Making the Chain Guard

 Creating the chain guard

I selected 5 polygons starting from the first one up against the bike frame and then copied the polygons onto their own objects to start forming the chain guard. I then scaled the object down and moved it to the front edge of the sprocket

Created a cylinder for the back of the chain guard about the same size as the circle in the reference image. I then attached the cylinder to the 5 polygons to make them one object. Next I selected the 5 polygon strip and 5 other polygons on the top left side of the cylinder and bridged them together and set the segments to 4. Using the edge tool I then selected the edges of the segments of the bridge piece so that the piece had  cure to it. I then moved the very far left edges so that they entered the sprocket.

I applied NURMS and then used swift loop to add some definition to the top edges of the chain guard. Next I selected the polygon in the middle of the right cylinder and inset it twice, extruded it inward, and inset it one more time to add a crease.

Created a cylinder inside the circular inset on the chain guard and then selected all the polygons except for the top middle two and deleted them. I then bridged the gap between the polygons in the front and the back so that the geometry was completely solid. I then used the swift loop tool to add resolution to the four-sided pieces on either side of the triangular segments. I then used the cut tool on the triangular pieces, cutting from one of the top side vertices, to the center line, and to the other top side vertex. I used the swift loop again to add some definition the the polygons I had just created using cut, and then used cut again, starting from the top vertices and going down to the middle line at the bottom of the shape.

I selected the two polygons in the middle of the two triangles and extruded them inward and upped the iterations to 3. I then copied the piece all the way around the circle using shift and the rotation tool. Finally I added a sphere so that the triangle designs look like they are secured by a pin.