SRAM's MTB component parts include
a PG-720 downhill-spec
7-speed sprocket compatible with HG freebody.
The tooth count is 11-13-15-17-19 in 2T increments,
then -22-25 in 3T increments,
making it an 11-25T configuration.
Even though it's called 7-speed, the tooth spacing
is the same as 11-speed, with the four cogs on the low end
filled in with plastic spacers.
Since it's an entry-level model, apparently,
the sprocket doesn't use spider arms—
it's designed with spacers and cogs alternating one by one.
The material is steel (chromoly according to the maker).
The retail price including tax is ¥6,600, and the stated weight is 224g,
but considering Shimano Dura-Ace R9200's
11-30T is listed at 223g
and 11-34T at 253g,
you could say it's light.
There's a higher-end version of this 7-speed sprocket concept
called the XG-795, compatible with XD freebody,
with a tooth count of 10-24T,
one tooth smaller across the board compared to the PG-720,
and the material is listed as chromoly with
extreme machining—a stated weight of just 136g, super lightweight.
But the retail price is a staggering ¥71,500,
making your wallet lighter too.
↑Like this (PG-720)
The rear hub is an FH-9000,
which uses 11-speed road splines,
so I've installed an 1.85mm spacer
before the plastic spacers.
After adjusting the shifting, I thought it might work
if I just screwed the low-side adjust bolt all the way in,
but as expected, it's beyond the range the rear derailleur was designed for,
and the bolt tip has come off the projection it's supposed to rest against,
so I need to do something about it.
When I tried to shift to the 8th cog from the top,
it easily tried to override the spacer,
so while I haven't actually done it, the chain could easily drop inward.
The rear derailleur is a Campagnolo Athena 11-speed,
and from the moment I opened the box (※), the parallelogram plates
have lightening holes on both inner and outer sides. The inner plate has
a round hole and a U-shaped cross-section,
so I'll use this to create a "bolt resting point."
※I didn't write "from the moment I opened the box"
With the rear wheel off, positioning the adjust bolt
roughly where the parallelogram would be
when shifted to low on 11-speed looks like this.
I installed the rear wheel and shifted to 7-speed low.
At that parallelogram position,
there's no projection to catch the bolt tip.
This is a non-standard longer bolt,
and as I tighten it, this happens.
The bolt's threads just barely scrape
the underside of the inner parallelogram plate.
I cut out the flat section from the end of
a modern Ridley fork column with D-shaped cross-section
(used for routing disc brake hoses)
↑and made something like this.
Two carbon plates were
glued together.
The stepped section is designed to fit
into the U-shaped form of the inner parallelogram plate.
Well, this was a failed attempt.
Materials are limited, but I still have some.
Colnago fork columns are also D-shaped,
but they have threaded holes for the top cap bolt embedded
and reinforcement material filled inside—
they're not hollow.
Either way, this type of D-section column has weak
clamping force against stem clamp bolt torque, and with a stem
that has a narrow clamp gap, you sometimes can't secure the stem
until the clamp is nearly completely closed.
↑Finished version.
The right part has unnecessary holes from
trial and error—the "error" part—but I'll lie and say
they're for weight reduction and to improve adhesive grip.
The left part is basically a huge square washer.
The back side of the right part looks like this
The low-side adjust bolt's threads are
standard M4, but I needed it longer than stock,
so I bought a stainless bolt and machined it.
As-is, the bolt head diameter was too large
to fit in the parallelogram hole, so I filed it down.
In the image above, left is original, right is after machining.
I secured the part with an M5 bolt.
Fortunately, for practical purposes,
there was a huge hole in the outer plate.
With it shifted to 7-speed low,
I tightened the adjust bolt until it hit the carbon plate.
The ErgoShift lever clicks into 8-speed, but the chain clatters,
so I can just shift back to 7-speed immediately—OK.
There's no way it'll go into 9-speed.
Unlike before I made this, the derailleur no longer tries
to override the plastic spacer.
The adjust bolt is doing its job.
↑This is shifted to top 11T.
The carbon plate's protrusion doesn't interfere
with any other part of the rear derailleur
except in low gear.
The reason I did all this
comes from a fanatical conviction that
road bikes are more honorable the less surface area they have—
a kind of puristic primitivism taken to extremes.
By the way, the crankside has a 44T single chainring,
which in terms of gear ratios equates to
39×22.1T or 34T×19.3T.
Unless the climb is a seriously steep pass,
you can generally still get up it.
The previous sprocket had 11T as the smallest,
then all 2T jumps after that:
11-12-14-16-18-
20-22-24-26-28-30T
on an 11-speed cassette,
but I barely used the 30T, mostly stuck to 26T,
and only occasionally hit 28T.
I cleaned the old sprocket.
From the top gear onward, it's all 2T jumps,
so the line of tooth tips forms
a single straight line.
Not a sombrero shape.
I must have done quite a bit of climbing,
but looking at the valleys of the low gear,
the color hasn't rubbed off—
there's barely any wear at all.
↑Bottom right in the image is the 11T top gear,
but the three sprockets I use most on flat terrain—14, 16, and 18T—
show ω-shaped wear in the valleys from the chain rollers.
The reason I bought this sprocket in the first place was
because of a severe supply shortage of Shimano components—
everything from Dura-Ace down to 105
was completely unavailable—
but as a result, I got something interesting.
According to the maker, the high-durability chromoly material
supposedly offers 2.5 to 3 times the durability
of Dura-Ace sprockets.
If that figure was comparing to just the titanium low gear section,
I thought that was a sleazy way to phrase it,
but in actual use, even comparing steel sections to steel,
the durability is clearly at least double.
This is from a past post, but this sprocket
only engages the splines at the very back and front.
This hub's freebody is titanium,
but if it were aluminum, it might wear in an unpleasant way.
I removed the sprocket.
Directly below the sprocket's hollow section,
black metal dust mixed with chain oil is caked on,
but it wipes away easily.
As for what this dust actually is,
I believe it's mostly chain roller shavings worn away
by the sprocket's superior hardness.
The sprocket's high durability is real,
but the chain roller wear rate is definitely not normal.
Shimano and other component makers could presumably
make harder sprockets—
third-party makers manage to do it—
but they presumably select an appropriate hardness
for a mutually balanced wear relationship with the chain.
The same applies to brake shoes and rims;
brake shoes harder than the stock specification
accelerate rim wear.
I'm not saying that's bad—
SwissStop BXP's rain braking performance
is outstanding compared to other shoes.
But complaining about rim wear as the price for that
would just be wrong. That's all I mean.
The crank is a Campagnolo Ekar,
and the chainring is the stock narrow-wide 44T,
but I've installed a homemade chain watcher.
Ekar chainrings are resistant to dropping,
but on the Osaka side descent through Yamagata-Kei valley,
at a specific gap midway down, the chain
almost always drops, so I installed this.
After that, in that one situation where it used to drop,
it never drops, and it hasn't dropped
in any other situation either.
But looking at this,
it's clearly doing heavy work...
↑This is with it on 11T top gear,
but when in low 30T, the chain's upper run
comes much closer to the watcher.
That gap that almost always causes drops—
the chain doesn't drop if you're running it in low gear territory
because there's high chain tension.
But shifting to low gear on a descent isn't normal.
This time, the low gear went from 30T to 25T.
When I checked by virtually tightening the chain until it was taut,
I could shorten it by 7 links.
So if I were to cut the chain,
6 links (3 pitches) would be a bit tight,
but 4 links (2 pitches) I could definitely remove.
The fact that 25T isn't at the normal low-gear position
but 7th from the bottom might also be related
to how many links can be shortened.
With the Ekar crank, the chain originally
barely dropped even without the watcher
in all but extreme specific situations,
so instead of "44×11T chain length calculated for 30T,"
with "25T as the actual limit on 44×11T chain length,"
there's a possibility it'll work fine without this.
So I decided to remove it.
If I ever reinstall it,
it'll be mounted lower than before.
With a round seat tube lacking a direct-mount bracket
and absolutely nothing front derailleur-like attached,
it has a very clean appearance.
I wiped it clean and applied basic coating.
You can faintly see the band marks,
but if I really went for it, those could be removed.
↑The removed chain watcher
The carbon seat tube's outer diameter is 30mm.
So with an inner-outer diameter difference of 2mm,
I cut a seatpost shim with 1mm radius and thickness
and installed it inside a 31.8mm inner diameter E-type front derailleur band.
But when I actually tightened it,
the clamp was closing before full tightness,
so I applied thin aluminum tape
and adjusted it by sanding smooth.
Looking at this, I have a feeling
I'll be needing its help again pretty soon...
a PG-720 downhill-spec
7-speed sprocket compatible with HG freebody.
The tooth count is 11-13-15-17-19 in 2T increments,
then -22-25 in 3T increments,
making it an 11-25T configuration.
Even though it's called 7-speed, the tooth spacing
is the same as 11-speed, with the four cogs on the low end
filled in with plastic spacers.
Since it's an entry-level model, apparently,
the sprocket doesn't use spider arms—
it's designed with spacers and cogs alternating one by one.
The material is steel (chromoly according to the maker).
The retail price including tax is ¥6,600, and the stated weight is 224g,
but considering Shimano Dura-Ace R9200's
11-30T is listed at 223g
and 11-34T at 253g,
you could say it's light.
There's a higher-end version of this 7-speed sprocket concept
called the XG-795, compatible with XD freebody,
with a tooth count of 10-24T,
one tooth smaller across the board compared to the PG-720,
and the material is listed as chromoly with
extreme machining—a stated weight of just 136g, super lightweight.
But the retail price is a staggering ¥71,500,
making your wallet lighter too.
↑Like this (PG-720)
The rear hub is an FH-9000,
which uses 11-speed road splines,
so I've installed an 1.85mm spacer
before the plastic spacers.
After adjusting the shifting, I thought it might work
if I just screwed the low-side adjust bolt all the way in,
but as expected, it's beyond the range the rear derailleur was designed for,
and the bolt tip has come off the projection it's supposed to rest against,
so I need to do something about it.
When I tried to shift to the 8th cog from the top,
it easily tried to override the spacer,
so while I haven't actually done it, the chain could easily drop inward.
The rear derailleur is a Campagnolo Athena 11-speed,
and from the moment I opened the box (※), the parallelogram plates
have lightening holes on both inner and outer sides. The inner plate has
a round hole and a U-shaped cross-section,
so I'll use this to create a "bolt resting point."
※I didn't write "from the moment I opened the box"
With the rear wheel off, positioning the adjust bolt
roughly where the parallelogram would be
when shifted to low on 11-speed looks like this.
I installed the rear wheel and shifted to 7-speed low.
At that parallelogram position,
there's no projection to catch the bolt tip.
This is a non-standard longer bolt,
and as I tighten it, this happens.
The bolt's threads just barely scrape
the underside of the inner parallelogram plate.
I cut out the flat section from the end of
a modern Ridley fork column with D-shaped cross-section
(used for routing disc brake hoses)
↑and made something like this.
Two carbon plates were
glued together.
The stepped section is designed to fit
into the U-shaped form of the inner parallelogram plate.
Well, this was a failed attempt.
Materials are limited, but I still have some.
Colnago fork columns are also D-shaped,
but they have threaded holes for the top cap bolt embedded
and reinforcement material filled inside—
they're not hollow.
Either way, this type of D-section column has weak
clamping force against stem clamp bolt torque, and with a stem
that has a narrow clamp gap, you sometimes can't secure the stem
until the clamp is nearly completely closed.
↑Finished version.
The right part has unnecessary holes from
trial and error—the "error" part—but I'll lie and say
they're for weight reduction and to improve adhesive grip.
The left part is basically a huge square washer.
The back side of the right part looks like this
The low-side adjust bolt's threads are
standard M4, but I needed it longer than stock,
so I bought a stainless bolt and machined it.
As-is, the bolt head diameter was too large
to fit in the parallelogram hole, so I filed it down.
In the image above, left is original, right is after machining.
I secured the part with an M5 bolt.
Fortunately, for practical purposes,
there was a huge hole in the outer plate.
With it shifted to 7-speed low,
I tightened the adjust bolt until it hit the carbon plate.
The ErgoShift lever clicks into 8-speed, but the chain clatters,
so I can just shift back to 7-speed immediately—OK.
There's no way it'll go into 9-speed.
Unlike before I made this, the derailleur no longer tries
to override the plastic spacer.
The adjust bolt is doing its job.
↑This is shifted to top 11T.
The carbon plate's protrusion doesn't interfere
with any other part of the rear derailleur
except in low gear.
The reason I did all this
comes from a fanatical conviction that
road bikes are more honorable the less surface area they have—
a kind of puristic primitivism taken to extremes.
By the way, the crankside has a 44T single chainring,
which in terms of gear ratios equates to
39×22.1T or 34T×19.3T.
Unless the climb is a seriously steep pass,
you can generally still get up it.
The previous sprocket had 11T as the smallest,
then all 2T jumps after that:
11-12-14-16-18-
20-22-24-26-28-30T
on an 11-speed cassette,
but I barely used the 30T, mostly stuck to 26T,
and only occasionally hit 28T.
I cleaned the old sprocket.
From the top gear onward, it's all 2T jumps,
so the line of tooth tips forms
a single straight line.
Not a sombrero shape.
I must have done quite a bit of climbing,
but looking at the valleys of the low gear,
the color hasn't rubbed off—
there's barely any wear at all.
↑Bottom right in the image is the 11T top gear,
but the three sprockets I use most on flat terrain—14, 16, and 18T—
show ω-shaped wear in the valleys from the chain rollers.
The reason I bought this sprocket in the first place was
because of a severe supply shortage of Shimano components—
everything from Dura-Ace down to 105
was completely unavailable—
but as a result, I got something interesting.
According to the maker, the high-durability chromoly material
supposedly offers 2.5 to 3 times the durability
of Dura-Ace sprockets.
If that figure was comparing to just the titanium low gear section,
I thought that was a sleazy way to phrase it,
but in actual use, even comparing steel sections to steel,
the durability is clearly at least double.
This is from a past post, but this sprocket
only engages the splines at the very back and front.
This hub's freebody is titanium,
but if it were aluminum, it might wear in an unpleasant way.
I removed the sprocket.
Directly below the sprocket's hollow section,
black metal dust mixed with chain oil is caked on,
but it wipes away easily.
As for what this dust actually is,
I believe it's mostly chain roller shavings worn away
by the sprocket's superior hardness.
The sprocket's high durability is real,
but the chain roller wear rate is definitely not normal.
Shimano and other component makers could presumably
make harder sprockets—
but they presumably select an appropriate hardness
for a mutually balanced wear relationship with the chain.
The same applies to brake shoes and rims;
brake shoes harder than the stock specification
accelerate rim wear.
I'm not saying that's bad—
SwissStop BXP's rain braking performance
is outstanding compared to other shoes.
But complaining about rim wear as the price for that
would just be wrong. That's all I mean.
The crank is a Campagnolo Ekar,
and the chainring is the stock narrow-wide 44T,
but I've installed a homemade chain watcher.
Ekar chainrings are resistant to dropping,
but on the Osaka side descent through Yamagata-Kei valley,
at a specific gap midway down, the chain
almost always drops, so I installed this.
After that, in that one situation where it used to drop,
it never drops, and it hasn't dropped
in any other situation either.
But looking at this,
it's clearly doing heavy work...
↑This is with it on 11T top gear,
but when in low 30T, the chain's upper run
comes much closer to the watcher.
That gap that almost always causes drops—
the chain doesn't drop if you're running it in low gear territory
because there's high chain tension.
But shifting to low gear on a descent isn't normal.
This time, the low gear went from 30T to 25T.
When I checked by virtually tightening the chain until it was taut,
I could shorten it by 7 links.
So if I were to cut the chain,
6 links (3 pitches) would be a bit tight,
but 4 links (2 pitches) I could definitely remove.
The fact that 25T isn't at the normal low-gear position
but 7th from the bottom might also be related
to how many links can be shortened.
With the Ekar crank, the chain originally
barely dropped even without the watcher
in all but extreme specific situations,
so instead of "44×11T chain length calculated for 30T,"
with "25T as the actual limit on 44×11T chain length,"
there's a possibility it'll work fine without this.
So I decided to remove it.
If I ever reinstall it,
it'll be mounted lower than before.
With a round seat tube lacking a direct-mount bracket
and absolutely nothing front derailleur-like attached,
it has a very clean appearance.
I wiped it clean and applied basic coating.
You can faintly see the band marks,
but if I really went for it, those could be removed.
↑The removed chain watcher
The carbon seat tube's outer diameter is 30mm.
So with an inner-outer diameter difference of 2mm,
I cut a seatpost shim with 1mm radius and thickness
and installed it inside a 31.8mm inner diameter E-type front derailleur band.
But when I actually tightened it,
the clamp was closing before full tightness,
so I applied thin aluminum tape
and adjusted it by sanding smooth.
Looking at this, I have a feeling
I'll be needing its help again pretty soon...