Electric Bicycle Frame for a Rear Hub Motor and Down-Tube Battery

The problem

Converting a standard bicycle to electric often overloads its dropouts, because a hub motor reacts its torque through the axle flats. Down-tube batteries also need mounting bosses and a stiff tube. A frame designed for the hub motor and battery from the start avoids spun axles, cracked dropouts and loose battery mounts.

Objectives

• Lay out frame geometry for an example rider size with a rear hub motor and down-tube battery.
• Model the frame with Weldments and add dropouts sized for motor torque reaction.
• Integrate battery mounting bosses, a controller bay and internal cable routes.
• Check the frame under example seated, pedalling and braking loads.
• Produce a drawing with a cut list and dropout detail.

What you need

Key concepts

Torque reaction at the dropout

A hub motor's stator is fixed to the axle, so motor torque is reacted by the axle flats bearing on the dropout slot. Weak or thin dropouts open up or crack, which is why torque arms are often added.

Frame geometry

Seat tube angle, head tube angle, stack and reach define how the bike fits and handles. They are set from rider size before any tube is drawn.

Custom weldment profile

A 2-D sketch saved as a library feature part in the weldment profiles folder. It lets you use oval or tapered-look sections not in the standard library.

Bicycle frame test standards

ISO 4210 and EN 15194 describe frame fatigue and electric-assist bicycle requirements. Use them as a reference for load cases and check the current edition for values.

Step-by-step guide

1. 1

   Define rider and component inputs

   Pick an example rider of 1.75 m height and a total system load of 120 kg including bike and cargo. From datasheets, record hub motor axle diameter and flat width, dropout spacing (for example 135 mm), rated motor torque and the battery outline. Set geometry targets: 700C or 27.5-inch wheels, head angle around 70°, seat angle around 73° and a step-through or diamond layout. Put all values in a Design Table or Global Variables.

   Checkpoint: An input table where every frame dimension traces to a rider measurement or a component datasheet.

2. 2

   Sketch the frame geometry

   On the Right Plane, sketch the frame centre lines: bottom bracket, rear axle, head tube and seat tube, with dimensions for chainstay length, wheelbase, stack and reach. Add wheel circles and a tyre clearance circle. Drive head angle, seat angle and chainstay length from Global Variables. Draw a top-view sketch for chainstay and seatstay spread so they clear the motor body and tyre.

   Checkpoint: Changing the rider size variable updates stack and reach while keeping the rear axle and bottom bracket positions defined.

3. 3

   Create custom tube profiles

   Create a custom oval profile for the down tube that fits around the battery rail, saved as a library feature part in the weldment profiles folder. Use standard round tube profiles, for example 31.8 mm for the top tube and smaller sizes for stays. Set profile sketch dimensions so wall thickness can be changed later. Check that the profile appears in the Structural Member dropdown before going further.

   Checkpoint: The oval profile is selectable in Structural Member and sweeps correctly along a test line.

4. 4

   Build the frame weldment

   Place structural members on a 3D Sketch skeleton made from the 2-D layout sketches. Use separate groups for the main triangle and the rear triangle so tube ends trim correctly. Use Trim/Extend to cope each joint and add an End Cap where stays meet the dropouts. Add the head tube and bottom bracket shell as separate solid bodies with the correct bores.

   Checkpoint: All bodies appear in the cut list with no overlapping members and the frame rebuilds without errors.

5. 5

   Design the dropouts for motor torque

   Model the rear dropouts as machined plates with an axle slot matching the motor flat width plus a small clearance. Increase plate thickness and add a torque-arm feature or bolt-on arm so motor torque is reacted over a larger area. Use the Hole Wizard for the brake mount and torque-arm bolts. Calculate the bearing stress on the slot faces by hand from rated torque, flat width and plate thickness.

   Checkpoint: Hand-calculated bearing stress on the slot faces is below the allowable for the dropout material with a factor of at least 2.

6. 6

   Integrate battery and cable routing

   Add threaded bosses on the down tube matching the battery rail hole spacing, using the Hole Wizard with an insert or rivet-nut option. Model a controller bay, either inside an enlarged down tube or as a bolted box under it. Cut cable entry holes and model a sweep for the motor cable route from the dropout to the controller. Check bend radius of the cable route against the cable datasheet.

   Checkpoint: The battery model mounts on the bosses in the assembly and the cable route keeps the minimum bend radius.

7. 7

   Check clearances in the assembly

   Assemble frame, hub motor wheel, front wheel, fork, crankset and battery. Run Interference Detection and check tyre clearance at the chainstays and seatstays, chainline clearance at the motor and heel clearance. Use Mass Properties to find bike mass and the combined centre of gravity with a rider block. Move the battery along the down tube if the centre of gravity is too far rearward.

   Checkpoint: Interference Detection passes, and tyre and heel clearances meet the targets in the requirement sheet.

8. 8

   Analyse frame load cases

   Run a Static study with members as beams. For a seated case, fix the rear dropouts with a Fixed Hinge, support the head tube with a Roller/Slider and apply rider weight at the seat cluster. Add a pedalling case with a vertical force on the bottom bracket and a front braking case. Separately, mesh the dropout as a solid and apply motor torque as a moment on the slot faces.

   Checkpoint: Factor of safety is at least 2 in every case and the dropout stress agrees with the hand bearing-stress calculation within about 20 per cent.

9. 9

   Iterate and document

   Change tube diameters, wall thickness or stay shapes to meet the targets with the least mass, recording each change in an iteration table. Make a frame drawing with a cut list table and a separate dropout drawing with slot width tolerances and a flatness requirement. State in the report that a physical fatigue test against a bicycle frame standard is needed before riding, and list the load cases you would test.

   Checkpoint: A drawing pack for frame and dropouts and an iteration table showing the final mass and factor of safety.

Testing & validation

• Every frame dimension traces to a rider measurement or component datasheet.
• Dropout bearing stress is checked by hand and by Simulation.
• Interference Detection passes with motor, battery and crankset.
• Each load case meets the factor of safety target.

Expected results

A parametric e-bike frame designed for a rear hub motor and down-tube battery, with torque-rated dropouts, battery bosses, cable routing, verified load cases and a drawing pack.

Take it further

• Compare aluminium and steel versions of the frame for mass and stiffness.
• Run a fatigue study on the dropout under repeated motor torque.
• Redesign for a mid-drive motor and compare the bottom bracket area.

Viva and review questions

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