Just in case anyone searches for the same thing later and finds this…
I’ve found some people saying the 2.1 RaRa and the ThuBu (why not?) come up the same in width, and another person giving these:
Racing Ralph Sizes
2.1
Casing width 1.88″
Tread width 1.95″
2.25
Casing width 2.04″
Tread width 2.12″
2.4
Casing width 2.27″
Tread width 2.39″
Measured on a DT 4.2d rim at 40 psi after 24 hours
DT 4.2d is 17.8mm internal, 24 x 1.95 = 46.8, call it 46.0 on 17mm rim.
Call that “on the safe side” due to smaller knobs on the ThuBu, and there should be plenty of room in forks measuring 59mm across the important bit. I reckon.
* = not that safe, due to random internetz initial input, but eff it, I’ll give them a try.