I've just read a review of Tom Zirbel's Jamis Xenith SL on Bike Radar and flicking through the pictures there is one of the head tube with the following caption:
Tom Zirbel's (Jamis Sutter Home) Jamis Xenith SL features a tapered 1-1/8 to 1-1/2in head tube - especially useful in a frame of this size
What benefits does a tapered head tube afford?
Lighter and stronger are the real gains. Also there is coolness factor. In a rare move I wish to cite Richard Cunningham from Road Bike Action for points 1 + 2:
Look, who has the longest unbroken history of carbon fiber frame making in pro road racing, uses the tapered fork steerer design to eliminate the narrow waist at the junction of the fork crown and steerer. This allows their all-carbon HSC6 fork to be laid up as one unit and in a manner that keeps the unidirectional carbon filaments that support the structure flowing from the fork legs, through the crown and up the steerer tube in a smooth, unwrinkled matrix. The more uniformly that the fibers are aligned where the crown flows into the steerer tube, the stronger the fork will be in its most highly stressed area.
Look’s engineers also developed a tapered lower bearing interface that eliminates the stress concentration that a conventional notched-type crown race would cause. The bottom line for tapered road steerers is that, done correctly, it streamlines the manufacturing process and makes for a better one-piece carbon fiber fork.
Regarding MTB: The taper allows for a bigger lower bearing while keeping the upper bearing smaller. The lower bearing receives a greater part of the load as most of the time it carries all the weight applied to the headtube and transmits it to the fork. The upper bearing mostly deals with side forces that try to "bend" the fork, i.e. cornering and braking forces.
It also allows for greater contact surface between steerer and crown, which translates into a much more resistant joint. This in turn allows for a longer travel fork that does not require an upper crown.
The final result is that you get a more aggressive front end, without the penalty of adding too much weight and also keeping the bike visually light (double crowns look like motorcycle stuff). These characteristics are preferred for All Mountain and Enduro bikes.
Another benefit of keeping the upper part of the steerer tube small is that you can still use regular stems and stem spacers, star nuts and top caps, which keeps compatibility with a huge range of components already available.
I have ridden both versions of head tubes/steerer tubes - tapered/non-tapered and it is my experience that the tapered version provides much more stability and with frame weights what they are today, more stability is always welcome!
