I’m leaning towards having a cross bike. If I buy a disc brake cross frame, can I just put all my existing running gear from another bike on it with a pair of 29er disc wheels I have spare, just buying a set of mechanical disc calipers such as BB7?

My 2 bikes I might strip have SRAM Rival 10spd and Shimano Sora 9spd.

Pretty much

You will probably find your 50T outer ring is next to useless on a cross bike though.

Wouldn’t go for BB7’s, the mechanical disc brake game has moved on… Go for TRP Spyres, or even better, the mech/hydro combination TRP HyRd instead. A caliper with only a single moving piston works ok on a road bike, but for anything that ventures off road into the dirt it’s less than ideal.

EDIT: FWIW use the SRAM over the Sora. I find that off-road, the fact that the brake lever doesn’t move is much more confidence inspiring when braking over bumps. Also SRAM 10spd road stuff is interchangeable with MTB, so you can run a 10spd SRAM MTB rear clutch mech which will then also give you the ability to run anything up to a 40T big cog cassette comfortably…

Thanks – those HyRd brakes look interesting!

I have a single ring set-up going spare on SLX cranks, so could use those maybe a 38/11-36 or similar.

Fwiw I have a 50/34 compact and 11-30 10 speed on my “cross” bike. It depends what you are using it for, if it’s going to live off road then the 50 is overkill but if you are going to use it like me for rides which are mixed on and off road exploring or maybe as a winter weather/lanes road bike with better braking then having a 50 big ring on it makes a lot of sense.

I don’t think I’ve ever dropped the chain from being bounced around on mine, your arms and eyeballs (& nerve) give out before that happens in my experience!

Single piston cable disc calipers work ok but are a bit annoying to have to keep adjusting. The HyRd option is good advice

All noted, thanks.

I’m hoping to get a cross bike with minimal new parts/outlay, and it’s looking like frame/calipers/tyres is all I need.

Interesting comment on the BB7/Spyres, as we’ve done far more swaps the other way when people of become fed up of the grit ingress into the Tektro’s, causing the arms to seize.
HyRd’s are amazing though, as long as you don’t want them on descents that are too long.

Also keep in mind if you go for BB7’s there is a difference in cable pull between road mechanical brakes and mtb ones, which is why there are road and mob specific versions with different cam lengths. Not sure if its the same on others such as the Spyres.