I'm afraid it won't work.
I run an oval 34 on it, that makes 15cm.
There would be enough space for 16, maybe 16.5cm but not more.
The chainline of the CX may be different, but I doubt you could add 4-5cm to it.
I highly doubt it will work with a 1x setup. For reference, I have the BSA bb with a SRAM GXP crankset / bottom bracket setup. The crankset is made for a 3x setup but I'm using the middle spot for the 32t ring and the outer spot for a bash ring. If I increased the diameter of the bash ring to match your 21cm measurement, it should work but will be very close. The downside is you would have to use the outer most option and the chain line really wouldn't work when you're in your granny gear. That being said, you might have to look at different frames or consider a different gear ratio as there is no way a 50t would fit in the middle ring option.
Thanks both of you for measuring it up!
That gives me at least the space I can calculate with - if I would stick with frame.
IMHO 50T is way too big to be running on that type of bike.
Im building up a similar style bike around one of these frames at the moment, actually most of my parts arrived today, just waiting on the workswell frame/fork 
Im going to be running 2 x 10sp XT, will have plenty of range for speed on fireroads etc and also to easily climb whilst loaded up with bikepacking bags etc.
Ive also got a touring/gravel grinder/cyclocross bike which is set up with 11speed shimano, currently have a 48t big ring and would not want to go any bigger on that, off road with decent tires (even on road on this type of frame) i think you'll find 50T waaaay to big.
Anyway thats just my 2c.
You are probably right.
50T is just the chainring I have on two of my bikes, making me believe I have to stick with it.
But with your comment in mind I did check on my bike to get my "normal-speed" gear and that ratio.
| | | Chainring | | Sprocket | | Ratio | | Feel |
| Fixed | | 50T | | 18T | | 2,77 | | mostly good |
| MTB | | 46T | | 16T | | 2,87 | | average gear |
| MTB | | 46T | | 14T | | 3,28 | | good speed - could go higher |
| Road | | 50T | | 17T | | 2,94 | | average gear |
| Road | | 50T | | 11T | | 4,54 | | Never really used |
My normal Ratio is somewhat around 2,9. Highest speed should be okay with something around 4.0.
Therefore I though I could think about an 42T up front, still having enough average-speed sprockets i can choose from (14-15T would result in 3.0 and 2.8 ), having the highest ratio at 3.81 on the 11T-Sprocket.
Still: Fitting an 42T as a Chainring still seems a lil bit tricky... But at least its one step closer
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