It occurs to me that we in the Slowtwitch office often assume, as they say in TV cop & lawyer shows, "facts not in evidence." In other words, we sometimes tell the second half of the story assuming you know the first. Perhaps that's the case when we write about how to miter the steer column of a carbon fork, and we know full well that half of you are scratching your heads and asking, "What's a steer column?"

We're going to publish some more fork overviews in the coming weeks, so we thought maybe this might be a good time to go back and write about forks starting from the beginning.

First, some nomenclature. The crown and the blades are fairly self explanatory, and the steer column is that thing that sticks up from the crown. It passes through the bike frame's head tube, and the headset contains a bearing above and below the head tube. These bearings are what affix the fork to the frame.

There are two ways that a stem can attach to a fork. A "threaded" fork has threads cut at the top of the steer column. The top cup and top nut of a "threaded" headset screw down on the steer column, and the "quill" of a traditional stem inserts down into the steer column and is affixed by an expander bolt.

The other method is for the steer column to be "threadless" and in this case the steer column extends above the top bearing of the headset, and all the rest of the hardware - stem and spacers - slip around the steer column or "steerer." The whole thing stays together because the headset parts, bearings, the bike's head tube, the spacers, and the stem are all held together in compression.

Each mode has its advantages. The threadless mode allows stems more adjustability, which is to say, you can move them higher or lower a further distance. Also, steerers and stems alike can be made out of lighter materials, and in general this way is considered preferable. But some prefer threaded steer columns and headsets because you can more closely adjust the pressure on the bearings.

The threadless type of headset was developed by Cane Creek / Dia Compe and while many other headset makers use this style you won't find Shimano using it. It hitched its horse to the old threaded standard, and now that pretty-much nobody is spec'ing their bikes with threaded stems Shimano just decided to cede that component to other headset makers. It's one of the very rare moments when Shimano has been outmaneuvered.

Steer columns come in various diameters, and its been the tradition that road bikes use one-inch steer columns mountain bikes use inch-and-an-eighth. At some point, however, road bike makers realized that there was nothing but advantages to the larger size, and now road bike makers also use the larger diameter.

I will point out, as an aside - and before I start getting emails - that yes, this is primarily a site read by triathletes, and there IS a downside BOTH to larger diameter head tubes AND integrated headsets (which I'll write about below) and that is in frontal profile of the bike. Head tubes are the first part of the bike frame that the wind sees, and making the head tube bigger is hardly going in the right direction if at the same time you're squeezing the down and seat tubes to make them more aero. Yes, some tri bike makers realize this, and Cervelo is one of them. Adjacent is a Cervelo P3 head tube and this is one of the very few bikes you'll find with an integrated headset and a 1" steer column. Cervelo is the ONLY company I've seen which has machined a head tube so that its diameter is actually the same as it would be in a bike with a traditional headset. This requires the head tube to start its life with the inside diameter of a traditional tube and the O.D. of a head tube taking an integrated headset. Then the center section of the tube is hogged out on a lathe to reduce its diameter. So far, pretty easy to do. The harder part is mitering the down and top tubes to fit nicely against the hourglass-shaped head tube. Very labor intensive. The only way to NOT have to do a difficult miter is to have the fat parts of your hourglass-shaped head tube well above and below the top and down tube, and that is what Litespeed does. Cervelo, though, in trying to keep the head tubes of its tri bikes shorter, employs the method described above.

What is an integrated headset? We have a separate article on this, and I'll just write that it is one in which the top and bottom headset bearings fit inside the frame's head tube, as opposed to fitting into headset cups that are pressed into the frame's head tube. Integrated headets are all the rage nowadays, and there are pluses and minuses.

With the advent of threadless headsets - and to some degree larger diameter steer columns - fork makers have been able to make their steer columns out of lighter materials. It's now common for a fork to be made entirely of carbon (dropouts and perhaps crown race seats will still be made of aluminum). These forks are wickedly light. Whereas steel forks used twenty years ago might weigh two to three pounds, these new forks are well under a pound.

But you give something up. Old-school bike makers used to make their own forks to go with their frames. They could build a bike with whatever head angle they desired, and they would "rake" the fork blades to make the bike steer in just the way they wanted. The fork blades of carbon forks are made in whatever rake they're made and that's it. Generally you get two options: say, 40mm and 45mm, or 38mm and 43mm, depending on the maker. You choose the rake or "offset" (the terms are synonymous) you want. This can be a bit of a problem if you're shopping for an aftermarket fork. What offset did the bike maker have in mind when your bike was originally made? Is your desired aftermarket fork available in that offset? If now, how differently will the bike handle with your new fork?

That's just the beginning of the problem. There is another, little known, measure which can alter the way your bike handles: the distance between the brake hole and the crown race seat. About a dozen years ago, when I owned and ran Quintana Roo, we started using Kestrel's EMS fork as an option in our tri bikes. As a cheaper alternative to this we offered a steel fork. But the EMS had an additional 8 mm of distance between the crown race seat and the brake hole. So if you simply put an EMS fork into our frame the front end was raised up 8 mm, like a hippy-style motorcycle chopper. Suddenly our steep-seat-angle tri bike wasn't quite as steep. It was hard to see with the naked eye, but it was there, and you could definitely feel the difference when you rode. So we decided to make two bikes: a frame that assumed our steel fork would be used, and another for Kestrel's fork.

Quite frankly, our hand wringing over this issue back then is not evident in today's bikes. Forks are spec'd by bike makers today without much thought given to how the bikes will ride, but on whether the buying public would rather have a Reynolds or an Easton or a Profile Design. Very few bike makers, it seems to me, actively build bikes around a specific fork design, because they're afraid of getting trapped into using one fork maker versus another. All this means bikes with today's forks are very light, but they won't necessarily ride with the sweet surety of an old Italian-built Masi or U.S. made Della Santa, both of which might weigh two to four pounds more than today's bikes.

Funny thing, fork geometry does not work the way you'd expect. You'd think that if you shallowed the head angle you'd compensate by lessening the rake. Not so. You'd actually increase the rake. Yes, this increases the wheelbase, and the plushness of the ride, and the weight distribution of the rider over the bike. The cockpits of tri bikes are pretty cramped, putting the weight over the front wheel. Therefore, well-conceived tri bikes often have a slacker head angle, and their forks have a bit more rake than you might expect. Whether the geometry of these bikes are well-conceived is another matter. It's unfortunately true that a lot of tri bike makers - especially companies that make tri bikes but don't have a strong history in this market - don't know what sort of fork and front-end geometries to use. Fortunately, riders can get used to just about anything.

I haven't mentioned comfort. In my own personal view, the most comfortable forks are still the old steel forks. But the carbon forks are comfortable as well. Forks are comfortable, or they aren't, based on their compliance. The fact that forks are compliant is evident in the way they're tested. Adjacent is a photo of Reynolds' testing machine, and if you look closely you can see the fork fixtured in it. There is a hydraulic piston that mimics bumps in the road, and the fork will fail after so many "cycles." There is no good standard for these testing machines, and the way fork makers often gauge the fitness and strength of their products is by testing a lot of forks made by various manufacturers and seeing how many cycles their forks last compared to forks made by others. In any case, it is axiomatic that forks flex - even straight-bladed carbon forks - because if they didn't they'd brake after one cycle in a machine like the one above.

Where do forks usually fail? Not where you'd think. Almost all well-made forks break right above the crown race seat, i.e., on the steer column just inside the head tube. Not to worry, though, most forks are made well and will not break at all. This is just where they break while on the testing machine after hundreds of thousands of cycles. As long as you follow the manufacturer's instructions for the use of your fork you've got little to worry about.

Having said that I've seen the insides of a lot of forks, and in general you get what you pay for. I've seen the insides of some cheaper forks made in the Orient and it's apparent that the process of impregnation of the resin into the carbon wasn't properly executed. If you're getting a carbon fork you may want to stick to a well-known brand that has U.S. connections. Reynolds makes most of its forks here in the U.S., and they're very well made. Kestrel, Easton, Kinesis, Profile Design and Giant all make most or all of their forks in the Orient (Easton makes one if its forks stateside), but they have engineering offices in the U.S. and/or in the Orient to help add a layer or two of expertise and quality assurance. The forks that most concern me are the nameless ones that get spec'd on bikes, and it's hard to know just what went into making sure the fork is properly made.