Bushwheel Fork and Tail Shimmy

[sierrasplitter]

I installed a Bushwheel Wide tail fork on my Pacer. Im on pavement most of the time so I really didn't need this but I had a bent fork on my stock Scott 3200 so I figured I might as well go with a wider tire to set up my plane for future dirt missions.

Anyway , I have a shimmy now. It shows up mostly as I slow down and when it starts it stays until I completely stop. I can do wheel landings only on pavement but I would like to try to get this thing to work.
Im using a 404 Glider tire at 60 psi

I have a stock Pacer spring but I had it re-arched using Gilbert's template . I did that when I had the small fork. It did not shimmy

So I had a shop remove my stock fork and install the wide fork. I also bought the rebuild kit for the Head from Alaskan Bushwheel.
Head was rebuilt and fork installed by a local shop.
Picked up the plane and started to Taxi back to my hanger and I believed the head was too tight. I could go right but would need brake to go back left. I could go left but needed brake to go right . Felt like the spring tension wasn't enough to change what ever direction the tire was traveling in.
I went back and had the shop loosen it some and felt good about the on ground operation.
Took Her up and did a 3 point and as I slowed it started to shimmy
Decided to go back to the hanger and loosen it up more.
Tested and it got worse. Was also running 45 psi at the time in the tire

Went back and tightened till the head wouldn't move then backed off so the motion felt smooth . Added more air to 60psi
Did four landings and all felt pretty good. No shimmy

Took a Buddy up yesterday and came in 3 point........shimmy.

Again the shimmy starts as I slow, maybe 25 mph and lower.

Im thinking about throwing in the towel and putting a stock fork on but thought I would post here.


Here is a pic of the Caster AngleIMG_9712.JPG

[Pacerfgoe]

To me it looks like your suffering the effects of poor tailwheel geometry.....

Check out your own post # 11 in your own thread about tailwheels

https://www.shortwingpipers.org/foru...l+castor+angle

[CamTom12]

Looks like you need more tail spring arch.

[sierrasplitter]

Yep About to order a Supercub tail spring Anyone install one on a Pacer ? Does the bracket between the Fuselage and the tail spring still fit ? Or do I need a different Bracket ?

Thanks For your help

[Gilbert Pierce]

Way too little arch. The pivot axel should be pointed aft at the top and forward at the bottom. The BW spring comes with everything you need. You throwaway the 1-1/4” spring adapter at the TW head.

[Harold Kroeker]

I think that the 404 glider tire is 11" high compared to the original 8" tire. The additional height probably changed the castor angle just enough to cause the shimmy. I was going to make the same change that you did but reports of shimmy after doing so on Backcountry Pilot convinced me not to.

[Steve Pierce]

I am not sure the new spring will solve your problem. Like Harold pointed out, the Baby Bushwheel raises the tail 3", with your smaller main taires that is a lot and don't know the the BW spring has that much more arch than your rearched spring. It is wider and stiffer so you can ditch the spacer and it comes with a new bracket for the tail post attachment.

[sierrasplitter]

What about a 4 leaf pawnee spring rather than the BW spring ? Im going with one or the other. Do I need to increase the bolt size to 7/16 dia in front ?

I forgot to mention I put 8.5's back on the mains. I had GY26's for one day. Got a hole in the tube from a defect in the tire so Im waiting for Aircraft Spruce to send a new tire/tube

[Steve Pierce]

If you are going to change the spring anyway I would use the Bushwheel spring, it will hold it's arch unlike the Pawnee spring from Univair in my experience. You still use 1/4" bolts for the rear bracket. I use an AN6-25A bolt with grade 8 washers and two AN4-16A bolts to hold the AK BW spring to the fuselage.

[Steve Pierce]

Here are some pictures comparing a 1 year old Pawnee spring from an FX3 Carbon Cub to a Alaskan Bushwheel spring. These are two bolt heads but arches are the same as the single bolt heads.

Pawnee vs AK BW Spring resize.jpg

Pawnee vs AK BW Spring2 resize.jpg