Titanium Parts

No discussion of materials is complete without considering the reasonable cost of improvement. When does improvement, in any material, fall so far behind its price to a consumer that it can no longer really be termed improvement?

Forks

The biggest hurdle to building a titanium fork that is as stiff as a steel fork and lighter than an aluminum fork is the steerer tube, as discussed earlier under Titanium Use and Abuse. There are other geometry restrictions that make titanium forks unattractive:

0. The compact shape of the conventional road fork crown was optimized for steel, and has been modified somewhat for aluminum. Duplicating the shape of an aluminum crown in titanium will make it stiffer, but not lighter. Removing enough weight from the titanium crown to make it competitive with aluminum involves considerable casting or forging complexities that raise the the cost significantly.
   
   
1. To compensate for the lower modulus of 3-2.5 (compared to steel), the fork legs need to be larger in diameter. This creates an opportunity to save weight, but tire clearance can become an issue.
   
   
2. The dropouts must be larger to fit the oversized fork legs, adding weight.

At best, then, a titanium fork can weigh about the same as an aluminum fork, with the stiffness of a steel fork, at a cost of five conventional forks.Unless the titanium fork can demonstrate some additional advantage, it appears to be a bad bargain.

Seatposts

The important properties in a seatpost are light weight, high strength, good failure resistance, and adjustability within the seat tube. Reliable aluminum mountain bike seatposts weigh as little as 220 grams. The lightest titanium post is around 195 grams. The titanium post will have better fatigue life, but it will also be more flexible.

A titanium seatpost is also very sensitive to head design and weld quality. Finally, if the titanium post is used in a titanium frame, it will gall, although proper lubrication can minimize the problem.

Chainrings

Chainrings must be light, stiff, and wear resistant. A titanium chainring of the same weight as an aluminum ring will not be as stiff for two reasons. First, aluminum's modulus-to-density is a few percent higher than titanium's. Second, to meet the weight restriction, the titanium ring must be 30% thinner.

A titanium ring of standard thickness could be more durable than aluminum, both in its wear properties and in its ability to survive impact damage from rocks and other trail debris. But this survivability comes at a significant premium in cost and weight.

Metal-matrix composites, whether aluminum or titanium-based, could be ideal materials for chainrings.

Brakes

Brake calipers need to be stiff, failure resistant, and light. Due to clearance issues and other design constraints, it is very difficult to make a titanium caliper that can match the light weight and stiffness of an aluminum equivalent. Aluminum or metal matrix composites appear to have the ideal properties here.

Bottom Bracket Spindles and Pedal Axles

The properties that are important in a spindle are failure resistance, precision, and light weight. A Shimano Dura-Ace or XTR spindle, made from heat-treated 4140 steel, has excellent fatigue characteristics, roughly twice that of current 6-4 titanium spindles. A 6-4 spindle can be considerably lighter, but its fatigue endurance is not acceptable.

An additional drawback is that titanium cannot be surface hardened to create a durable bearing surface. Thus, any titanium spindle must employ sealed bearings, leading to added weight, expense, and complexity.

Bolts

Lightweight titanium bolts, generally made from 6-4 alloy, have demonstrated excellent durability and strength in bicycle applications. Titanium's corrosion resistance is an added plus.

Titanium's lower modulus compared to steel is not a serious drawback, as virtually all bolts are used in tension against fully seated parts, where the bolt's flexibility is not an issue. However, titanium bolts will gall, or seize, when threaded into other titanium parts. This can be avoided by liberal application of anti-seize compound or other appropriate lubricant to the bolt threads before installation.

Handlebars

Titanium's high fatigue strength can be exploited to create mountain bars with excellent flexibility. The bars will transmit less shock and deliver a more comfortable ride. However, if the goal is to create bars of equal stiffness as existing bars made from steel or aluminum, then the weight of the titanium bars will be uncompetitive.

Stems

Forged aluminum road stems and welded steel mountain stems are light and rigid, and have a good safety margin. It is possible to make titanium stems as light, but rigidity suffers. Increasing the rigidity adds weight. A welded, one-piece bar and stem combination can be lighter and as rigid as any current equivalent; the only drawbacks are cost and adjustability.