Setting Collidebox Dimensions

[vTony]

Hello, I am trying to manually set the dimensions of an object's collidebox. As a simple test, I tried setting the dimensions of the collidebox to the size of the object's bounding box. This works for box.wrl, but with the file tree.3ds (located at https://www.stanford.edu/~tony516/tree.3DS), I get strange results. The tree on the left (which uses the default arguments for collidebox, behaves normally, but the tree on the right shoots up into the air.

import viz, vizact

def setOnGround(obj):
ymin = obj.getBoundingBox().ymin
pos = obj.getPosition()
obj.setPosition(pos[0], pos[1]-ymin, pos[2])

def enablePhysics():
viz.phys.enable()
ground.collidePlane()
tree1.collidebox()
tree2.collidebox(tree2.getBoundingBox())

viz.go()
viz.clearcolor(viz.SKYBLUE)
viz.MainView.move(0,0,-15)
ground = viz.add('tut_ground.wrl')
tree1 = viz.add('tree.3ds')
tree2 = viz.add('tree.3ds')
tree1.scale([.2]*3)
tree2.scale([.2]*3)
tree1.setPosition(-10, 0, 15)
tree2.setPosition(10, 0, 15)
setOnGround(tree1)
setOnGround(tree2)
vizact.onkeydown('1', enablePhysics)

Thanks in advance for any help!