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I presume there's a way I can work out
Yes, it's called maths. The pitch circle radius of a roller chain sprocket is very close to Pitch×Z/2π, then add 4mm to get the top of the teeth
(12.7×38/2π) +4=80.8mm, so your stays need to be 2×(chainline-4mm*) apart at a point 81mm from the BB centre*half the ring thickness + clearance for flex
gbj_tester-
maths
Yes, but I wanted the maths... AND the chainline of whatever mess I've bought.
Chainline is 52mm if this is to be believed:
https://bike.shimano.com/en-NA/products/components/pdp.P-FC-M8100-1.html -
your stays need to be 2×(chainline-4mm*) apart at a point 81mm from the BB centre
That would put the chainring miles out from the stays so I've probably done something wrong.
hippy
Cool. Well, I now have 3 sets of cranks for the Surly and a couple of BB and chainring options. :)
It says it wants BB-MT801 but is the 501 the same thing just cheaper or do I need a different BB too.
EDIT: Just looked and I did buy a 73mm MTB BB for the GRX cranks. Maybe I bought them at different times and just wasn't thinking straight at the crank purchase time.
I presume there's a way I can work out what my chainring size limit will be? ie. approx how wide will the chainring sit + diameter of 38T ring or whatever?