It’s a well-known fact in automotive circles that when calculating wheel rate from spring rate, you need to square the motion ratio. For instance, if a shock is mounted halfway along the suspension arm, the wheel rate becomes a quarter of the spring rate. This is because the force at the wheel is halved compared to the spring, and the spring’s compression is also halved for the same wheel movement. However, there’s another crucial aspect often overlooked when it comes to Car Shocks: the velocity of the damper shaft.
The Non-Linear Nature of Damping Force
While springs operate linearly with displacement, dampers behave differently. Their force output is based on the velocity of the shaft, and this relationship isn’t linear. In most piston-in-fluid damper designs, the force increases exponentially with velocity, often cited as a cubic relationship. This means that a small change in shaft velocity can lead to a significant change in damping force.
Bike Shocks vs. Car Shocks: A Matter of Velocity
Consider a shock absorber designed for a bicycle, typically engineered to operate at lower shaft velocities compared to a car shock. If you were to take a bike shock, designed for shaft velocities at a 1:8 ratio to wheel velocity, and install it in a car where the ratio is closer to 1:2, you’d place the damper in a much steeper part of its damping curve.
Let’s illustrate with an example. Imagine a bump inducing a wheel speed of 5 m/s. On a bike with a 1:8 leverage ratio, the piston speed would be 5/8 m/s. Now, if this same bump is encountered by a car with a 1:2 leverage ratio, the piston speed becomes 5/2 m/s.
Using the cubic relationship for damping force, we can compare the forces. If ‘X’ is the damping force at the bike’s piston speed (5/8 m/s), then the force at the car’s piston speed (5/2 m/s) would be approximately ((5/2)³/ (5/8)³)X, which is roughly 64X.
Implications for Car Shock Performance
This calculation, though simplified, highlights a critical point. While a car may require significantly more damping force than a bike, simply transplanting a bike shock onto a car, even if it physically fits, is problematic. The car shock would constantly operate in the “high-speed damping” range of the bike shock. This effectively negates the benefits of sophisticated damping adjustments often found in high-end bike dampers, as the car application would push the damper far outside its intended operational velocity range. Understanding this velocity factor is crucial for selecting and designing car shocks that perform optimally under the specific demands of automotive applications.
In conclusion, beyond motion ratios and spring rates, the velocity-sensitive nature of dampers plays a vital role in car shock performance. Recognizing the non-linear relationship between damping force and velocity, and considering the different velocity ranges in bikes versus cars, is essential for a comprehensive understanding of car suspension dynamics and effective shock absorber selection.
