If you're making a wheel, place the motor or hinge in a group with the wheel; this makes re-positioning easier. The same goes for most other objects connected to a main body.
If your joints are bending and flexing when you don't want them to, increase the mass of the objects they're connected to. You can do this with hidden geometry, set to 'ignore,' inside a group with the objects. This will strengthen the joints, and fix the problem!
Want a hole in a non-static object? Well, don't ask for an 'un-static' mesh, because it's just not possible. You can, however, group a number of objects, leaving a hole in the center. As long as the overall group's shape is 'default', this will work.
If you can't do something mechanically, you can often do it with a script. They are extremely versatile; they're what industry-standard games use to give the impression of a complex mechanism.
The mass of an object is determined by the bounding box/blue box around the group. If you need an object to have more mass simply add some more objects to the group and set them to be ignored and hide them. The further away from the rest of the group the larger the bounding box.
Be careful when altering the mass of the object. As well as making it appear heavier it will also alter the center of gravity (COG). this can be used to your advantage though. For example. You need your motorbike to stand upright without using the gyro joint. Change the bounding box so that the center of gravity (COG) is far below the motorbike itself, and it will stand upright.