I've got a 2019 Big Bro with a rigid fork (original part), and I'm considering a sus fork for it. But have no clue about steerer tube sizes.
From the headset specs
Headset: 44mm, SHIS ZS44/28.6 upper, EC44/40 lower
Can anyone tell me if THIS fork will fit with a) the bearings as specced above, or b) different sized headset bearings.
Thanks
Yup, will do, that's standard tapered steerer size. This might help understand(and the example given is the same as your dimensions)
I've no idea what a Big Bro is, or what you use it for but if you're a stocky rider or ride hard (or with a load of luggage!) then it may be quite a flexy fork, but otherwise, crack on.
I think that will work well, I run/ran a Rockshox SID 120mm on my Big Bro and it was really good but 100mm travel probably would have been more suitable.
Mine is a slightly later model I think and definitely Boost spacing on the fork/front wheel, it's worth just double checking that yours also has a boost fork/front wheel already in case they used to be 15x100?
Yeah, mines the non boost 100mm fork. I'm already looking at spacer kits for the wheel if I get a boost fork. Hope say their kit (Pro 2 Evo) needs a re-dish on the wheel, so I'm looking at 2 hub spacers to keep it central and one for the rotor
OK all in hand then! I'm sure I ran a Hope Boost kit on a Pro 2 Evo in my old hardtail without doing any dishing, it might not have been the 'right' way to do it, but it worked fine and I don't think I died.

