An automotive manufacturer asked us to redesign the torsion springs in the steering mechanism of its sports car series. About 15% of customers had complained about steering responsiveness and fatigue, and the springs were found to be underperforming under high-stress use.
The Problem
The original springs lost about 20% of their torque after 50,000 steering cycles. In a steering mechanism the driver feels that loss directly: the return force weakens and the steering becomes less precise. The new spring had to last longer and stay consistent, without changing the feel and feedback of the steering.
Requirements
- Service life: more than 100,000 full-range cycles without significant loss of performance.
- Consistency: stable torque through the whole life of the spring, for both driver feel and safety.
- Environment: heat from the engine bay, cold weather and moisture.
- Fit: the spring had to sit precisely within the existing steering assembly, with dimensions held to ±0.2 mm.
Our Solution
Material
We selected SAE 9254, a chrome-silicon spring steel supplied as oil-tempered wire. It combines high tensile strength with very good fatigue resistance, and it holds its load better than plain carbon steel when it gets warm. Those properties are why it is widely used for highly stressed automotive springs, and they suit the cyclic loads and vibration of a steering system.
Design
Torque loss over time is a sign that the spring is working too close to its limit. We revised the wire size and coil geometry so that the stress at maximum deflection leaves a safe margin for fatigue, and tuned the spring rate so that the torque builds progressively as the wheel is turned. That kept the steering feel the customer wanted while removing the cause of the early degradation.
Surface Treatment
The springs were finished with a manganese phosphate coating and oil. The coating protects against corrosion and reduces wear at the points where the spring contacts other parts.
Results
- Over the following six months, the failure rate fell to below 3%.
- Driver feedback pointed to better steering feel and control, particularly at higher speeds.
What This Project Shows
A spring that passes its first load test can still fail the customer two years later. Designing for fatigue from the start, with the right steel and enough stress margin, is what keeps a steering spring feeling the same at 100,000 cycles as it did at the first.
If you need a torsion spring for a high-cycle automotive application, contact our engineers through the contact page, or learn more about our custom torsion springs.
