Bike Fender Part 1: Bike Saddle

Introduction: Bike Fender Part 1: Bike Saddle

About: I'm an inventor / maker / designer based in the Bay Area. My background is in residential architecture, film set design, animatronics, media arts, exhibit design, and electronics. I use digital design and fabr…

In this lesson, we're going to get started on the Bike Fender project. We will demonstrate the process by making a 3D copy of the bike saddle and seat post we're designing the fender we just sketched to fit.

Here are the steps covered:

  • Importing images as guides for creating 3D geometry.
  • Units and measurement.
  • Using primitive solids.
  • Manipulating geometry using the Move command.
  • Creating sketches based on imported images.
  • Using symmetry in sketches.
  • Using the extrude tool for creating, cutting, and joining 3D forms.

Step 1: Measure the Bike Parts

To measure anything accurately on a small scale, you need a set of calipers.

I recommend buying a good set of digital calipers because they make it easier to quickly measure all kinds of things.


There's no need to painstakingly measure every part of the complex geometry of the saddle. All we need is the diameter and length of the seat post, the angle of the seat post, and the basic shape of the saddle. The only things that need to be really precise are the seat post measurements because this is the feature we're going to attach the fender to.

To get the diameter of the seat post, I use the calipers like a claw clamping down on the sides of the tube. I get 30.98mm, so I'll round that up to 31mm. It's usually a good idea to go with whole numbers for simplicity's sake- it makes your life easier because you can quickly do math in your head as you're designing the rest of the project.

To get the length, I use the tips of the jaws to touch the bike frame tube on the bottom and the end of the seat post on the top. For this, I get 128.45, so I'll round that down to 128. This measurement doesn't need to be perfect because the fender will attach only at the top end.

To get the angle, a photo is sufficient. The fender is going to slope down at the end of the saddle, so if I'm off by a degree or two it's not going to make a difference in the functionality or the look of the piece.

To measure things like this in photos, I use Autodesk Graphic. It's kind of a stripped-down version of Adobe Illustrator with all the features you need and none that you don't. It's a one-time cost of $30 as opposed to Illustrator's $20 / month- kind of a no-brainer.

I also got a rough measurement of the length of the back of the saddle from the end of the seat post. This will help me make sure I'm not designing the fender too close the end of the saddle. The width and length of the saddle marked in the top-view came from info I found online.

Notice that I'm taking a side-on closeup and a top-down closeup of the saddle and seat post. These will come in handy later when I'm modeling.

Step 2: Modeling With Primitives

There are many ways to create geometry in Fusion 360. To create the seat post, I make a cylinder to match the dimensions I measured and tilt it to the proper angle.

Create > Cylinder

Select plane for cylinder base

Draw circle on model origin point

Enter height value for cylinder

Right-click > Move

Set Rotation > Select center of cylinder bottom

Click "transform" > Rotate -20º

    "id": "quiz-1",
    "question": "What's the difference between extruding a circle in a sketch and creating a Cylinder Primative?",
    "answers": [
            "title": "Nothing!",
            "correct": true
            "title": "The Cylinder Primative is more precise.",
            "correct": false
    "correctNotice": "You got it! They both have exactly the same result.",
    "incorrectNotice": "Nope! They both have exactly the same result."

Step 3: Using Attached Canvases

In Fusion 360, I can use 2D images in 3D space to help create a 3D model that more closely matches the object being photographed. For our project, I'll use the side and top views of the bike saddle.

Insert > Attached Canvas

Select side plane

Choose local image file

Rotate, move, scale with manipulator

Construct > Offest Plane

Create new attached canvas on horizontal plane

Step 4: Creating Sketches

To begin our saddle model, I'll start by sketching the top profile of the saddle. The saddle has a complex 3-dimensional shape, but I can model it in steps by working on it in steps in 2 dimensions.

Sketch > Create Sketch

Sketch > Line

Sketch > Offset

Sketch > Offset

Sketch > 2-Point Circle

Sketch > Spline

Sketch > Mirror

Sketch > Trim (trim off excess lines)

Step 5: Extrude Tool

Sketch > Create Sketch > Project

Sketch > Spline

Create > Extrude (symmetric)

To create the base mass for the saddle, I'll extrude the top profile I just created. I'll refine it by creating another profile sketch in the crossing 2D plane, then extruding that profile to cut it out of the original mass I just created.

Step 6: Model History

Right-Click on Sketch > Edit Profile Sketch

Because Fusion 360 works with a historical timeline, I can go back and edit objects we previously made that affect objects that depend on them. I will alter the side profile sketch that I made before and watch it affect the shape of the body. With one more profile sketch for the underside of the saddle, I'll finish the rough model of the bike saddle that will serve as the basis for the fender design.

Step 7: Homework

Before the next lesson, find an object to model. Measure it using calipers and photograph it from the sides and top as needed so you can add the images to the canvas and check your work. I highly recommend modeling a solid manufactured object that's not too complex for your first go at 3D modeling. A/C adaptors, USB sticks, simple tools like screwdrivers, etc. make for good practice objects.

If you're feeling confident and already have a project in mind that needs a base model like the seat post + saddle example from this lesson, by all means try something a bit more complex.

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