The time trial.
The ultimate test pitting human and machine against the clock.
Every gram and bit of aerodynamic drag scrutinized and shaved down to the absolute minimum.
Every second marked by the clock is the difference between victory and defeat.
There was a time when the importance of reducing drag was overlooked.
Most bicycle designs looked fairly similar, and aside from new materials, racing bicycles
were largely unchanged for decades.
The never-ending search for a competitive advantage ushered in a new age of innovation,
with new streamlined shapes being used for helmets, bicycle frames, wheels, and even
water bottles!
Before any of these items can head to mass production, they first need to be tested extensively.
But to run a computerized simulation in programs like ANSYS Mechanical, Fluent, or Discovery,
we first need the 3D model.
Models made up of complex geometries may require more advanced techniques to be created.
Cuts and extrusions will not simply be along straight lines anymore.
The profiles used to create the geometry will often need to be made using more precise sketches.
This exercise will focus on some of these advanced operations to give you a better idea
of how to create the 3D model.
We will create the model geometry in a few separate phases.
Breaking the design up into segments will help us avoid having to work around tricky
intersections and transitions.
We will first create the rim, then the rough shape of the spokes, next we will define aerodynamic
profile of the spokes and complete the model adding the connecting hole at the wheel center.
Now, let's get started with the model.
Sketch a circle 622 mm in diameter.
Place the center point at the world origin
Use the Pull tool with Pull Both Sides option and pull 25 mm.
While not a requirement, creating your design symmetrically about the world origin
provides an easy reference for measurements and orienting the model.
Create a sketch on one of the flat faces
Click Offset Curve, then select the outer diameter of the geometry.
Create a circle that is offset by 65 mm toward center. This will define the depth of the rim.
Return to 3D mode.
Select the inner circle and use the Pull tool to cut through the entire body.
With the Pull tool still active, hold Ctrl and select the top and bottom edges of the
rim’s inner radius.
Create a Full Round by clicking and dragging toward the edge briefly, then reversing direction
and dragging away from the edge.
The result should be one continuous radius.
Locate the solid body in the structure tree.
Right click and select Rename.
Call it Rim and hide it.
If the world origin is not displayed, go to Display > Show and enable World Origin.
Create a new sketch on the same Plane of the rim.
In our case the XZ plane.
Start the Rectangle tool Place the mouse cursor on the world origin
and tap the Shift key.
Offset the starting point of the rectangle by 50 mm along the horizontal Axis.
The rectangle’s final dimensions should be 100 mm x 300 mm, with the midpoint of the
base on the world origin
Activate the Move tool and select all 4 sides of the rectangle Anchor the Move tool at the origin and enable
Create patterns from the Move options Rotate the rectangle in the plane until you
have 3 instances spaced evenly at 120 degrees
Press T to activate Trim Away and eliminate all overlapping curve segments at the center of the sketch.
Now activate the Pull tool and use the Both Sides option to extrude the profile 25 mm
Rename this new solid body to Spokes and hide it.
Complex designs sometimes require a specific order of operations.
It’s important to examine the design specifications and come up with a preliminary strategy for
how to approach the model creation.
Basic designs can usually be defined as one continuous body, but that may not always be
the optimal way to go.
For example, our bicycle wheel will be created as 2 separate parts and then combined.
It’s now time to start defining the aerodynamic profile of the spokes.
Create a new sketch on the same plane used for the rim and spokes Plane
Sketch a circle with diameter = 600 mm.
This will serve as a reference.
Sketch a vertical line from the edge of the circle to the center point
Sketch another line from the center point to the edge of the circle at an angle of 120 deg
from the first line segment.
Use Create Rounded Corner to add a 100 mm radius at the center vertex
Now, use the Trim Away to eliminate the entire circular outline.
We will refer to the curve we’ve just created as the Trajectory.
Return to 3D mode.
Select 1 end point of the newly created Trajectory curve and start a new sketch.
In the Display tab, disable the option Clip Scene Above Grid to show the entire geometry.
Take a moment to review this step.
Whenever we select a curve end point or any point along a spline and create a sketch,
the sketch plane is oriented at a 90 degree angle what engineers refer to as “normal”
to the curve at the selected point.
This can be quite useful, so make a note of it for future designs.
In the Sketch tab, click Construction Line and enable Define line from center.
Place a vertical line at the end point of the curve and make it 25 mm long.
Construction lines are one of the best ways to create references for sketches that require
very precise shapes.
Create a perpendicular construction line to act as a tangent reference.
Begin at one end of the construction line created in the previous step and extend to the convex
or “outside” of the original curve we first created.
The length is not important.
About 5 mm is enough.
Repeat this step at the opposite end point of the vertical construction line
Now, create a perpendicular construction line that is 50 mm long.
Place the starting point at the origin.
Make sure it extends to the concave or “inside” of the original curve.
This curve should be pointing in the opposite direction of the previous 2 reference lines
Activate the Spline tool Beginning at one end of the vertical 25 mm construction line,
hold the Alt key and place the starting point such that the tangency
condition indicated by 2 small lines appears parallel to the horizontal construction line.
Move the mouse cursor towards the construction line to make the tangency option identify
the correct reference.
Place the next spline point at the end point of the horizontal 50 mm construction line
Finish the spline by returning to the other end of the 25 mm line and creating a tangent end point.
Return to 3D mode and start the Pull tool
Activate the Sweep option and double click the Trajectory curve.
The entire set of curves should turn blue.
Click the Select icon in the Pull tool menu and click the U shaped spline we just completed, then press Enter.
The spline will be swept along the curve to create a surface with the same profile that
bends along the trajectory.
This surface will help define the profile of the spokes.
Select the new surface and create a circular pattern with 3 instances using the Y axis
as rotation axis.
Unhide the solid we labeled Spokes and start the Combine tool.
Select Spokes as the target body, then from the Structure Tree select Pattern as the cutter.
The combine operation should separate 6 segments from the solid body.
Hide Spokes from the structure tree again, then click and drag a box around the remaining
solids to eliminate them.
We can hide or delete the patterned cutting surfaces as well.
Zoom in to the center of the spokes.
Notice that there are 3 small notches where the geometry was not intersected by the cutting surfaces.
Click and drag from left to right to create a selection box around the notch and click
Fill to remove it.
Repeat this step for the 2 remaining notches.
It’s now time to put the entire design together.
Unhide the solid we named Rim from the structure tree
Start the Combine tool and hold Ctrl, then click each body to merge them.
Select the top and bottom edges of the outer diameter and use Pull to apply a full round.
There should be a small planar face at the center of the wheel.
Place a sketch there.
Create a hole at the center of the wheel with diameter = 30 mm
Now, make sure to clean up the structure tree and keep only the bike wheel.
Then save the model.
This completes the exercise on creating advanced designs.