Choosing your new mountain bike forks
New forks are one of the most popular upgrades for mountain bikes, and as it's easy to spend more on a fork than your frame (or even whole bike) cost it's worth getting the right one. The first question is, of course, do you need a new fork? If you're upgrading an entry-level bike you might find it makes more sense to get a whole new bike. But having decided that a new fork is the way forward, here are some things to look for.
We're not going to talk about specific forks or brands because they change all the time - the idea here is to help you work out what you're looking for, explain the various features you'll come across and with any luck let you figure out a fork shortlist. Once you've narrowed your choices down, turn to the reviews and see which particular fork sounds like it'll suit you best.
The key to a successful fork purchase is identifying what you need. That's a function of the bike you'll be putting it on and the riding that you do. As far as the bike goes, travel is the number one consideration. The length of your new fork has to be within striking distance of the old one - too long and you're riding a chopper, too short and things'll get distinctly twitchy. There's a bit of leeway, though. Most bikes that were originally specced with 80mm forks will be OK with 100mm forks, just a little slower steering. Also bear in mind that not all forks are created equal - one manufacturer's 100mm fork could easily be significantly longer than another's. If you've got a full suspension bike then it's usually best to get roughly equal travel at both ends. Again, in most cases an inch difference from one end to the other won't usually hurt, but if you already find your bike a bit slow steering then don't go any longer. If you're really not sure then go for a fork with adjustable travel. This'll either be via an external dial or by moving spacers around inside the fork.
The next thing to consider is your use of the bike. If you're an XC racer then you'll be looking for a short, lightweight, well-controlled fork If you're going a bit freeride then you need strength, more travel and big-hit performance. Look at what your competitors or riding buddies are using. You're trying to work out your performance priorities - is weight more important than suspension action? Is stiffness more important than tunability? Do you want to be able to service the fork yourself? These are questions that only you can answer…
A suspension fork wouldn't be a suspension fork without springs. The whole idea of suspension is to let the wheel move away from bumps without causing the rest of the bike to move. The spring's there to push the wheel back where it came from ready for the next bump. You'll encounter three possible springs. The simplest is an air spring - the fork will have a sealed chamber with air in it. As the fork compresses, the chamber gets smaller, compressing the air. Air doesn't like to be compressed and will try to expand again, thus extending the fork. Air springs are good in all sorts of ways. Obviously, they're light, but they're also easily adjustable. To work well, the spring has to be set correctly for the rider's weight and riding style and with an air spring that's as simple as adding or removing air from the chamber with a suitable pump.
A downside to air springs is that they need tighter seals to maintain air pressure which can lead to a stiffer fork action. Air springs are also progressive - the further you compress them the harder they get to compress any more. A degree of progression is desirable, but early air forks got a bit carried away with it. Most current air forks use a sufficiently big chamber that even at full compression there's a reasonable amount of space left so they're more linear in operation. Many air forks also incorporate a negative spring. This is an additional spring operating in opposition to the main one. The idea is to make it easier to overcome the static friction generated by the tighter seals that air forks require, making the first part of the stroke more sensitive. Sometimes the negative spring is another air spring, giving another degree of adjustment - if you want a less bobby fork, drop the negative pressure. If you want it more plush, raise it.
In contrast, most coil springs are inherently linear unless they've been specially made to be progressive. Coils are the spring of choice for freeride and downhill forks - they're consistent, they don't leak and until recently you could guarantee a plusher feel from a coil spring than an air spring. They're heavier, though, and to get a coil spring correctly tuned to your weight might involve changing the springs.
A number of forks use coil springs and air in a bid to get the best of both. By supplementing a coil with a low-pressure air spring, fork designers can get the plushness and reliability of a coil with the adjustability of air. This setup also lets you easily alter how progressive the fork action is - if it's running at the right amount of sag but bottoms out too easily, just add a bit of air. It won't make any odds at the top of the travel but it'll come into play at the bottom.
The other common spring is the elastomer bumper. These simple, lightweight chunks of foam used to be extremely common, but as fork travel has increased the limitations of elastomers have made them less suitable. They're still often found in budget forks, though.
Left to its own devices, a spring is going to re-extend fast after it's been compressed. For a bike fork, this isn't ideal - you'd hit a bump, the fork would compress and then rebound back at you. It'd be nearly as bad as just hitting the bump with a rigid fork. What's needed is some method of slowing the fork down so it extends in a controlled manner, fast enough to be ready for the next bump but not so fast that it upsets the composure of the rider.
Budget forks tend to rely on friction and/or the hint of internal damping that elastomers provide. On a short travel fork at low speeds this is fairly tolerable, but as you start riding faster and hitting things harder you'll start to find the limitations of this approach. "Proper" damping uses a piston running in a chamber of oil to control the spring. At its simplest, there'll be holes in the piston that the oil must flow through to get from one side to the other. It can only flow so fast, limiting the speed at which the fork moves.
The more you spend the more sophisticated the damping gets. Sprung shim stacks over the holes let the damper behave differently according to how fast the fork's trying to move, extra valves can open up lots of room for oil to flow if you hit something very hard, you can have compression damping as well as rebound damping, separate damping "circuits" for low and high-speed damping and adjustable everything. Think carefully about what you really need, particularly when it comes to adjustments - there's not much point having adjusters that you don't know what to do with. For most riders, adjustable spring and rebound damping are sufficient.
Lockout is a common fork feature - some sort of lever or dial that prevents the fork from compressing. It's useful on the road or on smooth climbs when you might find the movement of the fork distracting, so it's worth considering if you find yourself joining off-road sections with lots of road. Racey forks often have a bar-mounted lockout lever to help you eke that last bit of speed from the sprint to the line. Some forks also have lockdown, a method of temporarily shortening the fork to improve the steering on climbs. Long forks tend to wander a bit at slow speeds uphill, so by shortening it you can sacrifice some travel (that you won't need anyway) for improved line-holding ability.
Most forks on the market have V-brake bosses and disc brake mounts, although if you're definitely going to use discs you can search out a disc-only model, which'll be a bit sleeker.
Servicing is an important consideration. All forks need some degree of servicing, and some brands are better than others for DIY. You may find it worthwhile finding out what's involved in fettling the ones you're looking at. If you have no intention of working on your own forks then you might wish to go for something that you can get seen to locally, although many shops outsource fork servicing to specialist companies anyway.