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Nose Gear hypothesis.

frankh

frankh

Well Known Member
Morning all,

It has long been a theory of mine that for/aft oscillation of the nose gear is caused by the NG bearings being set too tight..This possibly even leading to NG collapse.

With the old style nose gear brackets its a real PITA to set the bearing tension because you have to do it with the wheelpant in place..Not easy. Add to this that you have to remove loosen the spindle bolt to flex the wheelpant to inflate the tire (tyre) then this is all leading to way too much of a chore...One day I might cut a hole in the pant to access the valve.

Now I have staked my wheel spacers so there is no reason to have the spindle set too tight and have never had the oscillations.

needless to say I just went through this whole procedure and for the first time I believe I can feel the oscillations.Uh ha!

So I need to re-adjust the spindle torque..Not a big deal but it would be a whole lot better if the wheelpant did not have to be in situ while the spindle bolt was tightened.

The new style of wheelpant bracket solves this issue.

So I was wondering if anyone has notched the old style wheelpant bracket to enable it to be slipped on after the spindle torque is set.

is there enoough meat on the rear bracket to be able to do this??

Frank
 
would a single piece axle help?

Frank - I can't help you directly, but I am watching the NG discussions with extreme interest. My engineering background has me convinced that the fore/aft oscillation is a very bad thing, and some of it may be caused by excess rolling friction in the NG system. It is my understanding that the friction can come from 2 places: the rubber seals on the bearings (this is supposed to be there), and the friction due to excessive preloading from the axle attachment.

Have you considered converting to the new Matco single piece axle ($43)?

It supposedly allows setting of the bearing preload independently from the wheel attach bolt torque. The idea is to avoid excess drag due to the compression of the bearings between the nose gear forks. The bearings are installed and preloaded off the fork, and then the assembly is attached to the fork.

I talked to them today and they are quite helpful about their product. I would love to hear from someone who had the dreaded fore/aft oscillation and found that this solves the problem. I'm trying to decide between this solution and a Grove wheel.
 
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Interesting

Yes my engineering background tells me the same thing!

As I have never experienced it before and the feeling of the oscillation (seems about the correct frequency) only happened right after I pumped up the front tire then I can only conclude its a function of tire pressure or bearing preload.

I'll look into the axle solution..if $43 takes the issue away then that is a no brainer.

Frank
 
Hmm

I just looked at the Matco website and it sure looks like this is the answer.

I already have a holes in my forks but it lloks like I could use one of my existing holes and drill/tap a new thread in the matco axle as there is no way my existing holes will be in the right place.

Thanks

Frank
 
Ordered..:)

The New axle head is solid and may therefore be drilled and tapped to suit the existing holes in the fork.

Looks like the way to go as it will be much easier to set the preload on the bench without the wheelpant in the way.

Frank
 
Just did this

Frank,

I just installed the new Matco Axle this past weekend. I needed a new nose wheel tire also, so I used the "new" one from Desser made by Aero Classic. Looks like a mini Flight Custom in 8 Ply.

The installation is straight forward, they would have you drill a hole in the fork to lock the axle but after reviewing it, once you torque the axle bolt down there is no way the axle can rotate (you'll understand when you do it). It was easier to set the preload with the wheel on the plane, nose in the air. You can adjust the bearing preload by turning the big threaded end in about 1/8 inch increments. Verify the seals aren't moving, then install the locking bolt for that.

As for landing.... is it any better? What I noticed the most was on wheel touchdown (around 40 mph for me) it was really smooth, way better. There was always this little downward jerk as the tire spins up that seems to be gone. I still have a shimmy during the slow down but I don't know if it's related to the nose wheel or the mains.

Quite happy with it and for the price, well worth it. :)
 
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Thanks Scott

I think you have the wheelpant bracket that can be installed after the wheel..I have the old style brackets..in other words the wheelpant is covering the wheel as your trying to set the preload.

This makes it much harder i think and I suspect I over tightened this time around..i will back it off this evening but I will look forward to setting the preload on the bench..and more to the point without the darn wheelpant covering the wheel..:)

Cheers

Frank
 
A question Scott.

I looked at Matco information about their parts.
Do I understand this correctly that the Vans two bearings and axle/bearing holder would be replaced by the solid axle?

Kent
 
Hi Kent,

I looked at Matco information about their parts.
Do I understand this correctly that the Vans two bearings and axle/bearing holder would be replaced by the solid axle?
Click to expand...

What the new axle does is replace the axle/bearing holder or "mushrooms" as I call them, from the original design. The Bearings are reused.

Before I took my wheel off, I tested it's spinability with the old setup. It did not spin freely at all. It took a fair amount of force to pull it through. With the new setup it moves very easy/smooth. I have the bearings loaded to the minimum that is required to keep the seal from moving. All this is in Matco's instructions that come with the axle.

I'm curious how much the new tire has helped with this? There is a lot of rubber on the new tire. The original one had 535 landings in 510 hours of service. ;)
 
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Matco description

kentb said:
I looked at Matco information about their parts.
Do I understand this correctly that the Vans two bearings and axle/bearing holder would be replaced by the solid axle?

Kent
Click to expand...
The Matco assembly looks a little like a dumbell. One end screws onto the center cylinder so that it's two larger diameter, flatter cylinders at the ends of a longer, smaller diameter cylinder. Because of the screw on part, the total length and thus the length between the inside surfaces of the two end cylinders is adjustable.

The two "pucks" get replaced, but not the actual bearings. You adjust the Matco assembly to get the correct pressure on the bearing, lock into place with a set screw and then put the axle bolt through the assembly and the fork. The entire assembly fits inside the fork. When you tighten the nut on the axle bolt you snug the fork to the assembly, but that no longer has any effect on the pressure on the bearings. I hope this helps.
 
more ideas & info

I'm glad this discussion started - great info here :D

I talked to Matco - they think their part is the answer - and they believe that the bearings with the integral seal are much better than the old school felt ring type. They also told me that they feel that the wheels need to be balanced (and they have stick on weights for sale).

I also talked to Grove - they attribute the real problem to the integral seal and thus think they have the best solution!

It's cool that in EX-AB we have options like this, but it sure is difficult to decide which way to go. The Grove stuff looks beautiful, but is $160 more!

I do agree that the felt rings aren't that great - I have them on the Skipper (Cleaveland nose wheel).

On a related note, I learned that there is an inexpensive nosewheel mod that Matco commonly does (about $30) that allows the use of a standard 500-5 aircraft tube rather than the lamb tube. The mod consists of moving the hole for the valve stem and it changes the part number from a 50125 to a 51125 (the modified wheel is a standard part). This mod sounds like a good idea too. He wasn't sure if you could get this wheel through Vans.

I'm leaning towards the Matco wheel, modified for the stardard tube, riding on the Matco axle with the new Aero Classic tire.... or maybe a Grove :eek:

This is harder than manual vs. electric trim!
 
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Fore/aft oscillation continues

I hate to burst anyone's bubble, but the fore/aft oscillation continues even though my 9A front wheel spins freely & there's 30 lbs of pressure in the tire. Thanks to the custom length steel spacer I inserted between the two outer sections, you can tighten the shaft as much as you like and the wheel still spins freely.

My guess is that undulations of the surface of the runway cause a flexing of the gear leg.

nosewheelspacer.jpg


The fellow who said he lifts his front wheel off ASAP, and allows it to land as late as possible, has it right.

You can find spacers like this at any auto air conditioning shop or NAPA.
 
The basic Van's design is actually very elegant from an engineering standpoint. It has minimum weight, cost, and complexity. I would not describe it as a "real" axle. It's more of a "virtual" axle. To function as an axle, the long, small diameter, center bolt must put a lot of clamping force on the mushrooms, which then convince the bearings that they are riding on a proper axle. If the clamp load is too low, the bearings can shake, rattle, and wobble. If the clamp load is too high, the bearings can be overstressed and bind. A lot of us obviously just can't come to grips with the concept of slight bearing preload and/or slight seal drag being good things (they are). Others just don't have the tools, skills, or inclination to get the clamping force right.

In the real world, filled with real customers, the new Matco "real" axle may be the real answer. It's probably a lot more forgiving--I ordered one. Hey, it's cheap.
 
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Barry:

I'm assuming you're sitll using the Matco wheel? How did you eliminate the drag caused by the rubber seal?

I have the Grove wheel and noticed the for-aft shimmy stop after I installed it (verified by watching long shadows cast while fast taxiing late in the day).

I still have my Matco wheel with tire mounted. I may try to rig up a camera and fly both wheels to do some comparative testing with the for aft shimmy on both wheels.
 
Ah, Barry & Jamie, thanks for piercing the noise. I too have a new grove wheel and Dresser tire, statically balanced, and also have what I suspect (since I haven't filmed it) is only a small to moderate fore-aft shimmy with rotation nice and loose and breakout force very tight.
 
Springs in a Series

We tend to think of the gear leg as being the only spring in our nose gear systems. That is not really so, and because each of our experimental airplanes is different, the true spring rate varies with each of our machines. That means the natural frequency of each nose gear system is different. That explains why some nose gears will vibrate on a given surface (input) and others will not.

Some of the things which function as springs include:

Tire

Wheel

Axle/bearing

Fork and pivot

Gear leg (primary)

Gear leg to mount attachment

Engine mount

Engine mount to fuselage attachment and related structure

Some of the stuff I put on the list is extremely stiff and can normally be ignored--unless they are loose, cracked, or have wallowed out holes. The big problem here is that all these things work as springs in a series. You don't just add up the rates of each to arrive at a final system rate. You have to do some fairly simple magic math to get the real rate, which is dramatically lower. What this all means is that if you have, let's say, some combination of a slightly cracked engine mount, slightly loose gear to engine mount attachment, loose wheel bearings, and low tire pressure, your natural frequency may be down to where a few gentle ripples in the runway can make your gear do wild and crazy things.

See: http://www.physicsforums.com/showthread.php?t=97241
 
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Fitted the Matco axle

A much better method than trying to tension the bearings with the wheelpant in place.

I found it best to set the bearing pre-load with the wheel in the fork and without the pant in the way was very easy to do.

I had to drill and tap my own anti rotation hole as I already had one in the fork..No big deal.

Flew it without the pant to test for shimmy and came back and put the pant on..Job done!

Nice solution..

Frank
 
I recently installed and have flown with the Matco axle on my -7A.

I closely followed the Matco instructions, which say to tighten the collar just enough so the seals don't spin when the wheel spins. This is the pre-load setting, which I thought still had more resistance to spinning the tire by hand than I thought it might or should. The nose wheel spins a little easier than it did using the Vans "mushrooms".

The nose wheel and axle essentially are assembled as a unit so its easier to re-install them in their proper place in the nose fork. Re-installing and tightening the axle bolt through the wheel pant brackets has no effect on the pre-load setting mentioned above.

During the landing roll, I had always (300 or so landings) held the nose wheel off as long as possible, typically 40 knots or less before touching. I guess the Matco axle works better.

At least, now I don't have to contend with the "mushrooms", which I couldn't seem to adjust them well even after "staking" them and trying the various ideas mentioned on this forum. That makes for easier nose wheel maintenance.
 
Nose Gear debate

Of the incidents that resulted in failures of the strut, how many had the
wood stiffeners installed? Just curious as to if this could make a difference
in the outcome.
Walt RV-6A
 
Yep, it works well.

.....since I've done 3 landings with it and no oscillations at all. I set the pre-load with the wheel off the airplane and locked it with their screw so it can't move....big improvement.

Regards,
 
Yup same here

Takes all the guess work out of setting the preload.

No oscillations either!

Frank
 
I installed the new nose wheel axle today.
Very well built and easy to adjust.

The nose wheel turns a lot easier.
My new type fork was pushing in on the
bearings too much. Now it is out of the
equation. The axle was worth the $.

I won't get to test fly it until Saturday.

Tom
 
I tested this new axle some more today... it's a huge improvement... very noticable.. this is a must-have item IMHO...
 
Side benefits

I used to tow the plane with two hands on the towbar. With the
new Matco axle it is effortless with only one hand.

Taxi power is reduced to lowest rpm available.

Shimmy upon letting the nose gear touch down is un-noticeable and
there is zero forward lurch as wheel spins up much faster.

I'm sure that my nose gear tire is going to last longer.

This new axle is a must have option for me.
 
IowaRV9Dreamer said:
I'm glad this discussion started - great info here :D

snip
Click to expand...

Dave, this discussion has been ongoing for quite some time. Here are a couple threads for your reading pleasure:

http://www.vansairforce.com/community/showthread.php?t=23475

http://www.vansairforce.com/community/showthread.php?t=18885

Additionally, if one searches for "nose" and "gear" and "bearings" one will be busy for quite some time. The RV10 nose gear tire in a picture in one the above threads could be considered the best smoking gun IMHO.
 
Can be done with existing wheel and axle!

My solution to this problem was as follows:
  • I staked the mushrooms with countersunk screws, assuring that they can?t turn.
  • Hit the mushrooms a few times with a center punch before installing the bearings, assuring the bearings will fit quite tight and can?t turn on the mushrooms.
  • Stretched-out the fork with a car jack, just enough, so there is no pre-load from the fork on the bearings.
  • Cut off the inner lip of the rubber seal of the bearing (leaving the outer lip of the seal to seal the bearing).
  • Had the wheels balanced by a specialist mechanic shop (could not find a tire-shop that had equipment that could balance such small wheels)
  • Installed the wheel and tightened the axle bolt, so the bearing is without play but the wheel turns freely, (1/2 to a full turn, run out, when swung by hand).
I am not flying yet, but the wheel is turning without excessive friction. At this moment I see no need for a different axle or wheel. I would like a stronger nose gear strut though! I will be installing wood gear-leg stiffeners, hoping that one day, when I have a bad landing, it will just be that little bit stronger and have extra dampening force, to prevent a flip over.

Regards, Tonny
 
I think

with the new style nosewheel pant brackets you can adjust the wheel bearing preload without the pant in place.

Now if you can't (and this is the case with the old style brackets it is very difficult to get the right preload because the wheelpant is in the way....This makes it very difficult to get right.

i have been swinging wrenches for many years and even with staked mushrooms etc it is hard to get right..One time I didn't and got the oscillations.

thats was enough to convonce me to get the new axle.

Frank
 
I received & installed the new Matco assembly today.
It seems to have a more stable touch feeling to it.
Well worth the money and time to install.
Took about 1.5 hours to install.
 
Matco or Grove?

AlexPeterson said:
Dave, this discussion has been ongoing for quite some time. Here are a couple threads for your reading pleasure:
(snip) RV10 nose gear tire in a picture in one the above threads could be considered the best smoking gun IMHO.
Click to expand...

Hi Alex - thanks for the thread links. I agree with you that that the dynamic operation of the Vans seal / bearing / mushroom design is an important part of the overall problem. Likely the seal drag supports or enhances a fore / aft divergent oscillation that in some cases ends badly. I think on smooth pavement there should be little or no fore/aft oscillation, so I'm going to try for that design goal in my airplane.

So it appears that the seal drag can be reduced with either the Matco axle (small $) or the Grove nosewheel (bigger $$, but you get a nicer wheel....). The Grove wheel uses the old school felt discs (like in my Beech) which are certainly not great seals, but they do have a proven track record in certified aircraft.

The Skipper originally had the seals open to the outside, and the felt discs did seem to attract some dirt. But ever since I put hubcaps on, that has not been a problem. I think the tight fitting Vans wheelpants would serve the same purpose, so I'd have no concerns about felt disc seals. After all, you check them every year, right!

Now to decide which one... $300 to Grove or $40 to Matco....

To complicate my decision I found out that Matco will modify the standard wheel to use regular 500-5 tires for a very nominal fee (IMHO a good thing) but Van's doesn't give you much money backif you don't take the stock wheel in the finishing kit.
 
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