Bicycle suspension is the system, or systems, used to suspend the rider and bicycle in order to insulate them from the roughness of the terrain. Bicycle suspension is used primarily on mountain bikes, but is also common on hybrid bicycles.
Bicycle suspension can be implemented in a variety of ways, and any combination thereof:
- Front suspension
- Rear suspension
- Suspension seatpost
- Suspension saddle
- Suspension stem (now uncommon)
- Suspension hub
Bicycles with only front suspension are referred to as hardtail and bicycles with suspension in both the front and rear are referred to as full suspension bikes. When a bicycle has no suspension it is called rigid. Bicycles with only rear suspension are uncommon although the Brompton folding bicycle is equipped with rear only suspension.
Even a rigid bicycle has some suspension as the tires, wheels, and even the frame absorb some shocks. Although a stiffer frame is usually preferable, no material is infinitely stiff and therefore any frame will flex a little bit. Some bicycle designers intentionally make frames in such a way that the frame itself can absorb some vibrations.
Besides providing comfort to the rider, suspension systems improve traction and safety by helping to keep one or both wheels in contact with the ground.
- 1 Front suspension
- 2 Rear suspension
- 3 Floating drivetrain
- 4 Seat suspension
- 5 Suspension hub
- 6 Terminology
- 7 Mountain bikes
- 8 Road bikes
- 9 Recumbent bikes
- 10 Softride and Zipp
- 11 See also
- 12 References
Front suspension is often implemented using a telescopic fork. The specifics of the suspension depend on the type of mountain biking the fork is designed for and is generally categorized by the amount of travel. For instance, manufacturers produce different forks for cross-country (XC), downhill (DH), freeride (FR) and enduro (ND) riding which all have different demands for amount of travel, weight, durability, structural strength and handling characteristics.
Suspension fork design has advanced in recent years with suspension forks becoming increasingly sophisticated. The amount of travel available has typically increased. When suspension forks were introduced, 80–100 mm of travel was deemed sufficient for a downhill mountain bike. This amount of travel is now common for cross-country disciplines, whereas downhill forks typically offer 200 mm of travel for handling the most extreme terrain.
Other advances in design include adjustable travel, allowing riders to adapt the fork's travel to the specific terrain (e.g. less travel for uphill or paved sections, more travel for downhill sections). Many forks feature the ability to lock-out the fork. This completely eliminates or drastically reduces the fork's travel for more efficient riding over smooth sections of terrain. The lockout can sometimes be remotely controlled by a lever on the handlebars actuating the lockout via a mechanical cable, or even through electronics.
As with all shock absorbers it usually consists of two parts: a spring, and a damper. The spring may be implemented with a steel or titanium coil, compressed air, or even an elastomer. Different spring materials have different spring rates which have a fundamental effect on the characteristics of the fork as a whole. Coil sprung forks keep an approximately constant spring rate throughout their travel and act linear. The spring rate of air sprung forks however increases with travel, making them progressive. Titanium coils are much lighter but also come at a significantly increased cost. Air sprung forks are generally lighter still.
Air springs work by using the characteristic of compressed air to resist further compression. As the spring itself is provided by the compressed air rather than a coil of metal it is much lighter; this makes their use popular in cross country designs. Another advantage of this type of fork design is that the spring rate can easily be adjusted by changing the air pressure within the fork. This allows a fork to be effectively tuned to a rider's weight. To achieve this in a coil sprung fork one would have to swap out different coils with different spring rates. However air pressure naturally controls both spring rate and preload at the same time, requiring air forks to have additional systems to adjust preload separately, adding to its complexity. Another disadvantage of air sprung forks is the difficulty in achieving a linear spring rate throughout the fork's action. As the fork compresses, the air held inside is compressed. Towards the end of the fork's travel, further compression of the fork requires ever greater force. This results in an increase in spring rate and gives the fork its progressive feel. Increasing the volume of the air inside the spring reduces this effect but the volume of the spring is ultimately limited by the need to be contained within the fork. The use of two air chambers within the system has allowed a more linear feel to air suspension, this is achieved by having a 'reserve' chamber that becomes connected to the main chamber when it reaches a certain amount of compression. Once achieved, a valve opens and effectively makes the chamber larger. By linking the two, the force needed to compress the air in the chambers is reduced which reduces the exponential spring rate feel traditionally associated with air systems when approaching the end of the suspension's travel.
The amount of preload on coil sprung forks can generally be adjusted by turning a knob on top of one of the fork legs. Air sprung designs have various ways of dealing with preload. Several systems have been designed to influence preload such as separately pressurizing different chambers or systems that automatically set sag after changing the air pressure.
A damper is usually implemented by forcing oil to pass through one or more small orifices (also called ports) or shim stacks. On some models the damper may be adjusted for rider weight, riding style, terrain, or any combination of these or other factors. The two components are often separated by housing the spring mechanism in one of the fork's legs and the damper in the other. Without a damper unit the system would rebound excessively and would actually give the rider less control than would a rigid bike.
Some manufacturers have tried other variations to the telescopic fork. For example Cannondale designed a shock absorber build into the steerer tube called HeadShok, and a single-sided fork with just one leg, called Lefty. The stanchions of both systems are not round but have flat faces machined onto them which slide on needle bearings instead of bushings, this prevents the wheel from rotating in relation to the handlebars. Both of these systems claim to offer greater rigidity and better feel, with lighter weight. Others such as Proflex (Girvin), Whyte and BMW, have made bikes utilizing suspension forks that employ four-bar linkage systems instead of relying upon telescopic fork legs, much like BMW's Duolever. The Suntour Swing Shock fork is based on a coil cantilevered swing set-up, and suspension is given by a coil spring which is located within the steerer tube and can be accessed from the top, technology which was originally used for suspension on early motorcycles.
To prevent water and dirt from damaging the suspension, gaiters have been used to cover the fork's stanchions. However even when properly sealing the stanchions and sliders, the gaiters have to have small openings in them to allow air to move in and out of the cavity between gaiter and stanchion as the fork moves through its travel. Some water and grit may find its way in through these holes, staying trapped inside and accumulating over time. Since modern dust wipers and seals keep out water and dirt adequately enough by themselves, and since gaiter-less stanchions are generally regarded as more aesthetically pleasing[by whom?], gaiters have fallen out of favor.
Bicycles with rear suspension typically also have front suspension, recumbent bicycles with suspension are an exception and often employ rear-only suspension.
Mountain bike suspension technology has made great advances since first appearing in the early 1990s. Early full suspension frames were heavy and tended to bounce up and down while a rider pedaled. This movement was called pedal bob, kickback, or monkey motion and took power out of a rider's pedal stroke — especially during climbs up steep hills. Input from hard braking efforts also negatively affected early full suspension designs. When a rider hit the brakes, these early suspensions compressed into their travel and lost some of their ability to absorb bumps. This happened in situations where the rear suspension was needed most. When braking efforts cause the suspension to compress it is referred to as brake squat , when braking causes the suspension to extend it is called brake jack.
Problems with pedal bob and brake jack began to be controlled in the early 1990s. One of the first successful full suspension bikes was designed by Mert Lawwill, a former motorcycle champion. His bike, the Gary Fisher RS-1, was released in 1992. Its rear suspension adapted the A-arm suspension design from sports car racing, and was the first four-bar linkage in mountain biking. This design reduced the twin problems of unwanted braking and pedaling input to the rear wheel, but the design wasn't flawless. Problems remained with suspension action under acceleration, and the RS-1 couldn't use traditional cantilever brakes since the rear axle, and thus rim, moved in relation to the chainstays and seatstays. A lightweight, powerful disc brake wasn't developed until the mid-1990s, and the disc brake used on the RS-1 was its downfall.
Horst Leitner began working on the problem of chain torque and its effect on suspension in the mid-1970s with motorcycles. In 1985 Leitner built a prototype mountain bike incorporating what became known later as the "Horst link". The Horst Link is a type of four-bar suspension. Leitner formed a mountain bike and research company, AMP research, that began building full-suspension mountain bikes. In 1990, AMP introduced the Horst link as a feature of a "fully independent linkage" rear suspension for mountain bikes. The AMP B-3 and B-4 XC full-suspension bikes featured optional disc brakes and Horst link rear suspension very similar to the Macpherson strut. Note that the sliding piston in the shock absorber represents the fourth "bar" in this case. A later model, the B-5, was equipped with a revolutionary four-bar front suspension fork, as well as the Horst link in the rear. It featured up to 125 mm (5 inches) of travel on a bicycle weighing around 10.5 kg (23 pounds). For 10 years AMP Research manufactured their full-suspension bikes in small quantities in Laguna Beach, California, including the manufacture of their own hubs, rear shocks, front suspension forks and cable-actuated-hydraulic disc brakes which they pioneered.
A soft tail (also softail) relies on the flexing of the chainstays of a regular diamond frame to create suspension travel, sometimes incorporating a specific flexing member within the chainstays. A shock absorber (or elastomer) is placed in line with the seat stays to allow the chainstays to move up and down, and for shock absorption. As the suspension moves through its travel the seat stay and shock absorber move out of alignment. This misalignment creates a mechanical lever for suspension forces, causing torque on the joint between chain- and seatstays. This is an inherent structural disadvantage of the soft tail design and severely limits the amount of travel possible, typically around 1 to 2 inches. Soft tails have few moving parts and few pivot points making them simple and requiring little maintenance. Some notable examples include the KHS Team Soft Tail, Trek STP and the Moots YBB. The Cannondale Scalpel is a four-bar suspension design where one of the pivots is replaced by a flexing link and has 4 inches of travel.
Unified rear triangle
The unified rear triangle or "URT" for short, keeps the bottom bracket and rear axle directly connected at all times. Suspension action is provided between the rear triangle, which unites rear axle and bottom bracket, and the front triangle, which unites seat and front axle. This design uses only one pivot, which keeps the number of moving parts down. The fixed length between bottom bracket and rear axle gives the URT the advantage of zero chain growth and consistent front derailleur shifting. Additionally the bike is easily modified into a single-speed. However as the URT's suspension moves, the distance between seat and pedals changes, detracting from pedaling efficiency. Furthermore, when the rider shifts any weight from the seat to the pedals, he or she is shifting weight from the sprung part of the bike to the unsprung parts. As such, part of their weight would not be suspended by the suspension system anymore. Since pedaling itself is a shift of this weight, the design is very prone to suspension bob.
The single pivot is the simplest type of rear suspension. The rear axle is held by a swingarm which is connected to the frame via a single pivot located near the bottom bracket. When the suspension moves through its travel, the path the rear axle describes is a circle around the pivot point. The rear triangle can be directly attached to the rear shock for a fairly linear leverage ratio between wheel travel and shock absorber travel.
The main advantage of the single pivot design is its simplicity. It has few moving parts, few pivot points, is relatively easy to design and has good small bump compliance. Challenges with this design are brake squat and chain growth.
Due to its simplicity, many inexpensive department store bikes use this design.
Linkage driven single pivot
Another implementation of the single pivot design attaches the swingarm to the shock via additional linkages, typically creating a four-bar linkage actuating ("driving") the shock to create a more progressive leverage ratio between wheel travel and shock absorber travel. This designs is referred to as linkage driven single pivot, colloquially called faux-bar. It employs a four-bar linkage but the rear axle is held in a swingarm and still describes a circular axle path. The four-bar linkage serves only to actuate the shock and has no role in governing the axle path. Manufacturers of the linkage driven single pivot often use the word "four-bar" in their marketing campaigns, which gave rise to the design's nickname "faux-bar".
High single pivot
This variation of single pivot suspension places the pivot in front of and above the bottom bracket, at a height above the smallest chainring or higher. This gives the design a significant amount of anti-squat when pedaling in smaller chainrings, which helps reduce loss of energy due to squat. This is particularly of importance on steep climbs, when one would use the smaller chainrings. However, this is a trade-off since the pivot's placement causes the design to suffer more from pedal kickback. Notable examples are Santa Cruz' Superlight and Heckler frames.
The split pivot design is a special case of linkage driven single pivot in which one of the four-bar's pivot points coincides with the rear axle. This allows for the disc brake caliper to be mounted on the floating linkage (also called coupler) instead of on the swingarm. As a result of this the braking torque now interacts with the suspension via the floating linkage. The linkages can be designed such that this has a positive effect on suspension performance under braking, typically reducing brake jack. Furthermore, the relative rotation between brake disc and brake caliper as the suspension goes through its travel is different from that in single pivot designs. The four linkages in a split pivot design influence how braking torque is transmitted, how the brake caliper moves in relation to the disc and influence the leverage ratio between wheel travel and shock travel. Since these influences may have a different optimum linkage design, the bike's design has to strike a balance. Dave Weagle designed a split pivot which he named "Split Pivot". Trek Bicycle Corporation also released a version of the split pivot design called "Active Braking Pivot" (ABP) in early 2007. Cycles Devinci has released a licensed implementation of the "Split Pivot" design which Dave Weagle managed to patent.
"Horst Link" suspension is a type of four-bar linkage suspension. It is characterized by having both connecting links pivot on the seat tube, with the lower pivot located above the center of the bottom bracket, and the rear axle being located higher than the pivot connecting floating link and lower connecting link.
Specialized has bought several of Leitner's patents which they use for their "FSR Suspension". Several manufacturer have licensed the design from Specialized. Some European manufacturers, such as Cube and Scott, do use the same suspension design, but can not import it to the United States. Norco, Canadian bicycle manufacturer also licensed the design from Specialized but further optimized the FSR system, calling their own system Advanced Ride Technology (ART).
Although not necessarily sharing any general suspension characteristics, these designs all share a certain structural benefit. Because the connecting links are so short, the floating link comes in the form of a rigid rear triangle big enough to encompass the rear wheel. This allows the rear triangle on both sides of the wheel to be rigidly connected together as one piece, before attaching to the connecting links of the four-bar. This significantly increases lateral and torsional stiffness of the rear, often a weakness of four-bar designs, and reduces the load on the pivots, connecting links and joint between rear axle and frame.
Virtual Pivot Point
The "Virtual Pivot Point" or VPP, is the name given to a four-bar linkage suspension with relatively short links connecting the rear triangle to the frame. It is characterized by having a chainstay lengthening effect and an 'S' shaped axle path. Some variations may have the links rotate in opposite direction as the suspension moves. The instant centre of rotation, as found in all linkage systems, is also called virtual pivot point. The "Virtual Pivot Point" suspension was developed by Outland Bicycles in the nineties and the associated patents are now owned by Santa Cruz Bicycles. VPP suspension is also licensed to Intense Cycles.
Dave Weagle's "DW-link" is another four-bar suspension system with two relatively short links, here they typically co-rotate. It is characterized by an anti-squat response that varies with suspension travel and is designed to reduce loss of energy resulting from squat during pedaling. It typically has higher anti-squat at the beginning of suspension travel, and less thereafter. The DW-link was originally used by Iron Horse bicycle company. After they went out of business in 2009, the DW-link suspension design was up for grabs.The DW-link is now licensed to Ibis, Independent Fabrication, Turner Suspension Bicycles, and Pivot Cycles.
Another variation using short, co-rotating, links is employed by Giant Bicycles called "Maestro".
Yet another variation of short link design is the "Switch link", found on some Yeti bikes like the SB-66 designed by Dave Earle. The rear triangle is connected to the frame using an eccentric pivot, effectively creating a very short lower link whose length is that from the center of the eccentric to the attached pivot. The links counter-rotate.
Trek Full Floater
Trek bicycles' "Full Floater" is a system where the rear shock is only attached to the rear triangle. The chainstays pivot on the seat-tube, and then extend into the front triangle, and the lower shock bushing attaches to the chainstays. The seatstays are attached to an "Evo" link, which then attaches to the shock top bushings. This system enables finer tuning of suspension kinematics, as the shock angle relative to the linkages during the travel changes in a more linear fashion. Trek also combines "Full Floater" with its "ABP" rear axle, where the rear suspension pivot is the rear axle, which reduces pedal feedback and prevents the braking forces on the wheel form altering the suspension movement.
The "Equilink" suspension system was developed by Felt Bicycles for their full suspension line. The system employs a "Stephenson-style" six-bar linkage suspension system. Equilink gets its name from the dog bone shaped bar that ties the upper and lower links together. Early models included a pivot between chain- and seatstay whereas in later, carbon fiber, models the chain- and seatstays are one piece, creating pivoting action by flexing. Felt contends that the system holds its pedaling efficiency in any gear combination.
Bikes with a 'floating drivetrain' or 'floating bottom bracket' can use any type of suspension system to suspend the rear wheel from the frame, but use linkages to connect the crank assembly to the frame and rear suspension. Since the linkages are connected to the rear suspension, suspension movement causes the crank assembly to move as well. The floating drivetrain is often employed to compensate for the disadvantages of a particular rear suspension system so the design can make better use of its advantages.
The "Independent Drivetrain" (or "IDrive") is a four-bar suspension system for bicycle crank assemblies, the rear wheel itself is suspended as single pivot suspension. It was developed by mountain bike suspension designer Jim Busby Jr. and was a direct result of the limitations encountered with the GT LTS (GT Bicycles' "Links Tuned Suspension) four-bar linkage design used by GT Bicycles from 1993 to 1998. The IDrive attempts to maximize the efficiency of the transmission of energy from the rider to the rear wheel. The bottom bracket is placed eccentric in a bearing within the swingarm, the distance between the center of the bearing and the bottom bracket effectively creating a very short link, and the swingarm itself creating another. A link between bearing shell and frame then completes the four-bar linkage with the bottom bracket on the floating link and the linkage as a whole actuated by movement of the swingarm.
The "Monolink" made by Maverick Bikes and designed by ex-RockShox founder Paul Turner, is a licensed variant of the Independent Drivetrain suspension, and is a variation of the MacPherson strut. It uses three pivot points and the sliding action of the shock to provide the fourth degree of freedom. The Monolink places the bottom bracket on the link connecting frame and rear triangle. Any load on the cranks is partly unsprung since it is also a load on one of the suspension's parts itself, and actively works against the suspension. However, because of this there is less bob during out-of-the-saddle sprints. Once again it is an attempt at maximizing drivetrain efficiency, compromising other areas, Notable bikes using this design are the Maverick ML7, ML7.5, ML8 and the Klein Palomino.
The "Pendbox" is found on several of Lapierre's linkage driven single pivot bikes in which the crank assembly is hung from the frame using a 'mini-swingarm'; the Pendbox. A link connects the swingarm and Pendbox such that they form a four-bar linkage.
The effectiveness of saddle or seatpost suspension depends on the rider placing their weight on the saddle. For this reason, this style of suspension is most popular on more upright styles of bicycle, where the rider spends the majority of time seated.
In mountain biking the term 'suspension hub' was used in the 90s to describe hubs with oversized axle ends and a thicker axles than were customary at the time. These hubs were designed to stiffen up suspension forks, which were still somewhat of a novelty, by rigidly holding the fork legs in position relative to each other once the wheel was fitted, improving steering response in the fork. This term is no longer used as this function is no longer an exceptional requirement for mountain bike front hubs, so all current mountain bike hubs are suspension hubs.
Several terms are commonly used to describe different aspects of a bicycle suspension.
Travel refers to how much movement a suspension mechanism allows. It usually measures how much the wheel axle moves.
Preload refers to the force applied to spring component before external loads, such as rider weight, are applied. More preload makes the suspension sag less and less preload makes the suspension sag more. Adjusting preload affects the ride height of the suspension.
Rebound refers to the rate at which the suspension component returns to its original configuration after absorbing a shock. The term also generally refers to rebound damping or rebound damping adjustments on shocks, which vary the rebound speed. More rebound damping will cause the shock to return at a slower rate.
Sag refers to how much a suspension moves under just the static load of the rider. Sag is often used as one parameter when tuning a suspension for a rider. Spring preload is adjusted until the desired amount of sag is measured.
Lockout refers to a mechanism to disable a suspension mechanism to render it substantially rigid. This may be desirable during climbing or sprinting to prevent the suspension from absorbing power applied by the rider. Some lockout mechanisms also feature a "blow off" system that deactivates the lockout when an appropriate force is applied to help prevent damage to the shock and rider injury under high unexpected loads.
Bob and squat
Bob and squat refer to how a suspension, usually rear, responds to rider pedalling. Squat usually refers to how the rear end sinks under acceleration, and bob refers to repeated squat and rebound with each pedal stroke. Both are undesirable characteristics as they rob power from pedalling. Many suspension systems incorporate anti-bob, anti-squat, or "platform" damping to help eliminate bob.
Pedal feedback describes torque applied to the crankset by the chain caused by motion of the rear axle relative to the bottom bracket. Pedal feedback is caused by an increase in the distance between the chainring and rear cog, and it can be felt as a torque on the crankset opposite to forward pedalling.
Compression damping refers to systems that slow the rate of compression in a front fork shock or rear shock. Compression damping is usually accomplished by forcing a hydraulic fluid (such as oil) through a valve when the shock becomes loaded. The amount of damping is determined by the resistance through the valve, a higher amount of damping resulting from greater resistance in the valve. Many shocks have compression damping adjustments which vary the resistance in the valve. Often, lockouts function by allowing no or very little compression.
Unsprung mass is the mass of the portions of bicycles that is not supported by the suspension systems. At one extreme are road bicycles with no suspension in the frames, very little in the tires, and none in the saddles. By raising themselves off their saddles, riders may provide suspension with their knees, making their mass be sprung mass, but all of the mass of the bicycles remains unsprung mass. At the other extreme are full suspension mountain bikes. With front and rear suspensions the only parts unsuspended are the wheels and small parts of the front forks and rear chain-stays. Even then, as mountain bikes have large low-pressure tires which allow much more travel than small high-pressure road tires, the wheels are sprung to some extent as well.
In general, bikes are so light compared to their riders that travel is a much bigger motivator than unsprung mass in determining where to put the suspension and how much to use. The exception to this is that on recumbent and tandem bicycles where the riders are either unable to lift themselves out of their seat or unable to see in advance when that will be needed, the riders' mass can no longer be expected to be supported by their knees over road irregularities. These bicycles generally have some sort of suspension system to reduce unsprung mass.
Many newer mountain bikes have a full suspension design. In the past, mountain bikes had a rigid frame and a rigid fork. In the early 1990s, mountain bikes started to have front suspension forks. This made riding on rough terrain easier on a rider's arms. The first suspension forks had about 1½ to 2 inches (38 to 50 mm) of suspension travel. Soon after, some frame designers came out with a full suspension frame which gave riders a smoother ride throughout the ride.
Newer suspension frame and fork designs have reduced weight, increased amount of suspension travel, and improved feel. Many lock out the rear suspension while the rider is pedaling hard or climbing, in order to improve pedaling efficiency. Most suspension frames and forks have about 4-6 inches (100–150 mm) of suspension travel. More aggressive suspension frames and forks made for downhill racing and freeriding have as much as 8 or 9 inches (200 or 230 mm) of suspension travel.
Many riders still prefer to ride a hardtail frame, and almost all mountain bicycle riders use a suspension fork. Notable suspension fork manufacturers include Marzocchi, Fox Racing Shox, RockShox, and (to a lesser extent) RST, Suntour and Magura. Some bicycle manufacturers (notably Cannondale and Specialized) also make their own suspension systems to fully complement and integrate the bike set-up.
Although much less common, some road bicycles do incorporate suspensions, particularly the Soft Tail variety mentioned above. One example is Trek Bicycle Corporation's s.p.a (Suspension Performance Advantage) rear suspension, offered on some of their Pilot models, but the system was removed for the 2008 model year. Virtually all bicycles produced by Alex Moulton bicycles also have very effective full suspension, due to the low unsuspended mass of the small wheels and high pressure tires, a characteristic of the unconventional design of these bicycles. A recent design is the cantilevered 'swing shock' on some modern hybrid bikes.
Many recumbent bicycles have at least a rear suspension because the rider is usually unable to lift themselves off of the seat while riding. Single pivot is usually adequate when the pedaling thrust is horizontal - that is, forwards rather than downwards. This is usually the case provided the bottom bracket is higher than the seat's base height. Where the bottom bracket is significantly lower than the seat base, there may still be some pedalling-induced bounce.
Short-wheelbase recumbents benefit from front suspension more than long wheelbase recumbents because the front wheel (often small diameter which further magnifies the need for suspension) is taking a much larger portion of the loads than in a long wheelbase recumbent.
Softride and Zipp
The Softride Suspension System was launched at the Interbike 1989 bike show. The original SRS systems consisted of two foam filled fiberglass boxes bonded together with a viscoelastic layer. Originally intended for the use in mountain bikes, Softride produced its first full-fledged mountain bike, the PowerCurve, in 1991. During 1996 Softride released its first aluminum frame road bike, the Classic TT. The Softride Suspension System is used almost exclusively for triathlon racing. Softride ceased bicycle production in 2007 after the design was banned from UCI races.
A very closely related suspension design to the Softride is the Zipp 2001, a contemporary competing beam bicycle, where the suspension was in the hinge, rather than in flex of the beam itself.
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