Hardly an eyebrow is raised anymore over bikes with fat aluminum tubes. Indeed, for some it´s an "in" look that connotes strength and power.
  But Gary Klein´s reliance on welding large–diameter aluminum, which began as a Student project at MIT in ´75, relates to function, not fashion. Compared to steel, aluminum is lighter but weaker and less stiff. This means copying a steel tube´s diameter in aluminum requires much thicker walls to obtain sufficient strength and rigidity, which negates the weight advantage.
  To overcome this dilemma, Klein and other aluminum adherents increase the tubes´ diameter, adding rigidity but not weight. The result is an efficient frame that´s pounds lighter than an equally rigid steel one but won´t succumb to flex–induced fatigue failure.
  This is no secret—it´s the same principle that spawned oversize fork steerers. But through the years, Klein has been the most driven to whittle down bike weight with fatter diameters, thinner walls, and exotic componentry. The Adroit Ultra Light, at just 23 grams over 20 pounds for our 20–inch test bike, definitely nudges the lightness envelope. It uses butted 6061 T6 main tubes with a 1 3/8–inch–diameter seat tube, 1 5/8–inch top tube, and massive 2–inch down tube.
  Speaking of oversize steerers, the Adroit´s aluminum one is uniquely huge, with a 1 9/16–inch diameter that requires special aircraft control bearings instead of a conventional headset. They´re pressed into a vaguely hourglass–shaped, externally double–butted head tube. Into the steerer fits a similarly oversize aluminum Mission Control one–piece bar and stem that weighed just 406 grams, about equal to a typical welded chrome–moly stem.
  While the Adroit´s'bare frame was a laudable 3.7 pounds, the secret of its success on the scale was its 1.2–pound fork, which gets its reliability from massive 1 5/8–inch blades. They´re further strengthened via butting at the crown and brake bosses, proportional thickening that gives their inside diameter an oval shape to increase fore/aft rigidity, and by a hidden reinforcing wrap of boron/carbon fiber—a trick Klein pioneered on his megabucks custom aluminum road frames more than a decade ago. All this, plus tricks like drilling holes in the rear dropouts (which open rearward like a track bike´s, confounding quick, clean–hands wheel changes), pares half a pound compared to Klein´s previous flagship frameset, the Attitude.
  Our stock bike came with Klein´s Ultra Light package, which is highlighted by titanium brake bosses and crank and pedal axles, and a Campagnolo alloy freewheel with teeth laboriously hand–relieved with a Dremel tool to mimic the release profiles of Campy´s cassette cogs. lt functioned flawlessly with a SunTour XC Pro rear derailleur, Sedis SL chain, and GripShift controls (134 grams per pair). At least, it did for 3 rides. On the fourth, the freewheel seized and provided the character–building experience of fixed–gear riding along 5 miles of hilly singletrack. No coasting to level the cranks for clearing obstructions. Naturally, after reaching the dirt access road where spinning in high gear would have been easy, it started working.
  Drivetrain problems aside, how did it ride? About as different from the Steelman as possible. The Adroit had no detectable top tube or bottom bracket flex, and I couldn´t feel or see the massive fork blades move. This bike punishes you for overinflating its tires, and yet there´s also something mysteriously forgiving about the ride. Whether it has to do with aluminum´s much–ballyhooed damping (compared to steel), or whether it´s a function of the bike´s lightness, I´m not sure. I do know that a steel bike made this rigid would deliver a brutally harsh ride.
  On rough, downhill washboard, there was none of the random yawing between the wheels that scares me on some light–gauge steel bikes. The Adroit felt as solid as an oak log, but a little more compliant. Whether you appreciate, endure, or abhor such a rigid frame, there´s no doubt it succeeds in its goal of eliminating energy–wasting flex without an iota of extra weight.
  Klein recently added a 19–inch frame and lengthened the top tubes on all sizes. These changes address my previous criticisms of how his bikes fit. I usually ride an 18– or 19–inch frame, but the 20–inch Adroit´s length was about right. Because of its laid–back 72.5–degree seat tube angle, I didn´t need to slam the saddle back to get my preferred pedaling position, thereby lengthening reach to the handlebar. But with saddle height correct, the bar was too low for anything but high–speed pavement pounding. In raising it to the correct height for serious off–road work, I exposed 1.5 inches of unpainted quill and a portion of the wedge binder. This looked like hell and can´t be healthy for the thin–wall alloy steerer.