To turn a clip into a Groove Clip, you can use ctrl + L
 
Make sure the clip starts exactly at the start of a measure (e.g. 02:01:000) and has a length of EXACTLY 1 (or more) measures. now you can ctrl + L it and drag it out for as many measures as you want.
 
As was said above, having Snap to Grid set to 1 measure helps a lot in trimming clips to length.
 
You shouldn't need to bounce to clip or apply trimming because as soon as you invoke the Groove CLip, Sonar will flag up a warning about "Discarding Data" which you can say Ok to.

tenfoot

Kylotan
Yeah, he was aware of the ability to change a clip to a loop and I covered the trimming in my first reply. The issue is that if you expect each repeat of the loop to stay in sync when you edit one of them, Groove Clips won't do that.

I am not so sure - re-read the first post. OP states the problem is when he extends the clip it restores the sections he wanted deleted rather then repeating the loop. That doesn't happen when you activate groove clip looping correctly - that is its purpose.

The problem you are solving was not the OP's original query. I think it may have been introduced in your post. Could be wrong of course:)

Okay, I opened up Sonar and checked, and it seems that the original poster is perhaps doing something wrong, because it looks like Sonar prompts to discard that extra data when you convert it to a loop. He said "I found the Loop function which changes the appearance of the clip (the corners get cut)" which implies he's correctly made it into a Groove Clip, but he's obviously not getting the warning that we get or having that extra data discarded.
 
So I was responding to the original query and trusting what was said, but something is obviously wrong somewhere.

Yep, looks like it.

Plenty of instructions there now Mike! Pretty certain once you get groove clip looping happening with any of the methods above it will be what you are looking for. Let us know if you have any issues.

Open the Groove Clip section of the Clip tab in the Track Inspector with the clip selected after converting to Groove Clip, and check/adjust the Beats in Clip value.
 
SONAR often guesses wrong when the clip doesn't end exactly on a bar line and/or the time signature is not 4/4.