When it’s time to swap out the shock absorbers that came with your ride, you may find yourself torn between installing monotube shocks or twin-tube shocks. There’s more to these two types than just the number of tubes they use.
Monotube vs. Twin-Tube Shocks
The most obvious difference between monotube and twin-tube shock absorbers is the number of tubes in the shock. But that second tube doesn’t mean the latter delivers twice the performance of the former.
Most shock absorbers achieve their dampening effect with a mixture of hydraulic fluid and pressurized gas. Monotube shocks feature a single chamber or “tube” where they keep both the fluid and gas. A free-floating piston keeps the oil and gas apart to prevent the latter from aerating the former.
In comparison, twin-tube shocks have two separate chambers. An inner tube holds the hydraulic fluid and the outer chamber stores the gas. They also have multiple valves that control oil flow from the inner tube. When your vehicle hits a bump or pothole, fluid oil moves between the tubes to dampen the pressure placed on the shock.
Aeration and Foaming
When hydraulic fluid passes through the shock absorber’s valve or valves, its movement generates a cavitation effect that can create pockets of air in the oil. The foam produced by this effect affects the shock’s performance because it’s compressible.
Unlike unaerated hydraulic fluid, foam gives way more easily to the pressure imparted by sudden jolts. As a result, the shock absorber becomes more unreliable.
Some shock absorbers reduce the foaming effect by injecting a charge of pressurized gas. However, these shocks have to keep the gas and the hydraulic fluid apart. Otherwise, the two substances will intermix and create aeration that their shock’s design is trying to minimize.
Twin-tube shocks are more vulnerable to foaming because hydraulic fluid flows between the tubes during operation. A sudden or forceful jolt can disrupt the precise operation of their valves and allow fluid to mix with the pressurized air. The resulting foaming will degrade their ability to dampen jolts. To compensate for this problem, many twin-tube designs use nitrogen, an inert gas that is less likely to cause foaming.
Conversely, you won’t have to worry about foaming that much with monotube shock absorbers. Their single free-floating piston does better at keeping the hydraulic oil and pressurized gas in their respective halves of the tube.
A shock absorber can also spring a leak and lose hydraulic fluid or pressurized gas. If it leaks gas, the shock becomes more prone to fading. As a shock puts on the years, the likelihood of a leak increases and fading becomes more commonplace.
Shock absorbers respond to the state of the road surface. The more responsive a shock, the faster and more precisely it adjusts its damping performance.
When it comes to responsiveness, monotube shock absorbers perform better than their twin-tube counterparts. Monotube shocks usually have bigger circumferences, giving them more room to distribute pressure and reduce the strain on a single area. They also rely on a single valve to spread the pressure evenly across the whole device.
Twin-tube shocks use multiple valves that must work together. Just one valve failing will negate the work of the other functional parts.
Monotube shocks enjoy several advantages over twin-tube designs in maintaining their damping effect. First, their single valve assembly distributes pressure much more evenly than multiple valves since the fluid only comes out of one valve. Next, they usually have a wider piston, which gives better damping performance thanks to its greater surface area and durability.
Third, they store a greater amount of hydraulic fluid, allowing them to dampen more pressure and get rid of heat better. And last but not least, their piston design keeps fluid and gas apart, increasing the precision of their damping performance and preventing aeration.
Monotube shock absorbers lend themselves well to DIY and professional installers alike. They keep all their parts inside a single shell, a simple design choice that makes them very rugged devices.
In comparison, installing twin-tube shock absorbers requires great precision. Make even just one minor error in aligning the shock and it won’t work properly.
Resistance to Fading
Fading is a recurring and serious issue that often affects shock absorbers. When a shock fades, it doesn’t dampen as much of the jolt as it should, so you will feel bumps in the road more. Common causes of fading in a shock absorber are foaming and overheating.
Monotube shock absorbers are far less vulnerable to fading. Their high hydraulic fluid capacity and greater surface area allow them to stay cool and avoid heat-related fading. Furthermore, they do a better job of keeping fluid and gas apart, preventing the foaming effect that also leads to fading.
Do Twin-Tube Shock Absorbers Have Any Advantages?
Twin-tube shocks cost less than monotube shocks. That makes them a popular choice as stock parts because the automaker can reduce manufacturing costs by using cheaper parts.
If you’re working on a tight budget or just want to replace the old or damaged shocks on your ride, you can settle for twin-tube shock absorbers. They’re not bad by any means and will do an acceptable job of smoothening your ride.
However, drivers who want more precise dampening and can pay for the increased performance should grab monotube shock absorbers. The superior dampening capabilities of monotube shocks will make off-roading and driving on rough roads a much more tolerable or even comfortable experience.