Mountain Bike Action Tech: Unlocking the Secrets of Platform Suspension
Different lines: Every rear-suspension design (and the execution of the design) has a different take on how to give the rider a firm suspension for pedaling while still softening up enough for taking the hits. Knowing how to ride each one can unlock their hidden performance and make you a better rider.
Longer-travel trailbikes have brought a new level of comfort and control to riding. With the extra inch or two of rear-wheel travel, however, there is always the possibility of too much of a good thing. Suspension movement due to pedaling efforts (instead of activation due to the trail surface) can have many riders questioning the logic of all that travel.
Luckily, almost every dual-suspension mountain bike priced over $800 has some type of technologybe it a shock setting or a suspension design with a mechanical advantagethat helps to minimize pedal bob.
“Pedal platform” is the blanket term for any of several strategies, both hydraulic and mechanical, that are used to prevent a bicycle’s suspension from compressing with each successive push on the pedals without restricting the system’s ability to react to bumps. To get the most performance and pedaling efficiency from your system of choice, you need to use a specific riding technique. Don’t be intimidated. Our techniques are easy to apply, and you are probably using some of them already.
Many early dual-suspension riders had one choice for adjusting their compression. They either ran an ultra-stiff coil spring or pumped air shocks to a seal-blowing psi level. They quickly found that while this kept the shock from being affected by pedaling forces, it also kept the shock from responding to bumps. It was like riding a hardtail, only heavier.
Shock designers worked around this anti-suspension option by using hydraulic trickery (usually a valved compression piston) that would resist low-frequency input, but would flow a large volume of fluid once it was popped open by a significant impact. Call it ProPedal or CTD (Fox), Motion-Control (RockShox), Lock Up (Marzocchi), Dual-Range Compression (Manitou) or simply compression adjustment (X-Fusion), these pedaling-platform shocks were developed so riders could run a good amount of sag without feeling like they were pedaling a pogo stick.
How to Tune Them
Most pedaling platforms are fixed at the factory. Some more expensive shocks allow you to select a level of plat- form (or compression resistance) with the flick of a lever or dial. Either way, start by setting the shock sag at 20 percent of its stroke using spring preload on coil-over shocks and air pressure on air shocks. If you don’t know how to set sag, grab your December 2011 issue (you keep all your issues, don’t you?). Our “Garage Files” explains how to set sag.
Free movement: Trek went to great lengths to design a balanced rear suspension that doesn’t need the mechanical advantage of complex linkages to produce a firm pedaling platform. Properly adjust- ed, this is a great pedaling and descending rear suspension.
Don’t be fooled in the garage. Test ride the shock to verify your sag, because the shock may not settle completely when it is static. Platform shocks can feel mushy with impossibly slow rebound, but this is a byproduct of the internal valving and should only occur when the bike is not moving and the damping circuits are closed down. Once open, the valving allows the shock to move freely. Tune the compression clicker to be soft enough to get full travel on big bumps at singletrack speeds and with the least amount of rebound damping that will still prevent bouncing when you land a jump.
How to Ride on a Platform
Pedal in the saddle as much as possible, and use a smooth cadence when you are out of the saddle. The compression valve is only able to resist bobbing when it is shut. Once it pops open, the high volume of fluid that escapes past the valve keeps it open until the next compression cycle. Learn to feel the threshold with your leg muscles and meter your power to keep from overcoming it.
Read the trail: There are times, like on a rocky ascent, when you should be less concerned with the pedaling platform and more concerned with the bump-absorbing suspension. Many climbs are easier to clear with no platform.
When you jump out of the saddle, try to balance your weight more over the middle of the bike instead of leaning over the fork and sawing away at the handlebar like a road rider. Keeping centered will distribute the downward thrusts of your pedal stroke between the shock and fork. Avoid leaning over the fork when you accelerate (an old-school trick that loaded up the fork and unweighted the shock to make both pedal firmly). You don’t need that trick if your shock is adjusted properly.
No-brainer: Specialized’s Brain platform allows the rider to just ride. The Brain Fade feature (the blue knob on top of the Brain’s cylinder) adjusts the amount of platform. This is a set-it-and-forget-it adjustment.
Specialized Brain Shock
Fox and Specialized developed an inertia valve that separates downward force generated by the rider from upward accelerations caused by bumps in the trail. The “Brain” shock is exclusive to Specialized bikes. The inertia valve is a spring-loaded weight that closes off the compression circuit. When the tire rolls over a bump, the motion slides open the weight, and the suspension is set free. A downward thrust from the rider, like a pedal stroke, slides the weight closed and makes the bike pedal as if it were a hardtail. A delay retards the speed of the inertia valve, so the suspension can compress over a bump while you are pushing down on a pedal. In this mode, the inertia valve damper still feels rigid, because the downward push
on the pedal is countered by the upward acceleration from the bump.
How to Tune It
The Brain shock will not compress when the bicycle is stationary, so you must ride around to activate the suspension. Then, make incremental adjustments to set the proper spring rate and damping adjustments. As with all stable-platform suspension, set the shock or fork to ride as supple as possible, just stiff enough so the suspension sags between 20 and 25 percent into its travel.
Some Brain shocks have a “Trail Tune” adjustment that allows you to dial in the sensitivity of the inertia valve. This is a rider preference, and the wrecking crew is all over the board on this adjustment. Some feel that you should run the Brain at the firmest setting, while others want a more traditional rear suspension and dial in more movement.
It is an easy adjustment that encourages experimentation.
The inertia valve tends to make the rear suspension ride high. Try to increase the fork’s ride height with additional preload or spring pressure to compensate for the tall-riding shock. Use an O-ring or a zip-tie to gauge your sag measurement and to ensure that you are achieving full travel. Avoid running the suspension too stiff to hedge against bobbing; the inertia valves do that job so well that even a moderately stiff spring setup may prevent full travel.
How to Use Your Brain
Just get on and pedal. The Brain shock rides hardtail-firm in any pedaling modefrom dainty spinning to big-gear mashing.
While the two previous examples rely solely on the shock to control suspension bobbing, many suspension designers have come up with mechanical solutions to keep bobbing in check. Brands who follow this route on some of their models include Niner, Giant, Pivot, Ibis, Intense, Turner and Santa Cruz.
Damage control: Staying in the saddle and spinning smoothly is great advice that no rider can follow all the time. When you do need to leave the saddle, stay centered (like Cody here) with your torso low.
While each design is unique enough to carry patents, they generally work with a pair of links instead of a normal swingarm pivot to move the rear wheel in an S-shaped path that angles backwards, away from the cranks. Chain tension created by pedaling counteracts suspension bob by opposing the rearward movement of the swingarm. Hitting a bump momentarily overwhelms the chain tension’s effect on the swingarm and compresses it past the center of the “S” path where the suspension is free to move through the remainder of its stroke.
The drawback to many mechanical platforms is that you don’t always have a way to tune around it if, for example, you are looking for a softer beginning or middle travel. Riders may also detect chain growth (a slight pushback from the pedals) on certain mechanical-advantage designs.
How to Tune It
Read the instructions. To properly set up a mechanical-advantage suspension, you must set the sag spot-on. Every linkage has a slightly different wheel path, so the sweet spotwhere the suspension rides right where pedaling pressure will resist bobbing, but not completely freeze up the suspension over bumpsvaries between models and brands. Pivot takes this so seriously that every bike is delivered with a sag tool to help measure the perfect setting.
Start where the bike company recommends in their manual, and experiment with softer and stiffer preload settings to determine which provides the least amount of movement when you are pedaling. Outside of sag guidelines, tune the shock the same as you would any conventional shock. That means you need to set the clickers to minimize compression and with enough rebound to keep the seat from pushing up after you smack a big low-speed bump.
The small amount that a mechanical-advantage design bobs at low-power outputs doesn’t amount to a measurable waste of power, but it can feel a tad sluggish. Use the linkage to your advantage. Maintain your momentum by standing up to power over short hills. When you are soft-pedaling, concentrate on a smooth cadence. Stay away from using the firm settings of pedaling-platform shocks. In many cases, you won’t need any type of platform shock to make this rear suspension work properly.
Again, read the owner’s manual. Companies that employ a mechanical-advantage rear suspension have spent a great deal of time testing the shock’s valving to work with their design. You have to set the shock to their recommendations to get the best performance out of the system.