Article courtesy of bmxbasics.org.
The more things change... you know the rest. Once upon a time, racers bought their frames and forks separately. This was because there were very few manufacturers of truly durable forks. Come to think of it, there were very few manufacturers of durable frames either, but for some reason they tended to be different manufacturers from the durable-fork ones, and so one would often see Bottema forks on a DG, or SE Landing Gears on a Webco, or some other such odd combination. Eventually, everybody figured out how to make a semi-decent unicrown fork, and from the early Eighties until the late Nineties, frames and forks were sold together.
Later on in those late Nineties, the clever people who run Dan's Competition came up with the idea of making you pay extra for the fork in a frame-and-fork set. They probably figured this would just result in everyone buying the "matching" fork and paying fifty bucks more than they used to, but an unintended consequence arose: the aftermarket fork market came back from the dead, and people began mixing and matching frames and forks on a scale not seen since Linn Kastan introduced the Redline fork in the Seventies. Sometimes this mix and match works; other times it does not. What we will be doing this month is taking a detailed look at how and why forks work. Although there's some science involved, everything you need to know you learned on the seesaw as a kid... (but if you are not familiar with the concepts of leverage, click here for my brief explanation.)
A fork has four main parts: the steerer, which is the tube you put through your head tube and clamp your stem to, the dropouts to which you bolt your wheel, the legs which go from the crown to the dropouts, and the crown which attaches the legs to the steerer. A traditional "unicrown" BMX fork doesn't have a separate crown; instead, the legs are bent and welded directly to the steerer, thus the name "unicrown". Your fork affects the way your bike rides in four different ways: geometry, weight, stiffness, and strength. Note that stiffness and strength aren't the same thing. We'll get back to that part later, but right now let's take a look at geometry.
The main way your fork affects your bike's handling is through rake. This is easy to understand. A fork with a lot of rake sticks the wheel way out in front, like an "Easy Rider" chopper. A fork with no rake has the wheel directly under the "fork line", like a unicycle fork. The "fork line", by the way, is an imaginary line drawn right down the center of the steerer tube. Rake is measured by looking at the distance between the dropouts and that fork line. Manufacturers take different routes to create rake. Some forks have the legs welded onto the steerer at an angle... these forks look "pre-bent" when you look at them from the side. That used to be very popular in the Seventies and Eighties. Other forks use a long dropout to put some space between the fork leg and the axle. That's what's hip now, especially in street forks which need some dropout room to fit front pegs on.
The way fork rake affects your handling is also simple to understand. The more rake you have, the harder it is to turn the bike. Think of a chopper motorcycle and how tough it must be to parallel park it! A fork with a lot of rake feels "floppy" in turns, like it doesn't want to change direction very much. Hmm... since we all have to turn in our races, wouldn't we want a fork with zero rake so we could turn in a hurry, easily? Not so fast. A lot of times you want your bike to be stable and hard to turn - like when you are jumping, or when you are coming out of the gate, or when you are bumping elbows with somebody in the rhythm section, or when... you get the idea. Nobody really wants a hyperactive race bike. What we want is a bike that is stable in the rhythm section and quick-turning in the berm. Since you can't really have both, you will have to choose a fork that matches your style. If you have top-notch bike control, go for the low-rake fork. If you have some "sideways trouble", like many twenty-and-thirty-something Expert riders, a higher-rake fork can help stabilize you. Don't forget, you can use a low-rake fork to make a slow-steering frame turn more quickly, and a high-rake fork to make a nervous frame "chill out" a bit. Once upon a time, there were a couple of "double-dropout" forks sold, with two axle slots on each side. It was a promising idea but the most prominent variety - the California Customs - showed a disturbing tendency to snap their dropouts right off their legs, no matter which axle slot you used. Thus an interesting concept was forever ruined by a single example of poor execution, and for neither the first nor last time in our sport.
There's another way your fork affects your handling, and that's fork height. What? You mean you thought all BMX forks are the same height, since they use the same size wheels? Think again. BMX forks can vary by an inch or more in effective height ,which we measure from the axle to the bottom of the headset. To begin with, you will want to make sure that your fork has enough height to clear your tire. For instance, there's less than half an inch of clearance between the top of my Snafu Knob Job 2.10 tire and the bottom of the steerer tube on my Odyssey Dirt forks. Any less space there, and I would run the risk of being "foot-jammed" by a stray leaf!
Next, you should consider the effect that fork height has on geometry. A taller fork slows down your steering; a lower fork speeds it up. To understand why this is so, imagine you have taken your fork out of your bike and you are holding it by the head tube, with the rear wheel resting on the ground. Lower the head tube. Presto! Your head tube becomes "steeper". Raise the head tube. Wow! It's "slacker" now. Since your fork height determines how high your head tube is off the ground, you can see that your effective head tube is changed as a result. (Note, also, that using a bigger tire raises your front end and slows down your steering - the real reason why many larger riders feel more comfortable on a large front tire.) Back in the days of matching frames and forks, fork heights were carefully matched to frame geometry; today, you will have to do the matching yourself. If you pair a quick-steering frame with a low fork, you may find yourself flopping over the front end in a tight turn... put a tall fork on a relaxed-geometry frame and you may have to plan your turns a couple extra feet in advance.
Now let's talk about how stiffness and strength affect your bike's handling. To do this, we need to visualize the different forces acting on a fork. The simplest one is also the strongest: the force coming from the axle, into the dropouts, through the legs, into the steerer, and from there into the headset. Every time you land a jump, every time you hit a bump, every time your bike is standing upright, force is being transferred in this path.
The other main force is the force applied by a turning handlebar, which turns the stem, which turns the steerer, which turns the legs, which turn the axle. This is not a trivial amount of force. To find out how much force is involved, stand your bike up on its back wheel, take the front wheel out, and try to hold the forks steady while a friend waggles the handlebars. Unless you are much, much stronger than your friend, you won't be able to do it. Why can't you stop your friend from turning the fork? It's simple; the handlebars and stem provide massive leverage. In fact, it's a leverage factor of as much as eight to one at the axles and thirty to one at the steerer, so ten pounds of turning pressure (which is a very light turn) twists the fork with three hundred pounds of pressure and applies eighty pounds of turn at the wheel. You need this leverage, because a bike and rider going straight ahead have some decent forward momentum and it's tough to change that momentum.
A fork designer has a tough job. Most riders want a little bit of flex in the impact force direction, because it's easier on their hands and wrists. A bit of landing-force flex also reduces the tendency to crash, because a very stiff fork tends to be "nervous" and transmit the irregularities in the track right to the handlebars, making the bike less stable. On the other hand, almost nobody likes a fork that feels "mushy" in the turns. The problem is that, by their basic design, forks are stiff in the impact direction and noodly in the turn direction. What can you do?
There have been two recent answers to this problem. The first one was to go back to the separate-crown design and use some exotic materials - the "gimmick forks" like what you see from Answer and Marzocchi. These forks can be "tuned" with a stiff crown (for flex-free turns) and slightly flexy legs (for easier landings). It's a heck of an idea, with just one problem: these forks appear to break more often, and more spectacularly, than traditional unicrown forks. After all, stiffness does not equal strength. A rubber hose is far less stiff than a tree branch of the same diameter, but try bending them and seeing which one snaps first. More pertinently, a steel fork leg can feel much less stiff than an aluminum one but hold up under much more severe impact.
The unicrown fork people have responded to the carbon/aluminum/Jell-O(tm) forks by introducing heat-treated forks with variable internal tube diameter, tapered legs, and pressed-in reinforcements. These forks turn well thanks to their thick steerer tubes and large-diameter legs, but use thin-walled tubing for impact absorption and weight reduction. A few years ago, Odyssey shocked the BMX world by introducing a thirty-six-ounce race fork with an unconditional guarantee, a heavy-duty internally threaded steerer tube, and a unique combination of tapering and minimal dropout design. Take a look at Odyssey Race Forks... they look sharp, plain and simple.
Since then, my pals at Supercross have managed to refine these same features into a sub-twenty-five-ounce race fork, which puts them head-to-head with the C/F and aluminum forks. I expect other manufacturers to follow suit - S&M is already experimenting with a lightweight dirt/vert fork and I wouldn't be surprised to see an "LT" race fork in the near future.
I vastly prefer these new lightweight unicrown forks to any multi-piece crowned fork out there. In my opinion, the safety and longevity issues associated with crowned BMX forks have not been properly resolved. When Marzocchi or Answer come up with a fork that weighs twenty-five ounces and can be used for race and light street use for a couple of years, I may change my mind... but right now I advise that you choose a Cr-Mo unicrown fork. If you are put off by the delicate looks of the Supercross fork, the Odyssey should suit you nicely; if your brand loyalties prevent you from choosing either, there are many excellent unicrown forks in the forty-five-ounce range. Better safe than sorry.
We've been talking about fork weight in the past few paragraphs, but how much impact does fork weight really have on your ride? Quite a bit, and not always in the manner you might think. It's often easier for a rider to feel an eight-ounce savings in fork weight than a sixteen-ounce reduction in frame weight. The reason goes back to leverage. When you pull your front end up, you are pulling on a lever. The pivot of that lever is... think about it... your rear axle. The majority of your weight is about halfway down that lever, slightly ahead of the bottom bracket shell. Remember that the longer the lever, the easier it is to pull a certain weight, which is why moving your bars forward often makes pulling up easier.
Unlike a seesaw, however, your bike isn't a long, flat board. It's an assembly of differently sized and weighted components, and having a lot of weight at the end of your "lever" - in other words, the front end of your bike - can make a suprisingly large difference, considerably larger than a similar weight difference further up the "seesaw". The major difference I always feel between my race and street bikes is the sheer weight difference in the front end - more than thirty ounces difference between the Odyssey Dirt forks on my Dirt Devil and the Supercross Race forks on my UL and Nomorea racers. The race bike's front end comes up on a whim, while the street bike needs a less subtle persuasion. For me, that's not necessarily a good thing. I have found over the past few years that I prefer the "autopilot" assistance a heavier fork offers, within reason. Since there is no doubt that a lighter bike accelerates faster, you will have to judge for yourself whether the additional "nervousness" of a light-fork front end is worth the benefits of pure speed. Younger riders who are not strong enough to throw a thirty-five-pound bike around with abandon should, of course, always run relatively light forks.
Obviously, forks are a lot more complicated than they look, and I hope you've learned enough to make your next fork decision an educated one. We'll close with a final reminder about sensible part replacement strategies. In the dirt-jumpa world there appears to be a kind of status thing going on with switching your forks a lot... in the race world the opposite is true, and I have seen many a rider go through a couple new frames while keeping his old forks. Be careful about that. Even high-quality unicrown forks can break, given enough time and wear. Fork failures are almost always very serious, usually leading to significant injury. Don't risk your season, your career, or your life on worn-out forks. The fork/frame pricing model most shops use today means there are often excellent original equipment forks available cheaply - I've seen solid brand name Cr-Mo forks on sale for $19.99 - so don't skimp on safety. For those of you who are interested in the fork industy's "missing link", I've included a brief appendix on the Tange Switchblade crowned Cr-Mo fork. I'll see the rest of you next time!
