Sorry for trying to give a sensible answer...
For Off-Road, there is a common set of Construction Rules from BRCA, through EFRA to IFMAR (unlike TC, where there are probably four sets of Construction Rules!!
) so the dimensions and drivetrain restrictions are quite clear.
What's more difficult, is the suspension design Rules - those factors that dictate what makes the best suspension design to control the contact patch of the tyre against the ground. For track cars, these design Rules are different to Rally cars, and then they vary again if you are using aerodynamics to generate downforce (like F1) or aerodynamics to control yaw and flight (like WRC).
I'm not sure what you want to do with these 'Rules' when you know what they are, so be aware that just because you know what the Rules are, doesn't mean you can design a car. Excellent car designs aren't derived from what the Rules say, they're derived from what the Rules don't say - thinking outside the 'box', or 'Rules', as Lee says - that generates the 'revolution'.
You won't remember Colin Chapman, although you may have heard of him. He founded Lotus, and had a long history of innovation in motorsport, some of which brought him into conflict with the authorities. His innovative 'twin body' Lotus 88 finally drained the fight out of him when it was banned, and from there on Lotus went downhill. When you read about Chapman (and the Wikipedia entry is a great place to start) remember two of his mottos - "If a part isn't doing two jobs in the car, get rid of it." - and "Simplicate, then add lightness." Look at every innovation he brought to racing cars, and you will find one, or both of these embodied in the idea.
Designing a car is 95% inspiration and 5% perspiration. If you have the mind for it, it can be very rewarding. If you just want to play at it, expect limited success! HTH