Dr Ian Plummer
There is confusion about when crushes can be committed. Hopefully the information below will assist players.
Laws 28.a.9 and 29.a.10 state that "a fault is committed if, during the striking period, the striker:
- strikes the striker's ball so as to cause it to touch a hoop upright or, unless the striker's ball is pegged out in the stroke, the peg when in contact with the mallet;
- strikes the striker's ball when it lies in contact with a hoop upright or, unless the striker's ball is pegged out in the stroke, the peg otherwise than in a direction away therefrom;"
These faults are generally called a crush - the mallet crushes the ball against the hoop upright.
Crushes are less frequent than people may think.
The Commentary on the Laws - the "Official Rulings on the Laws of Croquet" (ORLC) advises:
Law 28.a.9. This is the classic crush stroke but it is more difficult to commit than many referees seem to believe. Professor Stan Hall demonstrated that a croquet ball remains in contact with a mallet end-face for a very short time, and somewhat paradoxically, does so for longer in gentle shots. In any event, the longest distance that mallet and ball will travel in contact with each other is about 1 cm (less than 0.5 inches). Note that this does NOT mean that any ball within 1 cm from an upright is therefore a candidate for a crush. The distance that matters is that between the impact points on (a) the ball's circumference and (b) the upright's circumference. In practice, unless the striker is so incompetent as to drive the SB almost straight at the upright (in which case he will double tap anyway), this means that the nearest point of the ball must be within 1-2 mm of the upright before there is any real chance of a crush.
and
Law 28.a.10. This is the easiest way to commit a crush but should only occur if the striker is ignorant of basic physics or tries to play close to the forbidden line and the referee believes he transgressed it.
What Does This Mean in Practice?
There are both static and dynamic aspects to crushes.
For static situations you can readily define a range of safe angles that a ball can be hit through a hoop where it can slip through with no possiblity of being pressed against an upright.
In the plan diagram below the ball lies in the jaws of a hoop resting against one upright. The diagram is to scale with a 1/8" gap between the ball and the hoop.
The diagram shows that the ball can be played at an angle of up to ~20° out of the hoop without the possibility of a crush. The edge of the grey region lies on the tangent of the ball and the right hand hoop upright.
Consider however the situation below:
The stroke has to be played at no less angle than that defined by the perpendicular to the point of contact between the ball and the left upright in the diagram. Otherwise the ball will be forced into the left upright and be a crush.
At first inspection it appears that the ball will plough into the right upright - it does not have to be a crush though. The dynamic effects have to be considered.
As mentioned in the Commentary above, during a stroke the ball is in contact with mallet face for a brief time at the start of the impact and then springs away from the face. I have repeated Prof. Hall's measurements and confirm his observations. For a normal stroke the ball would 'stick' to the mallet face for ~ 0.25"; for a softer stroke a longer distance, 1cm = 0.4" is suggested.
The red line marks (approximately!) how far the ball needs to move before it makes contact with the right upright. Provided that this is greater than ~1cm and the mallet does not follow on then the stroke can be entirely legal.
Such strokes should be refereed since there is plenty of opportunity for things to go wrong. The likely faults would be a crush or multiple contacts between the mallet and the ball ('a double hit'), e.g. if the mallet catches up with the ball or the ball bounces back from the upright onto the mallet face.
Finally, crushes can happen in non-hoop running situations. For completeness an example is given below.
Here the yellow ball lies on the left hand wire. The red regions indicate where a crush would occur if the mallet is swung towards the centre of the ball. The green regions are 'safe' regions where the ball cannot be crushed - although near the upright the side of the mallet face would have to be used. The orange region is where it is also 'safe' to play although it would depend on the size of the mallet head as to whether it can get access through the uprights.