Trainers are really great when you can't get out. Knobbies tend to make a lot of noise and racket on a trainer, you can use knobbies but remember that the knobbies can be worn down by the trainer as well.
I use my wife's old Walmart bike to put on the trainer and put slicks on it so it don't make too much noise and doesn't rattle too much when I am using it. For an indoor trainer, a Walmart bike will do so you don't have to fuss with hooking and unhooking your good bike.
Now, there are two types of trainers. The one that mounts on the rear axle of your bike which has a roller that presses up against the rear tire and you have one that lies flat that you actually ride on. The first one you don't have to worry about balancing the latter you do.
Depends on how you want to do it. If you just want to pedal and enjoy a leisurely indoor ride, then the first one would fit the bill. If you want to work on maintaining balance, the latter would help with that.
Most stationary trainer manuals that I've read have cautioned against running a mountain bike on them. This is probably more due to the tire than anything. If you're gonna be in side all winter, throw a cheap slick on the back. Personally, I dislike riding the trainer, but it's a necessary evil if I want to ride for any length of time during certain times of the year. A basic magnetic trainer will be the cheapest. You can often find them used on craigslist or on close out somewhere. I have a 6 or 7 year old blackburn that has held up well to use that I paid less than $200 for.
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