Swingarm Design for a Mid-Drive Electric Motorcycle

The problem

In a mid-drive electric motorcycle the swingarm carries the rear wheel, reacts chain pull, brake torque and cornering loads, and sets the anti-squat behaviour. Its stiffness affects handling as much as its strength. Students need to see how pivot position, tube sections and bracing trade mass against vertical and lateral stiffness.

Objectives

• Lay out pivot, axle and shock positions from an example wheelbase and suspension travel.
• Build a tubular or box-section swingarm with Weldments structural members and gussets.
• Analyse vertical bump, braking, chain pull and a lateral cornering case.
• Measure vertical and torsional stiffness and compare two section choices.
• Produce a weldment cut list and drawings.

What you need

Key concepts

Swingarm pivot and anti-squat

The pivot height relative to the chain line changes how chain pull extends or compresses the rear suspension under acceleration.

Beam versus solid mesh

Weldment members can be meshed as beams for quick stiffness studies, but local stresses at joints and gussets need solid or shell meshing.

Torsional stiffness

Twist per unit moment about the arm's long axis. Low torsional stiffness lets the wheel lean relative to the frame in corners and hurts stability.

Weld toe

The edge of a weld bead where stress concentrates and fatigue cracks often start, so gussets and smooth transitions matter.

Step-by-step guide

1. 1

   Set geometry targets

   From an example 1350 mm wheelbase, 17-inch rear wheel, 110 mm rear travel and 520 chain, fix pivot height, swingarm length and shock mount ratio. Record the example rear load (say 120 kg static), a 3 g bump factor, a 1 g braking case and a 0.8 g lateral case as assumptions. Note the motor sprocket position because it decides the chain line and the best pivot placement relative to it.

   Checkpoint: A layout sketch in the frame assembly with pivot, axle and shock points fully defined from global variables.

2. 2

   Sketch the member layout

   Create a 3-D sketch with lines for the left and right arms, a cross-brace behind the tyre and shock mount links. Keep the cross-brace far enough from the tyre to allow for tyre growth at speed and chain clearance at full travel. Use symmetric relations about the mid-plane so one edit moves both sides, and dimension the sketch from the pivot and axle points rather than from arbitrary origins.

   Checkpoint: The 3-D sketch is fully defined and symmetric, and a tyre envelope body shows clearance at full bump.

3. 3

   Build the weldment

   Insert Structural Members for the arms using a round or rectangular profile, then the cross-brace, trimmed and extended to the arms with Trim/Extend. Add a pivot tube and axle plates as separate bodies, gussets at the brace joints and end caps on open tube ends. Apply an example steel such as AISI 4130 or an aluminium such as 6061-T6 from the library and confirm the cut list updates automatically.

   Checkpoint: The cut list shows every member with length and profile, and there are no overlapping bodies.

4. 4

   Check travel and chain line

   In an assembly, add a concentric mate at the pivot and a limit angle mate for travel. Drive the arm through travel in a Motion Study (Animation) or by dragging, watching tyre clearance to the brace, chain clearance to the swingarm and shock length. Measure chain growth across travel with a sketch-based measurement between sprocket centres, because it affects chain slack and tensioner design.

   Checkpoint: No interference at any position in travel, and shock length stays within the example shock's stroke.

5. 5

   Set up the vertical case

   Create a Static study. Fix the pivot tube bore with a fixed hinge and support the shock mount with a fixture or a spring connector. Apply the bump load at the axle slots as a remote load from the wheel contact patch. Mesh the structural members as beams first to get a quick deflection, then convert the joint region to solid elements to see local stress around the brace and gussets.

   Checkpoint: Axle vertical deflection and peak member stress are reported, and the beam result agrees with a simple cantilever estimate in order of magnitude.

6. 6

   Add braking, chain and lateral cases

   Create studies for braking torque reacted at a torque arm or caliper mount, chain pull at peak motor torque applied on the axle along the chain direction, and a lateral force at the contact patch that twists the arm. Keep fixtures identical across studies so the cases compare fairly. Record peak stress, location and factor of safety for each case in one table for the report.

   Checkpoint: All four cases solve and each has a factor of safety above an example target of 2 on yield.

7. 7

   Measure stiffness

   Apply a unit vertical load and a unit torsional moment about the arm's long axis and record deflection and twist. Compute vertical stiffness in N/mm and torsional stiffness in N·m per degree. Repeat for the round and box-section configurations, adjusting wall thickness so their masses are similar, and compare stiffness-to-mass ratios. This shows which section gives better handling per kilogram.

   Checkpoint: A table comparing vertical and torsional stiffness per kilogram for both section choices.

8. 8

   Refine joints

   Switch to a solid mesh with mesh controls at the cross-brace joints and gussets. Converge the stress in the weld toe region to within 5 per cent between meshes, ignoring sharp re-entrant edges that never converge and reporting stress a short distance from them. Add or reshape gussets where stress concentrates, smooth abrupt section changes, then rerun the critical load cases to confirm the improvement.

   Checkpoint: Converged joint stresses below the target, with gusset changes documented before and after.

9. 9

   Draw and report

   Prepare a weldment drawing with the cut list table, weld symbols, pivot bore tolerances and axle slot dimensions, plus an assembly drawing at full bump. Report geometry, load cases, the stiffness comparison, joint refinements and the limits of a linear static analysis. State that a physical stiffness and fatigue test on a jig would be needed before the design could be trusted on a vehicle.

   Checkpoint: A fabrication-ready drawing set and a report with load cases, stiffness table and the chosen section.

Testing & validation

• Full-travel sweep shows no tyre, chain or shock interference.
• All load cases meet the factor-of-safety target with converged joint stress.
• Beam-mesh and solid-mesh deflections agree within about 10 per cent.
• Cut list matches the member lengths measured on the model.

Expected results

A swingarm weldment with checked clearance through travel, strength results for four example load cases, a stiffness-per-mass comparison of round and box sections, and fabrication drawings.

Take it further

• Compare a single-sided swingarm for stiffness and mass.
• Plot anti-squat percentage across travel for different pivot heights.
• Run a Frequency study to check the first mode against motor and road excitation.
• Run a Fatigue study on the weld toe region.

Viva and review questions

Learn it at DIYguru

These DIYguru courses and programmes teach what this project needs.