Question on BB spacers
 

I am confused on the use or need of BB spacers?

On my gravel bike I have
68mm shell
SM-BBR60-68 HT2 BB
Tiagra 4700 Compact double
135mm 10 speed rear.

I have no spacers but from my reading, maybe I should have a 2.5 on each side.

I've got roughly 2500 miles on this combination so far (had FSA Omega cranks before that) and everything seems fine but I do notice the chain line APPEARS to be shifted a little more towards the bigger rear rings. Again this is just from looking at it but when I am in the big front ring the chain looks to be straight at maybe the 4th or 5th gear. I can use all gears and all combinations and I don't notice any areas being louder except for the extremes. I do have a relatively wide spaced frame and it can handle 45ish tires but I have adequate crank clearance, even for my cadence sensor mounted near the pedal spindle. Although my current combination works, I'd still like to explore any potential benefit of adding some spacers.

Measure .. to know
 

Generic , a measured, chain line, is 1) A center line through bike as its base..

2) Rear hub .. Center of gear cluster being 10 , an even number, the spacer between 5 & 6
distance to that center line is one end of the chain line ..

3) a parallel line equidistant to the rear,
should run through the ends of the crank spider that
being the spacer, between 2 chain rings,
It should match the distance to the rear chain line

single ring, or middle of a Triple , that line
runs through the chain-ring's plane..

So,... use the spacers, if needed, to dial in the front part of the ideal chain line ..

... ;)

Road cranks and bottom brackets do not come with, or require spacers. That's a MTB thing. Shimano has made no adjustment to crank width to compensate for wider hubs on road disc or gravel bikes. They don't seem to think it's a problem, and I tend to agree.