Tool 14 · Chassis
A robot that tips is a robot that stops. Feed in your chassis dimensions and where the mass sits, and this tool returns the angles you can survive, the slope you can climb, and how hard you can corner or brake before physics takes over.
Left wheel to right wheel
Front axle to rear contact point
Above the floor — measure to the battery if unsure
For the cornering speed limit
All dimensions must be positive.
Stability spec
tip angle = atan( half wheel spacing ÷ CoG height )
max acceleration before tipping = g × half spacing ÷ CoG height
max acceleration before sliding = g × µ
cornering speed = √( a_max × turn radius )
Every number here comes from one ratio: how wide the wheels are spaced compared to how high the mass sits. Stability is not about being heavy — doubling the robot's mass changes none of these angles, because weight appears on both sides of the equation. It's about geometry. A 500 g robot with a tall mast tips more easily than a 5 kg robot built like a paving slab.
The tool reports both failure modes, and which one wins matters. If the sliding limit is lower than the tipping limit, your robot skids when pushed too hard — undignified but harmless, and recoverable. If tipping comes first, the robot goes over. Designing so that grip runs out before geometry does is a genuine safety property, and the cheapest way to get it is a wider track.
The two fixes, in order of effectiveness: lower the mass (the battery is almost always the heaviest single item — putting it on the baseplate rather than on a deck is often worth 20° of stability by itself) and widen the track. Adding a longer wheelbase or trailing casters helps the fore/aft number specifically, which is what protects you when the robot brakes hard or starts climbing a ramp.
Finding your real centre of gravity height without CAD: balance the robot sideways on a straight edge — a ruler under the chassis — to find the balance point in each axis, then hang the robot from two different points and where the plumb lines cross is the CoG. For most builds, "the height of the battery's midpoint" is a good first estimate.
Only if it sits low. Mass added below the current centre of gravity pulls it down and improves every number here; mass added above makes things worse. This is why builders bolt lead or steel to the baseplate but keep decks light.
An extended arm moves the whole system's centre of gravity outward and upward — often dramatically. Run this calculation with the arm at full reach carrying its maximum payload, not with the arm tucked in, or your first successful pick will also be your first tip-over. The Servo Torque Calculator handles the arm's own moment loads.
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