3D Printed RC Car Is Geared For Speed

You can always go out and buy an RC car off the shelf. However, it’s readily achievable to print your own design that has many of the features of off-the-shelf models, as demonstrated by [Jinan].

[Jinan] set about creating a rear-wheel-drive design with a low center of gravity for good handling. Two large 5.2 Ah batteries slung low in the chassis help keep the car planted when cornering. [Jinan] also developed a double-wishbone suspension setup up front to handle bumps with ease.

With his eyes on top speed, [Jinan] needed a drivetrain that could handle sustained high RPM operation without failure. During the development process, [Jinan] spent plenty of time learning about the mathematics behind gear shapes before relying on a built-in CAD generator to do the job for him. Armed with proper gearing, he focused on making sure the driveshafts and other links wouldn’t fail at speed.

[Jinan] doesn’t shy away from diving into the engineering of his design, analyzing failures and improving on his designs along the way. It’s no surprise his design was able to reach 66 km/h (41 MPH) after his rigorous development process.  It’s compelling watching, and a great way to learn something.

7 thoughts on “3D Printed RC Car Is Geared For Speed

  1. The wide rear tyres are interesting.

    Wider tyres don’t directly translate to more traction as one would think.
    The only two real parameters that affect traction (on a solid road surface) is tyre compound and contact patch area (yeah, it’s a little more nuanced than that, but bear with me).

    Tyre compound is easy, softer rubber = better grip (but accelerated wear)

    Contact patch is determined by two factors and two factors only, and tyre width is not one of them.
    Weight on the tyre, and tyre pressure are the only two things that really affect contact patch area.

    But a wider tyre means a larger contact patch, surely!… Nope!
    A wider tyre alters the *shape* of the contact patch from long and narrow to short and wide. With the same weight on the tyre and the same air pressure inside, the area cannot change (beyond variances in the tyre flexibility, but that is not overly significant)
    Why would you want this? Well, a short contact patch means less sidewall flex and more time in the ‘relaxed state’, which means less heat being generated and more time to cool down as any given portion of the tyre rotates around ready to contact the road surface again.

    Wide tyres help keep your tyres from overheating under sustained high-speed use, that’s basically it. Oh and they look cool too. :D

    I wonder how much sidewall flex and heating is present in an RC car tyre? I have never considered it…. I think it would be really interesting to rig up one of those small thermal cameras to look at a tyre, then try different widths and tyre compounds to see what happens.

    1. Uh, no. A tire that is the same height but wider, most certainly increases contact patch. So does increasing tire diameter. Wider tires give more lateral traction, taller tires give more longitudinal traction. Pressure alters patch shape.

    2. There are geometric limits to that though, you can’t expect a bicycle tyre at very low pressure to have the same contact patch as e.g. a formula 1 car’s rear tyre.

      On typical automobile tyres the tread is also so rigid that at low enough pressures the contact patch begins to shrink again as the stiffness of the material causes it to bulge inwards.

      Smaller diameter tires have a lower moment of inertia for a given tread surface, and can spin faster for a given tensile strength. They also provide more driving force per torque. The extra width is then simply required to fulfill the geometric constraints (getting a big enough contact patch without having to crush the side wall to a third of its height, for example).

      Besides, RC car tyres are almost never pressurised, they are either foam or foam filled, so instead of the linear pressure model one would have to consider a most likely very nonlinear spring rate of the foam (and, if present, tire rubber).

    3. The (driven) rear wheels on various cars have often been wider for a good reason, and it doesn’t require a different compound or heat in the tires. The contact pressure isn’t the same as the air pressure, and the contact patch area is not determined solely by static load and air pressure either, although no-one’s saying that those two aren’t big factors. The larger rear tires are generally observed to make better contact when you put an acceleration on them, or corner with them, either way increasing the load past the static level. Plus, you could always run different pressures, which is common. My daily driver’s weight distribution is fairly strongly front-biased, so even though I’ve got a couple extra pounds of pressure in the front I see more deflection out of the front sidewalls than the back for identical tires at rest. I’m adjusting them the way I am for economy rather than traction though.

      The rubber of the tire isn’t a thin stretchy balloon; its material constrains things away from that ideal theory. Also, even a thin stretchy donut shaped balloon assumes the shape it does because a flattened shape would have a smaller volume and thus a higher pressure. If there was no difference, then inflating a tire would not cause it to change shape and would not cause the contact patch to change. So the deformation is an expression of the balance between forces, as mediated by the material which for a real tire is not particularly stretchy, though it can flex in a somewhat constrained way.

      I’m sure I could be more correct if I were a tire engineer rather than a driver, but the above should be at least mostly worthwhile.

    4. The tires are off the shelf “Pan Car” tires, made of a urethane foam rubber glued to a nylon wheel. They are quite wide compared to their diameter to provide more contact patch while maintaining a very low center of gravity. Look up pan cars if you’re not familiar with them, they are very low-slung and flat racecars that can achieve pretty incredible cornering speeds.
      There is some flex in the wheel, and a little in the tire foam itself, though in an actual racing application the foam is lathed down to be a very thin “skin” over the circumference of the rim with minimal material to deform. This projects looks to use the foam tires as they are bought, with a much thicker sidewall than is typically desired.

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