Yeti SB130
TLR T2 Complete

PSI / Spring Rate

MTB suspension must accommodate a wide range of rider weight. To achieve this nearly all forks and rear shocks use an adjustable air spring.

By varying air pressure riders set a desired sag, or percentage of travel that the bike “sits into” under neutral loading.

Modifying this sag percentage will change small bump compliance as well as bottom out characteristics.

Learning how to Pre-Charge your suspension pump will help you make finite adjustments to sag.

Typically 15% sag is for a firm setup and 20% sag is considered plush.

Air Spring Fine Tuning Control(s): Volume Spacers & Compression

Volume Spacers
Air springs ramp rate, sometimes called Spring Rate, is based on how air compresses in the containment cylinder. As the air is compressed the PSI increases according to the decreasing volume of the containment. This is done via non-compressible volume spacers in the air chamber.

Compression.
Hydraulic damping assists air spring providing support in mid-stroke and bottom out.

Opposing Control: Rebound.
Higher spring rate will drive the suspension back to neutral with more force than a lower spring rate. To keep the bike from skipping across trail rebound damping is utilized.

Volume Tuning

As suspension compresses (travel reduces) a piston moves closer to the end of a sealed container. By decreasing the volume of the container the PSI pushing back on the piston is increased.

By adding or removing non-compressible volume spacers riders modify the air spring’s ramp rate which directly changes the amount of support and bottom out feel of the fork or shock.

By removing volume spacers a rider increases the available volume in the suspension. The larger volume compresses less per mm of travel resulting in a lower PSI per mm of travel. This creates less support and is utilizes more travel.

Adding volume spacers decreases the available volume. With less air volume the PSI per mm of travel increases providing more support and greater resistance to bottom out.

Opposing Control: Rebound.
Rebound may need to be adjusted for volume tuning depending on how drastic of a PSI change is being tuned in or out of the suspension.

“Helping” Control: Compression.
Compression provides additional support and bottom out resistance.

Compression

Compression controls provide damping to slow suspension travel as a load is applied and the suspension is compressed. (easy one right?)

When riders have found sag and volume spacing preferences compression controls provide minute adjustments to dial in performance. Adding compression provides more support allowing the suspension to ride taller in its travel which is important when dialing in a bike’s personality in corners, etc.

With too little compression a bike will sit deep in the travel. This compromises cornering and braking force resistance. Headtube angle, bottom bracket height, front to rear weight bias, etc are modified as a bike goes through its travel. Maintaining control of the use of travel is paramount for good performance.

Since compression hydraulically slows the suspension’s use of travel it therefor lowers the air spring’s PSI. Properly setting compression controls will help ease the load on the rebound system by controlling the air spring’s push back onto the rebound circuit.

Too much compression will cause a bike to feel harsh and not use appropriate amounts of travel.

Generally, compression settings are fine tuned after sag and volume spacing have riders “in the ballpark.”

Low speed compression controls mid-stroke as well as support in cornering and against brake dive.

High speed compression helps with bottom out and high shaft speed inputs.

3 positions switches are a type of compression circuit with Open the most plush, Mid providing some additional support and Firm for climbing. If you climb in firm remember to put it back to plush for the downhill or you’re in for a potentially rough ride.

Rebound

Rebound damping controls a suspension’s shaft speed returning to a neutral position. Or, how fast the air spring pushes back as the load changes.

More rebound damping slows the suspension by decreasing the amount of fluid allowed to pass through the hydraulic design.

Less rebound damping allows the suspension to return faster with less hydraulic restriction on the damper.

Rebound setting is based on weight, ground speed, terrain and aggression. Setting the rebound properly means finding the right frequency or feel for your riding.

If your rebound is too fast, or doesn’t have enough clicks of rebound, the bike will tend to skip and suffer poor small bump compliance.

When the rebound is too slow, or you have too many clicks of rebound, the suspension may “pack up” creating a harsh ride as each bump uses progressively more travel forcing the suspension deeper into the travel, which will have higher spring rates.

Opposing: Air Spring PSI / Spring Rate.

“Helping” Control: Low & High Speed Rebound.

Some suspension is designed with 2 rebound circuits. The High Speed Rebound circuit is designed to provide additional control resisting increased PSI late in suspension travel.

Typically High Speed Rebound settings are used as the Low Speed Rebound controls edge towards closed. Example: you might not use any clicks of High Speed Rebound until you reach “X” clicks on the low speed.

Air Spring & Volume Spacers

The FOX 36 provides riders a range of setup options using air spring as well as volume spacers.

Suggested sag is 15% for a firm feel and 20% for a plush fork. On the stock 150mm setup 15% equals 22mm or about 0.9 inch of sag. 20% will use 30mm or about 1.2″ of sag.

Riders who choose the 160mm Lunch Ride option will find the sag settings quite close to the 150. 15% is 24mm while 20% is 32mm.

Volume Spacers

The 150mm FOX 36 is factory spec’d with 2 volume spacers (bike manufacturers may or may not change this). The 150mm fork can carry a maximum of 7 volume spacers.

At 160mm the factory spec is 1 volume spacers with a maximum of 6.

Do not install more volume spacers than the FOX advises. Installing more than the maximum volume spacers will result in  product damage and potential for injuries, etc.

FOX 36 GRIP2 Rebound Controls

The GRIP2 damper provides both Low (LSR) and High (HSR) rebound controls.

The addition of the High Speed or HSR control provides increased rebound control to account for the higher PSI produced by aggressive or heavier riders.

Typically riders will adjust LSR, Low Speed Rebound, to suit riding style and taste and refer to FOX’s guide for the appropriate HSR, High Speed Rebound, pairing.

FOX GRIP2 Compression Controls

The FOX GRIP2 damper provides Low and High speed compression controls to fine tune support.

Compression circuits hydraulically damp (or slow) the fork’s input assisting the air spring in providing appropriate mid-stroke and bottom out feel.

Low Speed Compression helps provide mid-stroke support. This allows a bike to ride taller in the travel in cornering while resisting brake dive, rider weight shift and other slow shaft speed inputs.

High Speed Compression works to fine tune bottom out feel as well as other high shaft speed inputs also known as square edge bumps. Example: if you sprint straight into a curb you’re likely to engage the High Speed Compression even if you’re not using full travel. This is due to the speed of the shaft moving oil to compensate for the hit.

Air Spring & Volume Spacers

The Yeti SB130 uses a 210 x 52.5mm shock or a 210 x 55mm shock for the Lunch Ride.

The spec’d Fox Float Factory DPX2 is the most popular based on its wide variety of terrain capacity. Enough shock for the bigger stuff while poppy on your trail adventures.

SAG is adjusted by PSI – typically Enduro riding styles gravitate to 25-30% sag. This would measure about 17mm for a plush setup.

Volume Spacers

Volume spacing provides fine tuning options to support the air spring.

By adding volume spacers, thus reducing the volume, you increase the air spring’s ramp rate for improved bottom out support and pop.

Conversely removing volume spacers produces a more linear feel as the air has more volume during shock compression per mm of travel.

Do not install more volume spacers than the FOX advises. Installing more than the maximum volume spacers will result in  product damage and potential for injuries, etc.

FOX Factory DPX2 Rebound Control

The FOX Factory DPX2 provides a low speed rebound control.

As rider ground speeds increase they will allow the rebound to “speed up”.

The higher PSI the more rebound control a rider will need to control the more forceful air spring.

Rebound should be set to allow the suspension to return to neutral before the next terrain feature without being so fast as to “skip” across the trail.

FOX Factory DPX2 Compression Controls

The FOX Factory DPX2 has a 3 position Open, Mid, Firm switch as well as external Low Speed Compression adjustment. The Low Speed Compression adjustment is located in the center of the 3 position switch and is controlled with an allen wrench.

The 3 way switch is great for small adjustments on trail. The Low Speed Compression control allows riders to supplement the air spring support with hydraulic damping. This produces a bike with more support in the mid stroke allowing it to ride taller in the travel. Riding taller in the travel is great for high speed, high g-force cornering as well as attacking steep terrain.