When playing CLASSIC mode, it is wise to play the AMATEUR/NORMAL Chamionship over and over again until you have purchased the best Bike/ATV available and all the best parts available. Don't waste money by purchasing only what you can afford.

Just keep racing and gaining cash until you can afford the best parts in each category. Once your bike is maxxed out, then it is much easier to win the higher level chamionships and the better you do, the more likely you are to win Prototype parts which can not be bought.

Once you win a Chamionship, just reset the Chamionship and win it again. You DO NOT lose any of your parts or money by doing this.
The best bike/ATV isn't usually the most expensive/powerful. Look for the bike/ATV that has the highest cornering ability and lowest weight as possible.

Lower weight is almost as good as horse power, but cornering seems to be what wins races in ATV Fury 4. Hope this helps, good luck.