3D printing technology is constantly testing new boundaries and discovering new applications. Following is a case study from our partners at Markforged about a 3D printed fiberglass-reinforced nylon application. Visit the Markforged site for more on this case study.
Charles Guan’s fiberglass-reinforced nylon steering assembly took 24 hours to 3D print and regularly withstands 800 N of force during Power Racing Series races. The materials required for the three parts are 500 cm3 of nylon and approximately 25 cm3 of fiberglass. The assembly easily handled the forces and a total system mass around 160 kg, including driver. The top speed of the Chibi-Mikuvan is 25 mph. The approximate radial load reaches 800 N per wheel. Estimated torque on the stub axle surface is 5 N·m. The nylon flexed as forces peaked but never failed. Charles won the Power Racing Series Road Course at the Detroit Maker Faire 2015!
Introducing Charles Guan and the Chibi-Mikuvan
Charles MIT pedigree and curiosity motivated him to learn additive manufacturing. Racing the light electric vehicles gave him a problem to solve with composites.
According to Charles, “I attended the Massachusetts Institute of Technology for my Bachelor’s Degree (S.B.) in Mechanical Engineering and worked towards a Masters (S.M.) thereafter.
I left the Masters program at MIT in Fall 2012 to take on an instructor & mentor role for undergraduates in Mechanical Engineering through my 2.00gokart experiment: concocting an electric vehicle design class for undergraduates with the end goal of forming a practical, multidiscipline design process foundation early on.”
Charles is also the creator and inventor of the Chibi-Mikuvan, the electric vehicle Charles’ races in the Power Racing Series. From the Chibi-homepage:
Chibi-Mikuvan is the latest technology and methodology demonstrator made from a hodgepodge of unrelated commercial and industrial parts, in addition to being my first foray into composite bodywork.
Combining a salvaged NiMh battery from a hybrid 2010 Ford Fusion, a 9″ angle grinder’s right-angle gearbox, a water-cooled R/C boat inrunner motor, and a 1/5 scale R/C car ESC, Chibi-Mikuvan is designed to superficially resemble a cartoony version of a 1987-1990 Mitsubishi “Van/Wagon” known as the Delica in non-US markets. The bodywork uses the fiberglass-foam composite sandwich construction method, and despite being decorative only, is still highly rigid.
Chibi-Mikuvan Components
| Motor |
Turnigy Aqua Star T20 motor (also sold under the TORO and Proteus brandnames, among others) |
| Motor Controller |
Turnigy Trackstar 200A |
| Battery |
Roe of Ford Fusion, 28.8V 16Ah NiMH chemistry |
| Gearbox |
9″ angle grinder gearbox similar to this model (4.09:1), 5:1 external #35 chain drive (12:60) |
| Electrical |
Arduino Nano on 2.007 Carrier (signal processing); Panasonic AEVS main power contactor; Hella 2843 kill switch |
| Wheel & Tire |
8″ Harbor Freight Pink Wheels for America |
| Brakes |
Front, generic e-scooter/e-bike disc brake calipers on dual 7″ custom rotors; rear, regenerative (electronic motor braking) |
Chibi-Mikuvan Specifications
| Top Speed |
25mph (as-geared, Y-termination) |
| Acceleration |
to 25mph in < 3 seconds |
| Braking distance |
< 30ft from top speed |
| Drivetrain |
RR layout, 1 speed, spool axle (no differential) |
| Dimensions |
50″ L, 28″ W, 24″ H |
| Curb Weight |
113lb with battery, without driver |
| Seats |
1, though if Chibikart was any indication to go by, up to 7 |
The Steering Knuckle Assembly Started as a CAD Drawing
The goal is to replace the heavy, failure prone steering components with lighter, just as strong fiberglass reinforced composites. Like most mechanical engineering projects it started in CAD (or Autodesk Inventor, to be specific). According to Charles,
Replacing Steel Frame Members with Composites
“The new steering knuckle, in light pink, will be printed flat to have large C-shaped sections of fiber holding onto the axle stub (a cut-down 5/8-18 bolt). The distance between it and the kingpin post is taken up on both sides by needle thrust bearings, compressed by a 4.5″ long bolt. Side loads on this assembly are handled by virtue of the fact that I’m rubbing large surfaces of nylon together.
The red steering follower link is a design compromise, since I needed high strength in 2 planes on this part. The proximity of the X-axis oriented long cap screw to where the ball joint mounts will hopefully aid in transmitting the steering force by relying on shear between the layers (which is a bit better than bending between them…). The closed fiber loops in this part run in the XZ plane (horizontally) to hold onto the ball joint.
The purple brake caliper mount should be seeing most of its loading in the XY plane, from braking, so printing that in the flattest configuration to get a bunch of fiber laps around the outside is easy.”
The design of printed parts is critical. While you can print copies of steel parts using a composite, the materials properties are different enough that it’s worthwhile to create a unique solution out of nylon. For example, this assembly could be printed as one piece but was split into three pieces to optimize the fiber direction. That geometric flexibility is a clear advantage over over traditional materials, like steel or machined aluminum.
Once the models were done in CAD Charles exported them as STL files. Eiger, the Mark One 3D printer software, imported those files for slicing and fiber layer addition.
3d Printing and Assembling the New Knuckle
The Mark One printed all three of the fiberglass reinforced parts in 24 hours. About 500 cm³ of nylon and 25 cm³ of fiberglass were consumed. In addition to the three-part knuckle assembly, Charles also printed a steering arm, see below.
3D Printed Steering Knuckles
Assembly was easy. The Kingpin post is a steel tube – the kingpin runs through it, a 4” long 10/32 bolt, with a conical compression washer that grips all of the layers distributing the stress and forces around the C-Shape. A few other bolts attach the ball joint and brake caliper mount to the steering knuckle. The only difficulty during the assembly was attaching the steering knuckle. Charles says, “It’s never had a failure, but there was a time when I couldn’t get the clamp tight enough. The plastic was so slick that I had to use 5 minute epoxy to hold the nut.”
Is the 3d printed steering knuckle strong enough? Yes! The largest forces in the system are on that stub axle. It has to deal with the road forces, a lot of Charles’ weight and the motion of the tires. Charles estimates the stub axle is handling 5 N·m or torque.
Stats
- a total system mass around 160 kg, including driver
- approximate radial load of 800 N per wheel
- Estimated torque on the stub axle surface is 5 N·m
- The top speed of the Chibi-Mikuvan is 25 mph
“I think that one of the reasons it survives is that the nylon is very flexible. It takes quite a lot of shock. If I steer very hard, the whole bracket moves a little bit. I am deforming it right now with about 40 pounds of force and the assembly takes it.
I drove this thing off a curb several times at full speed to try and break these parts. I caused version 1 of the kingpin bracket, not pictured, to buckle the fiber region where the part met the frame. It never failed, it just manifested itself as the wheel going more and more that way [more camber]
In Detroit, there was no failure and I drove like crazy in the race.”
