Contact points for undercarriage are described in some detail in the FSX (and P3D) SDK but what is missing are some illustrations to explain how undercarriage contact points work.
This is Milton Shupe's XA-38 Grizzly with the gear fully extended (keyframe 100):–
Notice there's a top-level object at the model's center which is at 0,0,0: everything else is linked to it via the model hierarchy. The top-level object is aligned to the Gmax world.
Make sure you're working in Decimal Feet for the viewport units.
For each contact point, pick a vertex at the correct position (bottom of each wheel) and note its position at keyframe 100. That gives the correct coordinates for that contact point. Also note the Z height at keyframe 200 (fully compressed) and you can calculate the maximum travel of the gear:–
The maximum travel is (frame 200 height) - (frame 100 height), in this case (-2.464) - (-2.712) = 0.248 feet for this example.
Each contact point needs a Static Compression value and a value for the Maximum Compression/Static Compression Ratio. The Static Compression is how much the gear compresses when the model is at rest on the ground.
Max/Static ratio = (maximum travel) / (Static Compression) so it follows that Static Compression = (maximum travel) / (Max/Static ratio)
The higher the Max/Static ratio is, the stiffer the suspension will be, but the higher the loads it can absorb before hitting maximum travel.
Here are two examples, using the same undercarriage model:–
You can see above that with a very high Max/Static ratio the gear deflects very little of its full range of movement when at rest. This is likely to loosen your virtual pilot's teeth, but might be appropriate for an F-18 Hornet doing crash-landings (sorry, carrier deck landings).
A much lower Max/Static ratio follows:–
Half the deflection available is used up when the model is at rest in this example. This gives a soft, forgiving feel when landing, but doesn't leave much travel for a heavy arrival or a large variation in payload.
A typical value of Max/Static is 2.5 and for the Grizzly this gives a Static Compression value of 0.248 / 2.5 = 0.0992 feet
The other things to note are how to determine Static CG Height and Static Pitch. Select the top-level object, move and rotate the model until it sits with all the wheels nicely on the grid zero at the correct keyframe for the Max/Static ratio.
correct keyframe for Max/Static ratio = (100 + (100 / (Max/Static)))
In this case correct keyframe = (100 + (100 / 2.5)) = 140.
Static CG Height is 2.261 feet.
Static Pitch is 14.2 degrees
Small tweaks can be done to give the impression of tyres compressing under load, but that's the basics.