It is a difficult subject in some ways. The human brain adapts so quickly that no matter how odd it might first appear, we soon get used to things and they become "normal". We often hear that Bückers are tricky to land, "but mine lands just fine." I suspect we get used to our own aircraft, we form a "muscle memory" and to some extent, our reactions become automatic.
But the landing gear on our Bückers is at the least "unconventional".
We will never know for sure what was in Anders Anderson's mind when he designed it, but there are certain things we can take a pretty good guess at. Perhaps the most basic is that the landing gear legs attach to the fuselage at the same points that the wing spars attach. This means that a separate reinforcement (hard point) is not required for the landing gear and so some weight is saved. A consequence of this is that to get the wheels in the correct location, the gear has to be angled forward quite a bit, giving the aircraft the look of an Eagle about to grab its prey, In my mind at least :)
This makes the geometry of the landing gear quite complicated. When combined with the wheel camber (the way the wheels lean outwards) we end up with a very dynamic geometry. The toe-in/out in particular is constantly changing. Let's start with what happens when the tail is lifted: Hold you hands in front of your face like this and estimate the camber angle.
Now without changing that angle, rotate your hands forward so that you fingers are pointing away from you. - That camber has now become toe-out. The right hand is pointing to the right, the left hand to the left, and so it is with the landing gear.
Here is a picture of Greg Stringer's Jungmann on jacks and in a level attitude
And here it is again this time in a three point attitude.
What else affects the toe-in/out? Perhaps surprisingly the gear extension does. This is more difficult to visualize, but the more weight is on the gear (the more it is compressed) the more toe-out the wheels will adopt. Similarly, the length of the spreader bar affects the toe. The longer the spreaders, the more toe-out you get.
I wonder if this is why some people say extended spreader bars help, while others say they do nothing. If your wheels have a little too much toe-in, extending he spreaders may correct that. If they do not, the longer bars will offer no improvement, and may even make things slightly worse. - I don't really know.
The reason for this change of angle with gear extension is the location of the tripod. Because the tip of the tripod, where the spreader bars attach, is located behind the wheels, the toe angle changes as the wheels move closer to or further away from the fuselage. If you don't believe that, try it in SolidWorks or AutoCAD and you will see.
Gear extension is affected by aircraft weight and by the condition of the springs. If you have 75 year old springs and a heavy engine/prop combination, there may be excessive toe-out.
Greg made a great movie of the way the angles change when the gear extends. Click here or the mage below to watch it
So what is the "correct" angle for the wheels that will lead to the best handling experience? That is a tough question to answer. Google "tail dragger toe in toe out" and see what you get. Thousands of opinions and just about 50% favoring toe in and 50% toe out. Supposedly the Husky, Pitts and Cessna 100 series aircraft specify a slight toe-in but obviously a lot of people disagree. By now you will know that there are so many variables at play in the Bücker landing gear that it is almost impossible to say. But based on measurements I have made of quite a number of original (unmodified) aircraft, I would say that a toe-in of between 1.0 and 1.5 degrees with the airplane resting on its gear, tailwheel on the ground) is about right. - Cars typically have a slight toe-in. This is what provides the stability that causes the car to continue in a straight line when you remove your hands from the steering wheel.Is the same true in an aircraft. - Again, I don't know :)
The theory is that setting it to a slight toe-in in the hangar will cause it to become about neutral when the aircraft is loaded with pilot(s) and fuel and/or when the tail comes up.
But, and it is a big but, not everyone agrees with this. I know of at least two people who say no, the landing gear toe should be neutral or even slightly "out" to provide the optimum handling. To me this seems to indicate that perhaps it is not that important, and that you feet really will adapt to almost anything. Again though, the majority of "original" aircraft I have measured show 1.5 degrees or so of toe-in.
This angle is pretty easy to judge. Standing in front of the aircraft, sight down the outside of the tire towards the rear of the aircraft. If the angle is about right, your sight-line will intersect the horizontal stabilizer somewhere between the elevator hinge and the tip, like this:
This view shows how that looks (on N28Bü).
Enough about the angles - There is another factor which I think affects the way the aircraft handles on the ground and I think perhaps it is something we often neglect. Returning to the car analogy, one of the most important components in a car suspension is the shock absorber or damper (shocks and dampers are not exactly the same, but close enough for this discussion). If the dampers are worn, the car will bounce wildly when it encounters a bump and the tire may even lose contact with the ground. Aircraft have dampers too. Sometimes, as on the RV series of homebuilts, it may only be the sidewalls of the tire and the temper of the gear legs that provide the damping, but in the Bücker, we have a real oil-filled damper.
The cylindrical tube that forms the landing gear leg is filled with oil and on the end of the landing gear piston, a brass collar slides along the tube through that oil. There are two grooves in the brass piston which allow oil to flow past it at a specific rate to control the rebound, or bounce of the gear.
If the fluid level is low, the liquid too "thin" or the brass collar/piston worn, the damper cannot do its job and the gear will be springy/bouncy.
It is interesting that some restorers chrome plate the inside of the gear leg tube and measure the collar to ensure a snug fit, while others may be using 75+ year old gear legs in which a lot of fluid makes its way past the collar.
I also find it interesting that the landing gear that Charlie Miller makes is noticeably superior in a strong cross-wind, and Charlie pays particular attention to the damping. He shared this interesting link: http://www.hangar9aeroworks.com/Aeroncastrut/Aeroncastrut.html It demonstrates Aeronca "no bounce" gear rather than Bücker, but the principles are the same.
Charlie shared some interesting facts about the gear. He has often been called upon to repair landing gear components and spreader bars in particular. He says that on a number of occasions "pairs" of spreader bars had quite different bolt angles, or even different construction methods and he found it difficult to understand why. Talking to José Martin, the importer of so many of the CASA aircraft in the USA, Charlie learned that when a person bought an aircraft from José, they picked all of the required parts from piles of disassembled aircraft so that although they picked two spreader bars, they almost certainly didn't come from the same aircraft.
He also explained that when he built landing gear, whenever possible the spreader bars were finish welded only after being installed on the properly loaded aircraft. That way the correct geometry could be ensured.
Notice the bolt angle(s)
Please let me have your thoughts on this subject. I would love to learn more and publish any suggested corrections to the above.