why not will?

Why can we use "going to" here, but not "will"?

"If interest rate are going to climb, we'll have to sell the shop."

NB. I know that "habitual will" could be used, but I'm thinking more about "future will".

I guess it's just normal convention to use "going to" in this sentence, but "will" in a sentence like:

"If interest rates'll climb, we can sell the shop."

Are there really any rules for this sort of thing, or is it something that you just have to learn by osmosis? All languages are full of things like this.

In American English this would normally be written as "If interest rates climb, we can sell the shop."

<<In American English this would normally be written as "If interest rates climb, we can sell the shop." >>

Ok -- how about:

"If interest rates'll just climb a bit, we can sell the shop."

You won't find it like that in writing, but in speech it's okay.