Anthropometric Model Configuration for Bicycle Ergonomics
The article explores how a SOLIDWORKS model has been configured to enable the rapid evaluation of novel bicycle concepts. The work was carried out as part of the BriefBike project, an Innovate-UK program to develop an entirely new class of folding bicycle that will effortlessly fold into a roller-case. The purpose of the model was primarily for ergonomic and clearance studies, but it may also be used for aerodynamic optimization using CFD.
Although anthropometric human models are commercially available, configuring them for fit on a bicycle is challenging. The work involved first parametrically defining the contact points and bicycle geometry, and then fitting the human model to this.
Parametrically Defined Contact Points and Bicycle Geometry
The contact points are represented by SOLIDWORKS parts, often containing only reference geometry such as points, planes and axes. Although sketches within the assembly could often more easily be used, this would mean that the position could not be animated or adjusted using the Mate Controller. Therefore, distance and angle mates were used to position these reference geometry-containing parts.
The following reference geometry was positioned in this way:
- Planes defining the width of each pedal from the centerline (Right plane).
- A crank length part which contains three axes representing the bottom bracket axle and the two pedal axles.
- Pedal thickness parts with an axis and a plane representing the top surface of the pedal.
- A Seat part with planes representing the seat tube angle and the top surface of the saddle.
- A Bar part with an axis to represent the handlebar.
- Orientating the handlebar grips requires two separate rotations for backwards and downwards sweep (Euler rotations). Grip_Sweep parts are first mated to define the position and backwards sweep. The actual grips are then mated relative to these, defining the downwards sweep.
NOTE: Angle mates have a defined direction. This can flip when an assembly is updating, leading to very unstable and unpredictable behavior, such as assemblies that fail rebuilds or suddenly jump into strange positions for no apparent reason. Defining a reference entity for each angle mate generally prevents this happening. Reference entities for angle mates are not created by default, although it’s usually as easy as clicking the Auto Fill Reference Entity button.
Kinematic Definition of Human Model
The human model may be considered as 19 rigid bodies and therefore has 114 degrees of freedom (DoF) before joints and other constraints are added. The bodies are the hands, lower arms, upper arms, clavicles, head, neck, thorax, abdomen, pelvis, upper legs, lower legs and feet.
These body parts are connected by one of two types of joint. Spherical joints remove three DoF, all three translations and no rotations. Revolute joints remove five DoF, all three translations and two rotations. These can be achieved most efficiently as follows:
- Spherical: Making two spherical surfaces concentric or two points coincident.
- Revolute: It is often achieved using solid geometry by mating two cylindrical faces and two planes which are perpendicular to the axis. However, this is over-constrained since both mates constrain the two rotational degrees of freedom. For a perfect kinematic arrangement, it is better to start with a spherical joint and then make two planes or faces parallel. Alternatively, two axes can be made coincident and then a point made coincident with a plane which is perpendicular to the axis.
The body parts are connected by the following joints, with the removed DoF given in parenthesis:
- Hand to lower arm (Wrist): Spherical (2x 3 DoF)
- Lower arm to upper arm (Elbow): Revolute (2x 5 DoF)
- Upper arm to clavicles (Shoulder): Spherical (2x 3 DoF)
- Clavicles to thorax (Clavicles): Universal (2x 4 DoF)
- Head to neck (Neck1): Revolute (5 DoF)
- Neck to thorax (Neck2): Revolute (5 DoF)
- Thorax to abdomen (Thoracic): Revolute (5 DoF)
- Abdomen to pelvis (Lumbar): Revolute (5 DoF)
- Pelvis to upper leg (Hip): Spherical (2x 3 DoF)
- Upper leg to lower leg (Knee): Revolute (2x 5 DoF)
- Lower leg to foot (Ankle): Spherical (2x 3 DoF)
These basic joints leave 43 DoFs unconstrained and additional constraints are therefore required to fix hands and feet to the pedals and handlebars, as well as further body positioning. Sections of the body will now be considered as independent kinematic chains to simplify understanding of this setup.
Kinematic Chain for Leg and Crank
The first kinematic chain we will consider is made up of four bodies: the crank, pedal/foot, lower leg and upper leg. This gives 24 DoF reduced by spherical joints at the hips and ankles and revolute joints at the knee, pedal axis and crank axle.
The three remaining DoF are:
- Crank position. This degree of freedom allows the pedaling motion.
- Foot angle with respect to the floor. Some cyclists keep the foot parallel with the floor, some extend the foot slightly at the bottom of the stroke, and some keep the toe pointed slightly downwards throughout the stroke.
- Rotation of leg around the line through the two spherical joints at the knee and the ankle. Assuming the leg is not completely straight, this can be considered as the width of the knee from the central plane.
Simplified model of leg-crank kinematic chain. Note that when the leg is fully straightened, the distance mate at the knee no longer prevents the remaining rotation of the leg.
Kinematic Chain for Body and Head
The pelvis is first fixed to the saddle, with an angle mate defining its forward lean. The chain then consists of the abdomen, thorax, clavicles, neck and head. It would be possible to set angle mates between each component, fixing them in series relative to the pelvis. However, this would make it difficult to control the overall lean angle.
It is therefore useful to introduce another component which contains only a plane to represent this lean angle, and an axis which is mated to the pelvis setting an axis of rotation. This lean angle plane can then be fixed with an angle mate, which is the only parameter needed for these body parts. The body parts can then be mated as follows:
- The dummy lean plane is set at an angle from the assembly top plane.
- A symmetry mate is set so that half of the forward lean comes from pelvis tilt. The front plane of the pelvis is the plane of symmetry. The seatpost angle and the forward lean planes are symmetric around it.
- The abdomen is set to be parallel with the dummy lean plane.
Kinematic Chain for Arms
The arm kinematic chain is considered to be just three ungrounded links, with 18 DoF through the upper arm, lower arm and hand. The clavicles and handlebar grips are considered to be grounded. The ungrounded bodies are constrained by spherical joints at the shoulder and wrist, and revolute joints at the elbow and where the hand grips the bars. This leaves two DoF, which may be considered as:
- Rotation of the hand around the handlebar.
- Rotation of the whole arm so that the elbow moves in a circular path about the axis between the shoulder and wrist joints.
One DoF can be removed by setting the wrist at neutral flexion with its top plane parallel with the forearm axis. The remaining DoF can be removed in various ways, but it is generally most stable to set a distance mate between a point on the elbow joint and the right plane of the assembly.
Bike Fit Positioning
Expert advice into to the model setup was provided by Mike Veal, author of the DIY Dynamic Bike Fitting guide. The following key points were applied:
- The pelvis was constrained so that the joint was on the plane of the seat post angle.
- Seat angles of 73° are generally ideal for bike fit, with steeper angles used simply to provide clearance for large wheels. However, time-trial aero positions require much steeper angles, which may effectively be up to 84°.
- The angle of the line between the hip and shoulder joints is typically between 45° and 55° from the horizontal. 45° to 50° is usual for a road bike, and 50° to 55° is typical for a more relaxed upright position. Dutch bikes can be from 65° to 90°.
- The angle of a person’s feet relative to the floor is quite personal, but a typical value is 15°.
- At the bottom of the pedal stroke the leg is never as straight as it seems. Typically, the angle between the upper and lower leg never exceeds 140°. A lot of literature suggests 150° is an ideal angle, but this is generally because a static measurement was taken with the foot parallel to the floor. When pedaling and the foot assumes a natural angle, the actual leg angle is lower.
The most challenging area is positioning the rider and handlebars to achieve neutral wrist angles. The wrist joint allows three rotations:
- Flexion/Extension: This can be assumed to be fixed at exactly the neutral position. Riders can readily adjust this, independent of their body position, by rotating the hands around the axis of the grip.
- Deviation: Sideways rotation towards the thumb is radial deviation and rotation towards the little finger is ulnar deviation. The neutral position does not mean that a gripped bar is perpendicular to the axis of the forearm, but rather that the third metacarpal bone is aligned with the forearm axis. One study found that a natural grip results in a mean angle of 65° between the grip axis and the third metacarpal, with the grip sweeping back as though the wrist was in 25° ulnar deviation. However, the standard deviation was 7°, due mostly to variation between individuals, suggesting significant adjustability may be desirable for this aspect of the grip position.
- Supination/Pronation: Rotation about the forearm axis is known as supination when the thumb is rotating towards the back of the hand and pronation when it is rotating towards the palm.
This parametric model allows anthropometric models representing different percentiles of the population to be fitted to bicycle designs to check for ergonomics. The setup means that only the key parameters need to be adjusted, such as the grip backsweep and down sweep, or how far out the rider places the elbows. This model will enable much more rapid development of innovative bicycles, suitable for a wide range of different people.
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