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The age of the 29'er is upon us. Not too many years ago, many of us turned our nose up at just the thought of riding a silly big-wheeled bike. The tables have turned, and many of us now swear by the 29-inch wheel. It's more than just a wheel size though. Geometry plays a big role, and the latest crop of slacked-out aggressive 29'ers we've been seeing over the last couple of years are really a whole new breed of mountain bike compared to 29'ers of the past. Transition has now entered this arena with the Sentinel - a buckwild, long, low, aggro 29'er sporting 140mm rear travel, designed around a 160mm travel fork.
The Sentinel utilizes a very unique approach to frame geometry, and Transition is calling it Speed Balanced Geometry (SBG). What's the scoop on SBG? Let's get into it - or if you don't care to endure the brain pain of learning the nitty gritty details, and you just want to know that it's awesome, then you can skip the whole next section called "What is SBG"?
What is SBG?
As bikes have evolved over the last several years, we've seen top tubes get longer, seat tubes shorter, head angle slacker, stems shorter, and bars wide. These are all arguably positive changes, and have helped to shape some of the most capable mountain bikes we've ever ridden. One area that hasn't changed is fork/steering trail. SBG incorporates the perfect balance of three key geometry traits, including reach, seat tube angle, head tube angle, and trail. Most of our familiar with the first three, but some of us may not be familiar with trail. What the heck is that?
Trail is a geometry dimension which takes into account both head tube angle and fork offset, which in turn affects steering feel and handling. It can be thought of as the tire contact point trailing behind the steering axis. Less trail equates to faster steering, effectively making the bike feel more nimble, but less stable at high speeds. More trail equates to slower steering, but provides more stability at higher speeds. With SBG, the head angle has been slackened, and the fork offset reduced, effectively providing more trail. That means the suspension fork on the 2018 Sentinel has a 43mm offset, which is 8mm shorter than a typical 29" suspension fork. The shorter offset fork brings the front axle rearward and under the rider, increasing front tire traction and provides more direct steering connectivity from the rider.
Head Tube Angle
Transition has slackened the head angle on all of their SBG bikes. The slacker head angle allows the fork to absorb impacts better at all angles, and reduces deflection. It also tends to reduce fork dive. positions the front wheel further forward in relation to the handlebars.
Seat Tube Angle
Seat tube angles on SBG bikes have been increased (steepened) by about 1.5 degrees. This puts your body in a more central location between your two tires while climbing, improving traction and reducing the amount of suspension sag while seated.
Transition's SBG bikes utilize a longer reach, but also are meant to be used with shorter stems, no longer than 40mm. Shorter stems provide a more direct steering path from your handlebars to your front wheel.
Transition believes they have achieved a balanced design here that effectively incorporates all of these factors, all while still achieving a nimble and responsive ride. This overlay effectively shows what an SBG bike looks like compared to Transition's previous model year bike.
That's it for SBG
The Sentinel utilizes metric shock sizing, as well as a Trunnion upper shock mount. Instead of a traditional DU bushing eyelet at the upper end of the shock, there are now two mounting ports on either side of the actual shock body that attach directly to either side of the main rocker link on the frame. Additionally, there are bearings housed in the frame linkage where it mounts to the upper end of the shock, reducing friction significantly compared to a standard DU bushing and traditional mounting hardware. The lower end of the shock uses a traditional eyelet and DU bushing. The Trunnion mount also effectively reduces the overall length of the shock pretty significantly, giving engineers more room to play with when designing frames, which is a big plus and will benefit us all in the long run.
The Sentinel Goes Carbon!
If there is one gripe folks have about the aluminum Sentinel, it's the weight. While alloy bike still rides great, there's no denying the fact that a nine pound alloy trail bike frame is a bit hefty. Transition's Sentinel Carbon frame weighs about 2.5 pounds lighter than the alloy version, which is a huge weight savings.
Other highlights include Boost 12x148mm rear dropout spacing, new rattle-free internal cable port covers, molded rubber downtube and chainstay protectors, as well as ample seat tube bore for compatibility with longer dropper posts. Boost dropout spacing has allowed for tire clearance up to 2.5". A threaded 73mm bottom bracket shell is another big plus in our book, as well as collet-style main pivot hardware makes for very easy maintenance. And for those who want to keep the weight off their back, the Sentinel has clearance for a full size water bottle inside the front triangle.
The Sentinel frame is backed by Transition's 3-year manufacturer warranty.
|Rear Shock:||Fox Float DPX2 Performance|
|Rear Suspension Travel:||140mm|
|Rear Shock Size:||205mm x 57.5mm|
|Rear Shock Hardware:||Trunnion Top / 8mm x 25mm Bottom|
|Rear Axle Width:||12x148mm Boost|
|BB Standard:||73mm BSA|
|ISCG Tabs:||ISCG '05 3-Bolt|
|Brake Mount Type:||Post-Mount 180mm|
|Max Rear Rotor Size:||203mm|
|Minimum Chainring Size:||28T|
|Maximum Chainring Size:||34T|
|Seat Post Diameter:||31.6mm|
|Seat Clamp Diameter:||37mm|
|Water Bottle Cage Mounts:||One|
cm / ft
|Rider Height (cm)||155 - 170||170 - 180||175 - 190||185 - 198|