Going mullet on the Amflow is a popular move, and for a 5'5" rider on a Medium frame it's probably the single most effective tweak you can make. Worth understanding what it actually changes, though, because "bringing the bike down half an inch" isn't quite the whole story.
Swapping the rear from 29" to 27.5" drops the rear axle roughly 12-15mm, which lowers the BB by about 6-8mm (less than the axle drop because the BB sits forward of the rear axle). It also slackens the seat angle very slightly and steepens the head angle by around 0.3-0.4°. So you get a touch more standover-feel at the saddle, easier rear wheel clearance on steep stuff, and a livelier back end in tight corners.
For your height this is genuinely worthwhile.
@Winford reckons the
29" rear makes the bike run wide exiting corners and 27.5" sharpens it up, and that effect is amplified for smaller riders.
@Plummet noted the
rear 29er causes "arse grind" on steep tech and drops, which is exactly the issue a shorter rider on a Medium frame will hit hardest.
A few practical points:
Cranks: consider dropping to 160mm to claw back pedal clearance. The BB will be lower and you'll thank yourself on rocky trails.
Spokes: @Winford and others have flagged the
stock 29" rear wheel as a weak point on the Amflow, so a custom build is killing two birds.
@Astro66's
30mm internal carbon 27.5 build saves around 300g over alloy if you want to go bougie.
Flip chip: the Amflow has a 4-position chainstay adjust that supports mullet, so you don't need to fudge the geometry. Set it to the mullet position and the numbers stay sensible.
What it won't fix: reach and stack. If the cockpit feels long, a shorter stem (35-40mm) and slight saddle-forward shift will do more for fit than the wheel swap. Mullet sorts the rear-end clearance issue, not the front-centre.
Honest take: do the mullet, fit 160mm cranks, and if the front still feels stretched, address the cockpit separately. Should be a noticeably better bike for you.
Happy to dig into specific wheelset options, tyre pairings, or cockpit setup if useful.
