Mountain bike suspension: What does SAG mean on an MTB, which terms are used for suspension forks?
SAG = negative suspension travel on MTB
The negative suspension travel - better known as SAG on mountain bikes - is the part of the suspension travel that is compressed by the rider's weight alone. Normally, the SAG of an MTB suspension fork and shock should take up between 15-30 % of the maximum available suspension travel. The general rule is: the less SAG, the firmer but also less sensitive the suspension of the bike. You can adjust the SAG via the spring rate (see below). You can find a detailed article on the subject of "How do I set my mountain bike suspension correctly" here here.
Spring stiffness: What is it?
On high-quality mountain bike shocks and MTB suspension forks, the spring stiffness can be adjusted to the rider's weight via an air pressure chamber. More pressure in the air chamber of the spring element means a firmer suspension. The optimum spring rate is a set SAG of 15-30 % (see also SAG/negative suspension travel above). Downhill or freeride bikes can also use steel springs instead of the air chamber to adjust the spring rate. To adjust the spring stiffness of steel spring elements, you can work with the preload of the spring to a certain extent. If the springs are completely unsuitable for the riding weight, the entire spring must be replaced.
Compression for MTB suspension
The compression stage (damping) should not be confused with the spring rate. The compression stage determines the compression speed of the spring element and is regulated via a separate oil circuit. At full compression, the fork or damper compresses very slowly, giving the suspension a firm, sporty feel and providing the rider with clear feedback about the surface. Inexpensive suspension forks and shocks have a preset compression stage. Expensive models allow the customer to adjust the compression damping themselves, usually using blue knobs. Enduro and downhill forks are also divided into high-speed and low-speed compression stages.
This is the low-speed compression stage
The low-speed compression damping is required when riding through large bumps, compressions or berms. It regulates the slow, soft loads on the suspension of the mountain bike. The harder it is set (rotary knob on plus), the firmer the MTB suspension remains in the situations described.
High-speed compression stage
The high-speed compression damping is stressed when the suspension is subjected to fast, sudden loads, e.g. when landing after jumps, fast root passages or when rolling over off-road edges. If you turn the adjustment knob for the high-speed compression stage towards "plus", the MTB suspension fork hardens in the situations mentioned. Racers prefer a high compression setting in order to create as much pressure as possible between the tyre and the ground. However, too high a high-speed compression setting in the suspension fork has a tiring effect on the upper body on long descents.
Rebound
Like the compression stage, the rebound stage is regulated via an oil circuit, determines the rebound speed of the MTB suspension and, like the compression stage, is part of the damping. As a general rule, the rebound of the MTB suspension elements must also be increased with a higher rider weight and the associated higher spring rate. If you compress the suspension fork when stationary and take your hands off the handlebars when compressed, the front wheel should not lose contact with the ground when the fork rebounds. If the front wheel still bounces slightly, the rebound should be increased (usually via a red knob on the underside of the suspension fork). The same applies to the rear wheel on MTB fullys. If too little rebound is set in the suspension, the bike bounces back and forth nervously on the trail and is much harder to control.
Platform on suspension fork/damper
When the platform is switched on, the compression damping in the shock or fork is significantly increased in order to prevent drive influences or bobbing of the suspension when riding uphill on an MTB. Not all suspension elements have such a platform mode.
Remote / Handlebar remote control
Remote is the term used to describe the handlebar remote control of suspension elements - but also of lowerable seat posts. A lever mounted on the handlebars can be used to lock the suspension - or lower a seatpost - via a cable or hydraulic line. If you can't find your way around the manufacturers' abbreviations, you can Here are the most common suspension abbreviations read more.
Rear suspension systems on mountain bikes
Single-link (supported single-link) rear triangle on MTBs
Fullys (= full-suspension mountain bikes), which essentially only have one joint around which the rear triangle rotates, are referred to as single-pivot bikes. If additional levers or bearings are installed that only serve to support the rear triangle but do not change its wheel lift curve, they are referred to as supported single-pivot bikes.
Four-link rear suspension for MTBs
A classic four-link rear triangle on a mountain bike is characterised not only by the number of four joints but also by the Horst Link. The Horst Link is a joint that sits in the chainstay below the wheel axle and its movement allows the rear triangle to rotate upwards during compression. In addition to classic four-bar linkages and single-bar linkages, there are numerous other rear suspension systems such as the VPP or DW-Link rear suspension. As a general rule, the number of joints does not indicate the function of the rear triangle.
VPP rear triangle / virtual pivot point
Fullys with a virtual pivot point (also four-link with Horst link) also have a reference point around which the rear wheel axle (A in the picture) moves, but this moves as the suspension compresses. If the levers that connect the rear triangle to the main frame are extended in an imaginary line, the virtual pivot point (IC in the picture) can be determined at the point of intersection. If the rear triangle compresses further, the centre of rotation moves backwards and downwards on a circular path. The position in the SAG area is particularly important for VPP systems of this type.
These standards exist on mountain bikes:
Chain line
The chainline describes the distance from the chainring to the centre of the bottom bracket. For triple drivetrains, this refers to the centre chainring, for double drivetrains to the large chainring. With a low chainline of around 47 mm, the chain has less skew, especially in the lower gears, making the drivetrain more efficient. The normal chainline is 49 mm. The 2015 introduced Boost standard has a chainline of 52 mm, together with the wider rear hub (cassette offset to the outside).
Thru axles
Thru axles are replacing the long-proven quick-release system for mounting wheels on mountain bikes. The precise fit and increased rigidity compared to the quick release are their greatest advantages. However, there is no uniform axle standard. On the front wheel, there are 110x20 mm, 100x15 mm and 110x15 mm (Boost dimension) thru axles. On the MTB rear wheel, the X12 axle with 142x12 mm or the new Boost standard introduced in 2015 with 148x12 millimetres predominate.
Direct Mount (front derailleur standard)
The direct mount standard is now the most commonly used front derailleur standard on modern mountain bikes. Here, the front derailleur is mounted on a special base on the seat tube with an Allen screw. The base prevents the front derailleur from twisting. Other standards for front derailleurs on mountain bikes are E-Type or conventional mounting using a front derailleur clamp, which is screwed around the seat tube.
ISCG (standard for chain guide)
ISCG is the abbreviation for "International Standard Chain Guide" and describes a mounting option for chain guides on the frame. The latest standard (as of 2015) is the ISCG 05 version. Here, chain guides can be attached to the frame with three bolts around the bottom bracket.
Q-factor for bicycle cranks
The Q-factor describes the width of the crank. Due to the wide tyres on fat bikes, wider cranks must also be installed to create space for the rear triangle. This is why they have an increased Q-factor. Normal values for the Q-factor on modern mountain bikes are between 156-173 millimetres. If you want to know exactly, you can find here a detailed article from BIKE.
MTB tyre lexicon: You should know these terms!
Carcass
The general structure of the MTB tyre - regardless of the tread and rubber compound - is known as the carcass. The carcass varies greatly depending on the area of use. For heavy enduro tyres, puncture resistance and therefore a particularly stable carcass are paramount. For cross-country tyres, significantly lighter carcasses are used to generate high comfort values and better rolling characteristics. If you are thinking about buying new tyres, you will find here a detailed buying guide to MTB tyres.
Rubber compound
It is decisive for the rolling resistance and the properties of an MTB tyre. Softer rubber compounds roll worse and wear faster, but have better damping and, above all, better wet grip. In general, front tyres on mountain bikes are ridden in a softer compound to build up grip. A harder compound is often chosen for the rear MTB tyre in order to keep rolling resistance and wear to a minimum. Some manufacturers use different compounds in different areas of a tyre. For more information, see "Compound".
EPI / TPI
The abbreviation for end per inch or threads per inch indicates how finely and tightly woven a tyre casing is. The higher the value, the finer the threads and the lower the weight. Typical values are 60 to 127 TPI. Enduro and downhill tyres generally have a coarse 60 TPI, while cross-country tyres have finer and more flexible casings up to 127 TPI.
PSI / BAR
Pounds per square inch is a pressure specification. In German-speaking countries, the specification in bar is more common. 14.5 PSI corresponds to a pressure of one bar.
Protective belt / Apex
A protective belt is designed to prevent punctures and usually consists of an additional layer of fabric or rubber under the tread or in the sidewall. The reinforced sidewall is intended to make the tyre resistant to sharp-edged stones, while the rubber protection in the sidewall (called the apex) is intended to minimise punctures on the rim flange.
Tubeless-Ready or Easy
In addition to the classic tubeless or UST tyres, there are more and more so-called tubeless-ready tyres. These mountain bike tyres can be ridden with a tube or in combination with a sealing latex compound. In addition to a specially shaped bead, a smaller inner diameter is intended to make fitting easier.
Tyre size
The tyres are usually marked with an inch diameter and width (e.g. 27.5 x 2.4) and a value according to the European standard ETRTO (62-584). The first value stands for the width, the second for the inner diameter of the MTB tyre in millimetres.
Rubber hardness / Shore
The Shore unit describes the hardness of a rubber compound. The lower the value (e.g. 40 A), the softer the compound.
Compound
Compound means mixture or combination. Some bicycle tyres have a multiple rubber compound within a tyre in order to better reconcile different requirements such as easy rolling, good traction and puncture protection. For example, harder rubber compounds in the central area of the tread are intended to reduce rolling resistance and wear, while softer compounds on the outer sidewalls increase cornering grip. While Schwalbe and Maxxis, for example, offer mountain bike tyres with three different rubber compounds, the first tyre models with four compounds are already available (Vittoria). In contrast, Continental relies on single-compound tyres.
Spoke angle and wheel symmetry
A stiff and durable wheel is always the result of several factors: spoke tension, spoke angle and symmetry are just the most important, along with the individual components such as hubs, spokes and rims. The angles of the hub flanges and the nipple holes form a kind of triangle. The two base angles on which the wheel is "supported" are very large, while the angle on the rim is very small. The greater the flange distance at the hub, the more "wide-legged" and therefore more stable the wheel is. And this is exactly what Boost does by creating the space to space the hub flanges further apart - by 10 millimetres at the front and 6 millimetres at the rear. However, because the cassette needs more space on the drive side, as do the brake disc sockets, the wheel is still asymmetrical even with Boost. The symmetry problem and the resulting unequal support is usually compensated for by tightening the spokes on the drive side at the rear and the spokes on the brake side at the front. However, unequal tension always has a negative effect on the durability of the overall wheel system.
What is the Boost standard on MTBs?
From the open standard Boost 148/110, which was originally Trek together with Sram for the 2015 season has become a new trend in mountain bikes and will be an integral part of modern bikes in 2023. This involves wider axle spacing - 148 instead of 142 millimetres at the rear and 110 instead of 100 millimetres at the front. The idea is that this increases the stiffness of (29-inch) wheels because the hub flanges move outwards by a few millimetres on each side and the spokes are supported more widely. The developers expect Boost to provide more precise handling, and the technology also allows more tyre clearance and shorter chainstays. Annoying for mountain bikers who still have older bikes in their cellar: Existing wheels are no longer compatible with Boost frames and suspension forks with Boost dimensions.
