Different Strokes - Decoding the marketing hype of rear-suspension designs

If you were to cruise through a bike shop, you would find the sales floor stocked with four distinct designs of full-suspension bikes. You might also encounter a few completely autonomous and divergent suspension systems, in addition to several “better mousetrap” variations of the major four designs. To hear the engineers, builders, or licensees of any single suspension system tell it, theirs and only theirs is the One True Path To Smoother Riding. After listening to this spiel a few times from different mouths, it is easy to feel, well, a little confused.What’s the real deal? Is a Horst Link the way to go? Should the letters VPP be in your bike’s surname? What the hell is a four-bar linkage? Does it matter? In this Gearhead, we’re going to look at four of the main players in suspension design, do a little nitpicking, and hopefully clear up a few things. There are more than four designs out there and we’ll save the discussion for systems such as the GT I-Drive and Maverick Mono-Link for later so we can focus on the stuff you will most likely find at your local dealer. The main thrust of this article will cover basic design parameters; we won’t get into which kind of shock works better, or who built what first—just sticking to these basics will be confusing enough.

Things That Go Bump
The main thing that all suspension designs have to cope with is the ground, and the bumps it throws at us. Those bumps are usually something more than a straight up-and-down impact, since the bike is usually moving forward when it encounters them.


Aside from terrain, there is the bobbing weight of a rider. Ideally, we could spin several pistons smoothly at hundreds or thousands of rpm. But we have only two pistons—our legs—and they weigh a lot and don’t spin very fast. As such, they tend to impart a degree of “bob” to any suspension. None of the designs we will talk about do much to counter this, and it is this force (or the fight to find some way of canceling it out) that has been such a primary factor in the development of platform damping systems (Gearhead, October 2004).
Then there are drive forces—namely, the effect that a loaded (as in being pedaled hard) chain has on the behavior of the suspension. Most suspension designs feature some degree of chain “growth.” This is a way of saying that the distance between the bottom bracket and rear axle gets longer as the wheel moves through its travel. When that happens, the loaded chain is fighting the action of the suspension trying to absorb a given bump. This is usually felt as feedback in the pedals.
And finally, there are brake forces—the effect that braking has on a suspension’s performance. People call it “brake jack.” Depending who you talk to, this is either a huge problem or it isn’t really noticeable at all. Suffice to say, some designs tend to stiffen under braking in a more pronounced manner than others do, and this tends to be more noticeable on bikes with longer travel. That said, a wheel that is being decelerated is going to track less smoothly and with less precision than a wheel that is rolling unimpeded, and whenever you grab the brakes you enact a forward weight shift that tends to unload the rear suspension. These two things are physical constants regardless of design, and every vehicle with wheels reacts adversely when this happens.
We should also point out at this juncture that no amount of good design or Bold New Graphics can overcome shoddy assembly, poor setup, or caveman maintenance. Get to know your suspension, whatever you might have. Make sure things that are supposed to be tight aren’t loose, and vice versa. Set the shock up right for your weight and style. And take care of it. It’s amazing how much better things work with a little maintenance. Okay, on to the different bits…

This is referred to by some as a “true” four-bar design, and is called a “Horst-link” in honor of Horst Leitner, the cagey German who patented it some 15 years ago. That patent is now owned by Specialized, and anybody who uses it has to pay The Big Red S to do so. Were it not for this, we’d be willing to bet that there would be boatloads more of these things being ridden. The design differs from the rocker link system in the location of the rear pivot. By locating the rear pivot on the chainstay in front of the axle, as opposed to on the seatstay, the axle is no longer required to follow such a strict arc, and it becomes relatively isolated from the twin evils of drive and brake forces.



This is good news for riders. Isolation from drive forces means that the bike doesn’t try to shorten its chainstays (causing either top-out or bob) under load, and can be pedaled neutrally. (It’s not totally neutral, though. There is still some degree of chain growth, especially if the axle follows a vertical or rearward linear path. But it’s as close to neutral as anyone has come up with yet, and the degree of chain growth is less than most other designs.) This in turn means that a manufacturer doesn’t have to compensate for rider input by over-damping the compression circuitry of the rear shock, since there is less rider input than in most other designs throughout the gear range. Brake forces, meanwhile, are very effectively neutralized, which is a joy to discover when railing into a washboard off-camber turn too fast on a big-travel bike.


The fact that the design has been around for more than a decade, and operates on basically the same physical principles it was first designed around, speaks volumes for how effective it is. Neutral, well-behaved, and active under both pedal and brake loads—it is a very good suspension design.
So why doesn’t everyone use it? Well, as mentioned, they’d have to pay to do so. There are also some design issues that can be bugbears. The first iterations of this design, back in the early 1990s, remembered mostly on AMP Design bikes, used a MacPherson strut approach and were woefully flex-prone. Flex doesn’t bother everyone, but it is a sure way to take a good-handling bike and turn things ugly when the rider gets big or speeds get high. More modern examples of the design use shorter seatstays and stouter linkages at the shock mounts, but they still need to be designed and manufactured carefully to avoid getting wiggly. Because the pivot is placed along the chainstay, at an area that is expected to transfer all the rider power from the cranks to the rear wheel (an area prone to high degrees of side-loading from impacts and wheel deflection), the design needs to be more carefully thought out and executed than something simpler and/or less active.
Still, those bugbears are easily worked around, and the design is still the standard to judge suspension performance by. Enough so that Specialized bought the patent, and high dollar/high performance brands like Turner, Titus, Norco and, until recently, Intense were more than happy to pay to use it.

Sometimes called a “four-bar” link design, sometimes a “walking beam,” this system consists of a swingarm that pivots from behind the bottom bracket, seatstays that pivot from above the rear axle, and a pair of rockers that attach the tops of the seatstays to the main triangle of the frame and compress a shock with their leading edges. It’s called a four-bar-link design because it consists of four separate, non-flexible, non-compressible components, all of which move in different planes to each other: the seatstays, the chainstays, the rocker arms and the front triangle of the bike. Unlike the Specialized licensed “Horst-Link” design, which features a pivot in front of the rear axle on the chainstay, these bikes run their rear pivots, as already mentioned, above the rear axle on the seatstay.
It may not seem like a big deal, but that different pivot placement has a pretty dramatic effect on the path the axle follows as it goes through its travel. The axle path on these bikes is identical to that of a behind-the-bottom-bracket-pivot, short-swingarm, single-pivot design—that is to say, it’s a tight arc that follows the radius drawn between the swingarm pivot and rear axle (usually upward and forward).


As such, these bikes share some traits with single-pivot bikes. They tend to bob with hard pedaling in the granny gear, and they also tend to exhibit a noticeable degree of brake-jack when the stoppers are squeezed (although we still stand by our contention that this isn’t really a big deal on short-travel bikes). However, the rocker link allows for a variety of shock placement options, which frees frame design parameters. And, depending on design, the shape of the rocker can determine whether the shock will be compressed at a linear, rising, or falling rate, which also allows for broader design concepts to be brought into play.


While not as simple as a single pivot, rocker-link bikes are solid designs that can be very sturdy and dependable. Because there is no hinge between the forward pivot and rear axle, swingarms can be light and stiff, and torsional loads are less likely to wear on them compared to a Horst-Link design. They are ubiquitous—nearly every manufacturer who wants more design freedom than can be pursued with a single-pivot suspension, but is unwilling to fork over the bucks to Specialized, has at one time built these bikes. As such, the design has stabilized into something eminently dependable, as evidenced by the hordes of kids on Vancouver’s North Shore jumping off cliffs on Konas and Banshees and Coves, all using this design.

This is the simplest rear suspension on the market. A rear triangle pivots from somewhere near the bottom bracket, causing the rear wheel to travel in an arc determined by the radius drawn between that pivot and the axle of the rear wheel. A shock is compressed by the upper, forward edge of the rear triangle—usually along the toptube or against the downtube. Single-pivot designs can be found propping up everything from the Santa Cruz Superlight, to the Fisher Sugar (don’t let that link-arm fool you, it’s still a single pivot), to the Cannondale Gemini, to the high-zoot, ultra-stealth Honda DH rig.


Single-pivot bikes are wonderfully simple. Not much goes wrong with them and they can last forever. They can be built as strong as houses, or as light as feathers, depending on application and intended use.


But there are drawbacks. They are probably the most susceptible to brake-jack of any design, and the suspension behavior can change depending on what gear you’re pushing. Put the bike in the granny ring and big cog, then pedal hard up a bumpy hill. Every time the chain is loaded, the suspension tries to top-out, because the chain is trying to pull the cranks and cogs together. In this granny/granny combo, the chain is running at a healthy tangent down and away from that line between the pivot and rear axle, and so ends up having a big effect on the suspension, pulling it taut.
That’s the bad news. The good news is that in the middle to high gears, when the chain is running almost parallel to that line between pivot and rear axle, these drive forces are minimized to the point of being negligible. Also, that inch-worming sensation in the granny ring is nowhere near as unpleasant to deal with as the marshmallow-y bobbing that some other designs exhibit in those same gears.

Some might contend that this design isn’t prevalent enough in its application to warrant being called a major design player, but given the burgeoning spawn of similar designs that have hit the market—from Giant, Haro, Marin and others—since Santa Cruz and Intense began breathing new life into the decade-old Virtual Pivot Point design a few years ago, we think it’s safe to say that the floating pivot point can stand on merit as a viable suspension platform.


Floating pivot point is the jargon used to describe the manner in which the rear triangle “floats” around a pivot area, rather than other designs, all of which are solidly anchored at a main pivot location. You could think of it as a solid rear triangle held apart from the main frame by a stubby pair of counter-pivoting link arms. The link arms allow the rear triangle to follow a path dictated by the separate arcs that each link pivots through. In the case of the Santa Cruz and Intense designs (who are the only two companies directly using the VPP patent now owned by Santa Cruz), this results in an axle path that follows a slightly “S”-shaped curve. As the bike settles into its sag point, the rear wheel actually moves slightly forward. Ideally, the sag point should be set at the forward-most point of that curve for the best chain tension for a solid pedaling platform. From here, the axle will follow an increasingly rearward path as it goes through the remaining three-quarters of its travel. Chain tension plays an active part in this design. At the sag point, chain tension holds the wheel under pedal load in the forward pocket of the axle path.


As such, it would be wrong to describe the design as “neutral.” It isn’t. There is chain growth and subsequent pedal feedback when climbing hard in the granny ring on rough ground—not as much as with a single-pivot design, but it is noticeable. And, while the bikes tend to work surprisingly well under brake loads, they are still affected somewhat by them.
Well, if it isn’t neutral, and it isn’t simple, why bother? Is that what you’re wondering? Good question.


Floating-pivot bikes may not be totally neutral under pedal loads, but when set up correctly, they are happily indifferent to them. We emphasize set up correctly. Too much or too little sag and they can be downright sluggish. Get the numbers where they need to be, however, and these bikes can climb and hammer as well as or better than anything else out there. By dint of the short, stiff links, they are also very stout designs in terms of lateral and torsional stiffness. The slightly rearward axle path past the sag point makes them able to eat up square-edged bumps and high-speed stutters with superlative ease. In a very short time, this new generation of bikes have won a lot of converts from the other design camps.

How Good Is My Mousetrap?

We’ve studiously gone out of the way of saying “this design is better than that,” but if I were forced to choose, I’d ride a single-pivot bike if it was always muddy at my house, one with a lot of tire clearance. And I’d ride a real beefy four-bar bike if I ever learned how to be a man and huck it like I knew what I was doing. And maybe a Horst bike for real technical climbing in rough and loose terrain. And a VPP bike with 6 inches of travel for everywhere else. But they’d all have to do really good wheelies.
Honestly. There are good examples of each design we’ve talked about here, and there are dogs in each category too. And, as mentioned already, the highest-tech bike in the world won’t make up for lack of skill or cross-threaded pedals. And none of them will ride themselves.

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