While it’s difficult to categorize full suspension bikes precisely, most fall into one of two broad categories – cantilever and linkage. There are examples of both types that are a dream to ride and others that are a nightmare.
Early full-suspension bikes were a cantilever design: the rear wheel is connected to a swingarm rotating around a single pivot near the bottom bracket, with the shock connected directly to the main frame. The design’s strong advantages – a single pivot and few moving parts – make the frame light, durable, stiff and strong without costing the earth. It is still in use – see the Santa Cruz Bullit. The disadvantage of the cantilever design is it is difficult to control the effects of chain reaction. Wherever you place the pivot, the rear wheel moves in an arc around it so that, as the axle moves through its travel, the distance between axle and chainset changes appreciably.
The addition of linkages adds extra pivots, changing the shape of the rear end of the bike as it moves through its travel. Linkage bikes come in a few different types, including, ’four bars’ and ’faux bars’ and Active Braking Pivot. All rely on smooth-running pivots, which appreciate regular cleaning and lubrication and suffer badly if subjected to jetwashing.
Faux bars are essentially link driven cantilever systems, as the chainstay or swingarm still moves around the main pivot in an arc. The advantage of adding linkages to drive the shock is that this allows the designer to fine-tune the leverage ratio – the relationship between shock movement and wheel displacement.
Faux bar bikes can be identified by the position of the rearmost pivot, just above the rear axle. Manufacturers currently using this design include Kona and Commencal.
Four-bar bikes have the rearmost pivot under the rear axle. This is often called a Horst link, after the designer who pioneered it. In this position, the linkage works as a parallelogram, giving the rear axle a near-vertical axle path. This effectively isolates pedalling and braking forces from the suspension action. An enduring example is
the Specialized FSR.
A newcomer design comes from Trek, and is called the Active Braking Pivot. In this set-up, the rearmost pivot rotates around the rearmost hub. This isolates the suspension even further from braking and pedalling forces, but requires a custom rear throughaxle or quick release.
Short link four bar designs utilize two very short linkages between the main frame and the rear triangle to create a parallelogram. Tinkering with the configuration of each link allows designers to customize the path the axle will take under vertical load, varying the extent to which braking and pedalling forces interact with the suspension reactions. Examples include VPP (Virtual Pivot Point) models from Santa Cruz and Intense, Giant’s Maestro system and Marin’s Quad-Link system.
Types of shock
As with suspension forks, rear shocks come in several different styles, while the two main components remain the same – some form of spring and some form of damping. The spring can be air or coil, and the damping can be air, oil or a mixture of oil and gas. An air spring is stiffened by pumping more air into the spring chamber. The volume of air is small, so adding a tiny amount makes a big difference: use a suspension pump with a gauge for the job. Bleed air off either by using the bleed valve on the pump (if you have one), or by removing the pump and depressing the pin in the middle of the Schraeder valve, then using the suspension pump to go back up to the right pressure. Don’t overpump the shock, you will damage it. The maximum pressure the shock will take is almost always printed on the shock body, otherwise check your manual.
Coil springs have a steel spring wound over a damping unit. The preload is adjusted by turning the plate that supports one end of the shock along a thread, squashing the spring. Springs don’t have a huge weight range and work much much better if you don’t put too much preload on them. Don’t be tempted to crank up the preloadadjuster on a spring that’s too soft to get the right sag. As a general rule, use no more than two turns of the preload-adjuster to get the right sag, unless your manual specifically says that you can use more.
Air shocks are slightly lighter than coil spring shocks, although using titanium springs rather than steel ones can decrease the weight difference. It is easier to adjust air shocks for a wide range of rider weights, whereas coil springs are only adjustable within the range of the spring – riders who weigh more or less than average will need to swap springs, whereas air can simply be added or released.
Coil and air shocks do feel different though. Coil springs are very supple and are often the preferred choice. If you don’t weigh much you might like air springs. If you carry a shock pump with you, it will cancel out most of the difference in weight between air and coil.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
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