Why a Heel Strike Curves Back and a Thin One Does Not

A player came in last spring convinced his driver was the strongest club in his bag and his long irons were unplayable. His strike pattern was nearly identical with both: consistently toward the heel, roughly a ball width inside the hosel. Same delivery, same miss, wildly different results. He wanted to know why.

The answer is gear effect, and it is one of the few pieces of clubhead physics that is worth explaining to a player in full, because it changes how he thinks about which clubs belong in his bag at all.

The Mechanism, Without the Diagram

When impact happens away from the centre of gravity, the head does not simply push the ball. It also twists around that centre of gravity. A heel strike twists the face open as the ball is still compressed against it, and the two surfaces are in contact long enough for that twist to impart sidespin — the way two gears turning against each other rotate in opposite directions.

The critical detail is the direction. A heel strike twists the face open, which drags the ball into draw spin. A toe strike twists the face closed and produces fade spin. That is backwards from what most players expect, and it is why a heel miss with a driver so often finishes in the fairway despite starting right.

Depth Is the Whole Story

How strongly this happens depends on how far behind the face the centre of gravity sits. A driver head is deep — the centre of gravity might be an inch and a half back from the face. That long lever arm produces a large twist and a large corrective spin.

A long iron has a centre of gravity sitting a few millimetres behind a thin face. The lever is tiny, the twist is negligible, and gear effect essentially does not happen. The heel strike loses ball speed, starts right, and stays right. There is no mechanism available to bring it back.

Gear effect is not a forgiveness feature that manufacturers add. It is a by-product of how deep the head is, which is why the deepest clubs in your bag are the most forgiving of horizontal misses and the flattest ones are the least.

Bulge and Roll Are the Other Half

Drivers and fairway woods have a face that curves horizontally — bulge — and vertically — roll. Bulge exists specifically to work with gear effect rather than against it. Because the face is curved, a heel strike also starts the ball further right than a centre strike would, and the draw spin generated by gear effect brings it back.

Those two effects are designed to roughly cancel. When they are matched, a heel strike finishes near the target, just shorter. When the head is wrong for the player's pattern, they do not cancel, and you get the overcorrecting hook that some players find on their toe misses.

Roll does the same job vertically. High strikes launch a touch higher and spin less; low strikes launch lower and spin more. This is precisely why the vertical penalty in our face mapping work is so much larger than the horizontal one — the vertical correction is working with spin, not against distance, and a low strike gets both a speed penalty and a spin increase at the same time.

What This Changes About Fitting

Three practical consequences come out of this, and they show up on build records here constantly.

Long irons should be earned, not assumed. If a player's pattern is scattered horizontally, the long irons in his set are doing nothing for him and a hybrid or a fairway wood will produce a materially better bad shot at the same yardage. This is the single most common recommendation on our hybrid records, and it comes out of the strike pattern rather than out of launch angle.

Bulge radius is a fitting variable. Not adjustable, but selectable. Heads differ, and a player who consistently overdraws his heel misses is often on a head whose bulge is too aggressive for the amount of gear effect his strike produces. Changing head family fixes it; changing loft does not.

Shaft length moves the pattern, and the pattern is the input. A driver that is an inch too long moves the whole strike map toward the heel. The gear effect will hide some of that from the player, so he never diagnoses it, and he keeps paying about seven per cent of his ball speed for the privilege.

A Note on Modern Faces

Face architecture has moved on. Variable thickness patterns and the flexible perimeters now common on the outer regions of a driver face do genuinely reduce the speed penalty of an off centre strike, and they interact with gear effect in ways that vary head to head.

We do not take manufacturer claims about this on trust. What we do is hit the head with a deliberate heel strike and a deliberate toe strike, log the speeds and the curvature, and see whether the behaviour matches what is printed. Sometimes it does. When it does not, the tape and the monitor are the two things on the record, and the brochure is not.