Answer:
I help engines spin and pants stay up.
I go up but never come down. What am I?
Brown I am and much admired; many horses have I tried; tire a horse and worry a man; tell me this riddle if you can.
Used left or right I get to travel over cobblestone or gravel. Used up I vie for sweet success used down I cause men great duress.
I hide in a dark tunnel awaiting my time. I can only be released by pulling back. Once released I may do unstoppable damage.
What do you purposefully put lots of in and on your body, but run away from when you encounter it outside?