My name is Matt and I ride mopeds – I’m not talking about the scooters or step through motorcycles that you see cruising around cities and college these days, I mean mopeds. Small, typically old, motorized 2-wheeled vehicles that have moveable pedals allowing a person to pedal the moped like a bicycle if they a) run out of gas b) need to go ‘stealth’ c) don’t feel like push starting the ‘ped or d) feel like exercising. This post and hopefully a few more will follow my very own product development design process as I restore and customize one of my vintage mopeds – the Kreidler MP 19 pictured in the slideshow above and the image below. This post explains the process I went through after deciding the seat needed some attention.

I had planned to have the original seat reupholstered, but after removing the cover and the foam I noticed that there was a bit of rust on the metal seat pan as well as some cracks in vital areas.

The Design Process

After seeing the condition of the old seat pan I decided it was time for the old to become new.  Rather than purchasing a new seat, I decided to use what I do day-in and day-out at EAC. I would design one and the try to build it on my own. I like to think that I’m a pretty good engineer and I have all the Creo software tools, so the designing wasn’t going to be a problem, but this will be my first real metalworking project.

Step 1: Design

The first step in my design process was figuring out what I wanted my new seat to be. Do I want it hinged like the original or have it rigidly attached to the frame? Do I want to tuck the taillight under the rear fairing? What gauge metal should I use, etc.? After getting my requirements I continued to think of my metal working capabilities as well as the tools available. Realizing that I don’t have much firsthand experience in metal fabrication and only basic shop tools, hammers, bench vise, angle grinder, and a welder, I knew I had to keep the parts and the design simple.

After many hours (and beers) contemplating my requirements and fabrication abilities, it was time to sit down and design the new seat in Creo Parametric. Inside of Creo, I used the sheet metal functionality to design my metal parts. The sheet metal functionality allowed me to design the parts the way that they would be built. For example, start with a flat sheet in the shape of the base and add a couple of 90-degree walls. Then add a couple of rounds and corner cutouts to get the base of the seat.

The other sheet metal parts were created in a similar fashion as the base. I decided to mount the taillight under the rear fairing which meant I needed an assembly model to make sure everything would fit together. Modeling the seat and assembly up in 3D was a life-saver. It showed me the original angle on the rear fairing was too steep and would interfere with the taillight. I flattened out the angle and adjusted the location of the taillight to get the correct fit and look. Below you can see the final design process.

With the design work completed, it was time to make sure everything would work in real life so I made a prototype. To create my prototype I printed the flattened state of the sheet metal parts and traced them on cardboard.  A little cutting, bending, taping and voila, a prototype. Building a prototype is something we strongly encourage our customers to make.  It’s should be part of every product development project. It made it so I could ‘place’ my design on the moped. It allowed me to see that I needed to make the seat just a little longer and a little wider. The original design just didn’t look right on the moped. Also, the slightly larger seat will be much more comfortable while cruising around on the winter-torn roads.  After I updated the design it was time to start cutting and forming metal.

Step 2: Fabrication

With each flat pattern done I created an assembly with a part that was the size of the blank piece of sheet metal and then I assembled all of the flat patterns to the stock piece to make sure I had enough stock material to cut out all of the parts. Knowing that I had enough stock material I went and traced out the parts on the actual sheet metal and started cutting. I used an angle grinder to cut out my patterns because the material was a little too thick for tin snips.

With the parts cut out, I laid the flat patterns back over the cutouts and marked the bend lines so I knew where to start and stop the bends. That is another perk of using Creo’s sheet metal functionality. It shows you the start and stop locations for bends with dashed lines. See the following picture for the bend lines.

Now that everything was cut out and marked I needed something to hold the metal flange walls in order to bend them on the base. Thankfully we have a bench vise in the workshop. The only problem was that the walls on the long side were longer than the jaws on the vise. To get around this I took a couple of 2x4s and placed them in the jaws of the vise and the sheet metal between the 2x4s. The 2x4s provided two benefits, support along the entire edge of the bend as well as a nice round edge for the sheet metal to follow. Thankfully I was able to create the bends with just my hands and body weight.

For the rear fairing and the front edge, the radii of the bends were so large that I could not use the same setup used to create the bends on the base. What I did was I found a steel pipe with roughly a two-inch diameter and clamped it in the vise. I then roughly placed the middle of the bend on the center of the pipe and pressed down creating a bend/crease in the sheet metal and repeated this several times. When I had the general shape bent out in the design process I took a rubber mallet and used that to smooth out the bend. One thing to note about the rear fairing and the front edge is that I left extra material on the ends in order to have something to hold on to while bending the parts.

Once all of the sheet metal parts were cut and formed it was time to start welding them together.  Welding seemed a little daunting since the only other time I welded, about 7 years ago, I set my pants on fire. Thankfully I had a coworker teach me a little about welding before I started. I also practiced quite a bit on some spare material before I started. This helped me get the welder settings correct for the material thickness. With the welder dialed in, I made tack welds to hold the parts in place while I made sure they were exactly where I wanted them. Once each piece was in its proper place I started to weld it all together. After the parts were welded together I ground down to smooth any excess weld and then went back and filled in any voids and re-ground as necessary. It was a learning experience. It was kind of like “lather, rinse, repeat” only “weld, grind, repeat.”

Below are pictures of the seat during different times of assembly as well as the finished product on the moped.

Lessons Learned:

Throughout this design process, I learned a lot.  For one thing, you can do a lot more with a can-do attitude than you think. Another thing I learned was that welding is not as daunting as I thought. As with anything in life, it takes a little practice and patience. Prototypes are amazing. They may cost some time and or money but they are worth it.  If I did not make a cardboard prototype in my design process I would have had a seat that was just a little too short and I would have had to remake the seat from scratch once it was finished. The sheet metal functionality in Creo Parametric is fantastic and really does work.  The bend lines on the flat patterns helped immensely to create accurate parts. One other thing I learned throughout the design process was that on my next project I need to take a lot more pictures along the way.

I’ll leave you with a couple more pictures of the finished product (minus paint and a little bit of leather and foam). Until next time…