First question, what are you spreading....wet corn, or already dried?

If you're spreading dried corn I'd  get one of the Spin-Level gravity spreaders. I believe their largest model shows being capable of a 36' bin. But, to me, that really doesn't matter. It just won't distribute corn (they don't "throw" corn anywhere) to the outer edges of the bin immediately. However, unless  you have a terribly shallow pitched roof, but the time the corn piles up under the center of the spreader when the bin is full, the corn will have neatly run to the eaves of the bin anyway. The gravity spreaders may spread a little unevenly in the beginning, depending on how you're feeding them, but as the bin fills, gravity helps even things out. The main thing is, the spreader will spread the fines & beeswings so they don't just accumulate in the center of the bin & block airflow or get hot. Electric spreaders do a good job of initially throwing corn to the outer circumference of bins, but they can keep piling the bin enevenly causing corn to run out the eaves (or vents) on one side, while being as much as a ring below the eaves on the opposite side. They don't allow gravity to help out much. Also the gravity spreader will probably spread for 100 yrs or more. Depending on your situation, you might check them for birds nests initially in the season, since that would block them & if there's no corn going thru, they won't turn to fling out the birds nests. Also, gravity spreaders turn on the minute you send corn to them. You don't have turn remember to turn on a switch, or check a c.b., or plug it in...it just goes. Also, you won't walk by the bin on a quiet day 2 months after filling it & wonder what that "humming noise" is that you hear coming from the bin, which will be the electric spreader that you (or probably someone else more blamable) forgot to shut off.

I'd go for the 36' gravity. I can almost say w/certainty, it will be the last spreader you buy for that bin. Now, the bad part, the company doesn't seem to have a good website, which is a mistake in this day & age. If they have one, they have it well hidden. So, here's a link to the smaller model with a photo, then a link to the 36' model from SloanEx ( in IL) w/a price but no photo. I also can't figure out why Sloan can't post a photo of the spreader. They certainly have the computer savvy to complete the job.

Good luck!

smaller model w/photo:
http://www.enasco.com/product/C09434N?ref=recentlyViewed

36' model from Sloan:
http://www.sloanex.com/cx54qy7wgw-national5000bushelhrmeduimspinlevelgrainspreader.html

A final "tip". You can't install the larger model from the top of the bin that I'm familiar with. You'll have to pull it up (they're not that heavy) after carrying it in thru the door. This means the bin will almost have to be empty.