Shimano 12-Speed Sprockets in the 17-27T Range Are the Same as 9-Speed

The Altegra 12-speed sprocket from the previous article was
11-34T, but

↑This is a Dura-Ace 12-speed sprocket I shot separately for an article: 11-30T.
With 12-speed, the low gear cluster is
4 sprockets for 11-30T, but 5 sprockets for 11-34T.
And what caught my attention seeing this was that the three sprockets below the lowest gear are
21, 24, and 27T—quite an interesting combination.


I've written out the six gear combinations available for the 7700-series Dura-Ace 9-speed sprocket.
Of these, the three cross-ratio versions have a 16T, and
the 12-21T additionally has an 18T as well.

The reason 11-23T doesn't have an 18T is
because the configuration was "when adding one more sprocket to the 8-speed 12-23T,
we chose 11T instead of 18T."

As you can see, 9-speed has the configuration "if you want 25T or 27T on the low end,
you have to sacrifice top 11T." Speaking in my usual manner, 27T is
a tooth count that's "uncool to use regularly except during
hill climb races like Tsugaike or Yatsugatake and immediately before them." Back then, compact chainrings weren't widely adopted
（PCD 110mm was originally a crankset spec for touring bikes），
and while some makers like Ritchey Logic advocated for them,
they never became popular because road-racers（not road bikes）would look uncool.
So aside from the front triple option,
the lightest gear ratio generally available was 39×27T,
or 38×27T using a non-Shimano 38T inner ring.
Incidentally, 38×27T is roughly equivalent to 34×24T.


Of these, the tooth count for "the widest ratio"←remember this part
12-27T is written out.


For this 9-speed sprocket,
if we set three wishes:
"I'd like 11T on top~"
"I'd like 16T in the middle~"
"I'd like 30T on the low end~"

Then with 10-speed, one of them would be granted,
with 11-speed, two of them would be granted,
and with 12-speed, all three would be granted.

And Shimano's 12-speed road sprocket
11-30T has exactly this same tooth count configuration.


And for the 11-34T case,
the only difference is
"we removed the 16T from the 11-30T and added a 34T."


In other words, the current Shimano 12-speed sprocket
even if you choose the cross-ratio version,
as long as you shift within the 17-27T range,
it's no different from the "widest ratio" sprocket
of the 9-speed Dura-Ace.
Compared to 9-speed Dura-Ace's 12-27T,
Shimano's 12-speed 11-30T overlaps in 5 sprockets within the 17-27T range,
but with the 11-34T, there are 9 overlapping sprockets.
Having 9 sprockets overlap with a 9-speed sprocket means
they're all contained within it.
So the 12-speed 11-34T has the configuration of
adding 11T on the top end and 30T and 34T on the low end
compared to the 9-speed 12-27T.

Generally people say "more sprockets make it easier,"
but in my view, it's hardly changed.

The term "easier" is ambiguous,
and while it's not wrong to say "having a larger low gear makes climbing easier（but slower）",
the meaning of "easier" as in "filling in intermediate tooth counts makes it easier"
has barely been realized in recent multi-speed designs.


I created a speed table assuming flat ground riding on the outer chainring
at cadences of 90, 95, and 100 RPM.
Assuming a hobby racer,
the front chainring is 50T.
The tire is calculated at a 25C equivalent circumference.

When 15-19T is a 15, 17, 19 arrangement with 2T gaps,
and you shift one sprocket while maintaining the same cadence,
the speed changes by roughly 4-5 km/h.
This is a pretty large change, and the pedaling resistance
either eases up significantly or tightens up noticeably.

50×15T at 90 RPM and
50×17T at 100 RPM,

50×17T at 90 RPM and
50×19T at 100 RPM

are roughly the same speed,
which means that to maintain the same speed
with one sprocket shift within the table's range,
the cadence change required is
roughly plus or minus 10 RPM.


I created a full ratio from 15-19T by inserting 16T and 18T in between.
What changes as a result is:


（Within this extracted 15, 16, 17, 18, 19T range）
"to maintain roughly the same speed,
the cadence change needed
per sprocket shift comes down to just 5 RPM."

This reduces leg fatigue during shifts,
widens the options for whether to maintain a given speed
by pushing hard（low cadence）or spinning fast（high cadence），
which is the other meaning of "easier."


Here, the speed range for 90-100 RPM cadence for
15-17T (shown above in red) and
17-19T (shown in blue)
are marked with arrows.

Because 16T was added,
the shift pattern changed from
15→17→15T to
15→16→17→16→15T,

and because 18T was added,
the shift pattern changed from
17→19→17T to
17→18→19→18→17T,

and if we're talking about which offers more benefit,
for hobby racers, the latter seems more significant.
Very few hobby racers can sustain flat cruising speeds of nearly 40 km/h solo.
And if they could, they wouldn't often choose the 50T outer ring anyway,
so the table speed is calculated for the 50T case.

Even with a 50T, can you really maintain 100 RPM on 15T
for even 30 minutes?
So there's no point making a table with 52T or 54T speeds either! That's my thinking.

So if someone said I could add either 16T or 18T
to an 11-speed sprocket with tooth counts of
11, 12, 13, 14, 15, 17, 19, 21, 24, 27, 30T
to create a 12-speed sprocket,
18T would likely be more useful for most people,
but because of the convention "if you're filling in teeth, do it from the top",
there's no actual 12-speed with 18T filled in this way.

It's like in a golf club set with a 14-club limit,
the difficult-for-beginners 3-wood takes up a slot,
preventing the inclusion of other useful clubs suited to one's ability.
Unlike club sets, with sprockets the fact that
you can't freely choose the configuration
is inseparable from getting that excellent shifting performance, so it can't be helped.

Putter, approach wedge, utility, 7-iron, 9-iron,
and whether to include one wood—
as for that cult-like sect claiming 5-6 clubs is plenty
for sprockets（→here）, go there.

And there's more to the 18T story...


12-speed sprockets don't have 18T!
Period!
There are probably exceptions, but
Shimano's 11-30T and 11-34T, and the 11-36T in 105,
SRAM's 10-28T（by the way, 16T is only in this tooth count），
10-30T, 10-33T, 10-36T,
Campagnolo's micro-compact evolution
Super Record Wireless aside from the conventional
12-speed 11-29T, 11-32T, 11-34T,
none of the component makers' 12-speed sprocket
tooth count configurations have an 18T option.
Campagnolo has three tooth count options, and in all of them
the top 8 sprockets are
11, 12, 13, 14, 15, 16, 17, 19T,
so for pro riders doing flat riding,
the feel is the same regardless of which tooth count you choose,
and they always have 16T—that's the configuration.
For pro racers on flats with outer 53-54T,
having 16T on the easy end is helpful,
12-15T to 12-16T feels "easier," and in the table terms,
the outer chainring teeth are different and
the flat speed range is outside the table's columns,
so whether 18T is available
isn't really an issue.


↑So here's the image for the second time in this article,
but it's 15-19T with neither 16T nor 18T.
15, 17, 19T is a 2T gap that could be achieved
not just with 9-speed but even with 6-speed sprockets.
With current 12-speed combinations, you can only escape this
if you choose the most cross-ratio（lol）11-30T,
so there's no way "more sprockets made it easier"!
That's what I want to argue.

Current 12-speed does make climbing "easier" with larger low gears,
but flat cruising at around 40 km/h with outer×15-19T hasn't gotten "easier."
By the way, if you change the outer gear teeth in the table above
from 50T to 52T or 54T, the table's range
becomes more like 40 km/h flat cruising rather than nearly 40 km/h.

Also, whether 21T onward uses 2T or 3T gaps makes
a pretty big difference,
and the shifting from 21-27T changes the leg fatigue a lot
between 21, 23, 25, 27T versus 21, 24, 27T.
If a 28T low gear is light enough,
then instead of
21, 24, 27, 30T for 21T onward
（30T is unnecessarily light but can't be helped），
doing 21, 23, 25, 28T（only the last is 3T）
would fit nicely in the same 4 sprockets.
With 12-speed this could be
11, 12, 13, 14, 15, 16, 17, 19, 21, 23, 25, 28T,
and if makers put out
11, 12, 13, 14, 15, 16, 17, 18, 19, 21, 23, 25T
removing the 28T and adding 18T,
that would be very useful, but...
The 10-25T Super Record Wireless with micro-compact plus 10T top
is configured by removing 18T from the above and adding 10T.
Hmm, close but not quite.
Of the Campagnolo Gipiemme-exclusive 9-speed sprockets,
the 11-19T and 11-21T, the former has
consecutive natural numbers filling it for full cross-ratio,
and the Campagnolo 10-speed junior specification
Veloce-grade only 14-23T
is also full cross-ratio.
In winter using the 17, 18, 19, 20, 21, 22, 23T range of this,
when you switch back to a normal sprocket from spring onward,
you get the feeling of "19, 21, 23, 25T and such? Way too gappy!"

By now there are probably people who "as a personal road bike owner
have only ever ridden disc brake models," and
people fitting that condition who
also race for a pro team, too,
but such people won't feel
any particular sense of strangeness about hard frame flex points on disc-specific framesets
or the different center of gravity compared to rim brake bikes,
and won't have trouble throwing the bike around,
or particularly mind that the handlebar ends are heavy
or the steering heaviness unique to bikes with internal brake hoses
routed in the handlebar and stem.
So they'll probably think of low gears having several at 3T or
ultimately 4T gaps as just "that's how it is."
Because they have no basis for comparison.
Not that there's anything wrong with that,
but as equipment gets more high-performance, some techniques get lost.
There are probably quite a few people who
"have never ridden a mechanical groupset on a personally owned road bike,"
and while it's not a common situation, mechanical front derailleurs
are very hard for the chain to climb
when shifting from inner to outer on uphill grades, so
you need to stop pedaling once and
then drive hard with sudden full force.
But electric front derailleurs use motor brute force to
just whiiir it up, so
that kind of unweighting pedaling technique is unnecessary.
So without bias, people who've started road biking recently
often tend to be unskilled or unconcerned about this point.
As for me, over the last 20 years or so
chain drops aren't zero
（since entering 2024 through now it's been zero），
but I've had no trouble with front derailleur or front shift issues.

Because I don't have a front derailleur installed at all.
↑You're probably thinking "over 20 years, even you must have lost
the front shift unweighting pedaling technique," right?

But I can probably still do W-lever knee shifts while standing,
↑making a strange boast piling on incomprehensibly
Plus knee shifts are such a crappy technique that losing it is fine anyway