There is simply no better way to make riding faster, safer and more fun than a quality full suspension mountain bike. Engineers have poured hours developing suspension that gobbles up bumps and maximizes traction. That said, how we talk about suspension is stuck in the 1990s; with almost every brand claiming they’re the only one to have solved suspensions’ achilles heel (whatever that is) with their new linkage or widget. This mismatch between marketing hype and engineering reality has led to a widespread misunderstanding of how suspension actually works. This post is our attempt to demystify suspension and, hopefully, give you enough information to make an informed purchase and unlock the benefits of your next full suspension mountain bike.
The Elements of a Full Suspension Mountain Bike
To start, mountain bike suspension is really the result of three mechanical systems working together. There are the pivots and linkages (engineers call the sum of these features kinematics) that turn rear wheel movement in one direction, into shock movement in another direction; there’s the spring that suspends the rider above the ground; and there is the damper which controls the speed of the suspension. All three need to work well together to make a great suspension bike.
The Instant Center – the Core of Bike Kinematics.
If you wanted to sum up suspension kinematics up with one variable, it is the instant center. The Instant center is the point in space that the rear wheel rotates around at any given point in its travel. On single pivot bikes, the instant center right on top of the main pivot. On linkage bikes, it lands at the intersection of two imaginary lines drawn from links and moves as the bike moves through its travel. The instant center holds the key to how the suspension responds to acceleration (pedaling) and deceleration (braking). The whole point is to locate the instant center to strike a good balance of the two while keeping the suspension sensitive to bumps. If you want to geek out on how this works, Seb Scott over at BikeRadar put together an excellent explanation.
Despite doing so in different ways, most every suspension design out there puts the instant center at or a few inches above the chain line when the suspension is at sag. Don’t sweat the marketing about ‘optimised’ or ‘virtual pivots; it’s nonsense. The perfect placement for the instant center varies with bike type, rider style, and even bike size. The good news is that the engineering behind this has been an open secret for the past decade, so pretty much every design performs reasonably well in this regard.
Leverage Ratio + Spring Rate: what makes suspension work.
For all the hype around pivot points, it’s how suspension moves through its travel that determines how a suspension bike will ride. The current consensus is that suspension should be progressive. It should start out sensitive for small bumps, be supportive in the middle of the travel to avoid wallowing and then firm up at the end to keep the suspension from bottoming. There are two ways to accomplish this. You can do it mechanically by changing the leverage ratio – how far the wheel moves for every unit that the shock moves – or you can change the spring rate.
To get a progressive leverage rate requires some kind of short link to compress the shock. This is why you’ll see links on some single pivot bikes even though they don’t affect how the wheel moves. It’s also why getting leverage rates on short link designs is so tricky; when you ask a link to control axel path and the leverage rate, there are only so many places you can put it. Normally, all this means is that designers of these bikes need to choose between the desired leverage rate and a place for a water bottle.
The most important thing to remember is that leverage rate has big mechanical advantage over the spring rate in determining how a bike rides. (Vorsprung Suspension explains why). Because the leverage rate is a function of the linkage design, whether a particular bike is progressive, linear or regressive is pretty much baked into the bike. Some bikes come with a chip system, like Rocky Mountain’s Ride-9, that allows riders to tweak the leverage rate of their bike. Other than that, there is a little bit of tuning that can be done by changing the volume of the air spring using different air cans or tokens, but these tweaks mostly add bottom out resistance.
Damping – The Icing on the Cake
Damping determines how quickly your suspension can react to different types of forces. At the most basic level, the damper has two jobs: 1) compression damping makes sure your suspension isn’t too sensitive without being harsh and 2) rebound damping makes sure your bike isn’t too bouncy but can still get all the way back to full travel. As dampers get more sophisticated, they can do these jobs with greater precision and greater tune-ability. Every damper comes with a “tune” that is set at the factory to match the need of a given bike. Better dampers can react to differences in shaft speed at different places in the suspension travel and allow riders some ability to fine tune these characteristics.
Damping is the icing on the suspension cake. While, great damping can make a great system even better it’s not going cover up a poorly designed system. Like icing, damping is somewhat a matter of personal preference; some people like a little, others a lot. The whole idea here is that great suspension should take advantage of damping, while still giving riders some room to adjust the way their suspension behaves. In a perfect world, you would get a bike that rides pretty well right in the middle of its damping settings, with a few clicks on either side to tune for your style or different conditions.
Surprisingly, this is one area where there are big differences between full suspension mountain bikes from different brands. While pretty much every brand works with suspension manufacturers to get a shock that is tuned to their suspension design, Bike Rumor points out that very few (Rocky Mountain being a notable exception) go the extra step of getting a size-specific shock tune for each frame size. Without a size specific tune – riders that are lighter or heavier than average will end up at one side or the other of the shocks adjustable range with little room left for tuning. Not an insurmountable problem, you can always drop some money on more-tunable, high-end shock. Unless of course the bike uses a proprietary shock – in that case, just stay away.
So if you’re shopping for a great full suspension mountain bike, it should be balanced, simple and tunable. Forget the marketing nonsense that suspension engineering is about unlocking some unknown secret that will solve all of suspensions problems. Mastering suspension, like geometry, requires an solid understanding of the basic systems involved and putting in the time to test and refine a design to achieve the desired ride. Even then, a great bike will leave room to fine-tune to the riders needs. While we tend to carry bikes that fit the balanced, simple and tuneable bill – the only way to tell if they will work for you is to swing by for a test ride. Of course once you do buy a full suspension mountain bike that works for you, we’re happy to help you out with a full suite of services, regardless of what kine of bike you choose.