Need help understanding IBUs

I've been trying a lot of IPAs recently, and noted some observations about the hop bittering/flavor. Some IPA's just have a real strong bittering component that kicks in during the aftertaste. Other IPAs however have some serious hop flavoring, but with very little bittering qualities during the aftertaste. In fact, just pretty smooth with a lot of hop flavor. That makes me reconsider my hop additions for IPAs, and what they actually contribute. I guess I'm wondering whether the same amount of IBUs in a beer will affect the drinker based on when it's added to the beer?

I have an equation for calculating IBUs based on water volume, sg, AA, etc. For my specific recipe, I have 2 oz of Simcoe (12% AA) boiling for 60 minutes to give me 53 IBU. That is my bittering hop.

Then I have 1 oz of Simcoe for flavoring which I add in with 15 minutes left to get about 14 IBU.

If I leave out the flavoring hop, I'll have 53 IBUs of bittering. Should have a decent bite. But let's say I leave out the bittering hops, and quadruple my flavoring hops to get roughly the same amount of IBUs. What will my beer taste like? I imagine it'll just be a smooth beer with a lot of hop flavor, but no bite.

Is this true? Maybe I need some help understanding the term IBU more.

This is called "hop bursting" or "late hopping" and is something lots of people do. Here is a good article about it.

I just recently hop bursted a blonde and while it was totally out of style it was really fantastic. There is still a bitterness due to the amount of hops you use, but it changes the flavors dramatically.

That article linked is pretty great and I'd read through it. And then I'd say give it a whirl.

I did almost the same hop additions as in the article linked in my Ginger Pale Ale (yet to be bottled). No hops were added until 20 min left in the boil. Last week when I racked it to the secondary, it tasted VERY bitter. Almost too bitter to even enjoy. Only a week later and it has a very nice hop taste and aroma. I'm excited about this beer!

I did an American IPA and did a continuous hop addition as called for in Dogfish Head clones, and dry hopped in secondary. I am very pleased with how this turned out. It was not a huge IPA in malt or hops, but everything was nicely balanced, and a great summer beer. Over the 90 min boil, I added hops every 7 mins or so.

There are six primary bittering compounds. Each hop has a different mix. Generally, hops with low cohumulone have less of a bitter aftertaste. Chinook & Columbus are two high-cohumulone hops and can be a bit harsh.