How To: Inspect Your Bike
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Air, Steel and Titanium – Mountain Bike Suspension Explained
Air fork and coil shock with titanium or steel coils? We’ll go over the characteristics of different spring media for mountain bikes and e-bikes.
These days, most mountain bikes and E-MTBs are equipped with air suspension. You probably see steel coils all the time, especially on downhill and enduro bikes. Is it as simple as riding with an air shock on your downcountry bike and a steel coil only on your downhill rig? Apparently there is disagreement about which suspension medium is best. In fact, all current options have their advantages, and deciding on which one is right for you is a matter of personal preference. It helps to know the basic properties of air suspension and metal coils, so we’ll give an overview here. Even though forks with steel coils have become rare, the technical aspects apply to both suspension forks and rear shocks. If you want to learn more about the many adjustment options of modern suspension elements, then be sure to check out our article on suspension parameters.
What does suspension stiffness, characteristic curve, progression and sag mean? What are the advantages of air suspension on MTBs? What is negative spring? What are coil springs and what advantages do they offer? Material Question: Steel, titanium or special steel coils? Finding the right suspension stiffness Special Case: Elastomers and carbon leaf springs Which suspension type for which application?
Air or steel? The decision should always be based on your area of use and riding style.
Find out here which advantages which technology offers!
Basic Terms: What does suspension stiffness, characteristic curve, progression and sag mean on the MTB?
When we speak of the spring medium, it is important to distinguish between suspension and shocks. Regardless of the spring medium, all suspension types perform the same task: they deform elastically when force is applied and rebound to their original shape shortly afterwards. In the process, they absorb energy and release it again. How far your bike compresses and rebounds under a certain force is determined by the suspension. How fast or slow it bounces is controlled by the shock. If you want to go more in-depth, have a look at our article on the topic: "How Does Suspension on a Mountain Bike Work? Springs and Shocks Explained.” All types of springs and coils are described by some parameters, which we would like to explain briefly in advance:
| The suspension stiffness (also called spring rate) determines how much force or weight must act on the spring in order to compress or stretch the spring element by a defined amount. |
| The characteristic curve represents the suspension stiffness over the course of the travel. In the case of a linear characteristic curve, this means that your suspension fork compresses twice as far when twice the force is applied. A spring with a progressive characteristic, on the other hand, requires more than twice as much force to compress it twice as far. |
| The sag quantifies how far your spring medium is compressed under your weight without any other forces acting on it. It is usually indicated as a percentage of the total suspension travel and allows your suspension components to rebound further when the load is relieved. For example, if you ride through a pothole, your wheels can keep contact with the ground. |
What are the advantages of air suspension on MTBs?
Suspension forks and shocks with air suspension are now widely used on mountain bikes of all kinds. The reason for this is due to two huge advantages. One is the possibility to precisely adjust the suspension stiffness to the rider's weight and other factors via the air pressure. You can also do this on short notice without much effort, for example to adjust your suspension for a trip into particularly steep terrain or to balance the luggage on a bikepacking tour. The other advantage is that air suspensions are significantly lighter: unlike metal coils, air weighs nothing. The basic principle of air suspension is simple: you fill a tight air chamber through a valve with an air pressure that keeps your weight statically in the sag. During compression, the volume changes: the air is compressed by a piston. Unlike coil springs, air suspension has a progressive characteristic curve. It thus provides a certain amount of breakdown protection towards the end of the travel. Since the progression of air suspension depends on the air chamber volume, you can individually adjust the characteristic curve of most high-end forks and shocks with volume spacers. RockShox calls these spacers "Bottomless Token" and at Fox Racing Shox they are called "Air Volume Spacer". More spacers reduce the air volume and thus increase the progression. Other systems, such as the "Total Tune Spring Curve System" (TTSC) by Öhlins or "Ramp Control" by MRP are somewhat more complex in design, but function more dynamically and promise even better fine-tuning options.
An air suspension system can be easily adapted to the rider’s requirements and the terrain without much effort. © bc GmbH
What is negative spring?
Practically all air suspension elements today have a so-called negative spring. This is not about negative spring travel (see also sag), but about eliminating a fundamental problem of air suspension: Since a simple air chamber is under pressure even when fully rebounded, a certain amount of force is needed to get it moving at all. Early air suspension forks therefore had a rather poor response and a stop was clearly noticeable when rebounding. On the other hand, modern suspension components make do with negative spring. It counteracts the pressure in the positive air chamber in the rebound state and virtually cancels it out. This allows your spring element to react to forces that are actually smaller than the air pressure in the main chamber allows. Since the negative spring is reduced, it only acts at the beginning of the spring travel and has practically no influence beyond that. Most shocks and forks use a second small air chamber as a negative spring. The air pressure is automatically balanced with the main chamber during compression. Some manufacturers use a small steel or titanium coil instead and thus manage with fewer seals.
What are coil springs and what advantages do they offer?
Titanium or steel coils are still found on mountain bikes for harder use – especially on the shock, where they are immediately noticeable. The spring medium is a so-called compression coil (also: coiled torsion spring), which sits on the damper and is compressed. The advantages of metal coils lie primarily in their performance on the trail:
- The response is particularly sensitive, which not only increases the comfort but also the grip of your bike.
- Since the system requires fewer seals than an air suspension, it has less friction and a lower breakaway torque.
- In contrast to air suspension, the suspension stiffness of a steel or titanium coil remains constant, even if the system heats up due to friction during a long descent or if the outside temperature and air pressure fluctuate. For example, in the downhill final you still have the same spring behaviour at the end of a demanding course. Air shocks can sometimes reach their limits more quickly in extreme situations such as racing.
- The less complex structure of a metal coil also makes maintenance easier. For example, fewer seals need to be replaced.
- Most coil springs have a linear characteristic, which helps you to make full use of the travel. This is assuming that the linkage design of the frame is compatible with a coil shock.
Material Question: Steel coil, titanium or special steel?
Most coil springs are made of spring steel. They are inexpensive, which is especially advantageous if you want to try out different suspension stiffnesses. Titanium coils, on the other hand, are expensive but much lighter. They are also rustproof – even without coating. For a few years now, some manufacturers have also been offering special steel coils that are lighter in construction and can often be seen on the MTBs of professional World Cup riders. They range in price between titanium and simple steel, but are often even lighter than titanium. An example of this are the coils with the name suffix "Super Light Steel" (SLS) from Fox.
Steel coil suspension systems are particularly sensitive and keep the suspension stiffness constant even after several hours. © bc GmbH
Finding the right suspension stiffness
The suspension stiffness of coil springs depends on the alloy, the diameter and the number of coils, as well as the cross-section of the wire. For mountain bike suspension components, it is usually given in pounds per inch (lb/in or just lbs for short). The number therefore indicates how many imperial-system pounds (1 lb equals about 454 grams) must act on the spring to compress it one inch (25.4 millimetres). Determining the right suspension stiffness for your bike depends primarily on your weight and the amount of sag you’re looking for. In the bike’s rear, the gear ratio of your frame, the suspension travel and the weight distribution between the front and rear wheel also play an important role. It is important to install a coil with the right stiffness, as you cannot adjust it afterwards. The preload does not change the stiffness, it only shifts the characteristic curve upwards and thus worsens the response. If possible, you should only use the preload to install your coil without play and rattle. If you feel that you are too low in the travel, then you should install a stiffer coil instead of increasing the preload.
Tip: Some shock manufacturers provide online calculators that make it easier for you to determine the suspension stiffness. If you are unsure, please contact us!
Special case of elastomers and carbon leaf springs
In addition to metal and air suspension, elastomers used to play an important role on mountain bikes. Today you will find such shock-absorbing elements made of plastic at most in minimalist systems on cross-country softtails, gravel bikes, on kids or folding bikes. Other niche components include forks with carbon leaf springs, but these are also hardly comparable with full-suspension mountain bike forks.
Which suspension type for which application?
The big question remains: titanium or steel coils or air suspension? As is often the case, only you can set your own priorities. Do you want to keep your bike light, or are you flexible with the setup? Then air is probably the better choice. On the other hand, do you attach more importance to the best response, maximum grip and having both of those at the end of a long, demanding stage in an enduro race or on a downhill course? Then it might be worth switching to a steel coil. When making your decision, however, be sure to follow the recommendations and approvals of the manufacturer of your bike. A linkage design intended for progressive air shocks might tend to rush and sag through the travel with a steel coil shock. In the end, apart from all the factual aspects, the decision is also a matter of taste. On the one hand, downhill world cups are now being won with air shocks, and on the other hand you see happy bikers on after-work tours with steel coil shocks on their downcountry bikes.
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