You have to turn up the fumble slider with the tackle slider. The higher it is the less likely a fumble. Not only that but I believe that fumble effects the ball carrier vision and the aggressiveness that they run with the ball. Put it on 100 for CPU and leave everything else the same and look how the RBs run differently. They run like they aren't thinking about fumbling. I also think it effects balance. I have it 100 for my RB as well and it allows to run by the blockers a lot better, I can still hit them and not get stopped or fall down as long as I just graze them. I feel like I have more control over the RB. RBs also fight forward more when tackled. I also play with Tackle on 100 because I read it effects how they decide to tackle. On 100 the players attempt to tackle properly and are not always just trying to run into the runner as hard as they can. They do all kinds of different wrap up tackles now. Love it.
If you read this thread you can see some good discussion
http://www.operationsports.com/forum...d.php?t=578899
also I'm working on a set if you want to try it on the last page of this thread.
http://www.operationsports.com/forum...d.php?t=599214
or just read through anyones sliders thread and there is a lot of discussion.