The Trotbot leg linkage is an eight-bar planar leg mechanism that converts continuous crank rotation into a foot trajectory mimicking a horse's trot. It solves the problem most walking linkages have — short stride and poor ground clearance — by producing a long, flat lower path with a high arched return. A pair of legs phased 180° apart keeps at least one foot planted at all times. Wade Wilgus published the geometry openly in 2015, and it now drives lasercut walkers, classroom kits, and small kinetic sculptures.
Trotbot Leg Linkage Interactive Calculator
Vary pivot clearance and moving-joint count to estimate Trotbot toe wander and see the D-shaped foot path respond.
Equation Used
This calculator estimates vertical toe wander caused by pivot slop in a Trotbot leg. The direct clearance stack is S = N*c. The article example says 0.2 mm clearance at each of six moving joints can create about 2 to 3 mm of vertical foot-point wander, so the calculator scales that range for other clearances and joint counts.
- All moving pivots have similar radial clearance.
- Trotbot link length ratios are otherwise held to the published geometry.
- Toe wander is scaled from the article example: 0.2 mm at 6 joints gives 2 to 3 mm foot wander.
- Planar linkage stiffness is assumed high compared with joint clearance.
Inside the Trotbot Leg Linkage
The Trotbot linkage takes a single rotating crank and routes its motion through eight rigid bars, two ground pivots, and six moving joints to deliver a foot point that traces a closed path on every revolution. The lower portion of that path is nearly straight and parallel to the ground — that is the stance phase, where the foot is planted and pushing the chassis forward. The upper portion arches high, lifting the toe well clear of obstacles before sweeping back to start the next stride. That arched return is what separates Trotbot from a Klann or Chebyshev walker — you get measurably more vertical clearance for the same crank radius.
The geometry only works if you hold the bar lengths to the published ratios. Wilgus specifies link lengths normalised to the crank radius, and if you stretch one bar even 3-4% off ratio, the foot path collapses — the toe either drags through what should be the swing phase or lifts during what should be stance. You will see this as a limp or a foot scuff every revolution. Pivot slop is the other common killer. A 0.2 mm clearance at each of six moving joints stacks up fast, and the foot point can wander 2-3 mm vertically by the time the error reaches the toe. That is enough to make the walker stumble on a flat desk.
Leg-pair phasing has to be exactly 180°. Off by 10° and one foot lifts before the other has planted, so the chassis drops between strides. Run a four-leg Trotbot and the two sides need to be phased 90° apart so the gait stays a true trot rather than a bound. Get any of this wrong and the linkage looks fine spinning in the air but falls on its face the moment it touches the floor.
Key Components
- Crank: The driven input link rotating continuously around the fixed crankshaft pivot. Crank radius sets the overall scale of the foot path — typically 15-30 mm on tabletop builds. Hold concentricity to within 0.1 mm or the foot path wobbles once per revolution.
- Ground frame (chassis pivots): Two fixed pivots on the chassis anchor the linkage. The spacing between them, relative to crank radius, is the most sensitive ratio in the entire mechanism. A 2% error here distorts the stance line into a curve and the walker rocks fore-aft.
- Coupler bars (six moving links): These six rigid bars triangulate the motion and carry the foot point at the end of the chain. Cut from 3 mm acrylic or 1.5 mm aluminium for desk-scale builds. Bar lengths must hold the published Wilgus ratios to within ±0.5% — a sloppy lasercutter kerf compensation will ruin the foot path.
- Pivot joints (six moving): Each joint typically uses a shoulder bolt or steel pin in a bronze bushing or 3 mm bore plain bearing. Joint clearance under 0.05 mm radial is the target. Above 0.1 mm the foot point starts buzzing audibly at speed.
- Foot point: The endpoint of the final coupler bar — this is the tracked locus that touches the ground. Adding a small rubber pad or shrink-tube sleeve here improves traction without disturbing the kinematics.
Industries That Rely on the Trotbot Leg Linkage
Trotbot shows up wherever a builder wants a walking gait that looks more animal than insect. The high arched swing makes it photogenic and forgiving on uneven surfaces, which is why it dominates hobby walkers and educational kits over the more familiar Klann and Jansen designs. You will not find it inside industrial machinery — it is a display and learning mechanism, not a load-carrying one.
- Educational kits: Wade Wilgus's open-source Trotbot plans drive lasercut walker kits sold through Instructables and shared on Thingiverse for high-school STEM classes.
- Kinetic art: Gallery walkers like the four-leg Trotbot sculptures shown at maker faires use the arched foot path for visible, theatrical leg motion driven by a single crank.
- Hobby robotics: Arduino-driven desktop walkers running an N20 gearmotor at 60 RPM through a six-leg Trotbot frame, popular in robotics-club builds.
- University demonstrations: Mechanical engineering kinematics labs use Trotbot alongside Klann and Jansen as a comparative case study in coupler-curve synthesis.
- Children's museums: Hand-cranked Trotbot exhibits where visitors turn a wheel to drive a transparent acrylic walker across a track, showing how rotation becomes walking.
- Toy prototyping: Walking-toy R&D shops use Trotbot for prototype gait studies before committing to injection-moulded production geometry.
The Formula Behind the Trotbot Leg Linkage
Stride length and forward speed are the two numbers that decide whether your Trotbot looks like it is walking or skidding. The stride is fixed by the linkage geometry — roughly 2.2 × the crank radius for the standard Wilgus ratios — and the forward speed scales linearly with crank RPM. At the low end of the typical 30-120 RPM band, the foot tracks cleanly but the walker creeps. At the high end, theoretical speed looks great on paper but the swing phase compresses below what the leg geometry needs to clear the ground. The sweet spot for desk-scale builds sits at roughly 60 RPM.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| vfwd | Forward walking speed of the chassis | m/s | ft/s |
| RPM | Crankshaft rotational speed | rev/min | rev/min |
| Sstride | Linear stride length per crank revolution | m | in |
| Rcrank | Crank radius (input link length from pivot to wrist pin) | m | in |
Worked Example: Trotbot Leg Linkage in a 6-leg greenhouse inspection walker prototype
You are prototyping a 6-leg Trotbot walker as a low-cost greenhouse inspection rover for a small agritech startup. Crank radius is 25 mm, the chassis runs on a 12 V brushed gearmotor with a target nominal crank speed of 60 RPM, and you want to know forward speed across the realistic operating band so you can size the gearbox correctly.
Given
- Rcrank = 0.025 m
- RPMnom = 60 rev/min
- RPMlow = 30 rev/min
- RPMhigh = 120 rev/min
Solution
Step 1 — compute stride length from the linkage geometry. With Wilgus's published ratios, the stance line spans roughly 2.2 × the crank radius:
Step 2 — at nominal 60 RPM, convert to revolutions per second and multiply by stride:
That is roughly 5.5 cm/s — a relaxed amble across a greenhouse aisle, fast enough to cover a 10 m row in about 3 minutes but slow enough that a Raspberry Pi camera can hold sharp frames without motion blur.
Step 3 — at the low end of the typical operating band, 30 RPM:
At 30 RPM the walker creeps at under 3 cm/s — useful for close inspection or vibration-sensitive imaging, but a bystander would have to watch carefully to confirm it is moving. At the high end:
11 cm/s on paper, but in practice you will see foot scuffing above roughly 90 RPM on a 25 mm crank build because the swing-phase duration drops below the time the leg needs to lift, arc, and re-plant cleanly. The sweet spot sits at 60-75 RPM.
Result
Nominal forward speed is 0. 055 m/s at 60 RPM crank speed with a 25 mm crank radius. That feels like a slow, deliberate walk — visible motion, smooth gait, and the chassis stays level between strides. Across the band, 30 RPM gives a near-imperceptible 0.028 m/s creep and 120 RPM theoretically gives 0.11 m/s but the foot scuffs above roughly 90 RPM. If your measured speed comes in 20% below the predicted 0.055 m/s, check three things in this order: first, leg-pair phasing — even a 5° error on a 6-leg setup shortens effective stride because the legs fight each other; second, foot-pad slip on the greenhouse floor surface, easily fixed with shrink-tube grip on the toe; third, gearbox backlash, which on cheap N20 units can swallow 8-10° of crank rotation per direction change and quietly steal stride length on every revolution.
Choosing the Trotbot Leg Linkage: Pros and Cons
Trotbot is one of three linkages a builder typically considers for a small walking robot — the others being Klann and Jansen. They look similar at a glance but trade off differently across stride, clearance, complexity and load. Here is how they stack up on the dimensions that matter when you are picking one for an actual build.
| Property | Trotbot | Klann linkage | Jansen linkage |
|---|---|---|---|
| Bar count per leg | 8 bars | 6 bars | 8 bars |
| Foot ground clearance (relative to crank radius) | High (~1.4×) | Medium (~1.0×) | Low (~0.7×) |
| Stride length (relative to crank radius) | ~2.2× | ~2.0× | ~2.4× |
| Typical crank RPM (desk-scale build) | 30-90 RPM | 30-120 RPM | 20-60 RPM |
| Build complexity / part count | High | Low | Highest |
| Load capacity (chassis weight per leg pair) | Low — display use | Medium | Low — display use |
| Tolerance sensitivity (link length) | ±0.5% | ±1% | ±0.3% |
| Best application fit | Animal-style gait, kinetic art | Classroom STEM kits, museum exhibits | Large-scale wind walkers, beach sculptures |
Frequently Asked Questions About Trotbot Leg Linkage
This is almost always laser kerf compensation on the bar lengths. A 0.15 mm kerf on a 60 mm bar is a 0.25% error per side — within tolerance — but if you cut all eight bars and the kerf compensation is set wrong on only the coupler links, you stack 1-2% error on the most sensitive ratios. The foot path then collapses asymmetrically and one toe sits 1-2 mm lower at the bottom of its arc.
Quick check: measure each bar pin-to-pin with calipers against the Wilgus spec sheet before assembly. Reject any bar more than 0.3 mm off. Recut, do not file to size — filing changes the pin-hole geometry and introduces angular error.
Static stability decides this. A 4-leg Trotbot has only two feet on the ground during the half of the cycle when the other pair is in swing, so the chassis pitches fore-aft visibly on every stride. That is fine for kinetic sculpture, bad for camera-carrying inspection work.
A 6-leg layout phased at 0°, 120°, 240° on each side keeps three feet planted at all times, eliminates the pitch oscillation, and roughly halves the vertical jitter the camera sees. Pay for it with twice the linkages to cut, tune, and align — but for any sensor payload, it is worth it.
Foot slip on the stance phase. The formula assumes the foot is a fixed pivot during stance — zero slip, all crank energy converts to forward chassis motion. In reality, a hard plastic toe on a smooth surface like polished concrete or melamine slips 15-25% per stride, and you measure forward speed proportionally lower.
Diagnostic: mark the floor under the foot with chalk at the start and end of one stride. If the foot mark moves backwards relative to where the chassis predicts it should plant, you are slipping. Fix is a 2 mm wrap of heat-shrink tubing on the toe or a small silicone pad — restores effective stride to within 5% of theoretical.
Partly yes, but not as far as you would hope. Stride scales with crank radius, so halving the crank from 25 mm to 12.5 mm and doubling RPM from 60 to 120 keeps forward speed the same on paper. The problem is that the swing-phase clearance also halves — you go from 35 mm clearance to under 18 mm — and the walker now trips on anything taller than a coin.
Rule of thumb: keep crank radius above 15 mm for any walker that has to handle real-world surfaces. Below that, you are building a display piece that lives on a flat acrylic stage.
Resonance in the coupler bars. Hand-rotation runs at maybe 0.5 RPM — far below any structural resonance. A motor at 60 RPM with six legs creates a 6 Hz excitation on the chassis, and 3 mm acrylic coupler bars on a typical desk-scale build resonate around 8-12 Hz. You catch the lower sideband and the foot point rings.
Two fixes: stiffen the bars by going to 4.5 mm acrylic or 1.5 mm aluminium, which pushes resonance above 20 Hz, or change RPM by 20% to step out of the resonant band. Aluminium is the better long-term answer because it also reduces bar bending under load.
For a first build, no. Klann uses six bars per leg versus Trotbot's eight, has wider link-length tolerance (±1% versus ±0.5%), and is more forgiving of pivot slop. You will get a Klann walking on the first power-up far more often than a Trotbot.
Move to Trotbot when you specifically want the high-arched horse-trot foot path — for animal-style gait studies, kinetic art where the leg motion is visible to the audience, or any case where ground clearance matters more than build simplicity. For pure locomotion, Klann is the practical choice.
References & Further Reading
- Wikipedia contributors. Linkage (mechanical). Wikipedia
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