Thank you! Have a great year too!
I’m still curious if you managed to fix that physics bug with excavator 
I’ve double checked my code which deals with passing some of the friction to tracks themselves - that’s what makes them roll on a slope if you don’t press breaks. The code looks fine but it has a heavy dependency on the weight of the track+sprocket+transmission. Currently, variable “Tread and transmission mass” is responsible for this weight and tweaking it makes a huge impact on the behavior of the tank. be default it’s set to 200 kg (don’t remember why) which is realistically very low for something like M113.
I had to do a ton of googling to get these numbers:
According to this source: http://www.tgl-sp.com/m113-track-shoe-assy
approximate weight of a single T130E1 track shoe is 20.7 lbs or 9.389362 kg, we have 64 of these on a side so 600.919168 kg is a full track. How heavy is sprocket I couldn’t find, but I got it’s part number [NSN: 3020-00-141-1154 Part Number: 11678255]
which doesn’t help much as most of the shops didn’t mention it’s weight. Anyway, let’s say it’s about 5 kg or maybe even 10, not much of the difference comparing to the weight of the track. The point is that ball park estimate weight is much higher than what is used right now. With the current setup if you set “Tread and transmission mass” to 100kg, M113 becomes very agile, if you set it to 50 you can start doing drifts 
I imagine that with about 610kg it will be barely controllable, but that’s a realistic weight!
There are two places where “Tread and transmission mass” is used, one is mentioned above and second one is calculation of Moment Of Inertia for transmission. We need it to convert engine/transmission torque to angular acceleration, so that we can figure out how fast transmission and sprockets and as a result tracks should spin. I need to properly calculate moment of inertia, most likely what I do right now is use simple formula for a cylinder I=1/2MR^2 with M of “Tread and transmission mass” and R of sprocket. Which is little bit too high More specifically it’s sprocket is made out of depleted uranium or something. Back to googling to get some idea how engine of M113 was connected to sprockets. Yeah and I don’t really have a good grasp on how to calculate moment of inertia for the track itself as it’s not rotating around it’s center but sprocket.
Forgot something else, so suspension on Aerosled is setup in such a way that it pushes only chassis. I was avoiding pushing both chassis and ski with it as I thought it’s going to give a hard time to solver to keep ski above the ground. Yesterday, I’ve tried to apply suspension force to both parts and to my surprise it works really well, Aerosled became much more controllable and ski are properly pushed away if it flips on the side. Force need to be tweaked a bit, as it technically became twice as large, but otherwise I’m very happy with results and will publish at least compiled version today.