Stingray Bike Build: Coaster Brake Hub

Sometimes the gears turn slowly. I started a build of a stingray bike (what kids used to ride before BMX bikes). I’m just now getting around to writing about it. I’ve been spending some time over at the SolidWorks forums, and I’ve noticed that in the last 5 years, there’s a mostly new crop of people asking a lot of the same old questions. Some of the old guard are still there, but the tone is a little different than it was… Anyway, I’m not here to talk about tone or customer relations. My point is that there is a whole new crop of people who are just figuring out the basics and some of the more involved ways of doing things that may not be exactly obvious.

As I built the coaster brake hub assembly, I came across several items that I kind of take for granted, but some of these new users might find interesting.

This is just a model for education and entertainment, so there might be some details that are not perfect, or up to production standards. I had to get this done in about 8 hours, which included figuring out how the mechanism worked, making the parts and assembly, and making sure the parts fit together. This assembly has 22 parts, 18 unique. None of them is very complex, but a couple of them were kind of tricky, and several include some interesting techniques.

The coaster brake is the mechanism inside the rear hub of mostly low-cost bicycles that provides brake when you back pedal, drive when you pedal forward, and coast when you don’t move the pedals.

The most interesting part of the mechanism are the brake shoes, which aren’t really solidly attached to anything else, they just kind of float between a couple of parts, and get jammed by a couple of angled surfaces into the i.d. of the hub to provide the braking. If you were going to make this assembly really “work” in SolidWorks with mates, you’d have to use contact, or possibly limit mates, because the parts just float in the grease between the bearing cone, the clutch cone, and the hub. The part was simple to model, but would be tough to assemble.

To model it, I used a simple revolve to make the main shape, and then a second revolve to make the first of the grooves on the outside, and then a linear pattern to make the series of grooves. It seems simple, but a lot of people have questions on how to make this kind of small detail that you find on real parts.

Another part that was simple to make, but took some thinking was the hub. The hub is actually made from multiple bodies. The spoke flanges are manufactured from softer metal. Because the braking and driving surfaces are integrated into the hub, the main part of the hub has to be hard metal. You know how you pounded on the brakes of your bike as a kid, right? Well to stand up to that, these parts are hard and heavy. But the connection to the spokes doesn’t need a hard material. So the flanges are pressed on, and offset angularly from one another so the spokes are evenly spaced. Plus, this saves manufacturing cost on these lower end bikes.

The body of the hub was modeled in the part, but the flange was imported using Insert Part. Then it was positioned, mirrored/copied, and placed on the other side. This gives us 3 bodies in the hub. This is an example of “inseparable subassembly”, which is a modeling technique often used to make a single part from multiple parts using multibody techniques.

How you would deal with this would depend on your role. If you are the original manufacturer, you would show the flange as a separate part with its own part number and drawing, and the hub body as a separate part with its own part number and drawing, then an assembly diagram showing the flanges  being assembled to the hub body. If you were the company building a wheel from a purchased hub, you wouldn’t care that the flanges were separate parts, you would just have a single part number for the assembled hub body with flanges.

If you have questions about how any of these techniques work in detail, leave a comment. I’ll go through some of the other parts in the hub in a later post. We’ll move through the bike and talk about interesting stuff. After the rear hub, I’m going to model the frame.