well... I think that Shout would do (if there is not any rust). Most of the dirt on spokes is mud, grease from the chain and a layer of dust on top (and maybe some cobwebs, leaf particles etc). Any sort of decent household degreaser will do. I use Ajax dish soap in water with a cloth (cheaper than Shout) and then dry them up with a dry clean cloth. I am sure that Shout would be fine. Caveat: as I said before: no rust.