Putting two different-sized wheels on your bike isn't anything new. The idea goes back as far as the mid 1880s with the Penny Farthing (or Boneshaker) that featured a wheel 60 inches or larger in the front with a 20-inches-or-smaller wheel in the back.
These original incarnations of the bicycle used bigger wheels because there was no freewheel at the time and pedals were mounted directly to the front wheel axle. This meant that bigger wheels allowed faster travel with a lower pedaling cadence.
Ultimately, progress ensued as chains and drivetrains came about. This allowed a bike with smaller (and safer--imagine the Penny Farthing dismount) wheels, which pushed the bicycle into further public acceptance as more people felt comfortable trying out the invention.
Skip ahead a century or so. Cannondale introduced their Beast of the East 26/24 featuring a 26-inch wheel in the front and a 24-inch in the rear. The idea gained merit, but ultimately disappeared from their lineup in the late eighties. Downhillers played with the idea too, and Specialized offered an 8-inch travel 26/24 downhill model, but the idea lost out due to race-driven product innovation that had to comply with national and international rules, which mandate 26-inch wheels front and rear.
The 29-inch movement we see today took awhile to gain momentum but after eight or nine years big wheels finally have a presence. And not far behind comes the radical idea of different wheel sizes, with individual riders modifying their 26-inch bikes to accommodate a 29-inch front wheel.
Dirt Rag editor Michael Browne documented his own experiment in a 2004 web-only Brain Fart, Beast of the Burgh, and received scores of emails from like-minded riders showing their creations and asking advice on set-up.
Now it's 2007 and companies both large and small are starting to offer production models. Davis Carver introduced the first, a few small builders (John Castellano, Steve Potts, Siren Bicycles and Titus to name a few) have stepped up and Trek is throwing their hat in the ring. Meanwhile, the internet, with its vast number of self-proclaimed experts, is abuzz with lovers and haters of the idea.
As with anything, the best advice is to try it.
But here's the best part--you don't have to wait until a demo tour comes to town to decide if you want to buy an entirely new bike, and you certainly don't have to wait for your friend to buy one first. You can even be the first guy on the block to have one. Follow these simple steps to modify your existing mountain bike using the newest old concept since bigger wheels.
The Theory
This project aims to squeeze a 29-inch wheel in the space of a 26-inch. By replacing a suspension fork with a shorter rigid fork, you'll be able to utilize the space that the telescopic shock required and replace it with the larger wheel. It's vital that you keep the bike's overall axle-to-crown and handlebar height the same as when you started.
By keeping these two heights the same, you will see few drawbacks. It's possible that you will encounter toe overlap (if your frame is small), a higher bottom bracket and/or a head angle change. The closer you stick to your original measurements, the fewer geometry adjustments you'll see.
The Big Question: Why?
A 29/26 combination allows many of the benefits of 29-inch wheels (traction through turns, easier ability to roll over obstacles) while negating some of the drawbacks (overall increased weight, sluggish acceleration and a longer rear end). All this with a bike you may already own.
Roll-over
In most cases, if the front wheel can pass over an object, the rear will follow. Rarely does the rear wheel hold up bike progress on the trail.
Traction
The same idea applies in traction--the bigger front wheel allows you to turn harder without washing out, whereas even a small rear wheel isn't particularly prone to sliding out in turns.