In annual Readers’ Rides issue tradition, I’ll once again be waxing eloquent about suspension tuning, but instead of worrying about super-low ride heights and suspension geometry or hellaflush wheel fitment and camber curves, we’re going to take a closer look at the inner workings of the most important part of your suspension: the shock absorbers.
But before we take a peek inside the automotive damper, and in particular examine the pros and cons of mono-tube and twin-tube shock absorber design, let’s quickly review what exactly a shock’s job is and how it goes about achieving it. At its most basic level, the shock absorber or damper is designed to absorb and dissipate energy, specifically to dampen or reduce spring oscillation. To visualize a shock absorber’s function, imagine a coil spring being compressed and all the stored energy this represents. Now imagine that spring uncoiling; without a damper to control all this stored energy, you’ll have a serious pogo-stick situation on your hands, causing your car to literally bounce down the road. So it’s the shock absorbers that smooth out all those bumps in the road, by converting the kinetic energy generated by the suspension and associated unsprung mass (brakes, hub, wheel and tire, etc.) as it moves up and down over bumps.
A side-by-side comparison of mono-tube (left two elements) and twin-tube (right three elem
In the case of modern hydraulic shock absorbers, heating up the viscous fluid and compressing the nitrogen gas inside the shock body is what absorbs and dissipates the kinetic energy generated during suspension travel. The oil in the shock is heated during a compression event (the suspension moving upward over a bump), where the shock piston and attached valve shims are pushed down into the shock body, forcing the fluid through orifices in the valves. This generates heat within the fluid, which is then dissipated to the shock body, where it can be transferred to the ambient atmosphere. The same heat generation, absorption and dissipation process happens in reverse on the rebound stroke, where the suspension travels back to its neutral position after encountering a bump.
In a nutshell, that’s how a shock absorber works, but when it comes time to upgrade your dampers to something better able to handle those weekends on the autocross course, racetrack or your favorite twisty bit of backcountry road, what should you be looking for when it comes to design and construction? More specifically, what are mono-tube and twin-tube designs and what are their relative pros and cons? To answer these questions, enter Bret Norgaard, product engineer at Bilstein and a genuine enthusiast and accomplished racing driver.
This cutaway of a mono-tube damper reveals the ported working piston and shim stack bolted
Starting with mono-tube dampers, Bret explains, “A conventional mono-tube damper is based on a single hydraulic cylinder design that incorporates a ported working piston with deflective valve shims to govern oil flow, a secondary floating piston and an inline high-pressure gas chamber. This type of damper design can be tuned with high precision due to the large surface area of the piston and valving shims. The nitrogen chamber pressure is a key element to this design. It prevents oil cavitation (foaming) and offsets changes in volume do to thermal expansion. Being that the nitrogen chamber is completely isolated from the oil by the dividing piston, a mono-tube damper can be mounted to a vehicle inverted (body-up) and maintain its damping performance. The mono-tube design has precise tolerance requirements that lend to its precise damping control, durability and long-term performance. This is in part why high-end motorsport dampers are based on this design, as is the fact that they’re also rebuildable and retunable to meet customer-specific needs [and explains why this design is used in the vast majority of Bilstein’s product line].”