What Is Deflection?

When you strike the cue ball directly through its centre, it initially travels along the line of the cue.

When you strike to the left or right of centre to apply side spin, the cue ball initially deviates away from that line. This is commonly referred to as cue-ball deflection or squirt. This behaviour is well established in billiards physics. Cue and ball deflection or “squirt” in billiards [physics.usyd.edu.au]

The amount of deflection produced can vary depending on the cue, its construction and the way the shot is played.

Why Does Deflection Matter?

Every player unconsciously becomes accustomed to the characteristics of their own cue.

If you've played with the same cue for years, you've learned how it reacts when using side, even if you've never consciously thought about deflection.

Change to another cue and that reaction may be different.

You could therefore have a new cue that appears perfect:

Right weight.
Right length.
Right tip size.
Right balance.

Yet when you actually play with it, something doesn't feel right.

You may find yourself consistently over-cutting or under-cutting certain shots when applying side.

The new cue may simply be producing a different amount of deflection to the cue you've become accustomed to.

Does a Stiffer Shaft Mean Less Deflection?

This is where I would change your original advice quite significantly.

Not necessarily.

People often associate a stiff shaft with low deflection, but shaft stiffness alone doesn't determine how much cue-ball deflection a cue produces.

Front-end mass is particularly important, while shaft construction, diameter, taper, ferrule and other characteristics can also affect the behaviour of the shaft. The Engineering Behind Carbon Fiber Pool Cues specifically identifies weight distribution, shaft construction, stiffness and front-end design as factors involved in deflection. [bizubilliards.com]

That's why you can test two apparently stiff shafts and find that they don't react identically.

Likewise, a shaft having some flex doesn't automatically mean that it will produce excessive deflection.

You can't reliably judge deflection simply by bending a shaft and deciding how stiff it feels.

Less Deflection Isn't Automatically a "Better" Cue

I think this is an important addition.

It's tempting to assume:

Less deflection = better cue.

I wouldn't necessarily look at it that way.

A player who has used the same cue for many years will have naturally learned to compensate for the way that particular cue behaves.

Give that player a cue producing substantially different deflection and they may initially struggle, even if the new cue could technically be described as lower deflection.

Consistency and familiarity are extremely important.

The objective is therefore not necessarily to find the cue producing the absolute minimum deflection.

It's to find a cue whose reaction is predictable and comfortable for you.

Accidental Off-Centre Striking

There's another reason I pay attention to this characteristic.

Players don't always strike exactly where they intend.

We're human.

If you unintentionally strike slightly away from centre, the reaction of the cue can influence how far the cue ball departs from your intended line.

A cue whose off-centre reaction feels strong or unfamiliar may therefore feel less forgiving to that particular player.

This can sometimes explain why somebody picks up one cue and immediately says:

"I can play with this."

Yet another cue with almost identical basic specifications simply doesn't feel right.

Why This Matters When Changing Cues

This links perfectly with the previous advice about replacing an old faithful cue.

You may accurately reproduce your old cue's:

58" length
18oz weight
9.5mm tip
29.5mm butt
17.5" balance

and the new cue can still behave differently when you start applying side.

That's because those measurements don't fully describe the playing characteristics of a cue.

Specifications tell us a lot about the size and physical setup of a cue. They don't tell us everything about how it will react through the cue ball.

Our Advice

When testing a new cue, don't only play straightforward centre-ball shots.

Try shots using:

• Left-hand side
• Right-hand side
• Controlled screw with side
• Running side
• Check side

You're not trying to find a cue with zero deflection. All conventional cues will produce some cue-ball deflection when side is applied. Research describes the cue ball as deflecting away from the cue's approach line when struck to the side of centre. Cue and ball deflection or “squirt” in billiards [physics.usyd.edu.au]

You're looking for something more important:

Can you predict what the cue is going to do?