Seat tube angle measures how far the seat tube tilts back from horizontal, usually ranging from 73° to 80° on mountain bikes. A steeper angle (76° or higher) positions you more directly over the pedals, which improves power transfer and keeps your front wheel grounded on steep climbs. A slacker angle shifts your weight back, feeling more relaxed on flat terrain but hurting climbing efficiency.
Most modern mountain bikes use steeper angles between 75° and 78° because climbing performance matters more than ever. If you regularly tackle steep, technical climbs, look for 76° or steeper.
One important catch: the "effective" angle changes as you raise your saddle. On many frames, taller riders running higher saddles experience a slacker effective angle than the published number suggests. Always check whether the listed angle is measured at a specific saddle height, not just the frame's actual tube angle—this distinction can significantly impact how the bike climbs for your height.