Bike frame size: why reach and stack beat S, M or L

No bike fit can fix a frame that's the wrong size — stem and seatpost can disguise half a size, no more. If you're buying a bike, this decision precedes every adjustment.

Forget S/M/L, look at reach and stack

Nominal sizes vary between brands: one maker's M is another's L. The comparable numbers are reach (horizontal distance from bottom bracket centre to the top of the head tube) and stack (the vertical distance between the same points). They state exactly where the cockpit sits relative to the pedals — which is what your body feels.

Height picks the range; your torso picks the point

The maker's height chart gets the range right (e.g. 171–178 cm → M). Within the range, your proportions decide: a long torso and arms point to the bigger size or a longer reach; a short torso, the opposite.

Arm-span trick: compare your arm span (fingertip to fingertip, arms out) with your height. Span greater than height → lean towards the larger size in your range; smaller → the smaller one. It's one of the inputs FitRide collects in your profile.

Caught between two sizes?

• Comfort/long rides priority: the smaller one, with a head tube that allows a higher bar.
• Performance priority: depends on your torso — don't pick the bigger frame 'to stretch out' if your flexibility won't follow.
• Check standover clearance, especially on MTB.

Already own the bike? Then the game changes: extract the best from the geometry you have — exactly what a good fit does.