Bushwheel Fork and Tail Shimmy

[sierrasplitter]

Thanks. Those look good. I think I have it , but went out to test fly Tuesday and had an issue of excessive rpm drop on my Electroaire Electronic ignition . Im working 12 hour days so I cant get back to it until Monday.

[Jeff Pralle]

They have always been heat treated. There is a stamped "HT" on the steering arm indicating so.

I have always seen the stamp on the old scotts and the Univairs. I have one out of a couple year old bushwheel I replaced recently I will try to post a pic tomorrow as an example. It has no stamp.

lately I have had good luck with ABW steering arms. So something changed...

[Jeff Pralle]

I took a very close look at this spring when I tore mine down. I can see a mark on each side that looks like it was stamped into the spring or a"notch" where the pawl would hit. It doesnt look like wear as it seems the be perfectly square and even on both sides. Can someone verify they are straight across on the edges or am I seeing a dent
Sorry, I wish I had taken a picture of the new one
I do have a pic of the one taken off the smaller stock 3200 fork that I replaced. It didnt shimmy
The indention can be see on this one between those wear marks

Attachment 17195

This is a wear mark. Look for pics tomorrow I have a new in package ABW spring I can photograph as an example.

i see these wear patterns develop quickly when there is a shimmy. I recently partially rebuilt an old Gar Aero wide fork with new parts put it on my pacer had a shimmy only made 3 landings took it off and had a notch on one side. Replaced ALL the internals rearched the spring and no more shimmy. Tail spring was 3 years old and about 250 hours a lot of off airport heavy ops on 31s with a Scott 3200 and on skis with a tail ski.

[Jeff Pralle]

They have always been heat treated. There is a stamped "HT" on the steering arm indicating so.

Here is a photo of a steering arm I removed from a ABW BBW assembly recently. No stamp HT. I have seen several like this glad to say it has changed abd the newer arms are lasting longer.

9934AA0E-53E4-4368-BA13-41E63EA00C24.jpegimage.jpgimage.jpg

[Jeff Pralle]

I took a very close look at this spring when I tore mine down. I can see a mark on each side that looks like it was stamped into the spring or a"notch" where the pawl would hit. It doesnt look like wear as it seems the be perfectly square and even on both sides. Can someone verify they are straight across on the edges or am I seeing a dent
Sorry, I wish I had taken a picture of the new one
I do have a pic of the one taken off the smaller stock 3200 fork that I replaced. It didnt shimmy
The indention can be see on this one between those wear marks

Attachment 17195

Here is a brand new ABW compression spring for comparison.

image.jpgimage.jpg

[sierrasplitter]

I FIXED IT ! Thanks for everyone’s input. Man I learned a lot about the Scott3200. No parts were broken. All springs are good. I’m running with 3 springs in my head and a 404 tire with the Bushwheel fork
My main issue ? Kingpin was too loose. Yes you can induce shimmy via the kingpin. If you have it too loose the springs in the head do not compress enough to lock the head on landing
Key Points if you have shimmy
1.Caster Angle. Per Scott - minimum 5 degrees positive unloaded and a minimum of 0 degrees fully loaded
2 Kingpin tightness - tighten until fork will not move then back
off 1/6 of a turn. If cotter pin does not line up try to go n the tight side
3. Tire roundness and inflation -I’m at 40 psi
For me 3 springs in the head work 5 will make it tighter - better for heavy tailed aircraft like a Cessna 180.
Pawl- another personal preference. I like the long one

That sums up what I know. Again Stoked It’s fixed and thanks to the experts on this thread.

[Wag-builder]

Glad you got it fixed and we all learned something along the way.

Good job!

[Gilbert Pierce]

Found a good note this weekend while searching the web

From Scott

DATE: 10-24-1950 S.N.L.
SUBJECT: Scott 3200 Tailwheel Shimmy Adjustment

"We have had some reports on Scott pneumatic tailwheels shimmying. The Scott 8" pneumatic tailwheel Is designed with a friction type shimmy dampenener. This friction is regulated by 3 small compression springs inside the upper casting. If the main king pin nut becomes too loose the shimmy dampener effect will be lost. When the shimmy occurs, it is recommended that the large nut be tightened in increments of 1/6 of a turn until the tailwheel starts to bind when rotated by hand. Then back off 1/6 o[ a turn for correct tension oil the shimmy dampener, The king pin nut is on the underneath aide of the lower housing directly above the tire. This procedure will definitely assure that compression springs in the dampener are acting again the friction dampener disc. We recommend a tire pressure of 30 pounds on both the 140 and 170 for the 8" pneumatic tailwheel. When connecting the springs and chains on this assembly, it is recommended that the stretch in the springs be from 1/8 to 1/4". The springs used with the 3200 assembly have a high tension rate and if stretched to any extent on assembly the mechanism may not release easily. No slack should be left in the chain and spring hook-up when both sides are connected."



Im going to start with 3 compression springs as the note above states

I also think my issue was from the kingpin being too loose

sierrasplitter
Can you post that Scott Service Bulletin?

[sierrasplitter]

I will see if I can find it again. It was posted at the Cessna 140 /120 forums. Let me dig around

[sierrasplitter]

Here is what I found

Re: Scott 3200 Tailwheel Shimmy Adjustment

by jezzicaz789 » Fri Jan 29, 2010 1:33 pm

N1478D wrote:My tailwheel shimmy went away when I replaced the old tire and tube with new ones. However, for those that might still have the problem, I found this in the Cessna Service Information Summary ( 1946 thru 1962 )
Date: 10-24-50
"We have had some reports on Scott pneumatic tailwheels shimmying. The Scott 8" pneumatic tailwheel is designed with a friction type shimmy dampenenr. This friction is regulated by 3 small compression springs inside the upper casting. If the main king pin nut becomes too loose the shimmy dampener effect will be lost. When the shimmy occurs, it is recommended that the large nut be tightened in increments of 1/6 of a turn until the tailwheel starts to bind when rotated by hand. Then back off 1/6 of a turn for correct tension on the shimmy dampener. The king pin nut is on the underneath side of the lower housing directly above the tire. This procedure will definitely assure that compression springs in the dampener are acting against the friction dampener disc. We recommend a tire pressure of 30 pounds on both the 140 and 170 for the 8" pneumatic tailwheel. When connecting the springs and chains on this assembly, it is recommended that the stretch in the springs be from 1/8 to 1/4". The springs used with the 3200 assembly have a high tension rate and if stretched to any extent on assembly the mechanism may not release easily. No slack should be left in the chain and spring hook-up when both sides are connected."

Thanks you for the post.
Hi guys, Im a newbie. Nice to join this forum.


Its on the Cessna International Forums.