Tailwheel Shimmy

Robert Eilers · Post by **Robert Eilers** » Fri Sep 15, 2006 3:57 am

When I purchased my 170, the previous owner recommended 29 PSI for the tailwheel. Someone on this forum recommended 34 PSI for the tailwheel. At 29 PSI I get little, or no, tailwheel shimmy. At 34 I get consistent tailwheel shimmy. Is it possible that tailwhell shimmy can be related to tire pressure? Has anyone experienced anything similar regarding tailwheel tire pressures?

Post by **Bruce Fenstermacher** » Fri Sep 15, 2006 1:45 pm

Robert I'd say that is entirely possible. It seems to me that tail wheel shimmy can be a delicate balance of several factors.

Two of the most important are the angle of the king pin and the friction internally caused by the springs and how tight the nut is on the king pin.

But I'm sure if other factors are on the edge then changing the tire pressure which would change the rolling resistance and bounce of the tire could cause shimmy.

Some people recommend heavily greasing the assembly while others practically run it dry. Again it's a balance thing and they don't want to change what has worked for them with this seemingly mysterious part.

I can't recall the official tail wheel tire pressure and never can which is why I just pumped mine up to 40 lb. which "looked" right. I figure eventually it will leak to the correct pressure what ever it is.

Anyway I didn't have trouble at the 25 lb. it was or at the 40 lb. it is now.

Assuming you believe your tail wheel assembly to be in good order and the king pin to be at the correct angle then I'd try tightening the king pin nut 1 or 2 flats and see it the shimmy goes away.

Like so many subjects this one has been thoroughly discussed on other threads and seem to recall perhaps even links or pictures to describe the correct king pin angle so a search would be in order.

Just did a search on TAILWHEEL SHIMMY which will being up several old threads on this subject. One of them has a link to an of forum white paper on the subject which has great pictures of the geometry, here is the link to that page: http://merlin.alleg.edu/group/eaa1194/graphics/tws.jpg

Post by **GAHorn** » Fri Sep 15, 2006 6:27 pm

Both Cessna and Scott address tail wheel tire pressure. Cessna recommends 34 psi for the 170, and that lies within the graph of weight versus tire-press. recommended by Scott as well.
One of the threads here that shows the same relationship/angle of caster is:
http://cessna170.org/phpBB2/viewtopic.p ... =tailwheel

1SeventyZ · Post by **1SeventyZ** » Sun Sep 17, 2006 7:16 am

Lower tailwheel pressure can lead to pinchflats in the tailwheel tire, or so I have heard. Lower pressure is probably dampening the oscillation in your shimmy by adding more surface area and friction, making it less noticeable.

I can't buy into the theory of under-greasing the tailwheel pivot. It's a pivoting part and needs lube, so I think running it dry is a bandaid fix. I do buy into the geometry theory though (more can be found on this at the link George posted above.)

Post by **GAHorn** » Sun Sep 17, 2006 11:10 pm

At issue for some owners, is tailwheel mainspring shape. New leaf mainsprings have bends in them that some owners have attempted to re-bend to some other conformation they've imagined to be better.
The mainspring is purposefully made with those two bends in it. It is tempered steel and should not be heate and/or bent. Install it as you received it from the mfr. It provides the correct caster.

Robert Eilers · Post by **Robert Eilers** » Mon Sep 18, 2006 12:37 am

I have finally gotten around to replacing the tailwheel main spring. Not the simple remove and replace job I was anticipating. The old tailwheel spring bundle was securely held in the tailwheel housing (an uneducated discription). I have the later model tailwheel housing on my 1952 C170B. After partially dismantling the tailwheel housing (at least the upper bolts) I was able to wiggle the tailwheel bundle out of the housing. I noticed, while working on removing the tailwheel spring, that where the main tailwheel sping is bolted to the tailwheel itself (I believe it is an old Scott), the main spring is not as wide as the slot, designed to accept it. As a result, the tailwheel pivotes approximately 1/4 to 1/2 inche either side - may be the cause of my tailwheel shimmy. I manufactured a sheetmetal insert to take up the slack on either side of the tailwheel spring. Reinstalling the tailwheel bundle looks like it may be a challenge. I will probably begin with the main spring and insert each subsequent spring on top of the main spring alligning the holes as I go - I will probably have to tap the top short spring in using a wood dowl. Then it appears as though I will have to compress the spring bundle again to the point where I can reinstall the tailwheel housing bolts, etc.

blueldr · Post by **blueldr** » Mon Sep 18, 2006 3:18 am

Robert Eilers,

From your description of your tailwheel and spring assembly it appears you are missing the spacer that reduces the 1 1/2" channel in the tailwheel casting to accomodate the 1 1/4" spring width. This spacer should be of a thickness that provides a tight fit of the spring in the casting. Any looseness will cause wear in the aluminum casting and eventually render the expensive casting unairworthy.

c170b53 · Post by **c170b53** » Mon Sep 18, 2006 3:33 am

Robert I think I joined you for lunch along with Jim R in Mabel Lake, really liked your plane.
When you removed your spring set was there a bushing at the forward end?
When I replaced my spring set the forward bolt went though a bushing that went through all the springs. It had not been looked at for sometime and the bolt looked like my index finger when its relaxed. I think that the bent bolt was due to the missing shim thats at the top of the spring set and general wear of the bushing and spring holes. IPC Ref figure 29 item #10 p/n 0510000-30. Just curious what you found.

Robert Eilers · Post by **Robert Eilers** » Mon Sep 18, 2006 4:26 am

Yes the spacer was there - the tailwheel was able to pivot from side to side inspite of the spacer. Perhaps it has worn over the years or been spread by the tailwheel spring - perhaps it is the wrong spacer. I manufactured a sheet metal replica of the spacer and placed it between the tailwheel and the tailwheel main spring. The sheet metal spacer closes the gap between the main spring and the original spacer for a snug fit.

There also was a bushing at the head of the spring bundle - which I forgot to mention. The bushing is in good shape and will require that I reinstall all the springs together rather than one at a time.

Robert Eilers · Post by **Robert Eilers** » Mon Sep 18, 2006 4:31 am

c170b53,

I remember the lunch at Mabel Lake and your 170 - very nice restoration as I remember. The flights out to Mable Lake were the highlight of the trip for me.

Robert Eilers · Post by **Robert Eilers** » Tue Sep 19, 2006 11:44 pm

I got the tailwheel back together today - quit a relief - I sometimes fear I am going to make things worse by trying to improve them myself. As a result, I learned a great deal about how the tail "system" is constructed. I also took advice I read here on the forum and tightened the King nut a bit while I had the tailwheel off. I also discovered the right rudder return spring is either broken or sprung to the point of becoming ineffective. I pulled the inspection plate on the belly and could see the right hand rudder return spring at the rudder end - the rest of it is out of sight. It appears to me that replacing the return spring is no easy job - is there a secret to doing it?

Bob

blueldr · Post by **blueldr** » Wed Sep 20, 2006 3:35 am

Bob,

You lost me a little bit when you mentioned the rudder return spring and "the rudder end". Those springs are below the rudder pedals and under the floor, running between the rudder pedal linkage and the firewall.
They are most easily replaced by disconnecting the rudder cables at the rudder horn so as to facilitate pushing the rudder pedals full forward. Some people work through the inspection plate on the belly, and some work through the lightening hole in the forward end of the tunnel. in my opinion, either way is a SOB!
If you disconnect the rudder cables at the rudder horn, be sure you attach a length of safety wire to the cables to facilitate pulling them back out.
The springs are relatively inexpensive and I personally would replace both of them at the same time to possibly preclude uneven tension between old and new springs.
Good lucK!

Post by **GAHorn** » Wed Sep 20, 2006 3:44 am

blueldr wrote:Bob,

You lost me a little bit when you mentioned the rudder return spring and "the rudder end". Those springs are below the rudder pedals and under the floor, running between the rudder pedal linkage and the firewall.
They are most easily replaced by disconnecting the rudder cables at the rudder horn so as to facilitate pushing the rudder pedals full forward. Some people work through the inspection plate on the belly, and some work through the lightening hole in the forward end of the tunnel. in my opinion, either way is a SOB!
If you disconnect the rudder cables at the rudder horn, be sure you attach a length of safety wire to the cables to facilitate pulling them back out.
The springs are relatively inexpensive and I personally would replace both of them at the same time to possibly preclude uneven tension between old and new springs.
Good lucK!

Will Pep-Boys carburetor-acelerator springs work?

Robert Eilers · Post by **Robert Eilers** » Sun May 20, 2007 12:47 am

Thanks to all the good advice I have been working throuhg the possible causes of my tailwheel shimmy. I have check the alignment of the King nut, tightened up my tailwheel chains, and double checked the play between my wheel and the forks, increased tire pressure. I still get a shimmy every third or fourth landing - no rhyme or reason other than possibly speed upon touch down. I considered replacing the entire tailwheel assembly with the Alaska Bush wheel. However, after looking at the replacement cost closely, I discovereed the fork, wheel and tire are not included in the $975 tailwheel Assembly price quoted by Spruce. Complete replacement would be more like $1800. So, rebuilding what I have is sounding better. I had the tail up today and noticed that with the weight off of the tailwheel there is some play between the fork and the assembly - about 1/2 inch either way. I suspect this may be the problem. Is there a guage as to how much play there should be between the fork and the assembly - when the tail is up I can wiggle the fork back & fourth.

Mike Smith · Post by **Mike Smith** » Sun May 20, 2007 1:24 am

The only thing I didn't read that you might have missed is the angle of the tailwheel (re: the picture above) plane of rotation. When I replaced my tailwheel leaf spring it fixed the shimmy on my 1950 170A. Even though you have an issue with the looseness in the fork, you may still get a shimmey if the angle (due to a worn spring) is bad.

Good Luck,