Steering changes the direction the wheels point; suspension lets wheels move relative to the body. Test them separately before building an obstacle course. A turning circle on a flat floor checks steering, and a small block under one wheel checks suspension movement and clearance. A car can steer well yet get stuck on a bump, or cross a bump yet need a wide space to turn.
Begin with a steering model
The Ackermann Soapbox exposes the front steering geometry without adding remote-control electronics. In a turn, the inside front wheel follows a tighter circle than the outside wheel. Ackermann steering turns the inside wheel more so both can follow their paths with less sideways scrubbing.
Read the parts checklist before beginning. During assembly, turn the steering gently through its available range. Stop if a steering arm meets the frame or a tie rod catches another part. Forcing it further can bend a connection or pull the linkage apart; a bigger steering angle is useful only while the mechanism can move cleanly.
Measure the turning circle
Choose a flat, dry floor and mark a starting point with tape. Hold the steering at one repeatable position. Push the model slowly through a complete circle, marking the outer wheel's path at several points. Measure the widest distance across that path, then repeat in the other direction.
Write down the wheel position and the measured diameter. Do not confuse diameter with radius: the diameter is the full width of the circle, while the radius is half that distance. A measurement taken from the body rather than the outer wheel describes a different boundary, so use the same reference each time.
Try it below before you go to the floor. Change the wheelbase, track and steering angle, switch between Ackermann and parallel steering, and drive a U-turn to see which wheel sets the width. Then see if you can fit three cars through a 60-stud street.
Interactive: Turning circle and Ackermann calculator. Change a car’s wheelbase, track and steering angle and see, from above, the turning-circle diameter of the outer front wheel and the ideal inner and outer wheel angles. Compare Ackermann with parallel steering and fit U-turns into a 60-stud street.
If the left and right circles differ substantially, check whether the steering is centred and whether a linkage binds on one side. Differences can also come from the way you push the car. Repeat slowly before changing the design. The steering page explains the basic mechanism.
Add a motor after the geometry works
The First RC Car (Servo) separates drive control from steering control. It adds a motor, receiver and servo, so it is a more involved build than the hand-pushed soapbox. Check the required electronics and wiring before deciding to assemble it.
Run its first test slowly in a clear space. Compare left and right turns using the same steering command. A servo does not automatically fix an off-centre linkage or a wheel rubbing the frame. Keep fingers away from powered gears, and turn the power off before adjusting a connection.
Now test suspension without steering
The Independent Rally Car is an advanced model with independently moving wheel arms, a differential, universal joints and Ackermann steering. It is useful for exploring how several systems share space, but start with its prerequisites rather than treating it as a first build.
With power off and the steering centred, place one wheel on a small, stable block. Look at the wheel arm and the body. Does the arm move relative to the chassis? Do the other tyres stay in contact with the floor? Does a spring compress, or is the entire car simply tilting? These observations distinguish suspension movement from a rigid chassis rocking over an obstacle.
The suspension page describes how springs support the body while allowing movement. Spring stiffness, available travel and how far the mechanism can move before touching the frame are separate considerations. A spring can be present without providing useful travel if another part blocks it.
Separate clearance from wheel travel
Ground clearance is the space under the body and drivetrain. Suspension travel is the range over which a wheel can move relative to that body. Increasing one does not necessarily increase the other. Larger wheels also change the geometry and the effective drive ratio, so they are not a neutral substitution.
For a slow bump test, use a small supported obstacle and keep its height fixed. Record whether the tyres climb it, the underside catches, or a wheel loses contact. Keep steering centred for this test. Turning onto the obstacle adds another variable and can hide why the model stopped.
Keep steering and suspension records separate
Use one result sheet for turning direction, steering position and circle diameter. Use another for block height, suspension movement, tyre contact and underside clearance. Make one change, repeat the relevant test, and inspect whether it improves that result without creating binding elsewhere.
A differential also matters during turns because the outside driven wheel travels farther. Balance affects how the body loads its wheels. Explore those ideas after the separate tests make sense. The creator videos below show independent, more elaborate mechanisms; these MightyCog guides provide smaller instructional examples. Our 3D animations show intended assembly and motion, not measured physical test results.
Interactive: Check yourself. A few quick questions on the ideas in this guide, each with an explanation.
- You measure the widest distance across the outer front wheel’s path: 48 studs. What is the turning radius?
- In a left turn with Ackermann steering, which front wheel turns more?
- Keeping the same steering angle, what happens to the turning circle if you make the wheelbase longer?
- Why keep the steering centred during the one-wheel bump test?
