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the mountain biking experience I had there inspired me to

pursue my future career. Little did I know how this ride would

lead me into the world of aluminum.That day, a local trail guide loaned me his GT Zaskar- Shortly

into the ride, I realized the bike rode far better than my own

clunker with its mismatched components. lt was more

responsive, lighter and extremely rigid, making climbing abreeze.

As an engineering student at Santa Clara University, I found

my young mind racing. What made this ride so well? How did

this frame's geometry compare to my cuffent bike? Did the

aluminum frame offer better power transfer than steel? Could I

design a better bike?After post-ride beers, I decided that my senior mechanical

engineering project must involve mountain bike frame design' I

set a lofty goal to build my own frame for graduation credit.

With two classmates and the fabrication skills of Paul Sadoff of

Rock Lobster Cycles, a successful California framebuilder, we

built an all-steel, full-suspension frame with five inches of rear

wheel travel. The success of this project led me to develop three

more generations of this frame design, and in I996, my small,

unofficial framebuilder business was born.For five years, I worked with Sadoff to build custom frames,

primarily aggressive trail hardtails, branded under the Rock

Lobster name. We worked with steel, the material of choice for

custom framebuilders at the time, until random chance

intervened. Santa Cruz Bicycles had purchased Outland

Bicycles, a small U.S.-based one-hit wonder, and through the

deal, Sadoff indirectly inherited literally

hundreds of Outland's Easton 7005aluminum tubes from Santa Cruz Bicycles.

It was a huge windfall of materials.Aluminum as a bicycle frame material had

intrigued me ever since that fateful Oregonride on the Zaskar.I knew it was time to

branch out into aluminum framebuilding.By 200f , I had officially launched AhrensBicycles with an immediate focus on

developing handmade aluminummountain bikes.

To understand aluminum frame-building more fully, I researched all viable

options for TlG-welded frame construction.Two options emerged: alloy 6061 as thefirst option, which gains its strength from

silicon and magnesium; and alloy 7005 as

the next option, which gains its strengthfrom zinc. Within the 7005 family,ultra-lightweight scandium derlvativeshave also been developed.

With respect to handmade frameconstruction, there are practical pros and

cons when dealing with either 6061 or 7005

aluminum. Alloy 6061 is readily available in

a wide variety of sizes and shapes formachining frame components. Conversely, alloy 7005 is only

offered in limited useful sizes. Both alloys are malleable and

easily machined by conventional methods.I'm not a metallurgist or material scientist, but I've gained

some practical knowledge over the years in aluminum frame-

building. Heat-treating 6061 after welding requires a high-

temperature quenching and age-hardening process that must be

tightly controlled for optimal results. During quenching, the

material's alloying elements are trapped in solution, which

results in a soft metal. During the impurity phase of the aging

process, the alloying elements form fine particles that impede

the movement of dislocations (defects in the metal's lattice

structure) and ultimately harden the material. Aging will occur

naturally over time or the process can be accelerated at elevated

temperatures (artificial aging).Heat-treating 7005 after welding is straightforward and

involves a low-temperature, age-hardening process. The 7005

butted tubes have already been quenched and aged at the

factory prior to frame construction and welding.

Stress corrosion cracking is a known failure mode with

7000-series aluminum alloys containing high amounts of

copper. My layman's understanding is that this failure mode

occurs at high-stress areas where crack propagation is likely to

occur. Corrosive elements, such as water, travel into crack

initiation sites and accelerate crack growth, which can lead to

failure. In practice, I have not seen a 7005 aluminum frame

failure from stress corrosion cracking, possibly because 7005

bicycle tubing has low levels of copper, and proven framebuild-

ing and design practices aim to minimize stress at critical joints.

Aluminum frame failures due to fatigue are well

documented, although I have not directly experienced this

failure mode. This means that at some point, after some amount

Ready for weld ing, an Ahrens rear t r iangle s i ts in an Anvi l f ix ture. The custom machined loweryoke, v is ib le in the lower r ight s ide of the p ic ture, prov ides ample t i re c learance and subt le tor-

s ional J lex and is jo ined to s tout rectangular chainstays for great accelerat ion.

of cyclic loads, the material will fail when it reaches its

endurance limit. In the bicycle world, this usually translates into

many years of riding on the same frame without failure. Some of

my earliest aluminum frames are still being ridden, and oneILIIi

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advantage of the tubing supply thatSadoff had acquired from theOutland sale.

Frame components such asdropouts, cable stops and bottom bracketshells were readily available from a localdistributor, and I assumed that moving tothis new alloy would be painless. Thatwas a bad assumption. The off-the-shelfframe components were poorly designedand quality was moderate at best.These parts had to be reworked by handto achieve a decent aesthetic. I wasclearly disappointed with the availableframe components and new versionsof these parts were required for myhandmade frames.

I encountered a new challenge whenI found that any weldable framecomponent had to be machined from7005 alloy using bar stock offered only ina l-inch by 4-inch format. This bar stocksize is not ideal and has limited designcreativity since each frame componentmust fit within this pre-defined outline.

What if I wanted to design achainstay yoke that was I.5-inch thickand 4.25-inch wide? These ideas werenot possible because the largest 7005aluminum blank size was too small. Andmy dropout designs were less thanL-inch thick, meaning extra materialwas wasted during CNC machining.Overall the transition to 7005 had asteep learning curve.

Tuning stiffness characteristics foreach rider is one of my primarymotivations when building handmadealuminum frames. Through extensiveR&D, I've discovered that aluminumframes can be highly tuned in a fashionsimilar to steel frames. In the fronttriangle, different styles of tubing can becombined to provide the desired stiffnesslevel, from race-specific applications toeveryday trail riding.

For example, changing the downtube's outside diameter and buttingprofile has an immediate effect on ridequality. In addition, tube-end ovalizationcan be further tweaked by hand toachieve different stiffness levels.

The most advanced tuning of myaluminum frames happens in the reartriangle by combining the custom-machined yokes I've developed withselect tubing shapes. Riders canperceive the aluminum frame's torsionalcompliance during trail riding.This means that the rear triangle "gives"

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