Magnetic RC Car Suspension System

Twisted Thruway is one of the busiest pieces on the MoMath floor. A visitor sits in a driver's seat, takes a wheel, and drives a small car around a suspended track while a camera on the car feeds a screen in front of them. Because the track is a Möbius strip, driving straight ahead eventually returns you to your starting point upside down, which is the point the exhibit is making.

That experience only works if the car keeps driving. When a car stalls or drops off the band, a staff member has to reach into the exhibit, and the demonstration stops being about topology. The redesign started from that failure mode.

Nothing about the vehicle could be sized until the surface it drives on was described properly. The band was modeled as a swept Möbius surface with the same width, wall height, and twist rate as the exhibit piece, which turns a hard to reason about shape into a set of numbers: the maximum curvature the car has to follow, the clearance between the walls, and how far the surface rolls beneath the car over a given distance travelled.

Those numbers became the design envelope. Track width set the wheelbase and track width of the car, wall height set how much the body could overhang, and the twist rate set how much suspension travel each wheel needs to stay in contact.

The car is held to the track by magnets in its wheels, so holding force and traction both depend on the same thing: the air gap between wheel and steel. A gap of a millimeter or two costs a large fraction of the holding force, and on a surface that is twisting under the car, a rigid chassis opens that gap constantly. Diagonal wheels lift, the magnets let go, and the car either spins in place or falls.

So the suspension is not there for ride comfort. Its job is to keep every wheel pressed flat against a surface that is never flat, across the whole loop.

The design lets each wheel move independently on its own spring along a constrained axis, with a coil spring setting how hard the wheel is pushed toward the track. Because the magnets are already pulling the car down, the springs are there to hold the wheel against the surface rather than to carry the vehicle's weight the way a car suspension does, and the spring rate is chosen so that a wheel following a twist stays loaded without the axle binding.

Keeping the motion pure was the constraining part. Each corner is guided so the wheel translates and does not cock in its slot, since a wheel that tilts breaks contact at one edge of the magnet and loses grip exactly when it is needed most.

How far each wheel could move, and how hard the spring pushed it down at rest, were worked out physically before anything was committed to a printed chassis. Linear rails stood in for the suspension axis, with the drive motor and its 90° gearbox mounted in place so the real hardware weight and packaging were part of the test, and both could be felt and changed quickly by hand. Measuring the mockups against the envelope from the track model kept the growing assembly inside the width the band allows.

From there the parts moved to printed structure: a body plate carrying the corner assemblies, halves that locate the drive and steering hardware, and mounts sized around the motor, servo, and camera the exhibit car has to carry.

The suspension only earns its place if the rest of the car fits around it. Drive and steering hardware were laid out against the same envelope, with the motor and turning mechanism packaged low and inboard so the corner assemblies keep their full travel and nothing catches the track walls at maximum curvature.

Iterations went in the direction of fewer parts and shorter load paths, since anything that flexes between the wheel and the body shows up as a lost magnetic contact somewhere on the loop.

The only test that counts is a full lap of the exhibit piece. Cars were run on the track itself, watching for the places where contact is most marginal: the steepest part of the twist, and the transition where the surface rolls past vertical and the magnets are carrying the car sideways rather than holding it down.

What the runs check is simple. Every wheel stays down, the car drives a lap without intervention, and it does it repeatedly, because on a museum floor the car has to do this all day.