[Lycosaurus]

Hello All,

While I already have the newer fork assembly (I think we got the finish kit in early 2006 ... has it been that long already?), the nose wheel service bulletin got me thinking about this again.

First, I had already drilled holes in the fork and bushings to prevent their rotation. Great idea (see earlier posts from other builders suggesting this).

When placing the wheel in the fork, I had to pry open the fork to allow the wheel/bushing assembly to slip in. It’s kind of a pain, with having only two hands. With the axle bolt inserted, I found it difficult to rotate the wheel. Now Vans says that this will loosen up with time, however I have read from others that rotation remained stiff. By stiff, I mean you cannot spin the wheel more than about a 1/4 turn.

My belief is that such a stiff wheel, when encountering some pavement irregularity, will impart this load to the nose-gear leg. Upon a landing, for instance, the wheel will be balking rotation and transferring the subsequent rear facing force to the nose-leg. Additionally, if you were to bounce the nose-wheel at the forks resonant frequency (or sub-multiple), this would magnify the forks travel. Under the right/wrong circumstances ... well, we know what happens.

Maybe my fork is a unique case, being so tight. If yours is like mine however, you may want to consider the following:

Here's what I did. I measured the fork internal width: 4.55". The wheel with bushings is approximately 4.65" in width (took some differential measurements) . With wheel removed, I placed an old car scissor jack (with wooden pieces to protect the fork), and opened it up to just a little larger than 4.65". It's tough to gauge, since the fork tongs are very springy. I had to try several times and stopped once satisfied with my width measurement.

Now I get to easily install the wheel with the bushings without having to pry the fork open. Put in my anti-rotation bushing screws, and finally the axle bolt. I can now have the tire rotate multiple turns. I then tighten the axle nut until I get the desired amount of rotation; I now have full control! I will have plenty of time to research the exact preload I need on the bearings (I think it is ¾ turn … someone can chime in with some values).

Maybe one reason that some nosewheel RVs are more prone to flips than others is due to some forks being tighter. I would be interested to know how many people have observed the tight fork when inserting their wheel/bushing assembly.

Just my thoughts. Comments appreciated, as I only know enough to get myself into trouble J

 

[steveciha]

Having built both a 6a and 7a, it is my opinion that the 6a wheel/axle assembly was actually better, in that the rotational friction was never an issue with the 6a axle. I now have almost 250 hours on the 7a and it is still stiff. I don't stake the mushrooms to the fork, just torque it to van's spec and the mushrooms don't rotate.

I too believe that the friction is a big contributor to the fore and aft shimmy issue. If i had it to do over again, i would use a wood fairing strip on the back of the nose gear leg and wrap that with fiberglass cloth. I never had an issue with fore and aft shimmy on the 6a but do experience it once in awhile on the 7a.

steve ciha

 

[videobobk]

I have a 9A, but the nose wheel is from a mid '90's 6A. I don't know if this was standard at the time, but there is a solid axle all the way through. The way you set the bearing clearance is with shim washers--fewer shims, tighter bearings. When you have it set, tighten the through bolt to proper torque for a 3/8" bolt. Nice and solid and no guessing. I will use the same system with the new fork I will be installing soon.

Bob Kelly

 

[robertahegy]

I wish Van's had used something like the Piper Cherokee axle. You could properly preload the bearings. I agree, the older 6A does look better. I staked the mushrooms on my 7A so I could set the preload and not be concerned with them spinning. Those rubber seals still keep the wheel from free spinning, though. But it's better.

Roberta

 

[Lycosaurus]

New Fork - only 1 need to pry

From the stats so far, I see 1 needed to pry the tongs (that's me), and 5 didn't. While I have one of the earlier ones, I am surprised at being the only one so far.

 

[yakdriver]

A little rotational friction on a full swiveling nosewheel is a good thing. It is one factor that prevents shimmy. Don

 

[db1yg]

I think Alfio is talking about rotational resistance at the nose wheel bearing, not the fork pivot.

Cheers,

db

 

[yakdriver]

Dave, I am talking about the wheel rotation. Don

 

[db1yg]

If you take a look at videos that some ingenious folks have taken (posted on this site) of the nose wheel shimmy on touch down you will see that rotational resistance can result in the gear flexing back on touch down as the wheel attempts to accelerate to the aircraft speed, followed by an unloading of the gear and a spring forward (in the best case--worse case is the fork friction washer catching on the ground and over she goes), followed by a reverse, etc. until the energy is disipated. This back and forth action sets up shimmy that is felt in the cabin. Add to this an out of round Chinese tire or one that is out of balance or both and you have mucho opportunity for shimmy. I think Lyco, Steve, and Roberta are correct in assuming the excess preload on the new axle assembly contributes to the issue on the RV A.

Cheers,

db

 

[Lycosaurus]

Lowering rotational friction

A little rotational friction on a full swiveling nosewheel is a good thing. It is one factor that prevents shimmy. Don

I am not proposing to eliminate the friction, just lowering it. In my case, the fork tongs were too tight, and were preloading the bearings. I have taken the tong tightness out of the equation, and can now preload the bearings with the axle bolt nut. I can now set it to the appropriate spec.

If my tongs were tight by 0.100", it is conceivable that some lucky builder out there may have an ever tighter one.

This is just one of the things that I hope will improve my nosewheel survivability:

• new gear leg / fork assembly (I already have that)
• widen the fork tongs
• stake the mushrooms with screws
• punch some dimples on the bearing side of the mushrooms, to prevent slippage when in contact with bearing rubber seals
• leave plenty of margin in the wheel pant opening to allow for a lower inflated tire to expand without having the sidewalls rub the fiberglass
• leave plenty of vertical clearance to prevent wheel pant from rubbing or dragging on the ground should the tire pressure be lower than normal, or rought terrain
• operationally: apply appropriate preload on axle nut, steering nut; tire air pressure
• oh, proper soft field technique at all times

Am I missing anything from the above list?

I know Vern Little has proposed a fiberglass skid plate under the the steering torque nut; I'll reconsider that one as I get closer to doing the wheel pants.

I am not familiar with the details of the piper nosegear, but I agree with Roberta that the nosewheel assembly could have been better.

By the way, I like the new U-713C wheel pant bracket, and will probably order a pair before my next shipment from Vans.

 

[Brockster]

I used Grove nose wheel

I chose to use a Grove aircraft nose wheel instead of the Matco provided by Vans. The bearing seal design is what I don't like about the Matco wheel. The Grove wheel uses a standard seal design found on Cessna's and Pipers so there are no issues of the seal causing excess frictional drag. I sent the new fork design to Grove and they fabricated a full axle that goes through the entire wheel and staked the bushings. One thing I think the full axle does is prevent the through bolt from flexing in the wheel that might cause uneven loads on the bearings. If you do a search for "grove" you should find a previous thread of mine with pictures of the axle assembly.

See "A model grove nosewheel help" in RV-9A forum, 2nd page

 

[Randy]

When I did the work for the nose fork service bulletin I read here about the spring clamping force of the fork on the bearings and the idea about springing it back out a bit with a scissor jack.

I did this and I can easily feel the difference. Before when the nose wheel toched down I could feel the friction from the bearing like brakes were being lightly applied up front.

I jacked mine open to allow easy slide in of the wheel with the mushroom spacers on. I also staked them. Now I can truly set the amount of pressure on the bearings like a normal wheel, I still must deal with a little extra friction from those seals though.

I think many people are confusing the clamping load from the too tight nose fork with friction from the seals. I have a much freer turning nose wheel after performing this mod.

This was a very easy change to make and it made a real improvement. I highly recommend it if you find your bearings are being clamped by the fork rather than the axle bolt clamping force.

Thanks for posting this great idea.

Randy C

 

[videobobk]

Question about grease

I will be installing the new SB fork in the next week or so. I have a fork originally on a 6A from the mid 90's and will use the old style through axle. I like it; it allows the wheel to rotate freely and is IMHO a more positive way of setting bearing load, although it does take more time. When I pulled the old fork, I found the previous owner had used grease that pretty much resembled bubble gum, both in color and consistency! No, I didn't try it for taste. I have never had any shimmy or steering problems at all and I can see that this stiff grease may have been partially responsible. It has to act as a shock absorber during quick rotational loads, sort of like a fluid head on a video tripod. The wheel bearings had normal wheel bearing grease, which I had of course replaced. What should I use for grease in the fork? Does the consistency really matter, or is this just an anti-wear thing? I couldn't find anything about what to use in Van's material.

Bob Kelly

 

[az_gila]

AeroShell #6

..... The wheel bearings had normal wheel bearing grease, which I had of course replaced. What should I use for grease in the fork? Does the consistency really matter, or is this just an anti-wear thing? I couldn't find anything about what to use in Van's material.

Bob Kelly

Bob... my Grumman Maintenance Manual states that MIL-G-7711 grease is the stuff to use.

This translates to AeroShell #6 - easily obtained at your favorite aircraft supply house.

The Grumman uses an identical system of Belleville Washers at the nose fork pivot.

gil A

PS Calls out AeroShell #16 for the wheel bearings.

 

[videobobk]

Thanks, Gil. Bet my favorite A&P has it on the shelf!

Bob

 

[Lycosaurus]

Seals - What a drag

I still must deal with a little extra friction from those seals though.

Thanks for providing an in-use feedback. It's nice when theory meets practice.

I was thinking about the frictional loss from the seals too. My thought is that when it is all greased up, the seals don't wear-in. What if the seal area was cleaned of grease, wheel rotated to created some wear on the seal, then clean and grease it all up again? The seals would then have had a chance to wear, and should create less drag. I'll have to think about this some more and maybe give it a try, by rotating the wheel with a drill or some other motive power.