Patrol Change in Wheel to Wheel Distance

[Steve W]

During the first 30 hours, I have noticed the gear has spread somewhat more than it was initially. After a conversation with Bob, the required distance center to center empty is 68”. I measured yesterday and with half fuel, the wheels were 72” apart.
My question is does anyone know what length of shortening the gear leg rod end will move the wheel in 1”? This would be helpful in knowing whether or not I have enough threads remaining on the rod end.
Thanks
Steve W P203

[spinningwrench]

I just finished setting my gear leg distance. I don’t have an exact number for you but a light fuselage ( no engine) about 9 turns in on the rod bearings gave me the 68”. It took very few turns to move the gear out to 72” so this is a pretty sensitive adjustment. If you are good at trigonometry measure the triangle of your gear leg and you could quickly calculate the gear spread change based on the rod thread pitch. I could probably do this for you over the next few days.

[Steve W]

I set the adjustment in the middle of the threads in relation to the specs laid out when I welded up the gear. I’m just not sure if I have enough distance left on the rod end to bring the wheels in 1-11/2” a side.
I’ll guarantee your shocks will be out from the fully retracted position with the engine and wings on.
If I need more room to adjust to the proper length/width then I might have to shorten the piston shaft. I’m just trying to gather information before I get too involved in lifting the plane and adjusting the shock length only to discover there isn’t enough room. The plane is an hour away plus I’d be looking for a warmer day!
Thanks

[Ed Welfred]

I do not know if I am remembering correctly but it seems to me that each tread will move the spindle about .170 " therefore 9 threads would move the spindle about 1.5 inches. It has been quite a while since I worked on my gear and my memory may not be correct so hopefully someone has these numbers written down and will post them. Ed

[Steve W]

Thanks Ed. I measured 9 threads on a 7/16” fine bolt and it was roughly 7/16” long. I might have that available for adjustment but will see. Thanks again

[Bcone1381]

My Patrol Plans shows 72" with the shock struts extended 1.25". Can you measure how much your strut is extended to get your 72"

Secondly.....I think (Key word is "think') you are saying that if the shock strut is fully retracted then the gear sits at 68".

If so, then its seems to me like the gear might spread out four inches with 1.25" strut extension. Using Higher Mathmatics that I learned in the third grade it MIGHT mean if the strut is unscrewed .325" it would move increases the spread by 1 inch.

Comment or correct my thoughts please. This is written in more of a brainstorm/ "Is this right?" thought process that needs validation as opposed to "Fact and good data".

Screen Shot 2020-12-24 at 8.56.49 AM.png

[Mark Goldberg]

68" to 69" spread at very light weights - widening to 72" at normal flying weights - to 74" (not to exceed) at max at gross. Mark

[Steve W]

Thanks to Brooks and Mark.
I think I have found the problem. The plans call for 1 1/4" of piston shaft "extension" when setting up the gear. When I did the set up, I'm quite sure I had only 1 1/4" of shaft protruding beyond the cylinder cap. I should have added the 1 1/4" to the length of the exposed shaft when it was bottomed out. Hopefully I have the adjustment to shorten the rod end by the static length of the piston rod from the cylinder cap.

[spinningwrench]

ert.jpg

I went and measured my Patrol today and did a quick CAD drawing as I have been wondering about the correct number of turns of the shock strut rod end bearings to change the landing gear track. Here are the results I got.

[Steve W]

Thanks for the information and time you took to discover the numbers. I have to lift a floor panel to get at the rod ends to see what threads are left. I went through old construction photos today hoping to find a clear view of the top of the shocks. It may be spring before I’m able to get at it. If I don’t have enough threads left, I think the simplest fix would be to dismantle the shocks and shorten the piston shafts.

[spinningwrench]

Minimum thread engagement I would be comfortable with is 1/2” or basically the rod end diameter although in theory 80% of the thread strength is obtained in the first three threads.

[Mark Goldberg]

Bob has told me that indeed 1/2" thread engagement (on the 1/2" diameter rod end shaft) is the minimum acceptable. I like to see 5/8" or a little more than the minimum. Mark

[Ed Welfred]

I was concerned that posting information from memory only, could lead to misinformation, so I did a check of spindle numbers for this post and for my own records. The plans do not state a specific length for the gear legs and the shock struts. These items are made to fit given other parameters, therefor other people’s build numbers may differ from mine but should be close.

I used some scrap material to build a mockup of my gear. At the top of the shock I drilled three mounting holes, the centre hole being my shock length and the other holes being offset one inch. I choose 1” because the rod end has 20 threads per inch.
Because of the geometry there are some things to note;

• The spindle movement is more than the actual tire movement in or out.
• The line that represents the spindle line on what would be the spindle weldment changes angle so the camber on the wheel will change accordingly.
• The spindle arc length and tire movement increases as the shock strut moves closer to vertical.

The first three pictures show the gear and shock at my base setting. The next three pictures show the shock length shortened 1” and the components swiveled according the change in shock length.

[Steve W]

Wow! Thanks very much for the workup Ed. I have been mulling this over in my head for the last three days and will have to make a trip up to the hangar to see what’s left for thread length. I will have to drop my remaining fuel to get a semi accurate empty condition to check the distances. I wish I had a picture or a number for the distance that the piston shaft protrudes beyond the cap when fully retracted. I have placed a small order to ensure I have material on hand in case I have to actually shorten the lower end of the strut itself. Brutal but maybe the only way to solve the problem.
I am very surprised with the amount of movement with one inch of length change in the setup. Fingers crossed.
Just another thought suggested to me by a fellow builder, any negative thoughts on a 1/4” to 3/8” spacer inside between the top of the spring and the cap?

[Ed Welfred]

I don't know Steve. It may depend on how much crush there is in the spring. You probably put the rod end threads in the middle so there is a good chance that you can get the adjustment that you need with the threads.. I would think it will be a major pain to lift, adjust, let the plane down then roll it to see the results and then maybe do it all over again. Hopefully these numbers will help you do the job once and be right on. Ed

[spinningwrench]

The angle of the gear relative to the ground works out to 37.4 degrees at a track of 68". You can simply use the cosine of this to get the change of gear track per change of shock strut length. Multiply this by two, one for each gear leg to get the change of track. Doing the math an initial check is that you need to shorten each shock strut by 5/8" to get a 1" narrower gear track.

[Steve W]

OK, so I went to the hangar and dropped the fuel from the Patrol. Instead of using the greased pad thing, I used two small sheets if 6mil plastic with dish soap between the layers under one wheel on plywood and centered the stance at empty weight. Not so messy. Distance was 70.5" which is too much. I have 6-7 threads remaining for "in" adjustment on the rod ends and did use a thin locking nut.
I called Bob yesterday and had a great conversation; as usual with Bob. So informative and knowledgeable. I asked him about placing a spacer inside the shock between the top of the spring and the cap to reduce the distance. He didn't like the idea and it was really interesting to hear the reasoning why. He wants the hydraulic shock to take the initial load before there is a heavier loading pressure on the spring. The shock is bottomed out in flight and basically unloaded. The initial contact with the ground on landing is to be against the hydraulic fluid before loading the spring. If there was a spacer introduced against the spring, then there would be a greater preload on the spring with more force acting on the assembly.
Initially, Bob suggested only to cut and shorten the bottom of the strut to get the distance I needed. I asked if Avipro shocks were identical to his plans and said yes but they don't reduce the inner diameter of the piston shaft beyond the threaded end. I have a scratch built fuselage and the Avipro shocks. I suggested shortening the piston shaft and continue the threads to get enough for the rod end to be properly positioned for strength. He agreed and said that was good idea and ensure there was at least an eighth of an inch of shaft remaining beyond the cap if you had to shorten it that much. Bob also said the lock nut was not necessary if I needed to use the remaining threads if I used a thread lock on the rod end.
Just posting this in case someone scratch building does the same not set the gear up properly.

[JimParker256]

Did Bob say "thread locker" or "LokTite" on those threads? The reason I ask is that LokTite (several different varieties that have different "removal" properties) is actually intended to prevent a bolt or nut from moving once installed. The so-called "thread locker" is just a painted witness mark that tells you were the bolt/nut was positioned after installation and torquing. If the paint seal is broken, the torque of the bolt/nut may have been lost. This stuff used to be called "torque seal" in the military, but I've heard lots of EAA guys calling it "thread lock" or "thread locker" which is a complete misnomer.

[Bcone1381]

Thanks so much for sharing what you and Bob talked about. It brings a deeper understanding.