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Old 02-24-21, 04:37 PM
Method to My Madness
Join Date: Nov 2020
Location: Orange County, California
Posts: 578

Bikes: Cannondale Synapse, Trek FX 2

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After finding this thread, I tried to stand on the shoulders of giants to make a custom Ultegra 12-28 cassette, but could not make it shift perfectly smoothly.

My Cannondale Synapse came with FSA 50/34 subcompact chain rings and an 11-speed Shimano 105 (CS-5800) 11-32 cassette. Like many, I do not recall ever using the 11T or the 12T cogs. The 32T cog mostly allows me to avoid shifting onto the small chain ring, but is hardly mandatory for my local terrain. An Ultegra 14-28 cassette omits the 32T cog and offers closer ratios in the middle, but is limited to a top speed of 28.5 mph at a cadence of 100 rpm. According to Strava, I have exceeded 30 mph occasionally, likely during GPS glitches when going downhill with a tailwind. A 12-25 cassette likewise offers closer ratios in the middle, but might make it a bit difficult to start uphill from a standstill in the big chain ring.

So a 12-28 cassette may be the Goldilocks setup but Shimano only offers a Dura-Ace version. It seems a little wasteful to buy one for a bike with a 105 groupset. There are also numerous online customer reviews complaining about the creaking when on the two biggest cogs in Dura-Ace cassettes. So I began looking into building a 12-28 cassette from other Shimano cassettes and found this thread.

According to the Shimano Dealer’s Manual (

Dura-Ace CS-R9100 12T-28T

12B 13B 14A 15A 16A 17C (19C 21C 23C) (25B 28A)

Ultegra CS-R8000 11T-28T

11A 12A 13A 14A 15A 17B (19C 21C) (23C 25B 28A)

Ultegra CS-R8000 12T-25T

12A 13A 14A 15A 16A 17C (18B 19D) (21B 23B 25A)

Combining the six smallest cogs from the 12-25 cassette with the five biggest cogs (on two spider rings) from the 11-28 cassette even maintains the 16A to 17C to 19C sequence in the Dura-Ace version. So I bought these two Ultegra cassettes to be the donors. The 12-25 cassette even came with 12B and 13B cogs, so that all cogs in the custom cassette perfectly match their counterparts in the Dura-Ace version.

Unfortunately, the above works better in theory than practice. The 17C to 19C shift misses about once every four or five times and leaves the chain rattling on the 17C cog. Another push on the STI lever moves the chain onto the 21C cog, skipping the 19C cog altogether. No issues with the 19C to 17C shift. Using the 17B cog (from the 11-28 cassette) instead of the 17C cog (from the 12-25 cassette) moves the problem onto the 16A to 17B shift, where the missed shift occurs slightly less often than the missed 17C to 19C shift. Again, no issues with the 17B to 16A shift. So it seems slightly better to have only a single tooth difference at the downshift from one donor cassette to the other.

I soon got tired of the missed downshift (between the donor cassettes) and the compensating effort to feather the shifter just so to pull off that downshift. The whole point of a custom cassette was to reduce disruption to my cadence. So now I use only the 12-25 cassette, leaving me with an extra, slightly used 11-28 Ultegra cassette. Yes, that means I usually have to shift onto the small chain ring before I come to a stop.

Last edited by SoSmellyAir; 02-24-21 at 05:13 PM.
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