Run a Tapered Fork on a 1.5" Cannondale
If you have bought a Cannondale mountain bike, you may be surprised at the limited number of fork options due to Cannondale's non-standard straight 1.5" headtube choice. Most bikes use a straight 1 1/8" headtube. Nicer frames use a tapered 1 1/8" to 1 1/2" design. The larger diameter is stiffer and provides for a larger bearing surface, so Cannondale wasn't being totally arbitrary with their choice, but it's nonstandard nonetheless.
If you're looking for a replacement, there are very few options. I got a 1.5" Rock Shox Recon Gold TK a while back, and it's a great fork for the money -- about $300-350 online. This was made by Rock Shox for Cannondale's top-of-the-line Trail SL 29er hardtail. It was significantly better at dampening rough trails and keeping the tire in contact with the ground than the RST Deuce that came with my Trail SL4.
As time passed and my bike habits changed, I wanted something sleek and light. Most of my riding these days is commuting to and from work, and a recent job change made it so that the commute was nearly completely on paved bike trails. My previous commute had about 8 miles of hardpack and gravel trails mixed in, so I was thankful for the front suspension. But the new commute opened the door to slick tires and a rigid front end.
Niner Bikes makes some of the best frames out there, and they offer a rigid carbon fork for mountain bike minimalists. The only problem for me though was that they offer it only in straight 1 1/8" or tapered 1 1/8 to 1 1/2" fitments -- no straight 1 1/2". After some searching, I found a solution that would let me run the fork on my Cannondale.
FSA produced a headset for a while called the Gravity One. It is designed to fit in straight 1.5" headtube frames, but allows you to use a tapered fork. It appears that it's not in production anymore, however I did manage to find it at a couple of online retailers. You can find it at Canondale Experts or Universal Cycles.
It's a nice looking piece, with clean edges and large bearings. The package came with everything I needed to install it on my bike, including the crown race.
To remove the existing headset cups in the frame, I made a tool using a length of 1.25" pipe (closet rod actually). Just cut a one foot length of pipe, then make two cuts at one end of the pipe so you can splay that end out. Park Tool sells this for $30 or so, but it's pretty easy to make.
Slide the pipe through the head tube so that the splayed ends engage one set of cups, then hammer the other end of the pipe to knock it loose. Repeat for the other cup. Simple.
To press in the new cups, I used a Bessey woodworking clamp to gently work them in and keep them square.
I used a length of 1 1/2" PVC pipe to set the crown race on the Niner fork. This took more effort than I expected it would, but it's a super tight fit now.
The remaining assembly from there was very straightforward. The carbon fork comes with a special insert to support the carbon steerer where the stem clamps on, along with some special paste to add friction to the steerer-stem clamping surface.
This change took nearly 1500 grams off the weight of the bike. The steering is razor sharp, and the ride isn't so jarring that my hands or arms are tired at the end of a long ride. Coupled with changing out to 28mm slick tires, the bike is a lightweight rocket now. It weighs 21 lbs now (it was 30 when new). The fork and tires seem to be good for a 1-2 mph improvement in speed.