AMR Chassis Wheel Coplanarity & Torsional Stiffness: What to Specify

An AMR chassis connects the wheel modules, batteries, sensors, controls and payload structure into one mechanical coordinate system. If the wheel-support interfaces do not relate correctly, the complete robot may rock, preload one module differently or reproduce poorly from one frame to the next. If the structure twists too easily under the defined load cases, sensor and payload interfaces can move relative to the wheel plane.

These are not solved by adding a generic overall tolerance to the weldment. The OEM should define wheel coplanarity and torsional stiffness from the mobile platform’s architecture, then translate those requirements into measurable datums, load conditions and acceptance evidence.

DRAmetal manufactures the released mechanical scope and performs the agreed fabrication inspection. The AMR OEM or integrator owns vehicle dynamics, stability, wheel loading, suspension behavior, steering and navigation performance, battery and electrical safety, functional safety, certification and final validation. Chassis measurements support that validation; they do not replace it.

Define the wheel-support architecture first

“Wheel coplanarity” means different things on different AMRs. A platform may use driven wheels with compliant suspension, fixed casters, sprung support modules or articulated bogies. Some wheel interfaces are intended to establish the primary running plane, while others are designed to float or carry load only after suspension is installed.

Before specifying the bare frame, identify:

• Which wheel or drive modules establish the functional support plane.
• Whether mounting interfaces are pads, bores, hole patterns, rails or welded brackets.
• Which interfaces locate module position and which only provide fastener clearance.
• Whether modules include adjustment, shims, compliance or spring preload.
• The approved chassis support condition during inspection.
• The installation sequence and required access for fasteners and alignment.

A drawing should not force every wheel bracket into one mathematical plane when the suspension concept intentionally uses offsets. Conversely, leaving all wheel interfaces under a general profile requirement may fail to control the relationships needed for assembly.

The correct specification begins with a functional interface map prepared by the design authority. This map becomes the basis for chassis datums, fixture pickup points and first-article inspection.

Release operating inputs before choosing stiffness targets

Torsional stiffness is a relationship between applied torque and angular deflection under defined boundary conditions. A stiffness target without a load case, restraint scheme and measurement locations is incomplete. Two teams can test the same frame differently and obtain results that cannot be compared.

The OEM or responsible structural engineer should define the relevant inputs, which may include payload states, battery configuration, acceleration and braking events, turning, ramp transitions, docking loads, lifting points, transport restraints and approved floor irregularity assumptions. Dynamic amplification, fatigue duty and safety factors also belong to the design authority.

For teams designing lightweight AMR structures, mass reduction should remain subordinate to released stiffness, strength, fatigue and service requirements. Removing a cross-member or changing a panel gauge may save mass while changing local wheel-pad rotation or transferring load into a battery bay. The fabrication RFQ should contain the released geometry and acceptance criteria, not an open request for the supplier to “make it light.”

The supplier can provide DFM feedback on bend access, weld sequence, joining details, tooling and inspection feasibility. Any structural redesign or acceptance of a different load path requires controlled OEM approval and subsequent validation.

Translate coplanarity into functional datums

A useful datum scheme recreates how the chassis is located during wheel-module assembly. The primary datum may be a defined set of wheel-support pads, dedicated inspection targets or another stable structural interface. Secondary and tertiary datums should control the chassis directions relevant to module spacing, axle relationships and sensor references.

Avoid selecting cosmetic outer edges merely because they are easy to probe. Bent covers and trim flanges often do not establish wheel geometry. Likewise, measuring each pad independently from a floor can conceal the relationship among all interfaces if the frame is supported differently for each reading.

The drawing should define whether coplanarity applies to complete pads, specified target areas, hole-axis intersections or installed module interfaces. If a pad will be machined after welding, identify the final-state requirement and the machining datum pickup. If shims are allowed, define their ownership, approved locations and how the as-built condition is recorded.

The robotics frame FAI and datum checklist can be used to assign characteristic IDs, measurement equipment, restraint state and deviation records before the first chassis enters inspection.

Specify torsional behavior as a testable condition

A torsional requirement needs a clear test setup. State where the chassis is supported, where the force or torque is introduced, which modules or ballast are installed, and where rotation or displacement is measured. Define the loading direction, any preload cycle, the evaluation range and the permitted result according to the OEM’s analysis and validation plan.

The bare weldment may be only one state. Covers, battery trays, bolted cross-members and payload frames can contribute to assembled stiffness if the design intends them to do so. The requirement should say whether the supplier checks a bare frame, a mechanical assembly or both. Fastener type, tightening condition and installed hardware revision then become part of the test definition.

Do not confuse a diagonal height check with torsional stiffness. A chassis can have an acceptable unloaded wheel-plane relationship but still deflect excessively under a specified torque. It can also be stiff but welded into a distorted free state. Coplanarity, unloaded geometry and loaded structural response require separate characteristics.

Control distortion through the released manufacturing route

Wheel-support geometry can change during forming, welding, stress redistribution, machining and coating. The process plan should identify when each critical feature is created and when it is verified. A hole cut before welding may not remain in the same relationship after the frame is completed.

Useful fabrication controls can include datum-based tack fixtures, balanced weld sequencing, controlled fit-up gaps, temporary braces, post-weld machining stock and final-state inspection. These are process options, not universal requirements. Their use depends on the released design, material, access, production quantity and agreed evidence.

Fixtures should locate functional features without forcing the chassis into an artificial shape. The supplier should document fixture pickup points and release checks so an unclamped frame is not accepted solely because it fit a rigid tool. If the drawing permits restrained inspection, the restraint condition must be defined and reproducible.

For build-to-print mobile structures, the robot and AGV chassis fabrication page describes the transaction scope for frames, decks, battery bays and sensor mounts. Broader stationary and mobile structures belong under robotics frame and chassis fabrication.

Choose evidence that answers the acceptance question

The table below separates common questions so the RFQ does not collapse them into one “frame tolerance.” Final methods and limits must come from the released drawing and approved inspection or validation plan.

Acceptance question	Possible evidence	Required setup definition	Owner of acceptance criterion
Do functional wheel pads share the intended relationship?	CMM data, height map or functional fixture result	Primary supports, pad target areas and restraint state	AMR OEM or integrator
Are module hole patterns located to the chassis datum frame?	Variable feature-position report or approved gauge	Datum sequence, simulator details and final manufacturing state	Drawing authority
Does the unloaded frame sit in the released free state?	Free-state geometry report	Gravity orientation and support points	Drawing authority
Does the chassis meet the released torsional target?	Load-versus-deflection test record	Supports, torque or force, assembly state and measurement points	OEM structural and validation team
Is production still consistent with the approved first article?	Defined sampling, fixture checks and retained records	Fixture revision, inspection frequency and reaction plan	Approved quality plan

Coordinate data should identify part and drawing revision, equipment, fixture, datum setup, measurement condition and actual results. Test records should also identify load application and the configuration of installed mechanical components. A simple “pass” without setup information is difficult to reproduce after a design change.

Separate first-article learning from production control

The first article is the point to prove that drawing requirements, process stages and inspection methods agree. It should include the complete characteristic set required by the buyer, plus any process evidence needed to approve fixtures or post-weld machining.

Production control can then focus on characteristics that indicate drift: fixture pickup points, wheel-pad relationships, key hole patterns, machined surfaces and selected weldment geometry. The buyer should define sampling, report retention and requalification triggers. A new chassis revision, fixture repair, process relocation or major weld-sequence change may require additional approval according to the quality plan.

Prototype structural testing is different from routine fabrication inspection. The OEM may use instrumented vehicles, representative payloads, floor transitions or accelerated durability testing to validate the system. Those activities remain with the OEM or its qualified test partners unless explicitly contracted and documented.

Related customer project: AGV Chassis: Wheel Alignment and Battery Access. Related resources: AGV chassis design guide.

Use a quote-ready AMR chassis RFQ checklist

An AMR chassis supplier cannot establish meaningful coplanarity or stiffness evidence from a model alone. Include the following with the RFQ:

• Released assembly model and part drawings with matching revisions.
• Wheel and drive module interface drawings and installation sequence.
• Functional primary, secondary and tertiary chassis datums.
• Wheel-support pads, targets, hole patterns and permitted adjustment or shimming.
• Free-state and restrained-state inspection requirements.
• OEM-approved loads, boundary conditions and torsional acceptance criteria.
• Bare-frame and assembled-state definitions for each check.
• Weld, machining, coating and final inspection stages.
• FAI format, variable data, fixture or gauge requirements and record retention.
• Production quantities, variants, packaging and revision-identification needs.
• Explicit boundary for OEM-owned dynamics, safety, certification and final validation.

Use the robotics industry hub to place the chassis within the wider platform sourcing plan. When the released mechanical package and acceptance inputs are complete, send them to DRAmetal for a project-specific DFM, fabrication and inspection quotation.