science of shift

bike 085bike 085

gearhead [by chris lesser]

THE SCIENCE OF SHIFTINGWe 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 slow-

motion button and you’ll find that what happens in that narrow win-

dow 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

BIKP-070700-GEAR lyot 4/27/07 7:49 PM Page 085

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gearhead

The Stakes are HighImagine 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,

hours and days through a mechanical symphony of

variable gearing, but it can’t handle more torque

than what it takes to push a second hand around a

dial. The mill, however, can handle incredibly high

loads, but its single-speed gear ratio is locked in.

A mountain bike drivetrain, by contrast,

demands it all: Rolex-smooth shifting that can

simultaneously withstand the brute, donkey-

strength torque generated by the pedal mashing

of an out-of-the-saddle climb.

One of the first attempts to balance the twin

demands of precision shifting and torque was

Campagnolo’s Cambio Corsa system, circa 1940.

To shift between four gear choices, users first

had to loosen the rear axle quick release—while

riding—via a seatstay-mounted lever, then use

an adjacent lever to guide the chain to the next

gear before clamping the axle into its slotted

dropout. It was a revolutionary first step,

albeit a sketchy one.

In the ensuing 70 years of shifter

development, changing gears has

become less of a death-defying

feat and more of the instinctual

action we take for granted. The

evolution of parallelogram

derailleurs, which articulate

along a highly specified arc

to guide the chain onto the

next gear, has played a

major role in the evolution

of today’s high shifting stan-

dards. But there’s another

critical, if relatively unher-

alded, aspect of the shifting

story: the gears themselves. The

untold engineering hours imbed-

ded 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 chain-

rings and cassettes from practically every other

gear-driven contraption ever conceived.

“Most gears in the world today

are still round,” points out Ric

Hjertberg, FSA’s new technology man-

ager. “They’re made by taking big stacks

of steel rings and a cutter, which goes up

and down and cleaves out one notch at a time,

and then the big stack rotates.”

But bicycle gears’ complex shapes demand

extensive programming hours on a CNC mill,

Hjertberg says. “Cross angles for roller chains?

Gears that aren’t uniform? Lassoing spinning gears

with a roller chain? Nobody else does this stuff.”

To scratch the surface of what goes into

engineering these gears, just look at a cassette

cog or chainring and consider that the posi-

tion, shape and size of every scallop, rivet, pin

and recess owes its existence to decades of

engineers wrestling with the challenge of get-

ting steel roller chains to simultaneously

engage solidly with, and float fluidly over, a

range of gear combinations.

Seeing these static shift features is one thing.

Understanding how they work on a trail is

another matter entirely.

Anatomy of a ShiftOne relatively constant factor in the grand equation

of shifting is the actuation of the shift itself. Re-

gardless of whether the “click” of an indexed shift is

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

same relative effect on the chain—

which is to say, it suggests the

chain jump to the next gear.

And this is where those

thankless hours engineering

chainrings come into play—

turning that “suggestion” into

an offer the chain can’t refuse.

In shifting from a 32-

tooth to a 44-tooth chain-

ring, 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

transfer through the shift. But if either of the two

rings in question were rotated a single degree in

Different strokes for different folks. Clockwise frombottom left: Truvativ Noir, Shimano XTR, Race FaceAtlas AM and FSA K-Force.

This 30-year-old SunTour six-speed freewheel (left) hasinterchangeable cogs, tall, uniform teeth and no pre-deter-mined shift gates. The Shimano XTR cassette, by compari-son, is engineered as a system to shift up and down onlywhere the chain will transition smoothly to the next gear.


either direction, it would spell disaster. The chain

would ride on top of the gear teeth and when the

rider mashes on the pedal, the chain would slip

off the rings and break away like a trapdoor—the

sudden release of torque sending the rider’s body

crashing forward.

Next, consider the reverse shift: Approaching

a hill in a high gear, our crash test dummy flicks

the front shifter to drop the chain from the big

ring to the middle ring. The spring of the front

derailleur is released and the derailleur cage

swings inward and thwacks against the outside

of the chain. But freeze the frame. The crank

arms are lined up horizontally and the leading

pedal is preloaded with as much leverage as the

rider can generate. Under such load, the chain is

not going to change gears easily, and if it did,

such a sudden change in gear ratio would result

in a violently altered cadence, sending the rider

crashing forward, again. But the teeth on the big

chainring that line up with the front derailleur

at this precise moment have been engineered

with a slightly taller profile to dissuade the

chain from dropping down just yet.

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

point when the crank arms are aligned straight-

up-and-down, and notice how the teeth on the

big ring that line up with the front derailleur

are chamfered slightly. There also is a recess in

the chainring itself, opening up just enough

space to let the spring of the clattering front

derailleur push through its stroke and slide the

chain over—its rollers falling cleanly into the

troughs of the middle ring.

To Clock A ShiftThe “strategy” by which shift features are inte-

grated into chainrings relies largely on an under-

standing of how riders deliver power to the ped-

als. Aside from a very few mutant cyclists who

have an even spin, the rest of us pretty much

just mash down on our pedals.

Engineers who pay attention to this stuff

refer to this as sinusoidal power input, which

simply means that the human body’s muscu-

loskeletal system generates varying amounts of

leverage at different points in the pedal stroke,

with maximum power coming when the leading

crank arm is at 2 o’clock, and with the least

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

gearhead

There are a limited number of places on the circumference of any two given rings where a chain can transitionsmoothly off one gear and neatly into the next. Finding those places and then getting the chain to move there iscritical to smooth shifting.


gearhead

points where they want shifts to happen—and as

importantly, where they don’t want shifts to

happen. Each chainring configuration presents

unique challenges, but mountain biking’s fairly

standard 22-32-44 combination provides a solid

model to work from.

Most chainring manufacturers build two

sets of shift ramps into the inner surface of

the 32- and 44-tooth ring, and position them

so they line up with the front derailleur at the

moment either crank is at the 2 o’clock posi-

tion, the point in a pedal stroke where an

upshift feels best from a biomechanical stand-

point. Because chains consist of alternating

inner and outer link plates, shift ramps come

in pairs to account for both “chain phase” con-

tingencies—engineer-speak that describes

whether an inner or outer link will line up

at any given point on a gear.

Ramps and pins from different chainring

manufacturers come in all shapes and sizes—

some are hardened steel plates, others are

machined directly into the chainring—but in

each pair of ramps, one ramp is designed to

pick up an outer link and the other is designed

to pick up an inner link. And by positioning

pairs of ramps across from one another on the

chainring, it shouldn’t take more than a half a

crank revolution for the chain to catch a ramp

and complete the shift.

The ideal time to initiate a downshift is

when the least amount of chain torque is

being exerted on the system, which in terms

of our power-clocking schematic happens

when the crank arms are aligned vertically—

by extension, this is also where the derailleur

spring has the most influence on bending a

chain laterally. Chainring engineers can help

induce a downshift by chamfering the edges of

specific chainring teeth, creating angled tooth

profiles and shaving away material where the

shift needs to happen.

Locating those shift ramps and recesses is

always a compromise because the best place to

engineer a shift one way might get in the way of

the shift features you need to let the chain shift

back the other way.

Why Chains SuckOnce we have the up-shift and downshift fea-

tures dialed, we’re only two-thirds of the way

home. The last third of the equation, according

to Garrett Smith, Truvativ’s resident chainring

guru, is the relative clocking of each chainring

with adjacent chainrings. The goal, he says, is to

get the chain to slide nicely into the next ring’s

teeth as it rolls from gear to gear.

Timed perfectly, in the instant between

shifts, a chain will be engaged on both gears

simultaneously. If the chain doesn’t line up

perfectly in the ring it is shifting to, it will ride

on top of the next ring’s teeth while staying

locked onto the primary chainring. Tension will

keep the chain bridged across the two rings

until it binds up underneath the bottom brack-

et, which will stop the rider faster than a

squirrel in the spokes.

“In the end, shifting on a bike is always a

compromise,” Smith says. “Because the ideal

clocking for making the perfect downshift will

not necessarily be the perfect clocking for a

perfect upshift, or for preventing chainsuck.”

Changing the relative clocking of two adjacent

chainrings by as little as a quarter of a degree

can make the difference between chainsuck or

a clean shift.

If these subtly varying shift ramps still seem

trivial, consider this: the new Shimano XTR

crank comes in two gearing variations, a 22-32-44

and a 24-32-44. The only difference between the

two appears to be the 22- or 24-tooth chainring,

right? Wrong. Because of clocking considerations

and because the whole system is interconnected,

changing the granny gear by just two teeth

means Shimano needed to engineer a whole

new middle ring, too. >

The secret to shift-ing doesn’t comeeasily, and compa-nies protect thedimensions of everylast gear tooth withextensive patents.


gearhead

Bringing Up the RearDespite 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 GoingShifting 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 thou-

sands 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 com-

pany 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 per-

formance 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 vari-

ous manufacturers and spread out across the world, are constantly test-

ing every combination of chainrings, cassettes and chains to see where

they can improve next. And ten-speed, they say, isn’t far away.


science of shift

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