A general-purpose parametric SolidWorks model has been created to enable rapid evaluation of novel bicycle concepts. It can be used for ergonomics, checking clearances, aerodynamics, kinematic and degrees of freedom studies, and product visualization.
Dr. Jody Muelaner, PhD CEng MIMechE
For this study in ergonomics, a set of anthropometric models from 3D Human Model were used as the starting point. Key bicycle geometry was defined using reference geometry in individual part files, located using distance and angle mates. This geometry includes the contact points at the saddle, handlebar grips and pedals, as well as the steering axis and wheel positions. Anthropometric models were then configured to fit to these contact points, with a few additional angle and distance mates that allow further adjustment of the rider position. The included models represent 5th percentile females, 50th percentile males and 95th percentile males. The model is stored as an assembly template file, enabling it to be easily reused as a layout for different design concepts. It was created for the BriefBike project, which is developing a new class of folding bicycle that will effortlessly fold into a roller-case.
Key bicycle geometry
There are three ways that references can be parametrically defined within an assembly. Using a sketch or creating reference geometry (point, axis or plane) is often more straightforward and, in the case of a sketch, allows multiple parameters to be defined in a single model tree feature. However, parameters defined in this way cannot be animated or adjusted using the Mate Controller. The Mate Controller is particularly useful within this model as it allows parameter sets for different riding positions to be stored independently of configurations. It is, therefore, possible to apply different standard riding positions to any configurations that a are created. In order to provide this greater flexibility, parameters must be defined using distance and angle mates. This means the reference geometry is first defined with a part file, and the part is then mated in the assembly file.
The parametric bicycle geometry definitions are:
- Planes parallel with the Right plane, defining the width of each pedal from the centerline (Right plane).
- The crank length part three axes – the bottom bracket axle and the two pedal axles. The crank length is defined by different configurations of the part file. The part is mated on the right plane with the bottom bracket axis on the Front plane and at a distance from the Top plane, representing the bottom bracket height from the ground. This leaves the pedals free to rotate.
- Pedal thickness parts contain a pedal axis and a plane representing the top surface of the pedal. They are mated to the pedal axes in the crank length
- A Seat part contains planes representing the seat tube angle and the top surface of the saddle. The seat tube angle is mated coincident with the bottom bracket axis and at an angle from the top plane. The top surface is mated at a distance from the bottom bracket axis.
- A Bar part contains an axis to represent the handlebar. It is mated at a vertical and horizontal distance from the bottom bracket axis.
- The orientation the handlebar grips is defined in terms of two sequential Euler rotations – the backwards and downwards sweep. A separate part is used to define each rotation.
- Grip_Sweep parts are mated to define the position and backwards sweep, with the following constraints:
- Two translations by mating the origin coincident with the Bar axis
- The remaining translation, the width of the grips, is defined by a distance mate between the part’s origin and the assembly Right plane.
- Two rotations are constrained by mating the part’s Top plane parallel with Top in the assembly.
- The backwards sweep is the only remaining degree of freedom. It is defined with an angle mate between the part’s Front plane and Front in the assembly
- Grip parts are then mated relative to the Grip Sweep part, defining the downwards sweep.
- All three translations are constrained by mating the origin coincident with the origin of Dum_Grip_Sweep
- Two rotations are constrained by setting the Front plane parallel with Front in Dum_Grip_Sweep
- The downwards sweep is the only remaining angle. It is defined with an angle mate between the Top plane and Top in Dum_Grip_Sweep
- Wheels contain an axle axis and a ground plane set at the wheel radius. Different configurations are used for different wheel sizes. Certain configurations may also include basic solid geometry to visualize the tire. The wheels are mated with the ground plane on the assembly Top plane and at distances forwards and rearwards of the bottom bracket axis.
- A Steering Axis part contains a plane to represent the steering axis angle and an axis to represent the line where this plane intersects with the ground. The axis is mated coincident with the Top plane of the assembly. A distance mate between this axis and the Front plane of the front wheel defines the steering trail. The steering axis angle is then set with an angle mate.
Care must be taken when using angle mates. The direction in which the angle is defined can flip when changes are made to the model, causing assembly rebuilds to fail or result in unexpected behavior. These issues can usually be avoided by defining a reference entity for each angle mate, which is not defined by default. This is normally just a one click operation, by selecting Auto Fill Reference Entity.
Kinematics of the Human Models
The human model has parts or sub-assemblies for hands, lower arms, upper arms, clavicles, head, neck, thorax, abdomen, pelvis, upper legs, lower legs and feet. These 19 rigid bodies have 114 degrees of freedom (DoF) without any joints or other constraints. Joints between the body parts are either spherical, removing the three DoF for translation, or revolute which also removes two rotations, constraining a total of five DoF.
When all of the joints are added to the body parts, the human model still has 43 DoF. Considering the kinematics of the model as a whole is, therefore, overly complicated. Luckily, it can be broken down into smaller kinematic chains that behave independently. For example, each leg forms a kinematic chain which also includes the crank, and each arm forms a kinematic chain between the shoulder joint and the handlebar grip.
An understanding of how a person should be positioned on a bike was provided by Mike Veal, who created the DIY Dynamic Bike Fitting guide.
- The hip joints should align with the plane of the seat post angle.
- 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 people’s feet relative to the floor is quite personal but a typical value is 15°.
- The leg does not completely straighten at the bottom of the pedal stroke. Typically, the angle between the upper and lower leg does not exceeds 140°. Although some literature puts this angle closer to 150° this is due to static measurements with the foot parallel to the floor. When pedalling, the foot assumes a natural angle which reduces the extension of the leg.
- Wrists allow three types of rotation and should ideally be in a neutral position for all of them:
- Flexion/Extension can be fixed at the neutral position, with the flat plane of the hand aligned with the forearm axis. It can be adjusted without significantly changing the position of the arms by rotating the hands around the axis of the grip.
- Deviation is sideways movement of the hand, towards the thumb is radial deviation and towards the little finger is ulnar deviation. The neutral position does not position a griped bar 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.
Kinematic chain from pelvis to head and shoulders
This section of the body is made up of the pelvis, abdomen, thorax, clavicles, neck and head. The pelvis is fixed at the saddle, but is free to rotate so that it tilts forwards. The clavicles are mated parallel with the front and top planes of the thorax, effectively forming one ridged body with the shoulders in a neutral position. Although the components could be mated in series, starting with the tilt angle of the hips and then setting the angle between each part, this would make it hard to set an overall lean angle. A reference part is therefore introduced with a plane that defines the lean angle. This part is mated with an axis through the hip joints and with an angle mate relative to the assembly Top plane.
A symmetry mate is used to apportion half of the forward lean to the pelvis tilt. The Front plane of the pelvis is the plane of symmetry. The seatpost angle and the forward lean planes are symmetric about it. The abdomen is mated parallel with the dummy lean plane, and the shoulder joints are set to be coincident with the forward lean plane.
Kinematic chain from hip joint to bottom bracket
Each leg can be considered separately, as a kinematic chain consisting of the crank, pedal/foot, lower leg, and upper leg.
These four bodies have 24 DoF, which are reduced by revolute joints at the knee, pedal axis and crank axle, and spherical joints at the hips and ankles, to leave three DoF:
- Crank position is intentionally unconstrained to allow pedaling motion.
- Foot angle from the floor varies between individuals but the toes are typically pointed downwards by an angle of approximately 15 degrees.
- The spherical joints at the hip and ankle also allow the leg to rotate about an axis between these joints, so that the knee moves in a circular motion. Assuming the leg does not lock out into a fully extended position, this can be constrained by making a point on the knee joint coincident with the plane defining the pedal width.
Kinematic chain from shoulder to handlebar grip
The kinematic chain for each arm consists of the upper arm, lower arm and hand. These three bodies have 18 DoF, reduced to just two DoF by spherical joints at the shoulder and wrist, and revolute joints at the elbow and the grip of the hand around the bar. The remaining two DoF can 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
Setting the wrist to neutral flexion removes one DoF. This can be achieved by mating the hand’s top plane parallel with the forearm axis. There are several possible ways to remove the remaining DoF. It was found that the most practical and stable is with a distance mate between a point on the elbow joint and the Right plane of the assembly.
Adjusting and configuring the model
The assembly template contains three different anthropometric models, all configured as described above. These models represent a 5th percentile female, 50th percentile male and 95th percentile male. They can be activated by simply suppressing or suppressing the associated folders in the feature tree.
Different pre-configured body positions are also included. These are defined using a Mate Controller for each percentile model.
The bicycle and human model template enables the rapid evaluation of novel design concepts. Applications include ergonomic studies, mechanical clearance checks, aerodynamic simulation and product visualization. The simple parametric definitions allow easy adjustment of all the relevant variables in the bicycle geometry and rider position.