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Upgrading the suspension on your mountain bike is one of the best things you can do to breathe new life into your bike. Whether it is a fork or shock, you will be amazed at the technology these days in suspension, and how that can translate to ride quality out on the trail. Not to mention the smile on your face!
When choosing a new fork, it can be complicated figuring out what fork fits your bike. In this blog we’ll go over the key things to keep in mind when making mountain bike upgrades with a new fork. There are tons of options (including fitment options) to consider, so that you can be sure the fork fits your bike. As you read further, we’ll go over the more common questions and discuss the more modern products (roughly ‘09 and newer). There are millions of bicycles out there in the world -- and all sorts of exceptions to “common” -- so please keep that in mind.
-There are lots of bike front forks out there -- literally hundreds. You’ll not only want to get one that fits your bike, but is also intended for your style of bike. This can almost always be determined by the travel of the fork. If you’ve got 80-100mm travel, odds are you’ve got an XC bike or general light trail bike; 120mm-140mm, you’ve got a modern-day trail bike; 150-170mm, it’s probably a modern-day enduro bike; and 200mm, you’ve most likely got a downhill bike.
-You always want to keep the same fork travel that your bike is made for. When frames are engineered they are designed specifically around a certain travel fork. If your bike is meant for a 100mm travel fork, stick with 100mm and you’ll be happy.
-If you desperately want to get a new fork with more or less travel, we NEVER suggest you go more than 20mm away from what the bike was designed around. If you do that, you are taking a major risk of disrupting the structural integrity of the frame and fork. You will also totally mess up the handling characteristics of your bike.
-Most common steerer tubes these days are tapered, which is 1.5’’ at the bottom and tapers to a 1-⅛’’ at the top. This is most common on newer bikes (2009+), but it is very rare or non-existent on older bikes. Older bikes often have a straight steerer, which is just straight 1-⅛’’ all the way. Certain older bikes might have a threaded straight steerer, but that is extremely rare.
-You need to be sure you get the right size steerer tube because certain frames only fit certain steerer tubes. If you are planning to keep your existing headset, those are only designed for a certain size as well.
-Keep in mind there are some other weird steerer tubes out there. Cannondale and Giant are two bike brands that have made some super confusing and proprietary steerer tubes for their bikes only. If you’ve got one of those… you may be in for a struggle.
-This seems obvious but as there are three wheel sizes these days (26’’, 27.5’’ and 29’’), it makes a big difference. So know your wheel size because bike front forks are specific to wheel size.
-To add a bit more complexity, you’ve got Fat bikes and Plus bikes that use one of the three common wheel sizes, but need a different fork because of the super wide tires. If you’re in this boat, you’ve got to figure out what you’ve got and get a fork with the same spec. Eg. 27.5+, 29+, Fat Bike specific, etc.
-This is only important for 29’’ wheeled bikes. Offset is how far the front hub sits out in front of the imaginary line that runs through the steering axis. All forks just come in the offset they come in, however, many forks made specifically for a 29’’ wheel are offered in two offset types. Standard and 51mm offset. The majority of modern day 29’r frames are engineered around a 51mm offset fork as the 51mm offset simply works better with the 29’’ wheel. In the some cases, there is a 29’r frame that is designed around a traditional offset, but that is rare and becoming more and more uncommon.
-If you have a 26’’ wheel or a 27.5’’ wheel, don’t worry about this at all. If you have a 29’r then you need to make sure what fork offset your frame is intended to use, very likely and most common it will be designed for a 51mm offset.
-If you want to geek out more on fork offset, PinkBike wrote a great article you can read here.
-You’ll want to get a fork with an axle that fits your front hub. Although 15x100 axles are very common, there are plenty of other “common” ones these days like the 15x110mm Boost axle, or the 20mm axle which is and always has been only 110mm wide, or the quick release axle also called QR or 9mm. Keep in mind a 15mm axle can also be called QR15, this means the 15mm axle has a quick release lever as shown in the pic below. Some 15mm and some 20mm axles have a QR feature and are often called QR15 or QR20 but that simply refers to the way you install/remove the axle from the fork. When you just hear "QR axle" with no reference to 15 or 20 then that means 9mm quick release style and not a thru axle. The 9mm QR axle is not a thru axle so the fork does not come with an axle and looks like the RockShox fork image shown below. 9mm QR is more old school and/or cheaper style. And of course don’t forget the ever confusing Fat Bike axles... but that is a whole other topic.
-Another confuser: The Predictive Steering hub. So far this is only available for RockShox RS1 forks, and if you have an RS1 fork you can only use the Predictive Steering hub. At the moment no other forks but the RS1 use this style axle/hub.
-Know your hub, know your axle and get a fork that matches. Or you can opt to buy a new front wheel/hub/axle conversion kit if that option is available.
-There are two mounting types (on modern day MTB’s made for disc brakes), post mount and I.S. mount. Post mount is where you only have two bolts that mount the brake caliper to the fork, and they thread directly into the fork. The I.S. style mount always needs an adapter, two bolts go through two holes on the fork and thread into an adapter, then two bolts come straight down from the brake and thread into that adapter. Post mount style is more common on 2010+ bikes, and I.S. is more common on older bikes. But there are always exceptions.
-The 160 post mount is the most common on 2009+ bikes/forks. This means you can mount disc brakes directly to the fork, no adapter is needed and it will be spaced for a 160mm rotor. However, some forks like the Fox 36 series use a 180mm post mount. Newer Fox 40’s use a 200mm post mount, where the old ones used a super confusing I.S. mount that worked with a 160mm I.S. adapter but was spaced for a 203mm rotor.
-If you’ve got an older bike or entry level mountain bike you might have V-brakes. These mount, work, and look entirely different than disc brakes. There is only one type of V-brake mount for mountain bike forks. So if you’ve got V-brakes and intend to keep them, you will need a fork with V-brake mounts.
-Brake adapters deserve an entire blog post of their own.(Or a dictionary-sized book!) The topic is seeming to get less complicated as time goes on but is still one of those things where lots of forks used different sizes and styles of mounts over the years.
Beyond all these little things, you’ve got loads of options as far as features you want on your fork: lockout, compression, rebound, air or coil spring, etc. These require their own blog posts as well. All in all, this stuff can be more than a bit confusing…
Our #1 tip - Let someone else figure this out for you. Our shop sells forks for mountain bike upgrades every day. We know all this stuff by heart, and we eat, sleep and breathe mountain bikes. Just contact us, send us a photo of your bike, call us, use our live chat, come into the shop etc. We help people figure out what fork is best for them and what fits their bike all day, every day. We enjoy doing it and are happy to help.
Once you know what you want and what fits your bike, check out our massive selection of all the best suspension forks out there. Get yourself some new suspension and go enjoy a better MTB ride. A better bike = A better life :)
Author: Jeff Cayley - Founder/Co-owner at Worldwide Cyclery