SHEAR FORCE
30lb Combat Robot
Final CAD
Render of Final Bot (Black Configuration)
3D Printed Prototype
Specifications
Objective & Role
Kaden ACE Picture
Project Overview
The objective of this project is to design and fabricate a 30 lb combat robot, “Shear Force,” that can be competitive in competitions (SCAR, NHRL, etc). The team operated on an accelerated six-month timeline from initial concept in January 2026 to a competition-ready bot by June.
My Role
I collaborated within a multidisciplinary team of eight across Chassis, Dynamics, and Electrical subteams. While officially on Chassis, my contributions extended heavily into Dynamics and software tool development.
Core Contributions
Drivetrain Optimization
Developed a custom mathematical model from first principles and converted it into a standalone .exe tool for rapid parameter iteration. This tool validated pulley ratios and motor specs to maximize acceleration.
Weapon CFD Analysis
Conducted Ansys Fluent simulations on the novel angular drisk to quantify parasitic aerodynamic torque. Used this data to analyze weapon spin-up time and tip speed.
Integrated Design & Fabrication
Contributed to Chassis CAD, 3D printing for rapid prototyping, and final fabrication of the Ti-6Al-4V components.
Overview & Tradeoffs
The drivetrain of a combat robot is a game of tradeoffs.
Torque vs. Top Speed: Pulley ratio choices define the machine's personality. A higher reduction provides the torque necessary for pushing matches but sacrifices top-end speed. Conversely, less reduction enables high-speed maneuverability at the cost of raw pushing power.
Wheel Size vs. Positioning: With the inherent nature of the weight distribution (shifted forward due to the weapon) and the need for armor/forks, there is a critical tradeoff between wheel size and placement. Smaller wheels allow for a more forward drive axis to improve turning geometry, though they inherently limit top speed. Larger rear wheels are utilized to ensure the bot is invertible—capable of driving even if flipped.
Our team developed an innovative 4WD system where the front and rear wheels are mathematically synchronized via specific pulley ratios and sizing. This enables a larger rear wheel and smaller front wheel to maintain identical tangential velocity. This configuration maximizes top speed while maintaining the 4WD traction required to utilize torque where the shifting weight distribution demands it most.
Drivetrain System Prelim CADComputational Modeling & Simulation
While typical 4WD models are common, the uniqueness of this drive system required deriving the dynamics from first principles to validate motor curve performance. The full derivations are shown in the embdedded pdf on the left.
I converted these derivations into a standalone .exe tool. Users can input parameters via a .txt file to simulate performance and export data to .xlsx files. The software includes safety logic to warn if input ratios will cause velocity mismatch and subsequent wheel slip.
Analysis and Results
Using software iteration, we finalized a 2.5in front wheel and 3.5in rear wheel. To reduce complexity, the motor shaft pulleys were kept at 30 teeth for both. The front utilizes a 20-tooth wheel pulley while the rear uses a 28-tooth pulley to achieve synchronization.
The result is a theoretical top speed of 15.51 mph, reaching 15 mph in 1.08 seconds. We are traction limited up to 9.65 mph, providing a built-in factor of safety for maximum pushing power at low speeds. We further limit ESC amperage to ensure full-throttle inputs don't cause unnecessary slip.
Objective
An angular drisk is a fairly novel weapon system for combat robots. Our specification for the weapon’s top tip speed was approximately 240 mph, keeping the design comfortably under the 250 mph SCAR limit. However, a geometry spinning at these speeds is notoriously difficult to model due to complex vortices and the uncommon "drisk" shape.
The primary goal of this CFD was to determine the parasitic aerodynamic torque induced on the weapon at various RPMs. This data allowed for a full system analysis, factoring in pulley ratios and motor curves to calculate the actual top tip speed and other relevant metrics, such as the time to reach 90% max tip speed.
Explanation of CFD Approach
The CFD was conducted in Ansys Fluent using a Rotating Reference Frame (RRF) approach. I utilized the SST k-omega turbulence model to accurately capture boundary layer separation and wake effects at high rotational velocities. Additionally, while performing the analysis, I monitored the residuals to ensure they remained small, typically less than 10⁻³.
By simulating a range of RPMs, I obtained the necessary data to generate a drag torque vs. angular velocity curve. This represents the parasitic "air braking" force the weapon must override due to aerodynamic effects.
Results and Analysis
Similar to a drivetrain system, analysis can be performed to find the RPM of the weapon blades and the tangential tip speed velocity. The key difference between the weapon system and the drivetrain is that the aerodynamic effects of rotation are significant.
In a drivetrain system, top speed occurs near the motor's no-load speed. For the weapon system, however, the top speed occurs when the torque the motor produces is equal to the parasitic torque caused by the aerodynamics at that RPM.
The analysis involves calculating the weapon axle’s τ-ω curve (the motor’s curve multiplied by reduction and a loss factor). By subtracting the parasitic torque from the motor torque, the net torque is used to calculate angular acceleration via the weapon's inertia.
As expected, the angular velocity and tip speed behave like a first-order system:V(t) = C₁ (1 - e-C₂t). To verify the simulation, I graphed the torque vs. ω for both the weapon system and the absolute value of the parasitic torque. Top speed occurs where the two curves overlap. Note that this model does not account for voltage sag or other frictional components.
