Will a straight-steerer fork fit a tapered head tube?
By Crankworthy · Published · Updated
Yes
Yes, with a reducer. A 1-1/8" straight steerer fits a tapered head tube using a lower headset cup that steps the 1.5" bearing seat down to 1-1/8". It is a common and safe conversion.
Why
This is the direction that works. The tapered head tube has more room at the bottom than a straight steerer needs, so the gap is filled with a reducing lower cup and a matching crown race.
It is often done to fit an older or cheaper straight-steerer fork into a modern frame. You lose some of the stiffness a tapered steerer would give, but nothing about the fit is compromised.