The Robot That Rolls Until It Has to Climb - by Jaimin
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The Robot That Rolls Until It Has to Climb<br>June 23, 2026 · Mobility & Field Robotics
Jaimin<br>Jun 23, 2026
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In last post we watched a robot dog give up most of its gearing just so its leg could feel the ground. Today comes the obvious follow-up question: should it have used a leg at all? A wheel, after all, is the most efficient way humans have ever found to move across flat ground, and a robot dog spends a lot of energy doing something a wheel does for almost nothing.<br>There is a clean way to measure this. Engineers use a single number called cost of transport: how much energy a machine burns to carry its own weight a given distance.<br>the energy cost of transport — how much energy a machine uses to move its own weight over a given distance.<br>Engineers often use a normalized quantity called Cost of Transport (CoT) :
\(CoT= \frac{Energy}{Weight \times Distance}\)
or equivalently:<br>\(Energy=CoT×mg×d\)
where:<br>m = mass (kg)
g = 9.81 m/s²
d = distance (meters)
It tells you: how many joules are required to move one newton of weight one meter.<br>This metric is very useful for robots because it removes the effect of size.<br>A 1-ton robot and a 50 kg robot can be compared directly.<br>Typical values:<br>Industrial wheeled robot: CoT ~0.01–0.05
Humans walking: ~0.2
Efficient quadrupeds: ~0.5–1
Early humanoid robots: often 1–5+
This explains a major challenge in humanoid robotics.
A wheeled robot is like a car: it can carry itself a long distance while spending energy equivalent to lifting itself only a few tens of meters.<br>A humanoid takes thousands of tiny controlled falls, repeatedly accelerating and decelerating its legs, stabilizing balance, and dissipating energy. It may spend the equivalent of lifting its body one to several times its own height every few meters traveled .<br>It is just power divided by weight and speed, which leaves a pure ratio you can compare across anything that moves.<br>The MIT Cheetah, one of the most efficient walking robots ever built, comes out around 0.5. In plain terms, on a flat floor a wheel moves for roughly a tenth of the energy a good leg needs, and a leg keeps draining the battery even while standing still. So why build legs?<br>How it actually works
The idea behind cost of transport is old. Two engineers, Gabrielli and von Karman, first charted it in 1950, lining up everything from cargo ships to helicopters on one scale. It has lasted because it ignores how big or fast a thing is and leaves one honest number behind.<br>A wheel is cheap to run for a simple reason. When it rolls on hard, flat ground, the body just glides along a level line. There is no lifting, no dropping, no slamming a foot down. The only real losses are a little friction in the bearings and where the tire squashes against the road. A leg does the opposite of all that. Every single step lifts the whole body up and sets it back down, swings a limb forward and halts it, and lands with a thud that turns motion into wasted heat. That is the gap between 0.05 and 0.5.<br>So a wheel should win everywhere. It does not, and the reason is shape, not energy. Picture a wheel rolling up to a stair. To get over the lip it has to pivot up and over the edge, and the taller the step, the more force that takes. By the time a step is about half the height of the wheel, an ordinary wheel is already spinning helplessly or stuck. Make the wheel bigger and it climbs more, but then it cannot fit through a door or turn in a hallway. A gap wider than its footprint just swallows it.<br>A leg has no such limit. It does not climb over an obstacle, it steps past it, picking a clean spot to land on the far side and ignoring whatever is in between. A curb, a staircase, a ditch, a heap of rubble: to a leg, each is just a different place to put a foot. That is the entire trade in one line. A wheel is a cheap way to cross smooth, continuous ground. A leg is an expensive way to cross ground that keeps breaking. What decides between them is how rough the ground is compared to the size of the wheel.
Once you see the trade that way, the natural answer is to carry both, and that is exactly where the field has gone. A growing class of robots puts a wheel at the end of each leg. On flat ground they lock their legs and roll, cheaply and fast. When the ground breaks up, they lift the wheels and walk, using each wheel as a foot. A team at ETH Zurich measured the payoff on one such machine: rolling instead of walking cut its energy cost by about 83 percent, while it kept the ability to step over things a plain roller never could. The clever part is no longer the body. It is the software deciding, second by second, when to roll and when to walk.<br>Sunday Robotics’ Memo robot uses a wheeled base and a height-adjustable central column to move, prioritizing passive safety, stability, and simplicity over the complex bipedal walking systems used by humanoids like Tesla Optimus...