Hello, my name is Rasmus and I'm 14 years old and I have question: Why no cars?
Good question. The simple answer would be that Trials HD is a motorcycle game. Better answer is that Trials HD is most of all a motorcycle game, that is fully driven by highly developed physics model.
As a gamer, no one should have to care that much on the technical solutions behind the game. But as we have anyway been discussing quite unorthodox things with Oskar and you Loading-blog-readers, lets go a bit deeper. The core thing that makes Trials HD is the physics model as discussed. Normally games do it in a way that you press a button, and something happens. What you see on the screen, are pre-made animations. You select what you do, where you go, when you do it, etc. There are great games that can be made like that. You have a lot of opportunities, but these opportunities are still limited. And you wont get that realistic feeling, cause things in real life arent driven by animations, but by the laws of physics. For any games, there is no need for that. For some games, the physics may change the whole genre.
When the game is based on realistic, and open physics, there are very few limits, if any. Very few things are pre-determined. As a gamer, you have the control. Eg, You play Trials HD, you get the real feeling immediately: Its you who really controls the riding. If you look the game very, very closely - you can ride one track in billions, trillions of different ways. Actually there are infinite ways. As it is the physics that in every fraction of the second decide what is happening, and there are always small differences. And when these small differences sum up into thousands of differences, the difference is not small, but huge.
Now, that opens up great possibilities for a gameplay. BUT, it is only fun, if everything is developed, tested, polished exactly the right way. If something in the physics works in wrong way, you dont know as a player why it is, but the playing doesnt feel fun. Considering that with Trials HD, the phycis of the bike, rider, environment, how they behave, and how the game controls affect that it all has to be exactly right so that the gameplay is really good.
Taking a concrete example, the bike and the rider in Trials HD, consist of 200 different physics parts, having over 500 parameters that define their relationship to other parts, and how they behave. As a result of all that we get the rider bike entity that feels fun to play. The aim is to make the game as good and fun as possible. The only way to do that, is to develop, change combinations, change parameters, test, and develop further. This alone, took us 6 months in Trials HD.
Having cars in the game, it would be fully possible. But having the playability in such a polished level, that we would ourselves accept, it would take some time.
Your question however is excellent, cause basing a car racing game on this sort of physics model, would be something very new. You would get the bit different feeling of controlling the car immediately. Even though not for the immediate near future - Thats still an interesting idea to consider cause who knows what the future brings.
And as a small hint, you should check out the DLC coming, there might be something interesting on cars there as well. Something different, of course.
Even after getting gold on the first extreme track and doing considerably better on Goin' Up than my previous best, AND spending like an hour on about half a dozen different mini games (Beat my best in all them too, I think) you're still ahead of me by a good 2 or 3 hundred thousand 
