Contact Wheel HELP

I am looking for another contact wheel. Need some help on durometer's. Like what are they? OK OK I know they measure the hardness of the rubber surface but what do you generally recomend for your knife grinder?

I currently use a smooth wheel and plan to stay with that but I don't know what durometer it is so I don't know what I what to get.

Thanks

Tony- I only know enough to be dangerous. I think 70 durometer is at the softer end, and 90 at the harder end, but why don't you direct a question to Steve Johnson- I believe there has even been a discussion of wheel durometer on one of his grinding threads.