Yes. And yes. Both preparedness and peace of mind come with a price.
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Personally, I would never entertain the idea of having a spare wheel/tire combination bouncing around loose inside of a rooftop aero-coffin.
And what would be your tipping point, trip distance, "decision-maker" be to warrant carrying that full-size spare? No different than motor vehicle collisions, the majority of tire punctures occur within 25 miles from the front door of your home. Best to prepare according to your worst
driving scenarios/conditions and not according to the duration or distance of your journey. The f*ck-up fairy could care less about your proximity to home base.
Well, thank you for picking my brain because I've been debating about this for weeks now
First, about putting the tire on the Cargo box...
The box has tiedowns but from what I've seen in pictures they don't look that rugged. What I'm thinking is crossing straps through the 4 channels the box has where you anchor it to the crossbars. That way I would strap the tire to the crossbars and not the box itself.
I've also considered going with a basket same as your setup, just a different basket, and I do like the look a lot. So I've considered the Thule basket with extensions, but what I don't like about this is I would need to put like a soft bag or something up there for cargo and I don't like that at all, I prefer the hard plastic lockable boxes like the Thule Motion which has a load capacity of 165 pounds. The box itself weights 42 pounds, the spare tire around 50, the crossbars around 10ish including the supports. You can put up to 176 pounds on the rails of the 21 Crosstrek, so that leaves me 176-42-50-10= 74 pounds of stuff I can still put inside the box.
Then a third solution would be a rear hitch spare tire carrier. This solution would really optimize cargo capacity on the roof, but I just can't wrap my head around the idea of loosing my back up camera. And a good hitch tire carrier is absurdly expensive. I haven't discarded this idea at all though.
Now about the tipping point...
I would say a radius 100 miles or depending on the terrrain I'm going to visit.
I live in Phoenix, a regular puncture lets you roll for a while and we always have on screen the tire pressures. So you can either get back home or get to a place where you can fix it, plus If it's a trip, even a short one I carry around my compressor (not when daily driving). Worst case scenario, a tire blowout, I would either have a family member bring me the tire, call my insurance and get towed or hop on an Uber and go home and bring the tire on one of my other cars.
About the terrain, for example we have this lake called Lake Pleasant about 40 miles away and it's a nice fun place to go and hang out, do a bit of off roading, but it has really sharp pointy rocks so for something like this I would definitely take the spare.