S

stepdaddy

Member
I have been addressing a shimmy issue since first flight. I have done all of the fixesI can find (breakout at 28lbs, tire pressure at 23 pounds, axle torque at 11 pounds, and all landings performed as soft field). I never drop the nose until it will no longer fly. I still have a tremendous shudder. I actually broke my nose rim this weekend. I forgot, new tires on all 3. Apparently i shimmied soo bad that the wheel flopped and folded the nose gear. I was doing 34mph at the time. I hear that these planes will have some shimmy, but come on!! I am within 1/4 inch on the main alignment which I know from car experience does not do this at lower speeds. Any help?
Daniel N914C
http://webpages.charter.net/db.kelley/my_foster_rv1.htm
 
Try balancing your nose wheel and tire as an assembly. Several years ago I had this problem on a Piper Cherokee 6. I had a new Airhawk tire with a new Cleveland wheel. When I checked the balance with a static balance tool, it took 8 each one ounce weights (the lead sticky back kind) to offset the heavy spot. Bottom line was that after the wheel and tire were balanced as an assembly my problem was solved.

Since that time I have checked many wheel assemblies. The wheel is usually pretty close to being in balance but I have never found a small airplane tire that was even close to being in balance. Lining up the red dot on the tire with the valve stem on the wheel is a good maintenance practice but it does not mean you don't need to check the balance of the wheel and tire assembly. Make sure the tire is inflated to its proper pressure prior to balance.

With no shimmy damper on the nose gear assembly, it is amazing that more people don't have this problem.

Good Luck.

Damian H. Weber Sr.
[email protected]
 
Tire Pressure?

I have never had the problem you describe with the breakout force at 23 pounds or better but I run my tires at 35 pounds or more. When I have let the breakout force get low the vibration is so bad I feel certain that I have blown the nose tire - I know that is not your problem. 28 pounds of breakout force doesn't seem too high but at what point does the castering function become impeded? My guess is you have worked your way up to this level trying to solve the problem so it is not the cause. Tire out of round or balance are possible causes. There isn't much more down there so you start thinking about the upper end of the strut and how well it is mounted. What you describe is not normal properly installed and maintained behavior.

Bob Axsom
 
I also run my nose wheel at 30-35 psi, never less. Break out force would be better at 14-20 lbs. I have found that the torque on the nose wheel bearings should be about 4 lbs. Torquing any higher seems to load the bearings too much. You really only want to remove any slop in the bearings. I have always questioned the torque Van's prescribes and I was experiencing some problems lately after repacking and retorquing my nose wheel bearings. The wheel never rolled freely after servicing the nose wheel last year during the annual. I ran it all year and it was still tight after after over 50 landings. I examined the bearings carefully and found no damage, but I still felt a strange, occasional, thump when the wheel was turned back and forth and the torque was set where Van's prescribed. I lessened the torque to 4 lbs and the thump was gone, the wheel rotated much better, and the shimmy or vibration when landing went away.

Hope this helps.

Roberta
 
Shimmies

Tire pressures, wheel bearing, and weakened dampers can cause shimmies, but only if alignment is an issue. I note that no one has addressed alignment. Toe out is by far and away the biggest culprit. When the above mentioned problems are included wheel shimmy is simply magnified.
A friend, who built a small amphip, experienced shimmy. The anidote was quickly applied. The master measument devices were two 2x4s. Result were an access of 1" toe out. Due to the construction of the feeble gears, a new spindle had to be installed. Due the wheel diameter, 3/4" toe in was applied. To date 300+ hours later, no shimmies, even with low tire pressure, or whenever wheel bearings are worn (fairly common with these little guys)
These birds also have weak landing gears, which by design are prone to shimmy, but with proper alignment, even this weakness becomes moot.
Tony Bingelis's first "How Too" book had a chapter on the subject. Note, total toe in was 1 3/4 inches.
T88
 
I think we are addressing nose wheel shimmy, which has it's own peculiar problems, usually unrelated to main gear toe alignment. Main gear toe alignment can certainly affect ground handling and even flight characteristics and can develope shimmy as well.

How does one adjust maingear toe alignment on RV-7(A) and 9(A) short of bending the gear legs? I have not found this to be an issue in my 7A. I have found incorrect tire pressure, tire/pant clearances, breakout force, and wheel bearing preload to have affect on the nose wheel performance.

Roberta
 
I'll second the balencing problem, but further, I'd say that if it takes more than 8oz of weight to balence the tire, try to reclock the tire to the tube (in 90? increments) untill it takes less weight. We had the same problem in our C177, and after two years of tracking down the problem, it turned out our tube was defective, even when reclocked, it took 12+ oz to balence the combo. New tube, problem solved. I also suggest AGAINST using airhawk (or any Aero Specialites) tires. When push comes to shove, I've found that Goodyear Flight Custom III's last over twice as long (on the mains), have a much greater skid resistance, and tend to be much easier to balence correctly. If your looking for a cheaper tire, Condors are also decent alternatives.
 
While on the question of tire brands, is there a better nose tire out there, than the one Van's provides? The mains Van's provides are not my first choice, but seem to be OK so far.

Roberta
 
Caution on the Goodyear tires

Van's documentation for my airplane RV-6A specifically says to avoid these tires because of sidewall clearance..."The suggested tire style is the common round tread variety rather than the "wide tread" premium grade tires. This is because of the close fit of the wheel fairings. The "wide tread" tires offer little sidewall clearance at the wheel fairing opening, and will rub when at low pressures and on rough surfaces". The same section (10) in my manual says to run an minimum practical pressure to reduce shimmy problems which conflicts with the way I run my tires. You may not like this note that I found in my manual: "...wheel shimmy is a rather nebulous thing; it occurs on some airplanes and not on others, and with varying degrees of severity." On my plane I run the tires at 35lbs. upon inflation but they do go down in a few weeks to 20 lbs. so I actually operate over a broad range - I like them best when first inflated. The only time I experience the shimmy problem is when the breakout force gets below 20 lbs. but as the manual says they are all different. Good Luck - it sounds like you are going to need it.

Bob Axsom
 
Nose wheel shimmy

I beg to differ with you that nose wheel shimmy is not entirely unrelated to mains. Misaligned mains can induce shimmy in the nose wheels. What set RVs apart is the fact that they are self castering nose wheels, which in itself is fairly easy to cure. Tires have to be extermely out of proportion as well as balance to affect self castering wheels. However, a self castering nose wheel, with marginal installations or conditions can and will at some point be induced to shimmy with misaligned mains.
The aircraft taxis on 3 wheel be it nose or tail dragger. It's surprising that all three are not, at least being inspected as a group rather than individually.
 
Tacchi88,

I believe I stated "usually unrelated" not "entirely unrelated". Sorry for the misunderstanding. Can you shed some lite on how toe is adjusted on RV type AC? Maybe this would help us all.

Roberta
 
Apology

Miss Roberta,
No apology necessary. I assure you, your comments were/are well received. Your contribution as well as those of others, are valuable. Agree or not, they spark the thought process, and in the end, we all benefit.
Thanks,
T88
 
Alignment

Miss Roberta,
Sorry, I did not read the second portion. I'm not certain how alignment can be adjusted with exception of perhapd the -8 which has the spring legs, I believe.
On my -10, I don't see how it's possible to alter it, if necessary, due to it's design and method for mounting. I had the fuselage on legs with "homemade " wheel for ease of movement, and I must admit, that I hope the holes at the gear mounts are in the proper place.
I built a Midget Mustang and a Coot Amphip. In both cases, the gear legs were bent before heat treating, if so requires. Both plane used flat spring legs and final adjustments ( they do "move after heat treating) were much like a Cessna, by using shims (tapered) in order to achieve toe in.
In any gear leg geometry, toe in is reqired, this is so due to natural forces which tend to "push" the gear rearward as the AC moves forward. Since we have "long" legs and tiny wheel, say as comparison to an automobile, movement (toe in/toe out) is greatly increased. Worse, should the leg it self be loose in it's fittings.
Automobiles all have toe in for the same reason. Toe out if found in some from wheel autos due to still another natural action, of the wheels wanting to turn into each other.
What ever the case may be, steering geometry can affect the "shimmy" in some way or another, particularly on ACs that have nose gears mounted directly onto motor mounnt, which is just about every nose geared AC. On TDs, I experienced wobbled on an other wise rigidly constructed main gear. It turned out that the bungees were worn, and the gears having loose movements in the gear leg slots. The tail wheel had loosened (46 T craft), causing the tail wheel to flop from side to side. It was not noticeable on grass, but on pavement, it was an eye opener (saucer pan eyes). I think we were sort of lucky to find both problems, since we decided at the onset, that due to the age, we should "check every litle thing".
After the fix, the T craft became even more responsive on grass. where as it was always "sloppy", but not so noticable as to cause the airframe to wobble.
I'm hoping that I will not have to face an alignment problem on the -10. I'm confident that Vans record of kit making is true to code.
BTW, we had a couple of experiences with a Piper and a Cessna (fixed gears). Both had cracked motor mounts, and transfered to the nose wheel which in turn "wiggled/shimmied"..In both cases, lots of time spent on the nose wheel. Finally, it was the "CELT", check every little thing, and found cracks on the mounts. Mounts fixed, shimmy gone.
T88
 
Thanks, T88. We all are doing our thing to help. I was afraid I may have stepped on your toes (no pun intended). Happy building.

You are very correct in pointing out the importance of main wheel alignment. I also built a seaplane (SeaRey) that was very dependent on perfect wheel alignment. Main wheel alignment, on 7s and 9s (and maybe 6s) is probably overlooked as a nose wheel shimmy inducer due to the inability to do any correction, short of bending. It can be checked and balancing should also be considered.

Most of what we nose draggers have been experiencing is the vibration, chatter (whatever) that occurrs after the plane is slowed and the nosewheel contacts terra firma. I guess that is why we focus on the nose gear itself.

I can only relay what I have personnally experienced and what I have done to correct it. I do not care to speculate.

I have had a number of people comment how stable my main gear legs are and that they do not exhibit the shimmy and wobble many are seen to do. I did use the wooden dampers and I balanced my wheel pants. I did not need to balance the wheels. They seem to b OK.

Roberta
 
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Shimmies

Miss Roberta,
I have a little experience with Seareys, in the alignment and landing gear department. They are neat little planes, but built like a UL. I reenforced the gear leags, made bushings for the legs, etc. all steel and believe it or not lost weight.
BTW, I made up the same mod for the fellow that bought yours. You were serious about braking.
The only self castering wheel I've had contact with is the Grumman Tiger. They are generally stable, but most complaints were from the wheel mount loosening in the leg it self. If it was "loose " without the benifit of any dampening, with the right pavement, it could begin to rumbha.
Looking at my -10. I can see that if the bushing is not closely fit, and the damper springs aren't doing their job, it could easily lead to wiggles.
Self castering wheel are simple, but a trip to the super market, will show that when the carts are new and "solid", they work well. When they've been banged around, they wiggle.
I would look into making sure that there is no play in the mounting, that is bushing is a close fit. Loose emough to turn, tight enough so as not to "flop" around. Dampening is the other issue. Dampers should move from side to side as if there is to change in drag. If for example, when moving to and fro, there is a "free" spot, there would be no dampening. If the fork bush is sloppy, tires out of round, bearing not properly preloaded, low pressure can induce shimmies.
As a result of this thread, I assure you that my nose gear will be carefully addressed.
When I was building the Coot, it's nose gear was subject to shimmy if not correctly dampened. This was well outline in the building instructions. I recall I purchased the leg material from Van's leg maker, Harmon Lange.
BTW, My compliments on your old Searey.
T88
 
Thanks

Thank-you for all the input. I am a little confused though. I was under the impression that low tire pressure was necessary to control shimmy in the nose, but now im hearing 35psi without problems. I just want to clarify that this number is in referance to the nose gear (as well as the mains). With the balance issue, how and where do you do this? these wheels dont fit on auto equipment? I would like to balance the new wheel when it arrives, just dont know how. Damper springs, on the 6A? Finally, nose strut torque issues. the smallest amount of lateral loading on the nose causes the strut to roll/twist. I didnt know how bad this was until I cross wind taxied after paint, and some of the glazing compound showed radial cracking through the new paint. Is this rolling typical or might I have an issue with the leg?
Dan
N914C
 
Tire pressures

The effects of low tire pressures are high wear, low tire life, possible damage to wheels and/or gears. In addition, low pressures help raise tire temperatures, which in turn accelerate wear and shorten life. Low pressures also cause movement of the inner tubes, which can tear a valve stem off the tube. Piper Cubs with 600X4 tires are notorious for this little bit of excitement. They typically run on 15 to 20#, and with a small rim size are ripe for the tire and tube sliding, which in turn can rip the stem.
However, there are cases where low pressures are warranted, but always at the risk of short tire life. Low pressures on AC, cars, motorcycles, etc, can cause "wiggles", which in turn affects gear /suspensiopn life as well as the tires well being. Considering tire sizes, for aircrafts for the weight and speed they must handle, low pressures are not the best situation.
In the case of Roberta's old Searey, as well as my buddy's, these little guys have no suspension of whatsoever, therefore, the wheels/tires are chosen to perform the task of suspension. These planes gross at around 1200+#, and for their sizes land on the fast side, certainly faster than most RVs. Now we have two problems, high speed and low pressure. Our Searey with just a tad over 300 hrs is on it's second set of tires.
In the auto world, where at one time low pressures were in vogue are now on the average of 32 pounds and up. Use low pressures, and tire life is vastly shortened, not to mention the risk of tire failure or some of the "bad habits" they can induce. such malidies as shimmies.
T88
 
More on pressures

Dan,
Low tire pressures will induce shimmy, and this is true with your auto or motorcycle, and most anything that runs above 25mph. The 35# number for 500x5 is indeed a good number, and no lower than 30. I plan to run 35 on my -10, and most likely the same on the main. On my old Arrow and most GAs with 500X5 the 35 number seems to give best tire life and least wiggles. On the mains about the same, at least more than 30. most GA mains will be 600X6.
On the balance issue, and I know I'll get static on this, it's not all that critical. I make a simple spin test, and is the mounted wheel stops at different points without rocking back and forth, it's good enough. In more than 22 years of AC owner ship, as well as servicing, I rarely had to "balance" any tire. Pressure, however, is an issue, and never below 30, except for the Searey without any suspesion.
If balancing is indeed required a simple, cheap bubble balancer is all that is required. Another simple balancer can be had from go kart shops. Stick on type weights will be necessary.
We have three 6s in our area and all are TD. I'm the only RV in our group with the training wheel on the nose, and I'm not flying it yet. With experience in the 6, I will say, that as I understand it, it's similar in design as the 10. The ten had 2 bellville washers, springs, if you will. These are pre loaded when asembled, and used as a damper.
Your suspicions about the small lateral movement may be wise to inspect closer. The gear leg as an assembly, should move up and down only. Should it move laterally, it can make for some wiggles. If nose wheel bearing are a tad on the loose side (not abnormal) or a tad lose wheel fork, add all three and even low tire pressure in the mix, and you'll have wiggles. Let us know what you find.
T88
 
It's interesting that the manual recommends against the 'wide tread' style. I have found that Goodyear's sidewalls are so much more stiff than Aero Specialites that the at the same pressure the Goodyears hold their shape better, not worse. Of course, how you cut out your fairings could solve any problems at all. Other options include Flight Special II's or the Condors which are both round tread. I don't see how an out-of-balance condition (if it is bad) will affect the castoring nosewheel any less than conventional nosewheels.
 
I run 25-28 lbs. front fork breakout force. When it falls below 20, I can get shimmy. This can be very severe and scary. I run 30-35 psi front and 25-30 psi on the mains. They leak down so I have to pump them up about once per month.

When I built my 6A I had a serious sympathetic vibration in the main gear legs at as low a 1 knot, just pushing the aircraft. On the fast taxi tests at about 40 knots, all of a sudden the whole aircraft shook violently and I thought it was going to tear itself apart. Lowering the tire pressure slightly improved the situation, raising made it worse. What followed to 95% eliminate this was experimentation with aluminum and wooden dampener strips taped to the main gear legs. I tried about 6 different setups until this almost completely disappeared and flew for about 75 hours, no problems.

After a forced landing bent the original gear legs, I replaced them and carefully aligned with 0 toe. I tried taxiing without any dampeners and have never had the same harmonic or any main gear problems again. I can only think that the factory did not align the main legs well when they drilled them (QB) and I had some toe on them but I never checked this originally when I assembled the aircraft. This was a vexxing problem.

I have to tighten the nose gear nut about once a year to maintain the breakout force. This is extremely important. I'd make this easy on yourself and install a ring to engage your fish scale on, sandbag the tail to get the tire off the ground and pull test this. There is no problem ground handling with 25-28 lbs. breakout force.
 
osxuser said:
I'll second the balencing problem, but further, I'd say that if it takes more than 8oz of weight to balence the tire, try to reclock the tire to the tube (in 90? increments) untill it takes less weight. We had the same problem in our C177, and after two years of tracking down the problem, it turned out our tube was defective, even when reclocked, it took 12+ oz to balence the combo. New tube, problem solved. I also suggest AGAINST using airhawk (or any Aero Specialites) tires. When push comes to shove, I've found that Goodyear Flight Custom III's last over twice as long (on the mains), have a much greater skid resistance, and tend to be much easier to balence correctly. If your looking for a cheaper tire, Condors are also decent alternatives.
Click to expand...

You are exactly right about the brand. I purchased the Airhawk because it was cheaper. Since then I have only used Goodyear tires. My theory is to try things before critisizing them. In that case I tried it and after I balanced it I ran the tire until it wore out. I replaced it with a Goodyear tire and I needed a little weight but not much.

Damian H. Weber Sr.
[email protected]
 
I be runnin GoodYear FC III's on my 6A with no problems. 45 psi in the mains. I run the cheapo Van's sells for the nosewheel. This is my first new set of tires since I wore out the orignals 5 years ago. I usually run FC II retreads from Desser but peer presure got me to try the FC II's. At 106 bucks a piece I don't think their worth it. I would like to find a replacement for the nose but have yet to do so. I do have a shake at 22.3 mph if I taxi to fast and while I decelerate from landing. Nothing major though and it never happens on takeoff.

Bob Axsom said:
Van's documentation for my airplane RV-6A specifically says to avoid these tires because of sidewall clearance..."The suggested tire style is the common round tread variety rather than the "wide tread" premium grade tires. This is because of the close fit of the wheel fairings. The "wide tread" tires offer little sidewall clearance at the wheel fairing opening, and will rub when at low pressures and on rough surfaces". The same section (10) in my manual says to run an minimum practical pressure to reduce shimmy problems which conflicts with the way I run my tires. You may not like this note that I found in my manual: "...wheel shimmy is a rather nebulous thing; it occurs on some airplanes and not on others, and with varying degrees of severity."

Bob Axsom
Click to expand...
 
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