Rear Shocks with Piggyback Reservoir

Is your next upgrade calling for a beefier shock, or is a standard shock enough for your trail pursuits? We break down the reservoir shock myth, clarify the transition from high-end rigs to budget full-suspensions, and examine if less can sometimes be more?

As riders, we know the feeling when our feet are planted on the pedals, the breeze tickles our face, and the trail stretches out before us – it’s pure magic. But to fully savor this experience, it’s crucial that the bike beneath us runs smoothly. Full-suspension bikes with 140mm of travel or more offer a technical sophistication that we often rave about in our mountain bike reviews: the piggyback reservoir.

But what exactly does this reservoir do, and does it really separate the wheat from the chaff? More budget-friendly models tend to skip this luxury, while it seems to be completely absent on full-suspension bikes with less than 130mm of travel. We delve deeper to find out how this small, unassuming cylinder impacts the ride and whether it’s truly worth shelling out some extra cash for.

The difference between damping and suspension is crucial in the mountain biking world. While suspension is adjusted through the air chamber or a coil spring and is responsible for absorbing bumps and impacts, damping operates through an internal oil circuit within the shock absorber. Damping is also known as compression and rebound tuning, and it regulates the speed at which the suspension element compresses and rebounds. A shock with a reservoir, also known as a Piggyback shock, is a significant upgrade in a bike’s damping system.

A reservoir is an integral part of the damping component, significantly increasing the oil volume within the shock’s damping circuit. This capacitive expansion, along with a larger surface area, has immediate positive effects on the thermal performance of the system. Both the increased oil volume and the extended surface area of the reservoir act as a buffer against heat, allowing the shock to dissipate and regulate heat more efficiently.

When you’re ripping a technical singletrack or conquering a gnarly downhill section on your full-suspension rig, your shock is working overtime. Every bump, every drop, every root – your shock absorbs them all, converting massive amounts of kinetic energy. The closer together those hits come, the more friction from the compression and rebound cycles generates heat in the shock’s circulating oil.

What many riders don’t realize: A shock overheating is not a myth – we’re talking temperatures that can easily exceed 50 degrees Celsius during brief descents. As the oil temperature rises, its viscosity changes. The thicker the oil becomes, the worse the damping performance. That’s because the speed at which the oil moves through the shock’s internal passages to regulate flow increases when the oil thins out.

The core issue is evident – the physical properties of shock oil change significantly with rising temperatures. It thins out. For us riders, this means that our finely tuned compression and rebound damping rapidly lose performance as the shock heats up. This isn’t just a marginal effect – we’re talking about a noticeable drop in performance that substantially changes how the bike responds on the trail.

An uncontrolled rebound is more than just annoying – it’s a potential safety hazard. Imagine you’re on a descent, going full throttle, and your suspension system becomes less effective because of temperature issues. The consequence? The rear suspension no longer behaves as you’re accustomed to. The risk of losing control during a descent increases significantly if the shock is rebounding and compressing without proper damping.

One of the most dangerous phenomena is when the rear end starts to “kick”. Let’s say you’re approaching a jump, ready for takeoff, and your shock is already overheated. Now, with rebound performance already diminished, your rear end propels you forward with an unwanted powerful burst as you glide over the jump’s edge. The outcome? Your landing becomes front-heavy. This not only robs you of your flow but also of control over your bike. A crash is a real possibility.

No rider wants to crash, especially not due to failing equipment. Therefore, it’s critical to secure a reliable solution that ensures the thermal consistency of your shock system, particularly on bikes designed for downhill aggression. Only then can you fully tap into a descent’s potential, pushing it to the max without your gear hitting its limits. A reservoir is a brilliant tool for this purpose – but let’s dive into the details.

The thermal stability provided by a reservoir is invaluable to riders and pilots tackling challenging terrain, where consistent performance from their gear is essential. The reservoir keeps the shock cooler, preventing oil from foaming, a phenomenon known as cavitation, which can compromise damping efficiency. This makes the piggyback shock a key component for those committed to nailing their descents with uncompromising precision and control.

For trail riders not squeezing every ounce of adrenaline from their descents and who aren’t constantly riding on the edge, the impact is less significant. On milder trails or with a more moderate riding style, the shock generates much less heat than on an enduro descent or in the bike park. Hence, bikes with 130 mm of travel or less often forego the reservoir to save on space, budget, and weight.

The disadvantages are quite evident. While a shock without a reservoir often weighs less than 300 grams, a comparable shock with a piggyback will add approximately 150 to 200 grams more to the scale. Particularly for tour and cross-country riders, who are notoriously weight-conscious, opting out of a piggyback shock is a common choice.

Moreover, the piggyback undoubtedly takes up additional space. This needs to be factored into frame design considerations. In some cases, the reservoir can interfere with bottle cage placement within the frame triangle. This is often an issue with smaller frame sizes.

Additionally, shocks with an external oil reservoir are always somewhat pricier than their non-reservoir counterparts. Thus, bike manufacturers looking to market an especially affordable full-suspension model often skip the piggyback shock to trim costs in the specs.

For riders who thrive on the challenges of off-road terrains and don’t shy away from lengthy downhills, this scenario is all too familiar: your rig is sporting a shock without a piggyback reservoir, and at some point on the trail, you start to feel the oil heating up. Here’s a golden rule that might just save your bacon on your next outing: If your shock doesn’t have a piggyback reservoir, it’s wise to turn the rebound dial (red knob) a bit more to the right at your starting point than you might think is necessary. It may seem counterintuitive, I know, but trust me, after the first 100 to 200 vertical meters of trail, your suspension will naturally speed up due to the heat build-up in the shock anyway.

This little tip ensures that your bike’s performance feels more consistent throughout the entire descent, even though you’re riding without the technical sophistication of a piggyback reservoir. Of course, it’s not a cure-all for the thermal challenges a shock system faces on long descents, but it’s a workable solution to get the most out of your setup without having to overhaul your entire suspension system.