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Air or Steel? Mountain Bike Suspension Explained
Mountain bikes and E-MTBs come with suspension elements that rely on air or steel as the spring medium - or even titanium. We'll explain the differences.
Most mountainbikes and E-MTBs today feature an air suspension: When compressing, air in the air chamber of suspension forks and shocks is compressed.
But MTB air suspension is not the only option. Especially in the demanding gravity segment, for downhill, freeride, and enduro bikes, suspension elements with coil springs (professionals simply say "coil") are still state-of-the-art. More precisely: helical compression springs made of steel or, less commonly, lighter and more expensive: made of titanium. Coil springs serve the same function as air springs. Both types of suspension have their specific benefits.
Which one is right for your MTB or E-mountain bike depends on your preferences and needs. In this blog post, we're shedding light on the complexities of suspension technology, clarifying basic concepts, and answering the question: "Which suspension for whom?"
If you want to learn more about the right setup for your suspension, best check out our article on damping know-how.
Basic concepts: What do spring stiffness, characteristic curve, progression, and sag on an MTB mean? What benefits do air suspensions offer on an MTB? What is a negative spring? What are Coil Springs and what benefits do they offer? Material question: Coil spring, titanium, or special steel? Finding the right spring stiffness Special case: Elastomers and Carbon leaf springs Which spring 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 concepts: What do spring stiffness, characteristic curve, progression, and sag on an MTB mean?
When we talk about the spring medium, it's important to distinguish between suspension and damping. Regardless of the spring medium, all springs serve the same function: They elastically deform under force, to then return to their original shape. In this process, they absorb and release energy. How far your bike compresses and rebounds under a certain force is dictated by the suspension. How quickly or slowly it does so is controlled by the damping. If you're interested in going into more detail, check out our article on the topic: “How Does Suspension on a Mountain Bike Work? Springs and Shocks Explained!”. All types of springs are described by some characteristics that we'll briefly explain:
| The spring stiffness (also spring constant) dictates how much force or weight needs to be applied to the spring element to compress or extend it by a defined distance. |
| The characteristic curve shows the spring stiffness over the course of the spring travel. In the case of a linear curve, it means that your suspension fork compresses twice as far when twice the force is applied. For a spring with a progressive characteristic curve, on the other hand, more than twice as much force is necessary to compress it twice as far. |
| The sag quantifies how far your suspension element is compressed under your weight without any other forces acting. It's often expressed as a percentage of the total suspension travel and allows your suspension elements to extend further when unloaded. For example, if you ride through a pothole, it helps your wheels maintain ground contact. |
What benefits do air suspensions offer on an MTB?
Forks and shocks (also called strut) with air suspension are now widely used in mountain bikes of all categories. The two biggest reasons for this are probably their advantages: On one hand, there's the possibility to precisely adjust the spring stiffness via the air pressure according to the rider's weight and other factors. You can also do this short-term, without much effort, to, for example, adjust your suspension for a trip into particularly steep terrain or to compensate for luggage during a bikepacking tour. On the other hand, air suspensions are significantly lighter: In contrast to metal springs, air weighs nothing. The basic principle of air suspension is simple: Through a valve, you fill a tight air chamber with an air pressure that statically holds your weight in sag. When compressing, the volume changes - the air is compressed with a piston. Unlike coil springs, air suspension therefore has a progressive characteristic curve: the further an air spring is compressed, the stiffer it becomes. Towards the end of the suspension travel, it thereby provides a certain bottom-out protection. Since the progression in air suspension depends on the volume of the air chamber, you can tune the characteristic curve in most high-end forks and shocks with so-called volume spacers. RockShox calls these spacers “Bottomless Tokens” and at Fox Racing Shox they're called “Air Volume Spacers”. More spacers reduce the air volume and thus increase the progression. Other systems, like the “Total Tune Spring Curve System” (TTSC) from Öhlins or “Ramp Control” from MRP, are a bit more complex in construction 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 a negative spring?
Practically all air suspension elements today have what's called a negative spring. This isn't about negative travel (see also Sag), but rather solving a fundamental issue with air springs: Since a simple air chamber is under pressure even when fully extended, it requires a certain amount of force to get it moving. Early forks with air suspension therefore had rather poor responsiveness, plus there was a noticeable stop when expanding. Modern spring elements get around this by using the negative spring: It counteracts the pressure in the positive air chamber when extended, essentially neutralizing it. This way, your suspension can respond to forces that are actually smaller than what the air pressure in the main chamber would allow. Since the negative spring is designed to be much smaller, it only acts at the beginning of the travel and has virtually 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 when compressing. Some manufacturers instead use a small steel or titanium spring, which requires fewer seals.
What are Coil Springs and what benefits do they offer?
On mountain bikes built for tougher use, titanium or steel springs – especially on the shock, where they are immediately noticeable – are still common. The spring medium is a so-called coil spring that sits on the shock and is compressed when the bike is compressed. Although not technically correct, it is colloquially often referred to as a spiral spring. The advantages of metal springs are mainly in trail performance:
- The responsiveness is especially 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.
- Unlike air suspension, the stiffness of a steel or titanium spring remains constant, even if the system heats up due to friction during a long descent or if the outside temperature and air pressure fluctuate. This way, for example, you still have the same suspension behaviour at the end of a demanding track in the downhill finale. Air shocks can reach their limits more quickly in extreme situations like race use.
- The less complex design of a metal spring also makes maintenance easier. For example, fewer seals need to be replaced.
- Most coil springs have a linear characteristic curve, which helps you fully utilize the travel – provided the frame's kinematics are suited for a coil shock.
Material question: Coil spring, 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 spring stiffnesses. Titanium springs, on the other hand, are quite expensive but significantly lighter. They are also rust-free – without any coating. In recent years, some manufacturers have also been offering special steel springs that are lighter in design and often seen on the MTBs of World Cup pros. They are priced between titanium and basic steel but are often even lighter than titanium. An example of this are the springs with the "Super Light Steel" (SLS) suffix from Fox.
Steel coil suspension systems are particularly sensitive and keep the suspension stiffness constant even after several hours. © bc GmbH
Finding the right spring stiffness
The stiffness of coil springs depends on the alloy, as well as the diameter and number of coils, and the cross-section of the wire. For mountain bike suspension elements, it is usually specified in Pounds per Inch (lb/in or simply lbs). The number indicates how many pounds (1 lbs is approximately 454 grams) must act on the spring to compress it one inch (25.4 millimeters). The right stiffness for you and your bike mainly depends on your weight and the Sag you want. At the rear wheel, the transmission ratio of your frame, the travel, and the weight distribution between front and rear wheel also play a significant role. Installing a spring with the right stiffness is important because you can't adjust it afterward. Preloading doesn't change the stiffness; it only shifts the characteristic curve upwards, thereby worsening the responsiveness. You should only use preloading to ensure your spring is mounted without play or rattling. If you feel like you're sitting too deep in the travel, you should install a stiffer spring instead of increasing the preload.
Tip: Some shock manufacturers offer online calculators to make it easier to determine the right spring stiffness. If you're unsure, feel free to contact us directly!
Special cases: Elastomers and carbon leaf springs
Besides metal and air suspension, elastomers used to play a significant role in mountain bikes. Today, such shock-absorbing elements made of plastic are found at most in minimalist systems on cross-country softtails, gravel bikes, on children's or folding bikes. Another niche is forks with carbon leaf springs, but these are also hardly comparable to full-fledged mountain bike suspension forks.
Which spring for which application?
The big question remains: Titanium or steel spring, or air suspension? As often is the case, only you can set your own priorities. Do you want to keep your bike light or be flexible in your setup? Then air might be the better choice. On the other hand, if you value the best responsiveness, maximum grip, and that too at the end of a long, demanding stage in the enduro race or on the downhill track? Then switching to steel might be worth it. But definitely pay attention to the recommendations and approvals from your bike's manufacturer. A kinematics designed for progressive air shocks might tend to blow through the travel and bottom out with a coil shock. In the end, besides all objective aspects, the decision is also somewhat a matter of taste. On one hand, downhill World Cups are now being won with air shocks, and on the other, you see happy bikers on leisure rounds with coil shocks on downcountry bikes.
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