# Material for suspension that stiffens in higher speeds?

I was hoping there exists a material like hard rubber used in bushings, but has the property to stiffen when exposed to vibration with high frequency.

So, my application is something like a small dampening suspension on a system with stiff wheels so in high speeds the dampening will get stiffer due to the higher frequency vibrations that occurs from the small wheel rolling on a surface like asphalt. So the system is soft in low speeds and stiffer in higher speeds. Do you guys know of any material or system with these properties?

Update:
Honestly, I was looking for something small and hopefully something not reliant on a power supply to change the ride characteristics of a skateboard/longboard truck depending on its speed. My theory is that if the bushing on the truck gets stiffer in higher speeds the ride is more stable and less prone to speed wobbles. ( Stiff bushings/dampener requires more weight to turn the board = less wobble in higher speeds)

I guess this is not that difficult to make using something mechanical with an electric speed sensor, but a better solution would be if such a material exists or if its possible to make a bushing molded in some material that stiffens when a small electrical current is applied.

Dampers.

Resistive forces are inertias (m), dampers (b), and springs (k), and the equation is generally written:

$$F_{\mbox{applied}} = m\ddot{x} + b\dot{x} + kx \\$$

That derivative term - $\dot{x}$ - means that the damper produces a force proportional to the speed of the object. Wind resistance doesn't do anything if you're not moving, but gets larger as you move faster. Dampers are the same way - they provide more force the faster something moves.

As I see it, there are a couple of ways to go about this. First is to change the existing truck bushing itself to add more damping. You can change the geometry of the bushing, the material or both.

Second would be adding some kind of secondary damping device to the truck. Rather than a complex fluid damping or magnetic system, you might consider something like the dampers used on washing machines which use a grease soaked compressed urethane foam between two sliding tubes. They're pretty simple to prototype and you can tune the design pretty easily.

This sort of thing could be accomplished using Magnetorheological (magnetic) dampers which are already used in a variety of vehicles (such as the Audi TT). These use shock absorbers filled with a fluid containing iron particles. When a magnetic field is applied these particles align making the response stiffer. The stiffness can also be varied by adjusting the strength of the applied magnetic field. You could use whatever input you want for the controller: speed, vibration frequency, etc. The downside is this type of system is relatively expensive/complex.

A more detailed description of how these work is available here

• Honestly, I was looking for something smaller and hopefully something not reliant on a power supply to change the ride characteristics of a skateboard/longboard truck depending on its speed. My theory is that if the bushing on the truck gets stiffer in higher speeds the ride is more stable and less prone to speed wobbles. ( Stiff bushings/dampener requires more weight to turn the board = less wobble in higher speeds) But thanks for the reply, I will check out the link :) – user1171426 Aug 2 '16 at 1:14
• @user1171426 you should edit your question to include those details. – agentp Aug 2 '16 at 12:29