What is the difference between G4 and G4D?

G4 and G4D look the same from the outside and run the same drills in Syncro. The difference is inside: how each gate decides which part of the athlete it has timed.

At a glance

	G4	G4D
Timing beams	1	2
Alignment laser (pointer)	No	Yes
How the torso is found	Longest break wins (correction in the gate)	Both beams broken at once (AND gate)
Single beam mode	Always	Can be selected

1. The alignment laser

G4D carries a third laser that does no timing at all. It puts a visible dot on the reflector so you can see where the gate is aimed while you set it up. G4 has no pointer; you align it by the beam itself and the strobe feedback described in Aligning your system.

2. The real reason for G4D: knowing it was the torso

A sprint time should be taken when the torso crosses the line. The trouble is that arms and legs get there first. A leading hand, a trailing arm or a high knee can all break a timing beam before or after the chest does.

How a single beam gate copes

A single beam gate cannot tell a hand from a chest, so it has to correct at the gate. It logs every break as the athlete passes, keeps the longest one, and takes the leading edge of that break as the torso. The reasoning is simple: the torso is wider than an arm, so it should hold the beam broken longer.

Most of the time that works. It is not foolproof. A forearm swinging along the line of the beam, rather than across it, can hold the beam broken longer than the torso does. The gate then picks the arm, and the split comes out early.

How a dual beam gate solves it

G4D has two timing beams, one above the other, and only counts a trigger when both are broken at the same time. That is an AND gate. An arm or a high femur can break one beam, but it cannot fill both. The torso is the only part of the body tall enough to do that.

So G4D does not have to guess from pulse width. It is not fooled by an arm moving along the beam, because however long the arm stays there it only ever covers one beam.

Which one do I need?

• G4 is fine for most testing, and the longest-break correction handles the usual arm and leg triggers.
• G4D is the choice where every split has to be trusted: high level squads, research, and anywhere athletes with a big arm action are being compared over short distances.

Both run the same Syncro drills.

Diagrams are schematic and not to scale. Traces are illustrative, not recorded data.