Front Fork Strength and Stiffness Analysis

The problem

The front fork carries braking, bump and steering loads and is a safety-critical part. On EVs with hub motors or heavier front ends, the loads can differ from conventional scooters. Poor modelling of the sliding tubes, crown clamps and axle often gives misleading results, so a careful analysis approach matters.

Objectives

• Model a simplified telescopic fork with crowns, stanchions, sliders and axle.
• Define braking, bump and lateral load cases from an example vehicle.
• Model contact between stanchions and sliders and fixtures at the steering head bearings.
• Report stresses, deflection, lateral stiffness and factors of safety.

What you need

Key concepts

Brake load transfer

Under braking, load shifts to the front wheel and the fork bends backwards. The front wheel force depends on deceleration, centre-of-mass height and wheelbase.

Bushing overlap

The length over which the stanchion sits inside the slider. Shorter overlap at full extension gives higher bushing loads and bending.

Bearing fixture

A SOLIDWORKS Simulation fixture that represents a bearing support on a cylindrical face, allowing rotation and, optionally, self-alignment.

Lateral stiffness

Force per unit sideways deflection at the axle. Low lateral stiffness affects steering precision.

Step-by-step guide

1. 1

   Define loads and geometry

   Take an example e-scooter with a stated mass, centre-of-mass height and wheelbase. Calculate the front wheel load when static and when braking at an example 0.8 g using the load transfer formula. Define a vertical bump case with a stated factor and a lateral case with a stated sideways force at the contact patch. Record fork rake angle, offset and stanchion dimensions from a measured fork or your design.

   Checkpoint: A load table with braking, bump and lateral forces at the contact patch, with all assumptions labelled.

2. 2

   Model the fork

   Model top and bottom crowns, steerer tube, stanchions, sliders and axle as separate parts. Model the bushings as short cylindrical faces created with split lines on the stanchion and slider bore. Leave out springs and dampers but note their absence. Create configurations for full extension and for a stated travel, changing the stanchion insertion. Assign aluminium or steel grades from the material library.

   Checkpoint: The fork assembly rebuilds in both configurations and mass properties give a realistic fork mass.

3. 3

   Hand-calculate the critical section

   Treat each stanchion as a cantilever from the bottom crown to the lower bushing, loaded by half the braking force normal to the fork axis. Calculate the bending moment at the bottom crown and the stress using the tube's section modulus. Estimate the axle deflection from beam theory. These give a target for the finite element model. Also check the steerer tube at the lower bearing, where the crown moment is reacted.

   Checkpoint: Hand-calculated bending stress at the bottom crown and axle deflection for the braking case.

4. 4

   Set fixtures and connections

   Apply bearing fixtures on the steerer tube faces where the steering head bearings sit, or a fixed hinge if you want a simpler model. Use bonded contact between crowns and stanchions for clamped joints, and no-penetration contact at the bushings so the stanchion can slide and tilt. Connect the axle to the slider dropouts with a pin connector or bonded faces.

   Checkpoint: The study runs without rigid body motion errors and the contact at the bushings transmits load as expected.

5. 5

   Apply loads

   Apply the braking, bump and lateral forces as remote loads at the tyre contact patch location, which sits below the axle by the tyre radius, connected rigidly to the axle faces. This reproduces the moments correctly. For combined cases use Load Case Manager or separate studies. Keep units consistent and check the force directions in the vehicle axes. Keep the contact patch location at the tyre radius below the axle for each configuration.

   Checkpoint: Summed reaction forces at the steering head equal the applied loads for each case.

6. 6

   Mesh and converge

   Use a solid mesh with mesh controls at the bottom crown clamp, bushing contact zones and axle dropouts. Run the braking case at three mesh densities and plot peak stress at the bottom crown away from sharp edges. Check that contact pressure at the bushings becomes stable with refinement. Look at mesh quality plots for badly shaped elements. Use the same probe points in every mesh so values compare directly.

   Checkpoint: Peak stress at the bottom crown changes by less than about 5 per cent between the last two meshes.

7. 7

   Review results

   Plot von Mises stress, displacement and factor of safety for each case and configuration. Compare the braking stress at the crown with the hand calculation. Find the fore-aft and lateral axle deflection and compute stiffness as force divided by deflection. Compare full extension with compressed travel and explain why overlap length matters. Report fore-aft and lateral stiffness separately because they affect braking feel and steering precision differently. Note where peak stress sits relative to the clamp edge.

   Checkpoint: Results tables per case, with the braking case within your stated margin of the hand calculation and stiffness values computed.

8. 8

   Improve the design

   If the factor of safety falls below the target, try a larger stanchion diameter, thicker wall, longer bottom crown clamp or longer bushing overlap. Use a parametric design study or configurations to compare options, recording mass and stiffness changes. Choose the option with the best stiffness-to-mass ratio that meets the strength target. Check that any change still fits the steering head geometry, brake caliper mounts and wheel clearance. Keep each option as a configuration for traceability.

   Checkpoint: A comparison table of design options with mass, minimum factor of safety and lateral stiffness, and a chosen design.

9. 9

   Report

   Write a report with the load derivation, geometry, fixtures and contacts, hand calculation, convergence, stress and stiffness results and design options. Be clear about limits: no spring or damper forces, static loads only, idealised clamps. Recommend fatigue and physical tests, such as a static bending test on a rig, before any use on a vehicle. Include images of the deformed shape for each case with the scale factor stated, so readers do not misjudge the deflection.

   Checkpoint: A report with traceable loads, validated results and stated limits.

Testing & validation

• Reaction forces balance applied loads.
• Braking stress agrees with the cantilever hand calculation within a stated margin.
• Mesh convergence at the bottom crown.
• Stiffness and factor of safety compared across design options.

Expected results

A fork strength and stiffness analysis with realistic load cases, contact at the bushings, convergence evidence and a design comparison that improves stiffness-to-mass ratio.

Take it further

• Add a fatigue study for repeated braking loads.
• Model a hub motor's mass and torque reaction at the axle.
• Compare a telescopic fork with a single-sided or leading-link design.

Viva and review questions

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