The Full Suspension Guide
The current crop of full suspension mountain bikes can be broken down into three general categories: cross-country, downhill, and freeride. There are also three general types of full suspension frame designs: cantilever, linkage, and unified rear triangle (URT). These are generalizations, but they can help to make sense of the huge number of different suspension bikes out there.
There several major points of consideration when evaluating a full suspension frame design:
The durability and smoothness of the pivots is crucial. Any pivot slop will be amplified at the wheel, and any friction at the pivots will prevent the suspension from operating smoothly.
Flex can be the downfall of an otherwise sound full suspension design. Flex will mean less control and feel, among other things.
“Chain reaction” is the tendency of a suspension design to compress or extend the suspension during pedaling, and/or to alter the chain length along the top run of the chain as the suspension compresses or extends, leading to pedal feedback.
Finally, sufficient and usable travel (the distance the wheel moves during the suspension compression) will make or break a suspension design.
The major differences between the three general categories can be seen in their intended uses, and the requirements of these intended uses. Cross-country full suspension designs need to be light, and the riders of this type of bike generally prefer a design that limits the amount of chain reaction during climbing or sprinting.
Suspension travel is a topic of debate among many riders, but generally, cross-country bikes run one to four inches of rear wheel travel, freeride bikes run four to eight inches of travel, and downhill bikes run six to 12 inches or more of rear wheel travel.
Some of the first popular full suspension designs of the mountain biking era were of the cantilever type, mainly due to this type’s simplicity in form and function. The cantilever full suspension design consists of a simple swingarm pivoting from a single point, actuating a spring/shock directly, without any linkages. The location of the single pivot is the key to success this design. Also, due to the lack of any linkages driving the spring/shock, this design does not readily lend itself to putting out more than about eight inches of travel. This design finds its roots in early road and off-road motorcycles immediately following the general demise of twin-shock rear suspension set-ups. A few modern motorcycles still use this configuration. Two examples of early cantilever suspension designs are the Mountain Cycle San Andreas, and the Cannondale Super V. The Orange Patriot and 222, and the Santa Cruz Super 8 and Bullit are examples of more current designs.
Linkage designs are some of the most popular suspension designs on the market today. There are a huge number of suspension designs that could be classified as linkage designs, and all are distinct from the next. Generally, a linkage design deviates from a cantilever design in at least one of two ways: the swingarm does not directly actuate the spring/shock, or the wheel is not directly attached to the frame via a single swingarm, but instead it is attached via a series of swingarms connected by pivots. This design can be found on many modern road and off-road motorcycles. The Mountain Cycle Moho CXS, and the Specialized FSR are two examples of linkage suspension designs.
Unified rear triangle designs (URTs), also known as floating drivetrain designs, are also very popular. The URT is distinct in that the entire drivetrain (bottom bracket, cranks, chainrings, front deraileur, rear cogs, rear derailuer) is located on the swingarm or swingarms. This translates into a design that does not have any chain reaction, and that stiffens when the rider stands out of the saddle. This design can be traced back to bicycles at the turn of the century, and is very bicycle specific. Two good examples of URT designs are the Klein Mantra and the Gary Fisher Level Betty/Joshua series.
The crucial aspect of any cantilever design is the pivot location. Assuming everything else in the design is properly executed (minimal flex, minimal pivot slop, smooth pivots, sufficient travel), the pivot location on a cantilever suspension design determines how the bike will ride.
The easiest way to understand how the pivot location affects a cantilever rear end is to imagine the rear end and drivetrain as a simple construction paper cutout pinned to a board with a simple pushpin, with a piece of string representing the chain. The pushpin would pin the swingarm to the board at the exact location of the pivot, while the string would attach to the swingarm where the chain would engage the rear cogset. Place a second pushpin in a location that would mimmick the location of the top of any of the three front chanrings, then pull the string from the swingarm around the pushpin and down. This type of modelling was apparently used by Horst Leitner when designing the now famous Amp rear suspension.
Now, if the swingarm pivot is located above the chainring selected, then when the string is pulled, the swingarm is pulled downward (extension). In the real world, what this means is that when a rider in the same selected chainring puts power to the pedals, the resulting force on the chain pulls the swingarm in a downward arc, or in other words, the rear wheel moves down with respect to the rest of the frame. This effect can be good or bad. It can be good when climbing in loose terrain because as the rider turns the cranks, the rear wheel digs into the ground, increasing traction. It can be bad when pedaling over bumps because when the rider turns the cranks, the rear wheel is driven into the bump instead of absorbing the bump, which is sometimes refered to "lock out" under power. The rear suspension will only begin to absorb the bump when the bump force exceeds the pedaling force. This makes for a less "plush" suspension system under power that climbs well. This type of suspension my "inchworm" under power, as each pedal stroke pulls the swingarm down and the rest of the bike up.
If the swingarm pivot is located below the chainring selected, then when the string is pulled, the swingarm is pulled upward (compression). In the real world this makes for what has been called a very "active" suspension system that does not lock out under power. Because the suspension is being compressed under pedaling, the suspension becomes more sensitive to bumps, as the effective spring rate is reduced by the force of pedaling. This makes for a more plush suspension system which may tend to "bob" under power, that is, the suspension will compress with each power stroke. This effect is generally more pronounced than the inchworming found in high pivot systems, because the weight of the rider is already trying to compress the suspension, adding to the bobbing effect. Some riders feel that this robs some of their pedaling power, and thus robs efficiency. Also, the bike does not react as quickly to accelleration inputs, especially compared to a hardtail.
If the swingarm pivot is in line with the chainring selected (use the same pushpin to represent the swingarm pivot and the top of the selected chainring), when the string is pulled, there is little to no effect on the swingarm (there is a bending effect laterally, but the swingarm would have to be very weak or the rider very strong for there to be any perceptible movement in this direction). This results in a suspension that is "fully active" under power, but does not bob or inchworm with each pedal stroke. This may sound like the perfect pivot location, and it is, but with today's archaic drivetrains there are three chainrings to deal with, so placing the pivot in line with one of them means it will be a comprimise with the other two.
Linkage style rear suspension designs differ from cantilever designs in that they use a series of "links" between the swingarm and the spring/shock and/or the swingarm and the rear axle.
Most modern motorcycles use a linkage style rear suspension using a conventional swingarm that mates to a spring/shock via a linkage or linkages. There are several reasons to use a linkage between the swingarm and shock. A linkage allows the leverage ratio between the swingarm and shock to be fine tuned, and can allow for a rising or falling rate of leverage on the shock. Fine tuning the leverage ratio will allow for more travel, and usually allows for the use of a smaller spring/shock. A linkage also allows the shock to be mounted in a better position on the bike other than just connected directly to the swingarm. Linkages can also cancel out flex in a design, as well. The Mountain Cycle Shockwave is one design that uses linkages between the swingarm and the shock.
Using a linkage between the swingarm and the rear axle is a bit of a different story. This type of linkage allows for a unique axle path compared to a conventional cantilever swingarm. Picturing the construction paper model of the conventional cantilever swingarm, the axle path describes a perfect circle (if rotated 360 degrees, of course), or, more accurately, an arc. This means that when the rear axle is below a horizontal line that bisects the main pivot, it moves rearward in relation to the main frame as it moves up. When the rear axle is above the main pivot, it moves forward as it moves up. This type of axle path is considered inefficient by many designers who feel that a consistent vertical or consistent rearward linear axle path is optimum. A vertical or rearward linear axle can be achieved using linkages between the swingarm and the axle.
This type of axle path has several advantages. Most notably, pedaling forces do not compress or extend the suspension as much, leading to what is commonly referred to as a more “active” suspension. Instead of trying to pull the swingarm up or down, pedaling forces are more or less cancelled out, much like when the main pivot on a cantilever design is located in line with the chainring being used. Also, a vertical axle path means that the length of chain between the cogset and the top of the chainring remains more constant compared to a cantilever swingarm setup, meaning less or no chain reaction through the pedals. A vertical axle path also means that the bike's overall wheelbase does not change as the rear suspension compresses and extends. The Specialized FSR is a good example of a bike that uses this type of linkage setup.
A linear rearward axle path is the route that Paul Turner (of Rock Shox fame) has chosen for the frames he has created. The theory goes that a rearward axle path moves more in the direction of bump forces, allowing the suspension to work better, and also moves in the exact same way that a conventional front fork moves, thus allowing for better balance between the two and a consistent wheelbase when both are compressed.
The main disadvantages of any linkage design are the number of pivots required, thus a higher possibility for pivot slop, and the cost and complexity of such designs. That being said, most of the better manufacturers have developed reliable, fairly slop free pivot systems that are not outrageously expensive. Linkage designs have basically become the mainstay of downhill and freeride frame designs, and are becoming more and more popular as cross country bikes as the popularity of URT designs wanes.
Unified Rear Triangle Designs
The unified rear triangle (URT), or floating drivetrain, is a rear suspension design whose popularity is waning in the current climate of downhill and freeriding.
The basic premise of this type of design is to isolate the rear suspension's effect on the drivetrain by placing the entire drivetrain on the swingarm itself. Thus, the various components of the drivetrain move together as the suspension compresses and extends. This eliminates any chain reaction and can also allow for a very clean, simple suspension configuration consisting of a single large pivot and a directly driven shock.
The inherent problem with this design is that the rider is in effect standing on the swingarm. This is less of a problem when the rider is seated, but the natural tendency when going over larger obstacles, rough terrain, or technical sections is to stand up, rendering the rear suspension almost useless. The flip side to this effect is that during sprinted or climbing out of the saddle, the suspension's lack of movement is considered a bonus, as less energy is wasted in suspension movement.
There have been several very popular URT bikes in the past, most notably the Klein Mantra, the Trek/Gary Fisher Y-bikes, and the Ibis Sweet Spot. There have also been twists on the URT design in the form of the GT iDrive and the Paul Turner desiged Maverick.
Which suspension design is right for you? Only with extensive reasearch, test rides, and many visits to your local bicycle dealer can you decide which full suspension bike is right for you. This guide is meant only as a basic breakdown of the different suspension designs in order to be able to understand what differentiates one full suspension bike from another. Consider what type of riding you do, what type of riding do you want to do (you may be an XC rider now but you want to get into downhilling), and make sure you get a well made, well designed bike that fits into the category that you choose.
But the biggest question may be, do you really want a full suspension bike? The first consideration here has to be budget. If you do not have at least $1000 to $1500 to spend on a bike, then it is a good idea to stick with a good hardtail with front suspension. Full suspension bikes below $1000 are generally not worth it if you plan to ride off road fairly aggressively.
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