Motor Mount Bracket Design and Strength Check

The problem

A motor mount carries the motor weight, the reaction to drive torque and road shocks, and it often cracks at bolt holes or weld toes when it is designed by eye. A short, disciplined loop of load estimation, simulation with correct fixtures and connectors, and mesh convergence shows how a real bracket is sized and why a hand check is still needed.

Objectives

• Derive load cases for motor weight, peak torque reaction and a vertical bump load.
• Model a bracket in sheet metal or as a machined plate with ribs and bolt holes.
• Set up a static study with bolt connectors, fixtures on frame tabs and remote loads at the motor centre.
• Converge the mesh and compare peak stress with a hand calculation.
• Iterate to meet a factor of safety of 2 or more at minimum mass and document the result.

What you need

Key concepts

Torque reaction

When the motor drives the chain or belt, an equal and opposite torque acts on its casing and is carried through the bracket to the frame.

Remote load

A force or moment applied at a point, such as the motor's centre of mass, and transferred to selected faces, so the motor itself need not be meshed.

Stress singularity

At a sharp re-entrant corner or a point fixture, stress keeps rising as the mesh refines. Such peaks are not real and must be identified, not reported.

Factor of safety

The ratio of the material's allowable stress, usually yield, to the computed stress. Fatigue may govern for vibrating parts and needs a separate check.

Step-by-step guide

1. 1

   Define load cases

   Take an example 6 kg mid-drive motor with 50 N·m peak torque and a 14-tooth sprocket. Define three load cases: static weight times a 3 g bump factor, peak torque reaction plus chain pull, and both combined. Calculate chain pull from torque and sprocket pitch radius. Record each case with its direction in vehicle axes in a table, labelled as example values, and choose an acceptance target such as a yield factor of safety of 2.

   Checkpoint: A load case table with forces and moments in vehicle axes and a written acceptance target.

2. 2

   Lay out the interfaces

   Insert a simple motor body and two frame tabs into an assembly at their real positions. On the bracket part, sketch the motor bolt circle and frame hole pattern by converting edges from the assembly, then break or lock the external references once positions are settled so the part does not change unexpectedly. Keep a minimum edge distance around holes of about twice the hole diameter, and leave spanner clearance around every bolt head.

   Checkpoint: Bracket holes align with motor and frame holes, confirmed by a concentric mate on each bolt.

3. 3

   Model the bracket

   Use Base Flange and Edge Flange in Sheet Metal for a bent bracket, or an extruded plate with ribs for a machined one. Start at 4 mm thickness of an example S355 steel or 6061-T6 aluminium from the material library. Add generous fillets at rib roots and bend reliefs at flange corners. Create configurations for 3, 4 and 5 mm thickness with a Design Table so you can compare them later without separate files.

   Checkpoint: Each configuration rebuilds cleanly and the sheet metal version flattens without errors.

4. 4

   Set up the study

   Create a Static study on the bracket alone. Restrain the frame tab holes with fixed hinge fixtures on the hole faces, or use bolt connectors to a foundation. Apply each load case as a Remote Load with a rigid connection at the motor's centre of mass, attached to the motor bolt hole faces. Use a separate study for each case, or Load Case Manager if your licence includes it, so results stay traceable.

   Checkpoint: The study runs and the deformed shape, scaled up, moves in the direction you expect for each case.

5. 5

   Mesh and converge

   Use a curvature-based mesh with a mesh control at the bolt holes and rib roots. Run three meshes with decreasing element size, recording peak von Mises stress at a probe on a real fillet, not at a sharp corner. Plot stress against element count. If stress keeps climbing at a sharp edge, add a realistic fillet or treat that point as a singularity and report stress a small distance away instead.

   Checkpoint: Peak stress at the chosen location changes by less than 5 per cent between the last two meshes.

6. 6

   Check by hand

   Treat the bracket arm as a cantilever from the frame bolts to the motor centre and compute bending stress from the bump case, using the section modulus at the root. Compare with the simulated stress a short distance from the fillet. Agreement within about 20 per cent is reasonable for this simplification; larger gaps usually mean a wrong load direction, a wrong fixture or a unit error in the remote load.

   Checkpoint: Hand and simulated stresses agree within the stated band, with any difference explained.

7. 7

   Iterate for mass

   Use the configurations to compare factor of safety and mass. Try ribs instead of a thicker plate, lightening holes in low-stress regions shown by the stress plot, and a different material. Keep a results table of thickness, mass, peak stress, factor of safety and maximum deflection. Pick the lightest option that meets the target with a deflection small enough to keep the chain or belt aligned under peak torque.

   Checkpoint: A chosen design that meets factor of safety 2 or more under all cases, with mass reduction stated against the 4 mm baseline.

8. 8

   Consider fatigue

   The bracket sees repeated torque and road loads, so note the stress amplitude for a typical riding cycle and compare it with an endurance limit for the material corrected for surface finish and size. If you have SOLIDWORKS Simulation Standard or higher, run a Fatigue study using an S-N curve from the material data. State that welded or bent regions need extra care and that a physical test would be needed before use.

   Checkpoint: A short fatigue note with the stress amplitude and a stated margin or a clear limitation.

9. 9

   Draw and report

   Produce a drawing with flat pattern, bend table, hole tolerances and ISO 2768 general tolerances. Write a report covering loads, assumptions, the mesh study, the hand check, the iteration table and the final choice. Include stress and deflection plots with scale and units shown on each, and list the tests that should be run on a rig, such as a static proof load and a cyclic torque test.

   Checkpoint: A drawing ready for laser cutting and bending and a report with every assumption stated.

Testing & validation

• Mesh convergence shows less than 5 per cent change in peak stress at a real fillet.
• Hand calculation and simulation agree within about 20 per cent.
• All load cases meet the factor-of-safety target.
• Flat pattern generates without errors and bend radii suit the chosen material.

Expected results

A motor mount bracket that meets a stated factor of safety under three example load cases, a configuration-based iteration table showing the mass saved, and a manufacturing drawing with flat pattern.

Take it further

• Run a Frequency study to check the first natural frequency against motor speed orders.
• Model the bolted joint with bolt pretension and check for joint separation.
• Run a Topology Study (Simulation Professional) to find an efficient rib layout.
• Compare a welded and a bent design for cost with SOLIDWORKS Costing.

Viva and review questions

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