You can't. LR will always Default to a known, used every day, ratio when it is so close to that ratio.
You'll need to use Photoshop.
Bummer. I hate that, when software tries to outsmart me. Offers a user setting then overwrites it.
Thanks for your reply!
I need the aspect ratio of some images to be 98:55. Yes, exactly that. But when I enter those numbers, LR automatically switches to 16:9 - somewhat similar, but not the same. How can I set an exact ratio and keep it?
This is a long-standing bug with a one-line fix that Adobe has decided not to implement (after a flawed analysis by a senior Adobe engineer). See here: Lightroom: I can't add (certain) custom crop ratios | Photoshop Family Customer Community
Please add your constructive opinion to that bug report, and be sure to click Me Too in the upper-right corner. In the past year or so, Adobe has been making an effort to fix old bugs, so it's possible they may revisit this one.
Meanwhile, you can use the Any Crop plugin to set an exact aspect ratio.
I wish I had known about this plugin years ago. It can do much more than just crop correctly.
Thank you for the hint!
I did add my 2ct to the report. Little hope that it will make any difference, constructive or not ... although, if you say Adobe is making an effort, that does give me a little hope. Thanks again.
This is probably close to the top of the list of poor design decisions Adobe has implemented in Lightroom and Adobe has steadfastly refused to fix. johnellis has even wrote the code fix for them. Hopefully the recent Adobe Lightroom team will fix this issue as they have been doing a pretty good job of fixing old issues lately.