About the Bias

January 11, 2009 by lawn bowls 

One of the main things that people will look for when buying lawn bowls is the bias that the bowl has. The bias is the part of the bowl that has been designed to allow the bowl to curve when it is thrown. The bias that the bowl will have will depend on the type of bowl and the manufacturer. Bowls can either have a large bias or a narrow bias and bowlers will choose one based on how they like to throw the bowl. A narrow bias will allow for bowls to get into small spaces and will roll straighter. Large biases are able to go around other bowls and will have a very large curve when they are thrown. How fast the greens are will also depend on the bias of the bowls. There are strict regulations pertaining to biases however, players still have a wide range of bowls with different biases to choose from.

Share and Enjoy: These icons link to social bookmarking sites where readers can share and discover new web pages.
  • Digg
  • Sphinn
  • del.icio.us
  • Facebook
  • Mixx
  • Google

Comments

19 Responses to “About the Bias”

  1. Ken Humhreys on February 11th, 2010 6:17 pm

    Re. “About the Bias” Can anuone tell me what are the rules that Bowls Manufacturers have to comply with so their woods qualify for the “Stamp” It seems to me that more and more “finer line ” bowls are being advertised and before long we will have some enterprising manufacturer selling bias less bowls.

    Your comments will be much appreciated and will go all the way to resolving many a discussion. Thanks.

    Ken Humphreys

  2. Tony on February 12th, 2010 4:42 pm

    Hi Ken,

    Understand there is a master ‘bowl’ that has the minimum Bias allowed that each menufacture has comply with!!

    Thr restamping every 10 years so Bowls can compete in major competitions is to ensue that the set of 4 bowls still are within tolerance of each other and still comply. More importantly it ensures that there has been no tampering with the bowls ie; shaving the bias off to have straighter lines then bendy woods as they progress. (first wood straight, 2nd not so straight etc)
    Hope this helps

  3. Ken Humphreys on February 13th, 2010 4:43 pm

    Hi Tony, Many thanks for your response. Close to resolving the problem except that a little more meat is required onto your first statement that there is a master ‘bowl that has a minimum ‘bias’ In other words who has set the standard and is this standard incumbent on all the manufacturers and all the various National and International bodies. My thanks in anticipation of the problem being resolved.

    Ken

  4. Keith on February 14th, 2010 9:57 pm

    If You Want to BOAR Bowlers in a quiz, TRY THIS, looking forward to see who holds the master Bowl; anyway see below;

    The trajectory of a ball in lawn bowls
    Rod Cross
    Physics Department, University of Sydney, Sydney 2006, Australia
    ~Received 6 November 1997; accepted 26 January 1998!
    The main objective in lawn bowls is to bowl a ball along a curved trajectory on a smooth grass
    surface so that it stops closer to the jack ~a small white ball! than the opponent’s ball. The curvature
    of the path is induced by shaping the ball so that one side is heavier than the other. Some of the
    properties of the trajectory are described to illustrate this interesting example of precessional
    motion. © 1998 American Association of Physics Teachers.
    I. INTRODUCTION
    Lawn bowls is a popular outdoor sport for both men and
    women, especially with more senior citizens, since the game
    does not require a particularly high level of physical fitness.
    The game is played at a leisurely pace on an immaculately
    manicured lawn and requires only an empirical knowledge of
    the laws of precession. Because the ball is weighted on one
    side, it travels in a curved path whose radius of curvature
    decreases with time until the ball eventually stops. The ball
    has a mass typically of about 1.5 kg, and a diameter of about
    12 cm, with small variations allowed to suit the individual
    bowler and the playing surface conditions. The ball is
    launched with a speed of about 4 ms21, at an angle of about
    10° to the direct line of sight to the desired end point, then
    rolls at a walking pace for about 14 s over a total distance of
    about 25 m.
    A weighted ball precesses like a gyroscope or a spinning
    top1,2 but its axis of rotation is not anchored to a fixed point.
    As a result, the precessional motion is combined with linear
    motion to generate a curved trajectory. In this respect, the
    path of the ball is similar to that of a wheel or a coin3,4 that
    735 Am. J. Phys. 66 ~8!, August 1998 © 1998 American Association of Physics Teachers 735
    rolls along a surface of its own accord. As the ball or the
    wheel slows down, the rate of precession increases and the
    radius of curvature decreases. A spinning top, or a rolling
    wheel or a coin precesses as a result of the torque associated
    with a tilt away from the vertical. A ball in lawn bowls tends
    to remain upright throughout its motion since the center of
    mass is shifted only slightly from the geometric center and
    since the surface is indented slightly by the ball, and the
    tendency of the ball to fall over as it comes to rest is resisted
    by a sideways reaction force from the ground. The ball will
    fall over when it is placed on a horizontal solid surface, and
    may also fall over at the end of its trajectory on a smooth
    grass surface, depending on its profile and the condition of
    the surface.
    The ball used in lawn bowls is not perfectly spherical and
    it is not deliberately weighted by any additional mass. The
    weighting or ‘‘bias’’ is achieved by removing mass so that
    the shape remains circular in a cross section normal to the
    axis of rotation @as shown in Fig. 1~b!# and is slightly elliptical
    in a cross section that includes the axis of rotation @as
    shown in Fig. 1~a!#. In the elliptical cross section, the minor
    radius on one side of the ball is slightly larger than the other,
    with the result that one side of the ball is heavier than the
    other. The essential physics can be adequately modeled, both
    theoretically and experimentally, using a solid spherical ball
    that is weighted by grinding a flat surface on one side. Such
    an experiment was described in this Journal by Guest5 in
    1965 as an interesting undergraduate experiment in precession.
    Guest used a small steel ball bearing on a horizontal
    glass or rubber surface and photographed the trajectory with
    the aid of a strobe light to measure the linear and angular
    velocities. However, he did not derive a formula for the precessional
    velocity and did not relate the experiment to lawn
    bowls. A simpler laboratory technique is to roll a ball or disk
    on carbon paper over a sheet of white paper to leave an
    imprint of the trajectory on the sheet.
    II. EQUATIONS OF MOTION
    The situation modeled in this paper is shown in Figs. 1
    and 2. A spherical ball of mass M and radius R is weighted
    by removing mass from one or both sides so that the center
    of mass is shifted a distance d from the geometric center, as
    shown in Fig. 1~a!. The ball is launched in the x direction on
    a horizontal surface, in the x – y plane, with an initial velocity
    v0 , and subsequently travels in a curved path from O to
    K as shown in Fig. 2. The main object of the following
    calculations is to determine the correct initial velocity and
    direction of the ball so that it finally stops at point K, a
    distance D from O and located at an angle D to the x axis.
    The problem has previously been examined by Brearley and
    Bolt6 and by Brearley,7 but their advanced mathematical
    treatment obscures the essential physics of the problem and
    is not suitable for any elementary presentation.
    The linear motion of a rolling ball can be described by the
    relation
    v5v02mgt, ~1!
    where v is velocity of the center of mass of the ball, m is
    the coefficient of rolling friction, g is the acceleration due to
    gravity, and t is the time. This relation follows from the fact
    that the frictional force acting to decelerate the ball can be
    expressed as F5mMg. Experimentally, it is found that
    m;0.032 on most bowling greens, independent of the mass
    or speed of the ball, but it can be as large as 0.038 on a
    slow green or as small as 0.025 on a fast green.6 In the
    following treatment, we ignore the initial sliding component
    of the motion since the ball starts rolling almost immediately
    after it is launched. An unbiased ball will move in a straight
    line path of length S5v0 2 /(2mg) and come to rest at time
    T5v0 /(mg). The coefficient of friction can be measured
    easily, without having to measure the initial velocity, from
    the relation S5mgT2/2. The same relation holds for the
    curved trajectory of a weighted ball, as shown below. It is
    interesting to note that the ball takes longer to arrive at its
    Fig. 1. Cross sections of a lawn bowl in ~a! the y – z plane and ~b! the x – z
    plane. The z axis is vertical, and the ball rolls in the horizontal x – y plane,
    initially in the x direction. For a rolling ball that indents the surface, the
    ground reaction force, N, acts as shown in ~b! to decrease both v and v so
    that v5Rv at all times.
    Fig. 2. Trajectory of a ball from O to K in the horizontal x – y plane. The
    ball is launched along the x axis at angle n to OK.
    736 Am. J. Phys., Vol. 66, No. 8, August 1998 Rod Cross 736
    destination on a fast green than it does on a slow green, since
    the ball must be launched at lower speed.
    The coefficient of rolling friction8–10 is not simply related
    to any other coefficient of friction, for the following reason.
    If a ball of radius R starts to slide without rolling, a friction
    force, F, acts horizontally at the point of contact, decreasing
    the linear velocity, v, of the center of mass and increasing
    the angular velocity, v, via the torque applied to the surface.
    Rolling commences when v5Rv, at which point there is no
    relative motion at the point of contact with the ground. It is
    observed that a rolling ball will eventually come to rest. One
    might assume that this is simply due to a friction force acting
    horizontally on the ball. While such a force would act to
    decrease the velocity, v, it would increase the angular velocity,
    v, and the ball would end up spinning on the spot like
    the wheel of a car stuck in the mud. Rolling friction arises
    from the fact that the ball or the surface is slightly rough or
    deforms in such a way that the reaction force, N, from the
    ground does not act at a point below the center of rotation,
    but it acts forward of the center as shown in Fig. 1~b! to
    reduce both v and v while maintaining the rolling condition
    v5Rv. Since the deformation and hence the reaction force
    is approximately proportional to the weight of the ball, m in
    Eq. ~1! is essentially independent of M.
    In order for a weighted ball to roll smoothly along a
    curved path, the axis of rotation must pass through the center
    of mass ~CM!, as shown in Fig. 1~a!. If the axis of rotation
    does not pass through the CM, then the CM will rise and fall,
    generating a wobble in the motion. For simplicity, it is assumed
    that the axis of rotation remains horizontal throughout
    the trajectory. In fact, the ball will tilt slightly but this has a
    negligible effect on the trajectory since the ball tilts only
    near the end of its trajectory and usually only by a small
    amount.
    A frictional force Fy5Mv2/r, shown in Fig. 1~a!, is necessary
    if the ball is to follow a curved path. In the absence of
    this component, the ball would travel in a straight line, rotating
    freely about a vertical axis through P due to precession.
    Such a result might be expected, for example, on a
    slippery ice surface. The torque component, t x5FyR
    2Mgd, acting about the geometric center acts to change the
    direction but not the magnitude of the angular momentum of
    the ball. This point is discussed in some detail in most elementary
    physics texts, in connection with the precession of
    a gyroscope or spinning top. The angular momentum is
    given by L5ICMv where ICM is the moment of inertia about
    the rotation axis passing through the center of mass. At any
    point along the trajectory, a tangent to the path makes an
    angle f with the x axis, and the change in angular momentum
    in the x direction, as a result of a small rotation df in
    time dt is dLx52Ldf. The positive x direction is into the
    page in Fig. 1~a!. The x component of the torque is therefore
    given by
    Mv2R
    r
    2Mgd5L
    df
    dt
    52ICMvvp , ~2!
    where vp5df/dt is the angular velocity of precession.
    Since v5rvp5Rv when the ball is rolling, Eq. ~2! yields
    vp5df/dt5MgdR/~I0v!, ~3!
    where I05ICM1MR2 is the moment of inertia about a horizontal
    axis through an edge of the ball. This relation differs
    from the usual expression for the precessional velocity of a
    gyroscope or spinning top in that the relevant moment of
    inertia is I0 rather than ICM . Equation ~3! accounts, within
    1%, for the experimental data given by Guest.5 The radius of
    curvature is given by
    r5
    v
    vp
    5
    v2I0
    MgdR
    , ~4!
    so
    dr
    dt
    52pr
    df
    dt
    ~5!
    where dv/dt52mg and p52I0m/(MdR). Equation ~5! can
    be integrated to give
    r5r0e2pf, ~6!
    where r05pv0 2 /(2mg) is the initial radius of curvature. The
    path length of the ball is therefore
    s5E0
    f
    r df5~12e2pf!r0 /p. ~7!
    The variation of f with time can be obtained by integrating
    Eq. ~3!, using Eq. ~1!, to give
    f5~ 2/p!ln~v0 /v!, ~8!
    indicating that f!` as v!0. From Eq. ~7!, the total path
    length is S5r0 /p5v0 2 /(2mg) which is independent of p and
    is therefore the same as that for an unbiased ball. The detailed
    behavior of the ball at the very end of its trajectory is
    not considered in this paper. Equations ~3! and ~4! indicate
    that vp!` and r!0 as v!0, but the ball is likely to tilt or
    topple in the last few mm of the trajectory.
    III. THE TRAJECTORY
    The x and y coordinates of the trajectory can be obtained,
    as a function of time, from the relations dx/dt5v cos f and
    dy/dt5v sin f, so
    x1iy5E0
    t
    veif dt5E0
    f
    reif df, ~9!
    where vdt5ds5rdf. Equating real and imaginary parts of
    Eq. ~9! gives
    x5
    r0
    11p2 ~p2pl cos f1l sin f ! ~10!
    and
    y5
    r0
    11p2 ~12l cos f2pl sin f !, ~11!
    where l5e2pf. The end point of the trajectory, where l50,
    therefore has coordinates Y5r0 /(11p2) and X5pY, so the
    range of the ball is given by D5(X21Y2)1/25r0 /
    (11p2)1/2 and the angle D in Fig. 2 is given by tan D51/p.
    A measurement of p can therefore be obtained by measuring
    D or X/Y, and the path length can be calculated from the
    expression S5(111/p2)1/2D. Since D is independent of v0 ,
    a ball must be launched at the same angle to the line of sight,
    regardless of the desired range, D. However, it is against the
    rules and spirit of the game to use a protractor.
    737 Am. J. Phys., Vol. 66, No. 8, August 1998 Rod Cross 737
    The radial distance, r, from any point (x,y) to the end
    point (X,Y) is given by r5De2pf, indicating that the trajectory
    is a logarithmic spiral, as noted by Guest.5 The ball
    reaches its maximum displacement in the x direction and
    moves purely in the y direction when f5p/2, at which point
    v5v0 exp@2pp/4#. The nature of the trajectory is determined
    primarily by the parameter, p. If ICM is taken to be
    essentially that of a sphere, i.e., ICM50.4MR2, then
    p52.8mR/d, which, for d;1 mm, is typically about 5 in
    lawn bowls, in which case r/r0;0.0004 and v/v0;0.02
    when f5p/2. Consequently, the ball approaches the end
    point in a relatively gentle arc and does not spiral inwards by
    orbiting the end point more than once, as it would if p were
    less than about 1. Typical trajectories are shown in Fig. 3 for
    cases where p54 to p510, which are representative of normal
    playing conditions. The rules of lawn bowls place no
    specific limit on p, since it depends on m, but balls are tested
    periodically against a master bowl. To pass the test, the bias
    of the ball must not be less than that of the master bowl. One
    of the challenges in lawn bowls is to allow for the fact that p
    can change during the course of a game since the grass can
    dry out during the afternoon and change the coefficient of
    rolling friction.
    The results shown in Fig. 3 were obtained by rotating
    the coordinate system through an angle D so that all trajectories
    start and end at the same points ~O and K, respectively!.
    In this rotated x8– y8 coordinate system, where the
    x8 axis is along the line OK in Fig. 2, one can show that
    the maximum y8 displacement is given by y8max
    5Dl/(11p2)1/2 and it is located at x8max5D(12py8max),
    where l5exp(2pD). For example, if p55, y8max50.0731D
    at x8max50.634D and if p58, y8max50.0459D at x8max
    50.633D.
    Almost identical results are obtained if the ball is allowed
    to tilt during the motion. The tilt angle, u, tends to remain
    small for lawn bowls, but the trajectory when u varies with
    time can be obtained by numerical integration of the equations
    dx/dt5v cos f, dy/dt5v sin f and df/dt5A/v
    where v is given by Eq. ~1! and A5MgdR cos u/I0 . The
    result of such a calculation is that vp differs from the tilt-free
    case only near the end of the trajectory, but the trajectory
    itself is almost indistinguishable from the tilt-free case. Discussions
    of the trajectory of a ball in ten-pin bowling, which
    slides rather than rolls for most of the trajectory, can be
    found in Refs. 11 and 12.
    From a teaching point of view, a rolling ball or coin is
    perhaps easier to understand than a precessing top or gyroscope.
    A gyroscope or top appears to have the somewhat
    mysterious property that it does not fall in response to the
    tilting torque, but moves in a direction normal to the applied
    force.1,2,13 The same situation applies to a rolling ball or
    coin, but a student is more likely to be at ease with the fact
    that the ball or coin does not fall, because its momentum
    carries it forward.
    1P. C. Eastman, ‘‘Painless precession,’’ Am. J. Phys. 43, 365–366 ~1975!.
    2P. L. Edwards, ‘‘A physical explanation of the gyroscope effect,’’ Am. J.
    Phys. 45, 1194–1195 ~1977!.
    3M. G. Olsson, ‘‘Coin spinning on a table,’’ Am. J. Phys. 40, 1543–1545
    ~1972!.
    4R. F. Deimel, Mechanics of the Gyroscope ~Dover, New York, 1952!, 2nd
    ed., pp. 93–98.
    5P. G. Guest, ‘‘Rolling precession,’’ Am. J. Phys. 33, 446–448 ~1965!.
    6M. N. Brearley and B. A. Bolt, ‘‘The dynamics of a bowl,’’ Q. J. Mech.
    Appl. Math. 11, 351–363 ~1958!.
    7M. N. Brearley, ‘‘The motion of a biased bowl with perturbing projection
    conditions,’’ Proc. Cambridge Philos. Soc. 57, 131–151 ~1961!.
    8J. Witters and D. Duymelinck, ‘‘Rolling and sliding resistive forces on
    balls moving on a flat surface,’’ Am. J. Phys. 54, 80–83 ~1986!.
    9A. Domenech, T. Domenech, and J. Cebrian, ‘‘Introduction to the study of
    rolling friction,’’ Am. J. Phys. 55, 231–235 ~1987!.
    10J. Hierrezuelo, B. R. Catolicos, and C. Carnero, ‘‘Sliding and rolling: The
    physics of a rolling ball,’’ Phys. Educ. 30, 177–182 ~1995!.
    11D. C. Hopkins and J. D. Patterson, ‘‘Bowling frames: Paths of a bowling
    ball,’’ Am. J. Phys. 45, 263–266 ~1977!.
    12R. L. Huston, C. Passerello, J. M. Winget, and J. Sears, ‘‘On the dynamics
    of a weighted bowling ball,’’ J. Appl. Mech. 46, 937–943 ~1979!.
    13J. B. T. McCaughan, ‘‘Teaching gyroscopic precession at the elementary
    level,’’ Phys. Educ. 17, 133–138 ~1982!.

  5. Tony on February 14th, 2010 10:01 pm

    What a post Keith!!!!!

    Real answer is that the World Bowls licnce the table and teating equipment to confir with tere ‘MASTER BOWL as below!!!!¬

    STAGE 6

    Final testing using the approved and licensed test table. All bowls are tested to the W.B.B. “Master” bowl. As well as having to comply with the even more rigorous requirements of the manufacturers. Thus the different models are tested to their model specifications to confirm that their performance will be optimal for given greens.

  6. Ken Humphreys on February 15th, 2010 12:02 pm

    The response from Keith made me check the date and finding it was not April 1 I can only assume he suffers from both verbal diarrhoea and basic comprehension.
    Thank you Tony for a succinct and literate answer.

    Ken

  7. morris mccullagh on August 16th, 2010 3:34 pm

    As a coach and keen mathematician I have always wanted to know the maths of the curve a bowls takes and so I found this exposition both interesting and bewildering. I wonder do manufacturers use mathematical formulae in order to determine the bias of a new bowl or is it hit and miss.

  8. Alan Glenny on October 30th, 2010 2:19 pm

    I’ve ‘read’ Keith’s entry on the motion of a bowl on the trajectory of a ball in lawn bowls by Rod Cross, Physics Department, University of Sydney, Sydney 2006, Australia.

    I also have an interest in this area (please don’t tell me ‘to get a life’), I do have one.

    Does anyone have access to the original article as the maths contained in it is somewhat garbled.

  9. Rob Buttrose on November 11th, 2010 5:33 am

    Hi, basic question from a newbie for all you bowlers.

    Given green conditions, wind, temperature and bowl bias how may paths will take a given bowl to the jack (or any desired location ?) Is it only one or range?
    Suppose you find the combination of speed and angle that gives you the perfect path. Now , if you bowl a narrower angle than that of the perfect path, the bowl has to have less speed (because it is travelling a shorter distance) . Will it , however, curve off the line or can it curve less? Suppose on the other hand you bowl at a wider angle than the perfect path. In this case, you have to have more speed because it needs to travel further. Can it curve back enough or will it stay wide?

    I’ve had a look at the above article by Cross. The answer may be in there somewhere, but I can’t find it. In any case, anyone who has bowled for a while (and has not been kidding themselves! ) will know the answer.

  10. Peter on December 16th, 2010 5:41 pm

    Keith’s response is a straight copy of a theoretical academic paper by Rod Cross published in August 1998 in the American Journal of Physics, for Physics Teachers.
    The rules and regulations for master bowls are determined by Word Bowls Ltd. For more information you can visit their website.

  11. Alan Glenny on January 31st, 2011 7:52 pm

    In answer to Rob Buttrose’s query about bowl delivery direction, I have looked at Rod Cross’s piece and had e-mail ‘discussions’ with him.

    What emerges from his work, amongst other things, is that to bowl to a given jack position, the angle of delivery (that’s the angle between the line between the mat and jack (centre line) and the initial direction of motion of the bowl) is constant regardless of the jack length.

    So if you bowl narrow with reduced weight, the bowl will cross the centre line and end up short, and vice versa.

    Arguments often arise about the statement ‘you need to give the bowl more green for a longer jack length.’ What Rod Cross’s maths shows is that for a constant mat position, the angle of delivery, therefore the ‘aimimg-point’ on the rear bank, is constant. True for a longer jack length the bowl will go further from the centre line.

    However, with a varying mat position and fixed jack length, again the delivery angle is constant, but the required aiming point moves inwards towards the centre line as the mat is moved up the green. This confuses some bowlers.

    I’m happy to explain this further if anyone is interested.

  12. Bill Huggins on February 27th, 2011 2:36 pm

    How do you decide what weight,size and bias suits you?
    I started with size 3 and wide bias and my son has bought me size 3 with a narrow bias.Both of which I was ok with but not consistent.
    I see other players with various sizes with heavy added,how do you know which is right for you to get consistency.

  13. PHIL ROBINS on July 13th, 2011 1:57 pm

    The important points to consider when choosing a bowl type (narrow or wide) assuming you have the correct size and weight are:-
    The amount of bias a bowl has and the type of curve it takes.
    The speed/s of greens you normally encounter i.e. grass, synthetic and carpet.
    The position you normally play in a rink.
    Remember when you use narrow bias bowls you have less tolerance with your weight (length of travel) than with a wider drawing bowl.
    With the narrow drawing bowl you have more tolerance with your green (width) than with a wider drawing bowl.
    On the slower greens you can get away with narrow bowls if you play at the ‘front end’, if your’e a ‘back end’ player you might be better off with a wider drawing bowl.
    There is a strong body of opinion that some modern narrow bowls are more affected by the wind than their wider counterparts.
    On the faster greens (15+ seconds) the ‘back end’ players prefer a narrow bowl because it has less or no hook at the end of it’s travel. This makes the ‘yard-on’ shot and it’s over draw weight deratives much easier to play.

  14. Shogun on September 25th, 2011 2:54 am

    Hi,
    Just for an experiment I am interested in instrumenting a lawn balling bowl with a 9 DOF IMU package. This would allow me to track and display the trajectory, speed, direction, ‘wobble’ and path of a ball in real time on a computer screen. The IMU has three 3 axis sensors, an accelerometer, a gyro, and a magnetometer. The gyro maybe limited to 2000 degrees per second , which tranlates into about 5.5 revolutions per second. Can anyone tell me the range of ball release velocities when balling. (I may be limited to about 9.8 mph using a 5″ diameter ball?)

    One could also use it to measure the ’speed’ of the green, the profile of the green observe the affect of bias etc.

    regards

  15. Zena on November 12th, 2011 5:45 pm

    I have just started playing bowls and am having difficulty deciding how to face my ball depending on the direction. Which side depicts the bias one way or another

  16. larry on January 10th, 2012 2:36 pm

    Hi,

    Where can I purchase a set of the lowest bias bowels?

    Thanks

    Larry

  17. David Irish on May 10th, 2012 8:58 am

    I have a question.
    In a singles game where each bowler delivers four bowls, do all four bowls have to be identical or is it legal to have different biases on each bowl?

  18. rose white on June 12th, 2013 7:50 pm

    As bowls are turned on a lathe there isn’t really any need for such fine calculations as the bias can be made simply by turning the wood or plastic rough blank to pure spherical with a spherical turning attachment – less the very small points needed to provide grip between centres of the lathe – and then both increasing the radius of the attachment to make a slightly bigger sphere while simultaneously moving it a set amount to one side – this will make the cutter remove material from the inner portion of that side without removing any from the diameter so that the second cut half has less material and when rolled will fall away or bias to the other side (I use ‘bias’ as a verb and a noun).

    Different biases can be made by different second cuts as the amount of material needing removal will be quite small which is why many people cannot tell a bowl is not a perfect sphere while old precision turners like myself can see that some bowls are definitely not spherical but just where and by how much eludes us.

    Before spherical turning attachments were available a bowl could be biased by making a semicircular template and shaping the bowl in a plain lathe – as commonly used to make bulbous table legs etc – and cutting the bowl by hand tools and eye until it fit the template then removing material at one side until a gap showed between template and bowl and that would be the lighter non-bias side and again variations in bias could be made by altering the gap width.

    These days a computer controlled lathe is used and the bowl designer can plot a path for the cutter to follow to make a bowl and can make the cutter move in a non-semi-circular path to ensure the outer rolling diameter is perfectly spherical but some part of one side is underspherical or flattish and the opposite side is overspherical or bulging and thus heavier and will be the bias side.

    Sets of flat/lawn/carpet bowls are intended to have different biases so that #1 will roll with slight bias, #2 with more, #3 more and #4 lots more so that as the jack becomes blocked by earlier bowls the later ones can be sent around the blockers to hit the jack.

    A lignum vitae perfectly spherical bowl may have a natural bias due to the density of the grain due to conditions when the wood grew or how big a tree it was cut from as in many trees the wood density can vary from outside to core.
    The variations or lack of variation in lignum bowls may have brought about the use of metal weights fitted to one side to try make pairs of bowls that biased the same.

  19. david soffe on June 19th, 2013 5:57 pm

    can someone tell me if there is a differance between the bias of a ball used for crown green and lawn green balls

Feel free to leave a comment...
and oh, if you want a pic to show with your comment, go get a gravatar!