I will post this again, as there are a zillion threads floating around on airlines and bike cases:
Regarding Delta
I’m a platinum medallion flier
I’ve talked to 9 different customer service agents/5 different ticket counters about this policy
The official policy is so convoluted, no one knows what it REALLY is
Here’s the best way (read: cheapest) to get it handled( a little shaky morally, but not that bad):
Dress reasonably well (I always wear comfy jeans, nice black work shoes, and a business blazer over my golf shirt, more on why in a sec)
Check in at CURBSIDE, once you’re at the counter YOU ARE FUCKED and will definitely pay AT LEAST $100 each way for the bike case)
When you walk up to the curbside checkin, have 2 $20’s clearly visible in your hand and ask the guys to help you out with checking in.
If they ask you, “Is that a bike case” respond “It looks like a TRADE SHOW DISPLAY to me” and visibly raise the 2 $20s so you are absolutely sure he can see them. Then ask, “You’re not going to make me check that inside are you?”
Every time I have tried this, they say something to the effect “Nope, we’ll take care of you out here”
The reason is that DELTA has no restrictions on TRADE SHOW BOOTH DISPLAYS, even oversized ones, as long as they roll and aren’t a pain in the ass.
So this method costs a total of $80 (or $40 if you only use 1 $20 each way) and the curbside guys all take great care of you.
Unethical? You decide, I don’t have the slightest hesitation with all the other zillion dollars I spend with Delta to exploit a hole in their own system.
EDIT: If you read a lot of the posts above, it involves arguing with ticket counter guys, who are notoriously unhelpful when it comes to this kind of stuff. Your chances of success are left up to their whims. In my scenerio, you are playing on the greed of the curbside guy, WHICH IS SOMETHING THAT I WILL ALWAYS BANK ON. 
On the return leg of a race like IMFL, you probably won’t have a lot of success (PC doesn’t even have curbside checkin at our little pissant airport), but hey at least it worked on the way out!
Good luck