High speed shimmy

[jetjr]

I have a J200.

 

Ive been having trouble with RHS wheel spat continuously breaking, cracking.

 

Thought it was just crook landings basically my fault

 

Recently though inspected brakes and repaired spat again!!!!. I was also checking for loose wheel on the stub, and was planning on fitting shims but its tight as it can get.

 

Sometimes when taxiing theres a clunk noise but nothing I can find, gear leg bolts are tight, as are wheel stub bolts, wheel spins fairly freely, brakes new and look and work OK

 

Then I noticed if I held the aircraft down a bit too long on take off or landed fast there was a fairly violent shimmy from that side of the AC

 

Seems to be just when weight leaves the gear or when I just touch down, ie gear is drooped

 

How do I align them? Any other ideas as to what may be wrong?

 

Cheers

 

JR

 

 

[quentas]

was there ever any `puncture goo` put in the tube?

 

 

[Guest brentc]

I get high speed shimmers, particularly with rigid spats that are very firmly attached. When I had my old original spats they were a loose fit and didn't shimmy. Since I fitted the new one piece units I have suffered far more shimmy on landing and takeoff plus certain speeds and condions (particularly cross-winds). Presumably they will crack soon.

 

Until I fitted my new spats I would have just said that you have a wheel alignment issue, but now I'm not so sure.

 

When building my 400 I used a laser level attached on the stub-axle extension and then filed off the undercarriage legs to align the axles to align my wheels. If your alignment is a long way out, you need to pack in washers and then when you are happy with the alignment, fill the void with fibre flock & epoxy - quite a bit job really.

 

I used to get the 'clunk' that you may be getting which turned out to be the undercarriage attachment bracket with the two main bolts. You need to ensure that you have the upgraded bolts fitted in the undercarriage as per Jabiru AD as the smaller ones stretch, clunk and eventually break.

 

 

[jetjr]

No goo, I replaced tyres, tubes, brakes, calipers, pads just after I bought it

 

I feel sure that correct bolts are fitted,

 

By new spats you mean the type fitted to 230's, the one piece ones.

 

Mine are still the fully enclosed 2 piece versions, they were off the plane for a long time before I refitted them.

 

JR

 

 

[Kenny]

Hi JR

 

I have a Savannah that vibrates through the rudder pedals on liftoff and could not work it out , however the guys at the strip where I fly from say that it is the nose wheel being out of balance. That makes a lot of sense to me and is the next job I will tackle

 

Regards Ken

 

 

[Guest ozzie]

Make sure that the tyre is fitted correctly, check for a mark on the side wall to indicate allignment with the valve. car tyres have a red dot. tyre is slightly lighter at this section to allow for the weight of the valve. you will be surprised at how many 'professional' tyre fitters get it wrong and the result is a heap of unnessary lead on the rim.

 

 

[Bruce Tuncks]

shimmy

 

If you could spin the wheel up on the ground, it would be possible to sort out balance from alignment...

 

An electric drill with a drum sander would probably not be fast enough to duplicate 50 kts, but it might show up bad imbalance.

 

cheers, Bruce

 

 

[youngmic]

Jetjr,

 

Seems to be just when weight leaves the gear or when I just touch down, ie gear is drooped

It may well be an out of balance issue, but your above comment elludes to an alignment issue to some extent, could of course be a combination, and supplemented by x winds if that is a consistant scenario.

 

Explanation is that as the weight reduces off the wheel it can then break traction and spring out/in side ways if it has been subject tracking misalignment. Simarly on landing but typically won't be as noticable if your landings are positive-ish.

 

A concrete floor a long straight edge and some chalk should be all you need for a confirmation execise.

 

Assume the leg is not loose.

 

M

 

(G'day Bruce)

 

 

[Guest ozzie]

Report in the RAAus mag last month about shimmy vibration on Vampire. eventually cured with a dampner on nose wheel steering connector. but then you are talking main gear.

 

other than out of balance. try the following

 

wheel bearings loose, worn

 

incorrect fitting of tyre. line up weight mark to valve. tubeless type tyre. do this even if tube fitted. bead not sitting on rim properly

 

flat spot on tyre.

 

bent stub

 

wheel halves not facing each other off correctly. face surface corroded. bolts not tightened in sequence or to correct inlbs.

 

also consider if the tyre/ tube is from el cheapo land.

 

your wheel and tyre can be balanced in situ with portable balancer strobe type. or on machine at tyre fitters.

 

hope this can help you out.

 

 

[facthunter]

Out of balance.

 

Check run-out of tyres, and balance. Out of balance on the drifter makes you think that the pod is falling off, when you get airborne initially. To check this get your passenger to apply the brake just after lift-off. YOU fly the thing and make sure that it doesn't touch the ground. Some tyres will get a semi-permanent set on the circumference (out of round) if they sit in one position for a while, ( I think this is called hysteresis) but this usually rectifies itself if you taxi for a while. (Tyre warms up). Nev..

 

 

[Guest Nelson Smith]

Vibrations Galore

 

Some weeks after returning from a flight to Birdsville, our Jabiru aircraft developed a severe vibration on the ground roll at about 50 knots (IAS). This was just prior to rotating. Landing seemed ok, off came the wheels and these were balanced. Although they were not far out of balance. Subsequent flights proved that the vibration was getting worse. To the extent that on landing and roll out at 40 to 50 kts (IAS) the vibration was severe.

 

A thorough check of the undercarriage revealed the l/h leg was slightly loose. Both legs were removed to inspect the mounting rubbers but these were found to be in good condition. So the legs were replaced and a wheel alignment followed.

 

Fast ground runs proved vibration was just the same. Bugger: Engine mount rubbers were removed and inspected and yes they needed replacing after 750 hrs. Well what now. It was at this point that Sandra (pilot of the Jab) mentioned the problem to one of our LAME’S at YLTV one Mr Gerrard Lappin. Without even thinking about it he suggested the problem was “bunching” or “scrunching of the tubes on landing. He explained that as the aircraft touches down and the wheels start to accelerate, the tubes may not pick up to the same speed as the tyre so the tube can bunch, particularly in smaller wheels.

 

Off with the wheels, out with the tubes and inspected same. No signs of bunching, but I am convinced this was the problem. Replaced tubes tyres and wheels and inflated them to 30 psi (they were at 24 psi) Then we went for a fast ground run. No more vibration Jexxx was I pleased. I suppose the moral behind this incident is to maintain tyre pressures.

 

Nelson Smith

 

 

[Guest brentc]

I use 40 PSI in my Jab tires as per recent Jabiru recommendations. Admittedly I sometimes find that I get more vibrations when at higher pressuers.

 

 

[Guest Nelson Smith]

Hi Brent,

 

We inflate the tyres to 35 psi now

 

Nelson

 

 

[Guest Rocko]

Hey Kenny

 

We had a huge issue with front wheel wobble on takeoff on my last plane

 

I made a jig up to balance the nosewheel, and problem solvered.

 

I put up a sketch of the jig in another part of the forum just yesterday, when someone else asked about it.

 

http://www.recreationalflying.com/forum/jabiru-users-group/16643-jabiru-wheel-wobble.html