shimmy, and more!

[stepdaddy]

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

[dhweber1]

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.
dhweber1@theairplanegarage.com

[Bob Axsom]

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

[robertahegy]

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

[tacchi88]

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

[robertahegy]

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

[osxuser]

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.

[robertahegy]

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

[Bob Axsom]

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

[tacchi88]

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.