If you go for a long cage then there in a risk of picking up debris and a tangling it in the rear cassette. I did this and snapped the rear mech , manggled the chain catcher and broke the front chain guard .
you can get away with a small cage if it's a newer model with the chain tensioner. The affine cage to the chain tensioner is not as low to the ground as a long cage .
Cassette hub make the chain line very close to big apples tyres if you fit them .
I think the chain will fall off a lot with a double front as jur reports and I found .You might get away better with a double and an IHG .
I do run an older birdie monocoque however .
just because something is available as standard doesn't mean its a good option .
Birdies are a flawed but fun design .
With compromises in other areas than other folders.
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