O-Chain Spider - Explained & Reviewed By Neko Mulally

Why Are Most Top World Cup Downhill Racers Using An O-Chain Device?

“Worldwide Cyclery has worked with Neko Mulally this past year with his Frameworks project where he sets out to design and race his own downhill bike at the highest level, he is experimenting with different suspension types and system. One of those is the O-Chain spider, a crank spider designed to give you a chainless feeling on the bike. After a season of racing and testing Neko has been kind enough to share his thoughts on the O-Chain and how it helped him achieve the perfect setup this past season. “ - Worldwide Cyclery

O-Chain devices are becoming increasingly common on World Cup downhill and EWS race bikes. Nearly half of the field in any given World Cup DH final are using O-Chain devices. So, what do they do and how do they work?

O-Chain states that their product gives a “Chainless feeling” which they achieve by isolating how the drivetrain forces effect the rear suspension. O-Chain is an active spider that allows the chainring to rotate independently of the crank. The spider takes the place of a direct mount chainring with options for most direct mount cranks and uses a standard BCD 104 chainring.

What Does It Do:

In general, as a mountain bike’s rear suspension linkage is compressed, the distance between the rear axle and the bottom bracket increases, at least at the beginning of the travel. As this happens, the chain tensions on the upper chainline, which pulls backward on the chainring, and on the lower chainline, which pulls forward on the derailleur cage. Basically, the tensioning of the chain due to chain growth creates an additional force or resistance that your bike needs to overcome as it moves through its travel. Decoupling the chainring from the crank arm allows the chainring to rotate backwards as the distance between the BB and rear axle increases, minimizing or eliminating the tension created during chain growth.

How Does It Work:

O-Chain uses a dual spring system allowing the spider to rotate up to 12 degrees. The first spring is a small coil spring that takes the O-chain through its stroke and returns it to the starting position. The second spring is an elastomer bushing responsible for absorbing the bottom out as the pedal stroke is engaged. Technically, the elastomer bushing is a spring because it deforms as it is compressed, but it does not add to the stroke or the rotation of the device, it just softens the bottom out. Without that, there would be a harsh metal on metal bottom out as the pedal stroke is engaged. The size of the elastomer bushing is adjustable on the current O-Chain design to allow for different degrees of rotation, or amount of travel, at 4, 6, 9 and 12 degrees.

Pedal Kickback:

The topic of pedal kickback brings a lot of debate. Pedal kickback occurs when the upper chainline pulls backwards on the chainring as the rear suspension system is compressed. This is obvious to see if you remove the spring from your bike and compress the suspension; the crank arm will rotate backwards. Using a standard chainring, with your foot firmly applying pressure to the pedal as you stand on your bike, the chain pulls the cassette forward until the free hub is engaged and chainring backward, creating chain tension that resists the suspension system from moving freely through its travel.

There is a theory that pedal kickback does not occur on the trail, which is based on the fact that the velocity of the chain tensioning would need to be greater than the forward rotation of the hub as the rear wheel is moving, i.e., the chain cannot pull backwards on the chainring unless the freehub is engaged, which can’t happen while your wheel is spinning fast. In fact, the speed that would need to be overcome to eliminate pedal kickback can be calculated knowing the axle path, gear ratio, and hub engagement. This theory is valid if you can always quantify the speed of your rear wheel while braking because many times under braking the rear wheel is not traveling at the same speed as the rider, so if the rear wheel is locked or moving slower than the rider under braking at the moment the suspension is compressed and the chain growth occurs, the calculation would need to be done at the speed the rear wheel is traveling, which can be hard to quantify.

Anti-Squat’s Relation to Pedal Kickback:

Anti-squat quantifies the suspension system’s mechanical resistance to compression under acceleration (chain tension from a pedal stroke). A bike with more than 100% anti-squat will extend under acceleration which a rider may associate with a firm pedaling platform. A bike with less than 100% anti-squat will compress under acceleration which a rider might associate with pedal bob. This number is defined by the upper chainline’s relation to the center of curvature. The consideration for trail and enduro bikes to have an “efficient feeling” for climbing has many bike manufacturers going for a higher anti- squat number, to make the bike feel firm while you are pedaling. Using a standard chainring, anti-squat, and pedal kick are directly related. The same mechanism that creates a “firm platform” as the chain tensions from your pedal stroke is creating harshness as your chain tensions from chain growth as your suspension is compressed on a descent. This gives bike engineers the challenge of balancing the amount of anti-squat and pedal kick they want for a firm climbing position, or on a DH bike, a firm sprinting position, and an uninhibited suspension system. O-chain decouples anti-squat from pedal kickback by maintaining the anti-squat that the bike would normally have as you are pedaling with the O-chain engaged while mitigating the pedal kickback as the O-chain can rotate as the chain grows while you are descending.

Ochain vs Idler Pulley:

Idler pulleys are necessary on high pivot bikes because the distance from the BB to the rear axle grows so much due to the rearward axle path that the chainline must be redirected towards the center of curvature using an idler pulley. Just as the chainline goes close to the main pivot on a standard pivot height bike, using an idler pulley it can do the same on a high pivot bike. With an idler pulley, the place where the axle is rotating around is also where the upper chainline originates allowing engineers to achieve the balance of anti-squat and pedal kickback they are looking for. The benefit to using an idler pulley design is that engineers have the freedom to choose the center of their pulley wheel, defining their chainline without being constrained to a chainring, which will always be directly linked to the bottom bracket as its center point. The downside to using idler pulleys is that they add some complication to the bike with more moving parts, create additional drag through the drivetrain, and generally come with a rearward axle path. O-Chain makes it possible to separate anti-squat from pedal kick without a rearward axle path and without the drivetrain inefficiency of an idler pulley. You pay for it in the form of a lack of pedal stroke engagement, 300g of added weight, and another wearable item on your bike that requires service.

Low Engagement Hub:

The difference between an O-Chain and a low engagement hub is that at the moment you compress the suspension and experience chain growth, your wheel is spinning and the pawls in your freehub body could be about to contact the next point of engagement on the hub shell, or it could be at the furthest point from the next pawl contacting the hub. It’s somewhat of a lottery to know the amount of chain tension you will experience with a low-engagement hub. The chances of a free suspension feel with a low-engagement hub are better, but depending on the exact time you hit a bump, the chain tension could be the same on a low-engagement hub as the tightest, most expensive hub you can get. With an O-Chain, you define the degrees of rotation you want at the crank, and it will always be starting from the same point. If you want to run your O-Chain in the 9-degree setting, you will always have 9 degrees of rotation, plus wherever your hub happens to have in relation to the next point of engagement.

My Thoughts:

I can’t prove that pedal kickback is real or not, and to me that’s not the important thing here. If you have ever ridden your bike without the chain, you may have noticed that the bike feels like it moves through the travel more freely than it did with the chain on. This could be due to removing pedal kickback from the equation, it could also be related to the force of the clutch on the derailleur cage that needs to be overcome as the suspension compresses. Arguments can be made that the chain whipping around as your chassis is upset by variations in the trail can create tension. I can’t say for sure what is causing it, but I can say confidently that most bikes, with the exception of some high pivot bikes with a properly positioned idler pulley, feel like the suspension moves more freely without a chain. O-Chain provides that “chainless feeling,” but still maintains the ability to pedal.

Using data acquisition on my bike, I have found that my average position, or dynamic sag, is higher with an O-Chain than without, meaning that the chain tension truly is adding resistance to the suspension moving freely. I generally go up in spring rate with when using an O-chain to get the ride height I want, which I believe gives me the best balance of support and bump absorption. We are all asking our bikes to do a difficult thing: Support inputs as we, the rider, push into the bike and use that energy for forward drive, and at the same time, absorb impacts from obstacles on the trail coming up from the ground up. I believe that running an O-Chain will take away resistance in the form of chain tension on the suspension system, which you can compensate with spring or damping, and get a balance of support and absorption that is more consistent and easier to quantify.

What Can You Expect From O-Chain:

While I believe that everyone will experience the benefits of using an O-Chain, they are not necessarily for everyone. If you haven’t heard of pedal kick back before, you probably don’t need one. Ignorance is bliss, don’t go looking for problems. If this article resonated with you and you can understand the benefits, I can guarantee you will notice the following things:

Pros: Little or no resistance from the drivetrain on your rear suspension i.e a smoother feeling as your bike absorbs bumps.

Cons: Lack of pedal engagement; 300g added weight; another serviceable item on your bike. (Service intervals are similar to most shocks)

To me, that Pro is huge, something we are all searching for, and outweighs the Cons. If you want to try out the feeling of an O-Chain before buying one, simply remove your chain on a descent, zip tie up your derailleur so it isn’t slapping around, and see if you notice any difference.

I use an O-Chain on most of my bikes. I am not sponsored by O-Chain, they are a small Italian company without much marketing budget. In fact, O-Chain does not sponsor anyone. Which makes the fact that you see them on so many pro rider’s bikes even more impressive.

Thanks for reading.