Bracing for (R)impact

It is clearly evident when taking a stroll through the Rimpact website that this is a business with a very specific focus. The UK company offers three very different products that are also very similarly aligned; tire inserts, tuned mass dampers, and the chain damper under review here. Rimpact’s business, then, is the business of damping. Not suspension damping in the way that most of us understand the word, but ride damping.

The idea of ride damping is to subdue harshness that is transmitted outside of a bike’s suspension system. Tire inserts change the characteristic of the tire’s air chamber and add a level of cushion and damping. Tuned mass dampers are used to absorb vibrations in everything from buildings to F1 cars, and with a mountain bike are claimed to defuse compression spikes, enhance chassis stability, reduce fatigue and improve ride feel. The premise behind the Rimpact Chain Damper is that it quells errant chain slap, and by so doing minimizes the effect that a flailing chain can have on suspension performance over rough ground.

Chain damping has been a topic of conversation in the gravity scene ever since 2015, when Aaron Gwin won a DH World Cup at Leogang without a chain. Pedal kickback is a well-known instance of chain/suspension interference, but drivetrain feedback in general – where the chain is slapping around when riding over rough terrain, whether pedaling or not pedaling – has been under increasing scrutiny ever since Gwin’s moment of chainless glory. Low engagement hubs and structured chain guides like the STFU system have been recognized as having a “quieting” effect on a bike’s behavior in the rough, and more recently the O-Chain damped chainring and E13’s Sidekick rear hub have taken a more defined and assertive role in attempting to mitigate how chain movement impacts suspension behavior and ride feel.

What Is It?

Enter the Rimpact Chain Damper. This is a damped chainring that bolts directly onto your crankarm, replacing your existing chainring/spider (current fitments include Shimano, SRAM 3 bolt, SRAM 8 bolt Transmission, Hope Direct mount and RaceFace Direct mount). It is available in 30, 32, 34 and 36t options, and is compatible with Boost 52, T-type Transmission and 55mm DH chainlines. It weighs 240g, and costs 372 USD/517 CAD.

What you get for that pile of scratch is a chairing that rotates between a pair of aluminum plates, riding on a set of thrust bushings and sealed by a pair of X-rings around the perimeter and an O-ring at the bottom bracket spindle. Four elastomers control the limits of chainring rotation, and a pair of steel coil springs determine the engagement preload. This is all sandwiched together with four 4mm hex head bolts, and the whole setup looks initially well sealed and well made. The system provides 8.6 degrees of rotation. This is a number that Rimpact have determined provides optimal performance at damping chain behavior without sacrificing pedal behavior.

Rider pedals forward, spring compresses, chainring pushes into elastomer and comes to a hard stop, bike moves forward. Rider stops pedaling, spring pushes chainring back, more or less decoupling it from the drivetrain,allowing it to float and react to chain flail or ground impacts that might otherwise be transmitted to rider's feet or to the suspension. This "give" is part of why some people favor low engagement hubs over high engagement ones, and is part of the hypothesis of why suspension works better on bikes without chains than it does on bikes with chains.

I am going to go on record here as regarding this all with a great degree of curiosity as well as a huge amount of skepticism.

So, with curiosity and skepticism both at the forefront of my mind, I installed a 32t Rimpact Chain Damper on my Starling Murmur. I chose the Murmur for the following reasons:

1. It’s a single pivot suspension, and as such, doesn’t have any fancy extra linkages to try and make it a hoverbike. There’s a fair bit of anti-squat, and that means that it does telegraph some degree of the ground feedback through the pedals when riding through chundery stuff.
2. It’s a bit of a softie. Meaning, the steel frame has a degree of flex to it that I can best describe as “harmonic.” It gives, but gives in a very predictable and reassuring fashion. But, it is also quite sensitive to suspension setup, and going too stiff on the preload or not getting the front/rear balance totally dialed has very pronounced effects on ride behavior. Same goes with trying to run really stiff wheels. Suffice to say – small changes are very noticeable.
3. It’s not exactly a flyweight, nor a rocket uphill. So, if I’m going to slap a chainring on a bike that adds weight along with a “to be determined” amount of drivetrain lash, this seems like a better candidate than some 26lb XC whip.
4. It’s the only longish travel FS bike I have.

Installation went as follows: Remove cranks. Curse the gods for not having the correct size tool to remove the damn lockring on my XT cranks. Go buy Shimano TL-FC41 socket at Vecchio’s in Boulder. Come home. Remove lockring. Remove Shimano chainring. Lightly grease crank mating surface and threads. Install Rimpact chainring. Reinstall cranks. The trip to Boulder aside, this took about 5 minutes.

What it does

My inner skeptic had a field day for the first few rips around the block. At the risk of separating the youth from the silverbacks here, my initial pedal strokes with the Rimpact installed threw me straight back to childhood days of riding bikes with buggered out cottered cranks. There’s this disconcerting dead spot in each pedal revolution when soft pedaling and when starting to pedal that – at the outset – feels ugly.

That feeling lasted about 15 minutes. A half hour later, at the top of a chunky few miles of climbing, I had decided that the tractability was worth the occasional extra few degrees of engagement when entering a techy bit, and had totally come to terms with the dead spot. An hour later, after another climb and some chunky mid-grade downhills, I was thinking that the juice was definitely worth the squeeze. By the end of a three hour initial ride, I was trying to calm down a growing sense of “damn, I had no idea” wonder and not let my confirmation bias get too far ahead of itself. So, I rode a bunch of flat pavement to finish the ride and conceded at that point that I could totally live with this on my long travel sled 24/7. Here’s what I noticed:

Pedaling - Those 8.6 degrees of takeup make themselves immediately apparent. When soft pedaling on flat ground, the dead spot pulse into power is very pronounced. Combined with the push into the spring when applying power and the release of the spring back when backing off, it’s an odd sensation that will totally reveal any sloppiness in your pedal stroke. Once I realized what was happening, it took a few minutes to adapt how I was pedaling and then ceased to be an issue, even on flat ground. On anything from low-grade consistent effort climbing to max-effort punchy steeps, it’s also a non-issue. Once power is applied, there is no flex or squishiness to be felt at all.

The float is there, though. So, just as with people who think super fast engagement hubs are crucial in technical riding, there will be people who just can't get around the slight wind-up of engagement with this damper. It adds 8.6 degrees of extra lag before engagement. That will totally be a deal breaker for some people.

Speaking purely for myself here, I got used to it super-fast, and then forgot about it everywhere but on flat terrain and super easy pedaling. And then, I reminded myself try and pedal like a cyclist instead of like a hiker and everything was fine.

I was trying to slap a GIF of the Chain Damper doing it's thing in here, but the whole deal went a bit sideways. So instead, here's Rimpact founder Matt Shearn on The Downtime podcast explaining what it does and why, from his perspective. It's worth a listen.

Everywhere Else - Maybe I am enamored of nuance. But this thing is sweet! The Starling is already a pretty “quiet” bike. Steel frame, coil spring, relatively soft Forge+Bond wheels. It is a smooth, tractable bike. That said, the Rimpact Chain Damper enhanced the smoothness and tractability everywhere – uphill and down. Square edged ledges and chattery rock sections felt less square, less chattery. The usual gentle tug on the pedals that would occur on mid- to big drops went away. The already plush initial responsiveness of coil sprung suspension felt plusher. I had not touched my damper knobs, but it felt like I had backed the compression adjusters out. Overall, my bike felt quieter, more tractable, and more comfortable.

Rimpact started out with tire inserts and a lot of what I experienced on my first ride with this chain damper mirrors a lot of what I felt the first time I tried CushCore. At that time, a friend described his experience with CushCore as “a better signal to noise ratio; more signal, less noise.” Even though this chain damper is working to subdue the 250 or so grams of chain that is wailing around on bumpy ground - as opposed to changing the way a tire compresses as it traverses bumpy ground – there are parallels that can be drawn between how these two different components affect ride feel. Both act as damping devices, and both have an effect on the overall behavior of a bike. Both serve to filter out an element of “noise” so that riders can focus more on “signal”.

In steep, rough terrain, I don’t really see a downside. My bike was markedly more composed everywhere from turn in feel on braking rut corners to straight lining through rock gardens. I suspect that the steeper and rougher things get, the better this feels. I felt less feedback through my pedals, my suspension felt plusher, and I was better able to find traction climbing, cornering, and braking.

The flipside to that coin is that I don’t really think that XC racers will find any use for this. It’s expensive, adds 100 grams or so to bike weight, and feels strange on flat ground or when soft pedaling. Extrapolating that out, riders who live where the trails are buffed smooth will probably not feel much benefit either. In smooth, undulating going, the Rimpact Chain Damper feels more like something that is loose in the drivetrain instead of all the smooth sweetness I was describing earlier. I should also note that in my case, this installation pushed my chainline out about 3mm. With the XT drivetrain I am riding right now, the knock on is not noticeable in terms of shift performance or drivetrain noise, but not everyone will be ecstatic about that.

I also wonder if I like it because I am on a single pivot bike; as in, I wonder if the benefits that I felt are as tangible on bikes that have more design sophistication invested in pedaling neutrality. Buuut then again, according to Rimpact, pedal feedback is only part of the equation. It’s overall mass of the chain flailing about that they are trying to address, because they believe that is what is having a greater effect on suspension and overall bike behavior. I can’t speak to that, and can only relay what I have experienced so far, in a handful of rides, with barely 10 hours invested.

But 10 hours in, here’s what I think: The Rimpact Chain Damper works well enough in rough terrain that I am more than happy to leave it on the bike and see how it holds up over the summer. I am quietly impressed, not nearly as skeptical, curious as to how it lasts, and willing to focus on keeping my spin smooth.

Rimpact Chain Damper