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Air Forks: Pros and Cons
A mountain bike’s suspension fork plays an important role in the handling and comfort of the bike. Air forks have been standard in almost every discipline of mountain biking for years. If you walk into a bike shop, you’ll find that almost all mountain bikes come with air forks. This guide outlines the pros and cons of using an air fork for mountain biking.
What is an air fork?
Air forks take advantage of the compression resistance of compressed air for shock absorption. The springs in the fork are supplied by compressed air sealed in an airtight chamber inside the fork arm. This part is called an air spring. When air is compressed, it resists further compression. Air forks are used on all types of mountain bikes, including trail riding, downhill, trail, enduro and freeride.
The air fork has a progressive spring rate. This means that the fork becomes harder to compress as it travels through its entire range. At the start of the range, the suspension is soft. As the fork is compressed, further compression becomes more difficult. For example, if the fork needs 500N to compress 50mm, it might take 1500N to compress 100mm.
The spring rate of an air fork can be adjusted by changing the pressure in the air chamber. Increasing the pressure increases the spring rate (making the fork stiffer). Lowering the pressure lowers the spring rate (making the fork softer). Adjusting the spring rate of an air fork requires a special high-pressure pump called a shock pump. You just connect the pump to the American valve on the fork and add or remove air. You can also adjust the fork preload and sag by changing the pressure. There is no preload dial like on the coil forks. Most air forks can also be locked so they don’t compress
Most air forks also allow you to adjust the process by changing the volume of the air chamber. This is accomplished by adding or removing plastic spacers within the air chamber. Adding spacers reduces the volume in the air chamber, while removing spacers increases the volume. Reducing the volume of the air chamber makes the fork more advanced or harder to compress early in the range.
The air fork also has a damping system. This smooths the ride and prevents the fork from bouncing after compression or rebound. Most models allow you to adjust compression and rebound damping.
Air fork advantages
1. Air forks allow you to set an accurate spring rate
You can adjust the exact spring rate or shock rate you want by changing the pressure in the fork’s air chamber. Increasing the pressure makes the suspension stiffer. Let the air out to make it softer. You can easily set accurate spring rates by changing the PSI in the shock using shock pumps and gauges like GIYO high pressure shock pumps.
The same adjustment sets the fork sag. This adjustment makes it easy to optimize the fork for different terrains, rider weights and ride feel. On the other hand, the coil forks cannot be set precisely. You need to replace the coil to change the spring rate.
2. Air forks allow you to adjust your progress
As mentioned above, air forks are progressive. This means they become hard to compress throughout the travel range. You can set the progression of the air fork by adding or removing spacers in the air chamber of the fork. These spacers change the volume of the air chamber. Adding spacers reduces the volume of the air chamber, making the suspension more progressive. Removing the spacer increases the volume of the chamber, making the suspension more linear.
This feature allows you to adjust the spring rate curve. In other words, you can control how the force required to move the suspension varies across the entire suspension range. For example, you can add spacers to make the fork harder to compress early in the stroke. You can remove the spacer to make the fork run almost linearly. More advanced forks are unlikely to bottom out. They also give the bike a bouncy and snappy ride feel. You cannot adjust the progress of the coil fork. They operate linearly.
3. Air forks are easier to adjust
To adjust the spring rate, you simply add or remove air from the fork. This is done with a special high pressure pump called a shock pump. You just connect the pump to the American style valve on the fork and pump the air in or out. The shock pump has a built-in pressure gauge that allows you to easily set accurate pressures. Adjustments only take seconds.
Most modern air forks have a valve to adjust both sides of the fork. Some older forks require you to pump each side separately. In this case, you need to make sure you have the same pressure on both sides to ensure even compression of the fork.
Because air forks are easy to adjust, you can change the spring rate and sag before each ride or even mid-ride if you carry your shock pump with you. This might come in handy if you want to tackle particularly rough trails. You can let some air out to soften the ride. If you’re trying to fit some heavy gear or bike bags on your bike, you might add a little more pressure.
Adjusting the air fork’s forward is also easy. You just check the starting pressure, let the air out, and unscrew the top cap to use the volume spacer. You can then add or remove spacers as needed. Gaskets usually just need to be screwed or snapped into place on the inside of the top cover. Then you screw the cap back on and pump the fork to your desired PSI and you are good to go. For more information on changing volume spacers, check out this guide.
4. Air forks are lighter
On average, you will save about 300-500 grams (0.66-1.1 lbs) by choosing an air fork over a coil fork. Air forks tend to weigh around 1500-1900 grams (3.3-4.2 lbs). Most coil forks weigh over 2000 grams (4.4 lbs). For cycling, a pound is quite a significant weight loss.
Air forks are lighter because the springs are provided by air and have no weight. On the other hand, the coil forks have heavy-duty steel coil springs. The lighter weight at the front of the bike makes the steering feel quicker and more responsive. It’s also easier to lift the front of the bike and jump.
A lighter bike is also easier to accelerate and climb. For example, a medium sized cyclist riding 2 miles at a 15% incline at 250 watts would reach the top in about 10 seconds faster with an air fork than with a heavier coil fork. This can be important if you climb a lot or ride long distances.
5. Air forks provide better bottoming resistance
This is thanks to the progressive spring rate provided by the air spring. As the fork moved through its range, it became difficult to compress. Near the end of the range, enormous force is required to further compress the air inside. The total force required to compress and bottom out an air fork is greater than a coil fork. This assumes both are set to the same rider weight. This reduces the chance of the fork bottoming out after a big drop, hard hit or jump landing.
Air forks also allow you to easily adjust bottoming resistance by changing the spring rate or progression. Increasing the pressure in the air chamber stiffens the suspension, making it harder to compress and bottom out. The fork is made more advanced by adding spacers to reduce the volume of the air chamber. This makes the suspension harder to compress early in the range, making it harder to bottom out.
This feature is important because bottoming out too hard or too often can damage your bike. For example, bottoming out too hard could bend the wheel or frame. You want to adjust your suspension, so bottoming out is rare.
6. Air forks are more versatile
The adjustability of air forks makes them more versatile than coil forks. For example, you can easily adjust the spring rate to match the type of riding you plan to do that day. If you plan to ride on rough trails with lots of drops and jumps, you can add some spacers to increase your progress to reduce the chance of bottoming out during landings. If you’re lending the bike to a friend who is light weight, you can let out some air to lower the spring rate, making the fork softer. Maybe you want to pack your bike in a bike bag and go camping. You can increase the PSI to make your fork stiffer so it can handle the extra weight.
To make these adjustments, all you need is a shock pump and a few minutes. If you want, you can optimize the suspension for every ride. Being able to easily change the spring rate before riding makes the air fork more versatile. You can use the same bike for multiple types of mountain bikes. For example, you can easily use the same bike for trail riding, bike packing, enduro and trail riding with just a few tweaks. To make the same adjustment to a coil fork, you’ll need to take the fork apart and replace the coil spring.
7. Quieter
With proper maintenance, air forks run very quietly. You won’t hear any crackling or squeaking unless you hit the bottom, which is rare.
8. More fork and bike options
If you walk into a bike shop, you’ll notice that most new mountain bikes these days come with air forks. There are also more air forks on the market if you want to buy a new fork for your existing bike. If you decide to go with an air fork, you have more options.
This is so because air forks are more adjustable, easier to adjust, and more popular. Bike shops are more likely to sell bikes with air forks because they can simply adjust the forks to fit any rider without having to carry a bunch of springs and swap them out. Makes it easier for the rider to fit the bike. Most mountain bikers also seem to prefer air forks.
Air Fork Disadvantages
1. Requires more frequent maintenance
Air forks have more seals than coil forks. These additional seals are required to keep the compressed air inside the air spring. Because there are more seals, air forks are more susceptible to dirt, dust and other contaminants than coil forks. The seals can become contaminated and air or oil can start to leak, causing the fork suspension to stop working.
Also, air forks generate more heat due to the tightly sealed friction. While this heat will not affect performance, it may accelerate the degradation of seals and fork oil. For these reasons, air forks require more frequent maintenance than coil forks.
You will need to service your air forks every 3-6 months, depending on how often you ride. Most manufacturers recommend that you perform basic maintenance every 25-50 riding hours. This includes cleaning and inspecting the struts and lower fork, and replacing the lower seals and dust brushes. You may also need to top up. You must perform this basic service 1-3 times between full rebuilds.
Every 75-100 hours, you should have your forks completely rebuilt by a professional. For most riders, this is about once a year. A full rebuild includes replacing all seals and oils, thoroughly cleaning all components and inspecting them for damage.
In contrast, coil forks can often be used for longer intervals without maintenance. They can run 50-100 hours between services and up to 200 hours between rebuilds. If maintenance is neglected, performance will degrade. You can also cause unnecessary wear and tear. For more information on maintenance, check out this great guide.
2. Air forks are less sensitive to small bumps
Because they need to keep pressurized air inside, air forks have more seals than coil forks. Some of these seals also need to be installed tighter. Friction between seals and moving parts creates friction in the system. Therefore, air forks have more stiction or stiction than coil forks. Sufficient force is required to overcome the stiction to start the fork moving.
The force required to overcome static friction is called the separation force. Air forks require more breakaway force than coil forks. This makes the air fork less sensitive to small bumps. For example, riding on rootlets may not generate enough force to break the stiction on an air fork. That collision can be transmitted through the fork into your arm. The same rootlets may generate enough force to break the stiction of the coil fork and be absorbed. For this reason, the ride can feel a little rough when you’re using an air fork. Also, the air fork doesn’t react as quickly when you hit bumps. The damping system also doesn’t work very well because it doesn’t react quickly to impacts.
There are several ways to improve small collision sensitivity. You can lower the spring rate. When you do this, the fork tends to blow through most of the travel and then rise quickly at the end. You can also opt for an air fork with a negative spring to help overcome stiction and improve small bump sensitivity. This negative spring is the opposite of the air spring and helps reduce separation forces. A good negative spring system can make an air fork almost as sensitive as a coil fork, but not very sensitive.
3. Air forks don’t provide as much traction
Since the air forks can’t absorb some small bumps due to stiction, they don’t provide enough grip. The reason is that the front wheel cannot consistently track the ground when it cannot absorb small bumps. The bike might bounce off some roots, rocks and bumps instead of absorbing them. Your tires don’t provide enough grip when bouncing.
The progressive spring rate can also make the fork feel unresponsive. So you might not be able to make such hard turns with the air fork on some surfaces. The front wheels are easier to wash off. Braking performance is also affected.
4. Less comfortable ride
Air forks feel a little more snappy than coil forks. The fork feels more springy thanks to the progressive spring rate. Some riders like the feel, while others don’t. Additionally, air forks have poor small bump sensitivity and can make the ride feel rougher on certain surfaces. For these reasons, air forks may not be suitable for people with back or joint pain. The ride is a little bumpy and not very comfortable.
5. Air forks are lower
Due to poor bump sensitivity, the air fork doesn’t provide enough traction, so you probably won’t be able to corner or brake as hard without losing grip. Also, the springy ride of an air fork can make the bike harder to control on very rough terrain. For example, if your bike is bouncing violently under you, it will be difficult to steer. So you can maintain a slightly slower average speed with the air fork.
That said, the difference in performance between modern air forks and coil forks is pretty minimal. On a typical downhill run, it might slow you down by a second or two. That being said, there are certain situations where air forks can allow you to ride faster. For example, air forks are better for riding paths with lots of jumps, drops, and rollers because they provide more support. This extra support allows you to overcome these obstacles faster and harder without having to slow down or risk bottoming out.
6. Air forks are less durable and reliable
Air forks have more parts that can wear out or fail. They don’t usually fail catastrophically, but they can leak if you don’t maintain them in time. For example, having more seals inside could become contaminated and start leaking. The top American valve may leak or fail. You may need to add some air before riding. If you’re far from civilization, it’s best to keep a shock absorber with you. Even if your fork is well maintained and in good condition, you may need to add some air every month or so.
With shorter maintenance intervals, it’s easier to overlook the fork and cause premature wear. Having said that, modern air forks are very reliable. If you maintain them properly, they are extremely unlikely to get you into trouble.
7. Air forks cost more to run
Generally speaking, air forks cost more than coil forks over their lifetime. The initial price difference is very small. In fact, air forks are often several hundred dollars less than comparable coil forks. Where air forks get more expensive is maintenance. Since air forks require more frequent maintenance, running an air fork for the average rider can cost an extra $100 per year.
Air forks typically require a full rebuild every 8-12 months, while coil forks may only need a full rebuild every 12-18 months. A complete factory rebuild costs $150 to $175. Between full rebuilds, the fork needs basic service to the lower part of the fork. Between full rebuilds, air forks may need 2-3 basic services, while coil forks may only need 1 basic service. A basic service costs $30 to $50. Doing basic services yourself can save you some money. You will have to buy some parts and tools.
Coil forks cost about the same to maintain, but at longer intervals. All this extra maintenance adds up. You will spend more money maintaining your air fork in the long run. Your air fork will get more expensive if you neglect maintenance. For example, if your originals get scratched due to lack of maintenance, the cost of a new prop can exceed $500.
8. Brake dive could be a problem with the air fork
Air forks tend to be softer at the start of the stroke and mid-range. When you brake hard, the front of the bike can drop into mid-travel. It can be difficult to steer when you’re descending this far. Worst case, you could fly over the handlebars.
There are several ways to solve the brake dive problem. You can add some air to increase the spring rate. You can also increase the low-speed compression damping if your fork offers that setting. The problem is that these tweaks reduce small bump sensitivity. Good technique can also help reduce brake dive. When you have to brake hard, move your weight back as far as possible.
9. Some suspension ranges may not be available
Since the air fork is progressive, the force required to compress the spring increases throughout the range. At the end of the range, it takes a lot of force to compress the fork further. This makes the last part of the suspension travel almost useless. End of stroke is only used on the rare occasion when the fork takes a particularly hard hit and bottoms out.
To make more suspension available, you can lower the PSI to lower the spring rate. When you do that, you blow over most of the suspension when you hit a small bump. When you reach the end of your trip, progress quickly ramps up. There may be some unused trips left. If you increase the spring rate, the suspension travel increases in the middle of the range. This gives you less usable range and sensitivity. Pauses also don’t work when some pauses are not in use.
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