


  Just curious, and haven't seen reference to it....are these shimmy/gearleg vibrations limited to "scratch-built" kits, or are the fast-build kits similarly afflicted? (Hoping that fix that Duane was mentioned considering gets included in my fast-build kit due in Jan).

  Bob


  -----Original Message-----
  From: Brian Michalk <michalk at awpi.com>
  To: Velocity Aircraft Owners and Builders list <reflector at tvbf.org>
  Sent: Sat, 19 Nov 2005 10:24:00 -0800
  Subject: Re: REFLECTOR: Shimmy and Nose Gear Pivot Bushings


  Alex Balic wrote: 
  > There is a viscous coupling that I had to spec years back for a machine 
  > design project- kind of like a tiny torque converter filled with oil- I 
  > have no idea if there is one available that could/would fit the nose 
  > gear, but basically, it allows relatively slow turning of the shaft, but 
  > resists rapid turning/oscillation ( shimmy) kind of the same function 
  > that a shock absorber has on the suspension of your car- will see what I 
  > can locate on it and report back........ 
  > > Alex 
  >  
  How about a magnetic particle clutch? 
  http://www.sdp-si.com/D790/HTML2/D790C13026_2.html 
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