Bulge
Bulge is the horizontal curvature of the clubface on woods — it steers balls struck off-centre back towards the target line.
Bulge is a slight rounding of the clubface from heel to toe, that is, across the direction of play. If you do not strike the ball in the middle, the so-called gear effect kicks in: the ball starts sideways but at the same time receives spin that makes it curve back. A strike at the toe produces hook spin, a strike at the heel produces slice spin. Bulge amplifies this correcting effect and thereby makes woods more forgiving. Irons do not have this curvature because their shorter shaft and construction need the effect less.
In practice you feel the bulge when your drive is not quite clean and the ball still lands respectably on the fairway. An example: you catch the ball slightly towards the toe of the driver. Without bulge the ball would drift straight to the right. Because of the curvature it starts to the right and pulls back towards the middle with slight spin. Exactly this physics explains why many off-centre escapes turn out mercifully.
The typical misconception: bulge does not heal bad swings. It softens small strike deviations but compensates neither a widely open clubface nor a wrong swing path. If you regularly produce big curves like the hook, you will find the cause in the swing, not in the equipment. Bulge is an engineering detail from the 19th century that is still built into every wood today — a quiet helper that you only notice when you think about strike points.
FAQ
Why does my drive sometimes still land on the fairway despite a poor strike?
That is the bulge effect. The horizontal curvature of the clubface gives a ball struck off-centre spin that makes it curve back towards the target line. A strike at the toe produces hook spin, a strike at the heel produces slice spin — and both times the ball bends back instead of drifting straight.
Do irons also have bulge?
No, the curvature is only found in woods such as the driver. Irons do not need the effect because their shorter shaft and construction react differently to off-centre strikes. That is why the clubface of an iron is flat.
Can bulge cure my slice?
No, that is a widespread fallacy. Bulge only softens small strike deviations; it compensates neither an open clubface nor a wrong swing path. If you regularly produce big curves like the hook or the slice, you will find the cause in the swing — not in the equipment.