This is the response to a reader's question on why we run a negative rise stem on the Project Titus 96er featured in the November 2006 issue. The reader also wanted to know how a 3.9-inch travel RockShox Reba 29er fork would work compared to the 3.1-inch model we originally used.
Answer:
Remember that a 29-inch wheel has a 1.5 inch greater radius. That means that not only is the front axle 1.5 inches higher off the ground, you also need to give 1.5 inch more clearance between the tire and the fork crown. All things being equal, the head tube on a bike with a 29-inch front wheel will be 3 inches higher off the ground than the head tube on a bike with a 26-inch front wheel. That's why 3.1-inch forks are so popular on 29-inch bikes--it lowers that 3 inch higher head tube by 0.80 inch. But if you still run the same bar and stem on both bikes the grips on the 29er will be 2.20-inch higher off the ground than the 26-inch bike. Flip-flopping a 5-degree-rise stem and running it in the negative position lowers the bar another inch. With no stem spacers, and by also using a low-rise bar, it was possible to achieve a 96er grip height that was only 0.25 inch higher than the current setup on our 26-inch Titus cross-country racer. You will notice that almost all 29-inch bikes run the stem in the negative rise position.
The 3.9-inch travel FRM Airway 100 Carbon 29er fork tested in December 2006's Thumbnail Thrash uses a fork crown that gives a little more tire clearance than the Reba. So even though the FRM fork gave 0.80 inch more travel, its axle-to-crown measurement is claimed to be only 0.60 inch more than the 3.1-inch travel Reba 29er. We say claimed because the actual measurement is even less; it's only 0.20 inch longer than the Reba. This is because the FRM fork runs negative sag, meaning that there is a spring that keeps the fork from fully topping out. That's why our axle-to-crown measurement was shorter than FRM's claim, which is a virtual reading if the fork were fully topped out.
The point of all this is that the 3.9-inch travel FRM Airway 100 29er fork only raised the head tube of the Project Titus 96er by 0.20 inch, a measurement that had no affect on the feel of the bike. The real beauty of the FRM fork is that it is the only 29-inch fork on the market with more axle offset. It's 0.25 inch more than other 29-inch forks, which all use the same offset as their 26-inch forks. FRM's increased offset reduces trail, which makes the steering quicker and much better suited to the more stable, slower turning 29-inch wheel.
The 3.9-inch travel Reba would slacken the head angle 3/4 degree more than the 3.1-inch travel Reba. Since it uses offset designed for a 26-inch wheel, it would turn too slowly.
A standard Racer X with a 26-inch wheel uses a 71 degree head angle. The reason Titus used a slacker 70.5 degree angle on our Project 96er is because they wanted to make certain there would be ample toe-to-tire clearance when turning. Our 96er gives a good 2 inches of clearance between the tips of Eddie Arnet's Sidis and Stan's The Crow 29er tire when the wheel is turned. (Toe overlap is a problem on some 29-inch bikes.) We feel that Titus should most likely have used a steeper head angle on our project bike. The bigger wheel is so much more stable and slower turning than a 26-inch wheel that it's possible to get by with a way steep head angle. Niner runs 72 degrees, and Intense uses a 73-degree head angle on their 29-inch bike.
But again, everyone is designing their bikes to use a fork that runs the same offset as a 26-inch fork. Only FRM offers a 29er fork with 29-inch correct geometry. And that forkmakes the 70.5 degree head angle of our Project Titus 96er perfect.
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