In the DH World Cup, the weight debate is currently going in the opposite direction: instead of saving grams, more and more teams are bolting lead weights onto the bike – deliberately and in order to achieve the lowest possible center of gravity. The goal: more composure and a calmer bike in high-speed sections.
Fittingly, Orbea are launching the new Rallon, the first bike to come stock with a mounting option for special weights on the lower bottom-bracket – right on the so-called Gravity-Link. You can bolt a main weight and two side weights to the DH-Link, adding a total of 583 g of ballast to the bike. The trend in the Downhill World Cup has already shown that this is far more than a flashy gimmick. But what’s the physics behind it – and what does it really deliver on the trail? Do average riders benefit, will we soon see this more often on production bikes, or will there even be dedicated aftermarket solutions? To answer this, we took the Rallon to the trail, discussed with experts, and explored when too much of a good thing is, well, too much.
The theory – More composure through inertia
The general aim is clear: more composure through inertia. Adding weight low and central increases the bike-and-rider system’s moment of inertia around the key axes. In physical terms, that means: for the same disturbance – like steering inputs, roots or braking bumps – the direction of motion changes more slowly and smoothly. The effect works like a filter: high-frequency chatter gets absorbed and damped, while your overall direction remains steady.
The core principle is simple: a system’s moment of inertia increases with both added mass and its distance from the axis of rotation. For steering feel and handling, the key is: with the same disturbing torque, a higher moment of inertia means a smaller angular acceleration. Or translated to the bike: additional, correctly-positioned mass calms fast, small movements without you having to countersteer actively. Many perceive this as more “planted”.
Yaw axis (vertical axis)
Yaw inertia determines how willingly your bike rotates around the vertical axis. On fast, rough tracks – e.g. Les Gets, Fort William or Andorra – braking bumps or small lips constantly generate tiny, lateral steering impulses. More mass low in the frame makes the system more resistant to these impulses – the bike fidgets less and holds its line better, especially at very high speeds. In other words, it increases straight-line stability.
Pitch axis (transverse axis)
When braking or when the front wheel drops over an edge, the bike gets a pitch impulse. Higher inertia around the transverse axis damps these motions: the bike bucks less, sits longer in its travel and keeps more grip at the front. This stabilises your body position over braking bumps and helps you brake later and more controlled in rough berms.
Roll axis (longitudinal axis)
Rolling means the bike tips into lean. However, your body mass sits high above the frame and thus represents the biggest lever, dominating the moment of inertia around the roll axis. A low-mounted add-on weight sits relatively close to the roll axis, so its contribution to roll inertia is lower than for yaw or pitch. There’s still an indirect benefit: lowering the center of gravity makes the bike tip into corners more calmly and with greater stability. Riders also feel more grip in lean, since the load acts more consistently on the tire contact patches. On tight, playful trails, however, you’ll need to steer more actively—the calmer bike responds best to deliberate inputs.
A lower center of gravity – what you actually feel
Weights in the bottom-bracket area lower the center of gravity of the bike-and-rider system. You’ll feel this in berms and fast corners as more support and reserves because the load “rocks up” less. At the same time, the front end remains composed for longer in steep sections, letting you choose lines later and with greater precision. A tangible example: eMTBs tend to feel more composed thanks to the heavy drive, which lowers the center of gravity significantly. On rough DH tracks and in bike parks with many braking bumps, additional mass acts like an inertia anchor: small, fast irregularities are less likely to knock the bike off course. The tires carry load more consistently and evenly, as the frame provides a stable reference point and the suspension stays in its optimal range. Most riders know this sensation from switching between a trail bike and a heavier eMTB: same speed, but a far more planted ride. In our test, the high yet well-positioned weight consistently improved tracking—though ultimately, distribution matters more than raw mass. Over small bumps, the vertical acceleration impulse is converted less into angular and longitudinal motion due to the greater mass. This increases tire contact time and therefore boosts traction – under braking but also when accelerating out of corners. The latter should be largely irrelevant for most. Theoretically, the braking distance on a trail isn’t automatically longer even though the system weight increases. That’s because the increased pitch-axis resistance often pays off here and gives the bike extra grip and traction.
Drawbacks & limits – what you buy when you add mass
First and foremost, a bike’s agility decreases, because more inertia requires more deliberate and forceful inputs. On tight, slow trails, with manuals, bunnyhops and whips, the bike feels more sluggish and demands more input. Accelerating also costs more energy and, for example, sprinting out of a corner or the start gate gets a little tougher. The braking distance is – as explained above – not longer per se, but more heat is generated under braking and material stress increases slightly, although rider weight has a bigger influence here.
Important: add-on weights belong to the sprung mass. More unsprung mass – e.g. weights on the chainstay or on the brakes – worsens suspension response and would be counterproductive. Reducing unsprung mass is one of the key benefits of gearbox-equipped bikes, as we’ve covered in a recent article. For the same reason, many World Cup teams, as well as ourselves, have also experimented with tuned mass dampers (TMDs) on the fork or frame. While the principle is different, the goal is the same: reducing high-frequency vibrations.
How we tested – the Orbea Rallon DH
The new Orbea Rallon provides the perfect basis for our test, as it’s the first bike that – alongside a host of other adjustment features – provides a mount for additional weights. With the GravityLink, the weights can be bolted directly to the lowest point of the frame – a 395 g main block plus two side plates of 93 g and 95 g, for a total of up to 583 g. Alongside Orbea’s system, we strapped about 1.0 kg of diving weights low on the frame to simulate greater mass increases and explore the limits more thoroughly.
We kept all other variables constant, i.e. identical tires and pressures, the same suspension setup and repeated runs on one trail. Alongside the DH version, we also carried out tests with the enduro model to cover a wide range of different trails – from fast, rough descents with high-speed braking bumps and rock gardens to tighter, flatter trails with lots of direction changes.
In parallel, we tested a tuned mass damper (TMD) to see where the differences lie. This allowed us to contrast “static” additional mass (BB weights) with “dynamic” mass (TMD) and to tease out the effects more clearly, even if that wasn’t the focus.
Our verdict on add-on weights
Add-on weights aren’t a miracle cure, but they do offer an effective and affordable way to tune your ride. Mounted low on the bike, 200–600 g noticeably increase calmness, traction and braking stability – especially on fast, rough tracks. The price you pay: less agility on tight, playful trails and extra mass when pedalling. However, if you mainly ride fast, rough trails with lift support, it’s worth experimenting with additional weights.
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Words & Photos: Peter Walker
