Aside from an occasional oddball, like magnesium or something from the kingdom Plantae, material choices for mountain bike frames are aluminum, steel, and carbon fiber. Each one has distinct advantages and disadvantages, and the amount of time that's been spent deviling into their nuances is unfathomable. But, based on common deductions, there are solid reasons why Niner would continue to offer an updated S. I. R. 9 among its carbon and aluminum 29ers. The S. I. R. 9, along with the alloy AIR 9, launched Niner into mountain biking's limelight. And it's hard to believe that the S. I. R. 9 is already approaching eight revolutions of the big yellow orb since it began humiliating bouncy bikes with lesser wheels. In that time, we've seen carbon eclipse ferrous and aluminum chassis due to gram counters' love affair with fibers and epoxy. But, steel still holds its own when it comes to ride quality and value. Most manufacturers pick either aluminum or steel for a price-point line. Niner chose to use both -- alloy for its entry level line, while reserving steel for the seasoned rider who can appreciate its distinct ride characteristics, and who doesn't need to drain a bank account on a carbon rig to chase KOM's. If you're not up to speed on the differences between steel and aluminum, the quick and dirty is that aluminum is intently stiffer and lighter, so it tends to accelerate quicker, but it can be more fatiguing due to vibration -- imaging holding onto a jackhammer all day. Steel, on the other hand, dampens trail chatter and has a natural spring-like nature that results in lively and encouraging handling on singletrack. In this case, Niner uses proprietary Reynolds 853 tubing with custom S-bend rear seatstays and chainstays. The tubing's wall thicknesses, including the down tube and top tube, are tweaked both externally and internally to produce the best possible ride quality for each frame size. Clean TIG welds provide the strength, while short chainstays and a st...