jim jewell
Joined: 09 Jan 2006
Posts: 82
Location: Kelowna B.C. Canada
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 Posted: Wed Feb 13, 2008 12:19 am Post subject: nose wheel fork, bobbing and shimmy issues |
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Wheel balance, out of round tire, Nose wheel fork breakout, axle nut torque setting (bearing pre-load), wheel pant balance, nose wheel strut mounting bolt torque, the actual castor angle and the presence (or not) of some form of strut stiffener all add up to a rather complex troubleshoot when nose wheel shimmy and or bobbing are suspected.
With the above in mind I offer this:
During the early test flights I observed my 6-A suffer severe nose wheel bobbing and shimmy during touchdown and rollout. This resulted in some damage to the wheel pant.
While investigating the cause I checked the "break out" force, the tire pressure and the torque setting of the axle bolt nut.
All were to spec.
With the wheel off the ground I did note that with the axle torque at spec. the wheel resisted rotational movement. The resistance to rotation was similar to having a partially applied brake.
Suspecting that the resistance had a direct relationship to the nose wheel "bobbing" I decided to reduce the axle toque to the point that the wheel turned more freely.
During the next series of test flights the nose wheel bobbing was all but gone and the shimmy was much less severe.
Lowering the tire pressure did not help the shimmy so I increased the pressure. with the tire pressure at about 33 lb. the shimmy almost went away. Increasing the "break out" a small amount got rid of the rest.
Van's nose wheel mod became an issue and I dismantled the nose wheel gear in preparation .
It was at this point that I found the outer bearing spacers had been turning causing damage to the old nose wheel fork. The damage was due to the reduced axle bolt torque. After a bit of research I found that others had run into this same condition.
I began a close examination of the axle bolt and all the related outer spacers bearings etc. without the wheel pant in place.
I discovered that with the axle torque set at spec. the resulting bearing pre-load contributed a large undesirable amount of drag to the nose wheel's rotation. With a relatively small reduction of axle bolt torque the seals and outer spacers could begin to turn with the wheel any further reduction of axle torque allowed freewheeling of all the various parts to occur.
With the wheel pant in place it was all but impossible to determine by sight whether the outer spacers etc. where turning or not.
Therefore the only way to be sure the outer spacers and bearings are not turning with the wheel is with the axle torque set to spec. I think this torque setting excessively tightens the wheel bearing pre-load. This then seems to induce negative nose wheel performance i.e. nose wheel "bobbing"
During my earlier research I learned of someone else putting a spacer between the bearings.
With the above in mind I made and fitted an inner bearing spacer. I reduced the inner spacer width dimension until the wheel had zero run out. I then reduced the spacer dimension a further five thousandths of an inch. This reduced size should allow for some bearing wear in (seating) and the seal material on the back of the bearing to crush a bit with age.
With the spacer in place and the axle bolt torque set to vans spec. the various axel parts are mechanically locked and do not turn with the wheel and there is a small amount of drag that is caused by the bearing grease seals interacting with the wheel.
I would expect that should bearing wear become an issue either bearing replacement or a further reduction in spacer width might be called for. The axle bolt torque could be increased without adverse effects to wheel drag but I doubt that doing so is or should be required
The aircraft is not yet ready to test fly after the modifications described above. Therefore I cannot predict the nose wheel performance that will transpire as a result of the additional spacer. If need be it can be omitted as no other changes to the nose wheel parts have been made.
I'm told there haave been or are more than one supplier of wheels for Vans products.
If inner spacers turn out to be effective it seems to me that a set of specific spacer dimensions could be established.
I am not suggesting that others copy or emulate what I am experimenting with.
however I am interested to have input in regard to the above,
Jim in Kelowna
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