We write a lot about making strong, end use parts with the Mark Two. It’s for good reason; the strength and reliability of the printers that we produce mean that printing previously un-printable parts is possible. We’ve pushed the limits of strength and toughness with our parts before, but there is one parameter we really haven’t tested with end use parts; the sheer size potential of a Mark Two printed part. For this post, we pushed the limits of size and weight on our printer to produce a custom headrest for Chase, a driver of a quarter midget race-car.
Quarter Midget Background
Quarter Midgets of America is a organization that facilitates short track youth racing around the United States. The uniquely shaped cars are derived from Midget race cars, which are short track vehicles with very high power to weight ratios. They’re raced by children aged 5 to 16 on tracks that are 1/20th of a mile long. There are many racing classes based on engine size and age, but most critical aspects of the car are consistent between racers. Racing is highly technical: entering into and exiting turns precisely makes the difference between winning and losing. Doing this is much easier said than done, especially at the 50mph speeds that some racers reach.
Despite the speed of the racers, Quarter Midget events are remarkably safe. This is due to the extensive harnessing system required in cars. It secures everything from the shoulders down, keeping critical body parts inside the safety of the roll hoop. The head, however, isn’t secured. While the races do wear neck braces, the sheer centripetal acceleration of the car means that kids’ heads often are forced away from the center of the track. Focusing on the inside of the track while their heads are being forced the other way is very difficult, and as a result most cars have some lateral head support system.
Head support systems come in all shapes and sizes. Because pit crews cannot alter the frame of the car, most solutions are simple and impermanent. Chase’s father employed a safety net for a sprint car stretched from the roll hoop to the front bracing of the car for his support. It worked alright, but was by no means a permanent solution. With that in mind, we worked with Chase and his father to design and print a superior head rest.
Making a Markforged Headrest
The Markforged headrest had to improve upon the safety netting headrest in a few key ways. First, it needed to be more solid than the the safety net that was previously used while maintaining a little flexibility. Second, the part needed to mount to the roll hoop of the car solidly, but without adhesives or welding. Third, the part had to conform to Chase’s helmet and look like it belonged on a race car.
Normally, getting a combination of solid and flexible parts is a pain. With the Mark Two it isn’t. In order to get the strong, solid part, we printed the main body out of Carbon Fiber reinforced nylon. This part is huge. It was one of the largest parts we can print on the Mark Two, taking over six days to print. This large, strong, singular part is almost completely stiff. To introduce the limited flex that we wanted, we printed clamping plates reinforced with kevlar. These plates flex, allowing the headrest to be slightly flexible in bending. The two materials combine to match the desired behavior of the headrest.
One major benefit of the safety net headrest was its ease of mounting. Small metal straps secured the netting adequately around the space frame, allowing for manual tightening and loosening of the headrest. While the restraint attached easily, it didn’t have the mechanical integrity that a more robust solution could provide. Enter tube clamps, aluminum rods, and rod ends. By using rod ends, we could clamp hard on the metal tubes, ensuring a more secure connection. On the roll hoop, two aluminum rods connected to either side of the roll hoop act as cross members on which the main body is clamped. Because both the aluminum rod mounts and the Kevlar clamp places are adjustable, the entire headrest assembly can be moved both vertically and laterally; this is critical to continually conforming to an ever growing driver. These clamps hold most of the force. However, using only them would leave the large headrest cantilevered in an unfortunate way. To better support the headrest, we printed a Onyx bracket with co-processed nuts. This bolts onto the end of the headrest, allowing a third aluminum rod to brace the part from the front bracing member. This three-point attachment system fully constrains the part, allowing only the small, desired deflections of the kevlar clamp plates.
Nylon and Carbon fiber are inherently more aesthetically pleasing than safety netting; however, the freedom in design that the Mark Two allows meant that the final design could be both customized for Chase and visually complementary to the car. The final headrest has a curve that matches Chase’s helmet, and is angled to negate centripetal acceleration and point his head towards the center of the track. His number is also printed into the part, so in addition to having the most functional headrest on the track, Chase looked the best too!
It’s hard to argue with results, and results are exactly what Chase got with the new headrest. In his most recent Quarter Midget race with the new headrest, Chase spotted turns excellently and won! With Markforged’s help, Chase is racing better than ever. If you’d like to examine the headrest for yourself, the main .stl file is available for download here.