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[by chris lesser]
THE SCIENCE OF SHIFTING We shine light on the crucial, untold secrets of shifting gears
MODERN MOUNTAIN BIKE TRANSMISSIONS LET USERS RIFLE THROUGH SHIFTS without missing a beat. That tactile sensation between clicks and taps of the shifters and the corresponding sound of the chain dancing across and slipping into exactly the right gear happens hundreds of times during a long ride, and for most mountain bikers shifting is an ingrained, if seldom examined action. But here’s the rub: There’s a lot more to shifting than just shifters and derailleurs. Break out the magnifying glass and hit the slowmotion button and you’ll find that what happens in that narrow window of time between shifting and shifted takes place within a moving mechanical microcosm of ramps, pins, chamfers, bevels, recesses and mind-numbing attention to detail—all working in concert to produce seamless shifts. >
PHOTOS: MORGAN MEREDITH
bike
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The Stakes are High Imagine if you will the wide world of gears, with the precision engineering of a Swiss-made Rolex at one end and a donkey-driven grinding mill at the other. The Rolex can synchronize seconds, minutes,
“Most gears in the world today are still round,” points out Ric
hours and days through a mechanical symphony of
Hjertberg, FSA’s new technology man-
variable gearing, but it can’t handle more torque
ager. “They’re made by taking big stacks
than what it takes to push a second hand around a
of steel rings and a cutter, which goes up
dial. The mill, however, can handle incredibly high
and down and cleaves out one notch at a time,
loads, but its single-speed gear ratio is locked in.
and then the big stack rotates.”
A mountain bike drivetrain, by contrast,
But bicycle gears’ complex shapes demand
This 30-year-old SunTour six-speed freewheel (left) has interchangeable cogs, tall, uniform teeth and no pre-determined shift gates. The Shimano XTR cassette, by comparison, is engineered as a system to shift up and down only where the chain will transition smoothly to the next gear.
demands it all: Rolex-smooth shifting that can
extensive programming hours on a CNC mill,
simultaneously withstand the brute, donkey-
Hjertberg says. “Cross angles for roller chains?
strength torque generated by the pedal mashing
Gears that aren’t uniform? Lassoing spinning gears
ting steel roller chains to simultaneously
of an out-of-the-saddle climb.
with a roller chain? Nobody else does this stuff.”
engage solidly with, and float fluidly over, a
One of the first attempts to balance the twin
To scratch the surface of what goes into
range of gear combinations.
demands of precision shifting and torque was
engineering these gears, just look at a cassette
Campagnolo’s Cambio Corsa system, circa 1940.
cog or chainring and consider that the posi-
Understanding how they work on a trail is
To shift between four gear choices, users first
tion, shape and size of every scallop, rivet, pin
another matter entirely.
had to loosen the rear axle quick release—while
and recess owes its existence to decades of
riding—via a seatstay-mounted lever, then use
engineers wrestling with the challenge of get-
Seeing these static shift features is one thing.
Anatomy of a Shift
an adjacent lever to guide the chain to the next
One relatively constant factor in the grand equation
gear before clamping the axle into its slotted
of shifting is the actuation of the shift itself. Re-
dropout. It was a revolutionary first step,
gardless of whether the “click” of an indexed shift is
albeit a sketchy one. In the ensuing 70 years of shifter development, changing gears has
made by the push, pull, dab or twist of a shifter, it has the same relative effect on the derailleur, and that derailleur has the
become less of a death-defying
same relative effect on the chain—
feat and more of the instinctual
which is to say, it suggests the
action we take for granted. The
chain jump to the next gear.
evolution of parallelogram
And this is where those
derailleurs, which articulate
thankless hours engineering
along a highly specified arc
chainrings come into play—
to guide the chain onto the
turning that “suggestion” into
next gear, has played a
an offer the chain can’t refuse.
major role in the evolution of today’s high shifting standards. But there’s another critical, if relatively unheralded, aspect of the shifting story: the gears themselves. The untold engineering hours imbedded in the gears used by modern mountain bikes—right down to the shape, angle, radius and offset of each and every tooth and trough—separate today’s chainrings and cassettes from practically every other gear-driven contraption ever conceived.
In shifting from a 32tooth to a 44-tooth chainring, for example, strategically placed ramps and pins built into the side of the big ring mate precisely with the outside profile of the chain, helping coax it onto the larger gear. Look close and you’ll see that certain teeth on the outer ring are scalloped ever so slightly to allow the chain to cross onto the bigger gear. Now, pause the frame in mid-shift. The chain, taut with torque, is simultaneously engaged on two rings at once, allowing for constant power
Different strokes for different folks. Clockwise from bottom left: Truvativ Noir, Shimano XTR, Race Face Atlas AM and FSA K-Force.
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transfer through the shift. But if either of the two rings in question were rotated a single degree in
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There are a limited number of places on the circumference of any two given rings where a chain can transition smoothly off one gear and neatly into the next. Finding those places and then getting the chain to move there is critical to smooth shifting.
either direction, it would spell disaster. The chain
point when the crank arms are aligned straight-
would ride on top of the gear teeth and when the
up-and-down, and notice how the teeth on the
rider mashes on the pedal, the chain would slip
big ring that line up with the front derailleur
off the rings and break away like a trapdoor—the
are chamfered slightly. There also is a recess in
sudden release of torque sending the rider’s body
the chainring itself, opening up just enough
crashing forward.
space to let the spring of the clattering front
Next, consider the reverse shift: Approaching
derailleur push through its stroke and slide the
a hill in a high gear, our crash test dummy flicks
chain over—its rollers falling cleanly into the
the front shifter to drop the chain from the big
troughs of the middle ring.
ring to the middle ring. The spring of the front derailleur is released and the derailleur cage
To Clock A Shift
swings inward and thwacks against the outside
The “strategy” by which shift features are inte-
of the chain. But freeze the frame. The crank
grated into chainrings relies largely on an under-
arms are lined up horizontally and the leading
standing of how riders deliver power to the ped-
pedal is preloaded with as much leverage as the
als. Aside from a very few mutant cyclists who
rider can generate. Under such load, the chain is
have an even spin, the rest of us pretty much
not going to change gears easily, and if it did,
just mash down on our pedals.
such a sudden change in gear ratio would result
Engineers who pay attention to this stuff
in a violently altered cadence, sending the rider
refer to this as sinusoidal power input, which
crashing forward, again. But the teeth on the big
simply means that the human body’s muscu-
chainring that line up with the front derailleur
loskeletal system generates varying amounts of
at this precise moment have been engineered
leverage at different points in the pedal stroke,
with a slightly taller profile to dissuade the
with maximum power coming when the leading
chain from dropping down just yet.
crank arm is at 2 o’clock, and with the least
Now, slowly advance the frame and see how, as the forward crank arm rotates past the 5 o’clock position, torque on the chain lessens. Inch forward another couple frames, to the
amount of leverage exerted when the crank arms are oriented in the 12/6 o’clock positions. Thus armed with an idea of how the motor is behaving, shift engineers design the specific
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The secret to shifting doesn’t come easily, and companies protect the dimensions of every last gear tooth with extensive patents.
points where they want shifts to happen—and as
always a compromise because the best place to
importantly, where they don’t want shifts to
engineer a shift one way might get in the way of
happen. Each chainring configuration presents
the shift features you need to let the chain shift
unique challenges, but mountain biking’s fairly
back the other way.
standard 22-32-44 combination provides a solid model to work from. Most chainring manufacturers build two
Why Chains Suck Once we have the up-shift and downshift fea-
sets of shift ramps into the inner surface of
tures dialed, we’re only two-thirds of the way
the 32- and 44-tooth ring, and position them
home. The last third of the equation, according
so they line up with the front derailleur at the
to Garrett Smith, Truvativ’s resident chainring
moment either crank is at the 2 o’clock posi-
guru, is the relative clocking of each chainring
tion, the point in a pedal stroke where an
with adjacent chainrings. The goal, he says, is to
upshift feels best from a biomechanical stand-
get the chain to slide nicely into the next ring’s
point. Because chains consist of alternating
teeth as it rolls from gear to gear.
inner and outer link plates, shift ramps come
Timed perfectly, in the instant between
in pairs to account for both “chain phase” con-
shifts, a chain will be engaged on both gears
tingencies—engineer-speak that describes
simultaneously. If the chain doesn’t line up
whether an inner or outer link will line up
perfectly in the ring it is shifting to, it will ride
at any given point on a gear.
on top of the next ring’s teeth while staying
Ramps and pins from different chainring
locked onto the primary chainring. Tension will
manufacturers come in all shapes and sizes—
keep the chain bridged across the two rings
some are hardened steel plates, others are
until it binds up underneath the bottom brack-
machined directly into the chainring—but in
et, which will stop the rider faster than a
each pair of ramps, one ramp is designed to
squirrel in the spokes.
pick up an outer link and the other is designed
“In the end, shifting on a bike is always a
to pick up an inner link. And by positioning
compromise,” Smith says. “Because the ideal
pairs of ramps across from one another on the
clocking for making the perfect downshift will
chainring, it shouldn’t take more than a half a
not necessarily be the perfect clocking for a
crank revolution for the chain to catch a ramp
perfect upshift, or for preventing chainsuck.”
and complete the shift. The ideal time to initiate a downshift is
Changing the relative clocking of two adjacent chainrings by as little as a quarter of a degree
when the least amount of chain torque is
can make the difference between chainsuck or
being exerted on the system, which in terms
a clean shift.
of our power-clocking schematic happens
If these subtly varying shift ramps still seem
when the crank arms are aligned vertically—
trivial, consider this: the new Shimano XTR
by extension, this is also where the derailleur
crank comes in two gearing variations, a 22-32-44
spring has the most influence on bending a
and a 24-32-44. The only difference between the
chain laterally. Chainring engineers can help
two appears to be the 22- or 24-tooth chainring,
induce a downshift by chamfering the edges of
right? Wrong. Because of clocking considerations
specific chainring teeth, creating angled tooth
and because the whole system is interconnected,
profiles and shaving away material where the
changing the granny gear by just two teeth
shift needs to happen.
means Shimano needed to engineer a whole
Locating those shift ramps and recesses is
new middle ring, too. >
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gear head Bringing Up the Rear Despite having three times as many gears in the rear as in the front, rear shifting is actually less complicated because engineers don’t have to factor in the varying torque influence of the crank arms. Just as with front chainrings, there are a limited number of places on the circumference of a cassette cog where the teeth line up to allow the chain to mesh cleanly from one gear to the next. By identifying these critical geometries, and by lining them up on an indexed freehub body, engineers can create “shift gates,” or sectors of corresponding cogs where two or three tooth profiles are manipulated just enough to encourage the chain to pass up or down the cassette. Look straight down on a cassette and you’ll see a series of wave-like scallops on opposing sides. These are the “off-ramps” and “on-ramps” where the chain will, by design, want to shift. If all goes according to plan, the shift will happen in the specified shift gate and the chain will line up evenly in the troughs of the gear it’s shifting to. But lining up nine cogs to work as a harmonious package is easier said than done. Shimano was the first company to figure this out back in 1989, with its Hyperglide system, and now a close view of a chain shifting through a cassette looks a lot more like a snake slithering through the gears than a chain riding up, over, and then down into the gears.
Where Shifting is Going Shifting may be one of the most overlooked aspects of cycling, but the technology that lessens the time in between shifts is one of the most scrutinized and hotly contested fiefdoms in the whole of cycling’s intellectual property landscape. A good portion of the thousands of claims Shimano has filed with the U.S. patent office relate to the nuances of how a chain is shifted between gears. Because Shimano’s engineers study shifting so closely, and because the company also makes chains, derailleurs and shifters to accompany its chainrings and gears, “Systems Engineering” is unavoidable—much to the chagrin of competitors trying to field compatible products into the marketplace. For the most part, other chainring and cassette manufacturers have had to reverse engineer what Shimano has done so that their components play nice with Shimano parts. But now that SRAM has enough brands under its umbrella to engineer its own systems, and now that FSA is reportedly working on its own shifters, competition should heat up. As mountain biking has developed as a sport, “standards” have come and gone as axle spacing dimensions and freehub body designs have evolved to make room for more gears. Where once there were five gears, now there are nine. Freewheels have given way to freehub bodies and, gear for gear, there is less real estate to work with. Chains are narrower, spacing between gears is closer and tolerances are tighter. Through this evolution, drivetrain engineers have had to not just keep up with shifting speed and accuracy, but also improve performance at the same time. While there always will be stubborn retro-grouches who swear by their 30-year-old SunTour friction shifters, and while others flat-out prefer the simplicity of single speeds, a cadre of shift engineers, employed by various manufacturers and spread out across the world, are constantly testing every combination of chainrings, cassettes and chains to see where they can improve next. And ten-speed, they say, isn’t far away.