Transition's New Bike: The Sentinel with Speed Balanced Geometry

Transition Bikes, located in Bellingham, WA, recently announced that they were introducing significant alterations to its existing frame geometry, later this year. They have invented an acronym, SBG or “Speed Balanced Geometry” to represent a new philosophy in the ongoing evolution of mountain bike geometry. The key to developing successful frame geometries lies in the ability to design frames that handle well on both climbing and descending, without severely hampering one or the other. In a world where mountain bikes are becoming more specialized, i.e., XC, AM, DH, and enduro, a one-bike-for-all-terrain is a stiff challenge to aspire towards.

Visit Transition Bike’s website and chances are you’ll be greeted by a deranged-looking, motley crew of Viking warriors, wizards, and priests that are passionate about building and riding mountain bikes. If you survive their gauntlet, you may be rewarded with glimpses of their fine array of assault vehicles; not quite your ordinary wooden long-ships, dragon-ships or knorr*, but sleek metal war machines designed to slay mountainous terrain. (*Knorr, could the ancient Vikings have foreseen the usage of knar to describe burly trail-riding?)

Vikings aside, let’s take a serious look at Transition’s SBG design philosophy, which centers around numerous points on a mountain bike’s frame and fork, with the goal of improving a rider’s positioning between the front and rear wheels to achieve increased traction, descending, and climbing, while offering improved pedaling and less front wheel slide/push.

Let’s define some terminology, which is not equally employed by one engineer to the next:

Head Tube Angle

Also called “steering axis angle” is the measurement taken from an imaginary line running through the center of the head tube, down to where it intersects a horizontal plane; you could use the center of your bike’s bottom bracket or center of the rear triangle’s axle as horizontal references for this measurement. Head tube angle in conjunction with fork offset is manipulated to influence the effort required to turn and lean a front wheel. A steeper, upright, head tube will require less effort to turn and lean the wheel and it will improve climbing. A slacker, less upright head tube angle will provide greater stability at higher speeds and on descents, but will be less responsive to steering and climbing. Of course, this is not an isolated element and is affected by fork offset, trail*, and other factors. (* Trail is discussed under fork offset.)

Fork Offset

Defined as the distance between the center of the front axle from an imaginary line drawn thru the center of the fork’s steerer tube. So, imagine looking in and through the center of your bike fork’s steerer tube, until that imaginary line reaches the same horizontal plane as the center of your front axle. Then measure that distance. Voilà, that’s your fork’s offset. However, the aforementioned is really the axle offset, as true fork offset is a measurement of the distance the fork’s crowns are situated in front of the steerer tube. You will see more engineers use the initial description versus the actual one, the latter.

Both fork offset and axle offset, with the addition of fork length, equal “total offset” relative to the bike’s head tube angle. These parameters are manipulated to create a bike’s “trail” which is a measurement based on running our imaginary line through the center of the steerer tube, all the way to the ground. Then, look at where your front tire initially contacts the ground, and measure the distance from the steerer tube’s central axis (where it touches the ground) to the area of the tire’s first contact with the ground, at the midpoint of that contact region. That’s your bike’s “trail”. In general, a higher trail number means increased high speed stability but at reduced speeds, greater wheel flop, which refers to the lowering of the front end of the bicycle whenever the front fork is turned. Wheel flop affects steering by allowing easier steering at low speeds. However, if there is excessive wheel flop, the bike will not hold a straight line when climbing. A bike with more trail will have more wheel flop, yet because of the increased trail, it will be more stable. On straight pathways, one neutralizes the other, but on cornering, where wheel flop is less prevalent, and trail stability shines brightly.

Stem Length

This refers to three parameters: height, angle or rise, and reach. Choosing a correct stem depends on style of frame, length of its top tube, type of terrain ridden, rider height and reach, angle or rise of the stem, and stem length. Stem length is the measurement taken from the center of the headset’s cap bolt to the center of the handlebar. Stem length has a profound impact on mountain bike handling. A longer stem will require more steering input, which will feel slower, more sweeping. With a shorter stem, the bike will provide more nimble steering with less input required to effect the desired steering. In addition, a shorter stem while descending will make it easier to shift your weight towards the rear of your bike, which will allow you to focus your weight on both tires, equally and over the bottom bracket. However, a shorter stem can make you feel more cramped. Whether you prefer a longer or shorter stem length, your bike’s top tube length will be a critical factor in what works best for you, as well as everything aforementioned in our discussion.

Seat Tube Angle

Determined by running an imaginary axis through the center of the seat tube, and extending it until it touches the ground. The angle formed between that point of contact and any horizontal plane that intersects that seat tube axis determines seat tube angle. A less or shallow seat tube angle moves the rider forward, relative to the bottom bracket, while conversely, a larger or steeper seat tube angle moves the rider rearward, past the bottom bracket. However, these positioning effects can be neutralized via fore and aft saddle adjustment, seatpost offset, and handlebar position. Yet, if excessive movement in either direction is encountered, bicycle handling will be hampered because overly steep seat tube angles places more weight on the rider’s shoulders and arms, making the bike harder to handle.

Frame Reach

Defined as a measurement  taken between a line running downwards and thru the center of a frame’s head tube, and extending until it intersects any horizontal plane, and a vertical line ascending perpendicularly thru the center of the frame’s bottom bracket until both lines intersect. This measurement indicates how the bike will perform while the rider is standing out-of-the-saddle. This method of measurement is appropriate for today’s aggressive trail and mountain-type riders, whom are out-of-saddle more regularly versus sitting on flat trails. Shorter reach equals shorter front axle to BB, and longer reach equates to longer front axle to BB. However, and like all things related to bicycle geometry, individuals parameters do not reflect the synchronous nature of all elements taken as a whole.

Transition's Philosophy On All Of This

Now, let’s move on and examine the claims made by representatives for Transition Bikes, which emphasized the five bulleted points that were explained above, while stating that on their upcoming new frames, slacker head tube angles will be incorporated, which will afford the front fork the ability to absorb shock better, irrespective of the angle of contact and moves the front wheel forward, relative to the handlebars, which improves rear wheel traction under breaking. Secondly, a shorter fork offset will be utilized, which like the shallower head tube angle, moves the front axle rearward to increase front tire traction. Thirdly, their bikes will feature shorter recommended stem lengths in order to keep the fork offset and stem length similar, which affords a slightly longer reach on the bike. Fourthly, seat tube angles will be larger/steeper in order to assist climbing traction by centralizing the rider between both tire’s contact points with the ground. And, finally, longer frame reach. Together, those five parameters will positively affect the bike’s handling capabilities, regardless of whether the rider is seated or in a standing position while pedaling. By increasing the bike’s length, while placing the rider more forward and centrally over the contact regions of the wheels, traction is enhanced.

The brain trust at Transition Bikes seems to appreciate the interrelationship between all aspects of frame building geometry, including the front fork, tire size, saddle positioning, and rider variability. Positioning the rider centrally between both tires, affords riders the ability to stand directly upon both feet, whether on the balls or flat footed, which will maximize core muscle strength where bodily movements originate from, and allow maximal range-of-motion of bodily appendages. It is from this posture(s) that bicycle control and power is predicated on. The so-called “squat” position on a bike with arms and legs half bent is very effective in terms of muscle function because a rider possesses greater muscle strength, balance, agility, and speed, when muscles are in a partially contractile state versus a fully extended state. This principle applies to all human athletic endeavors. We look forward to the introduction of Transition’s Sentinel bike in the Fall of 2017. It may provide a glimpse into the realm of where mountain bike frame geometries will be heading along the evolutionary journey of cycling, in the near future.