Hotspur and Oversize Titanium Tubing

• Mark V
• 17 years ago
• Categories: Blog
• Tags: Davidson

For the Hotspur frame, Bill Davidson decided that to use the Feather Tech oversize titanium tubing. The key feature is the custom milling that Feather Tech employs to create external butting on very large diameter (for titanium) tubing.

It’s simple statics that dictates that doubling tube diameter increases stiffness by a factor of 16 if the wall thickness is kept constant. So, what you do is increase the diameter by say, 20%, and decrease wall thickness until the weight is less than a standard tube but still thick enough at the ends to survive the welding process. Then you end up with a tube that is both lighter and stiffer than before. Large diameter, butted tubing is not a new concept. Steel bikes have evolved along these guidelines for over a hundred years, hastened by newer, stronger micro-alloyed steels and better tube-drawing methods that allow thinner walls to be created.

However, titanium is NOT steel. It’s material properties are such that higher strength ti alloys are not very suitable for seamless tube drawing. And not everyone can make titanium tubing; most of it comes from industries catering to aerospace companies. So, in other words, these companies are not interested in creating bicycle tubing. Smaller companies can then buy the tubing and modify the tubing. Feather Tech is one such company.

Steel is generally butted using a method called rolling, which doesn’t actually involve material removal (loss). This creates an internal butting profile, ie the change of wall thickness is only detectable on the inside of the tube, thus invisible on the finished frame. For titanium, Feather Tech removes material from the outside of the tube; you can see how the ends are thicker.

The Hotspur’s top tube is 38mm (versus the typical 32-35mm) and the downtube is a massive 42mm (vs 35-38mm). The seat tube is 35mm with a machined cap to bring the inside diameter down to fit a 31.6mm seatpost. Bill could have easily chosen the next size smaller for each of the tubes to create a lighter bike, but Bikehugger specifically requested a stiffer, more responsive frame.

Next Bill opted for the Reynolds carbon seat stay. Light, stiff, and clean, the Reynolds piece provides a little vibration damping where it can do the most good. And Paragon makes a ti dropout that elegantly connects to the bottom of the Reynolds seat stay. Bill then used Dedacciai tapered chainstays to stiffen the power transfer to the rear wheel. Bill thinks that that a metal connection from the BB to the rear axle is the strongest, most durable structure.

In the same philosophy of leaving material where it’s needed and removing it where it’s not, Bill eccentrically machined the head tube stock to thin the tube on the front and leave material on the back to reduce warp from the welding of the top and down tubes. The frame accepts a standard 1-1/8 headset, because there is no good reason to incorporate an integrated headset into a steel or titanium frame. It would just unnecessarily add weight (for reasons beyond the scope of this article, integrated works pretty good for aluminium and great for carbon).

The resulting frame weighed 1280 gr before paint in a 56cm equivalent size, just about 80 grs heavier than the ‘07 Trek Madone frame that it will replace. When combined with paint and a Reynolds UL carbon fork, the whole thing is just about the same as the Madone and its respective fork (within 5gr or so). Quite competitive, especially keeping in mind that ultra-light wasn’t the design goal. Bill could have built a lighter bike, but that wasn’t what Bikehugger asked for. The Hotspur is a bike for the roads you actually ride, and how you want to ride them.