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I have a bike with Tiagra 4700 hydraulic groupset and I want to lower the gearing from the present minimum 34/28. The one thing I'd not like to replace is the shifters:

  • These ST-RS405, while truly not being beauty contest winners, sit in my hands better than many others.
  • I do not want to move away from hydraulics and do not want to move to 11 speed (or to any non-Shimano shifters).

I know I can replace the derailer with any Shimano road 11 speed long-cage, which allows to use cassette up to 34T according to Shimano specs and I'm sure that it will handle 36T.

How can front gears be lowered? I absolutely do not need 50T. My preferred chainring combination would be 42/28, but:

  • FD-4700 is rated for min 46T larger chainring (and standard 16T difference). This might allow using 46/30, which I'd be fine with, but does anyone have practical experience with chainrings so different from the "native" 50/34?
  • What crankset could give me such combination not (much) spoiling the 43.5mm chainline? A road triple with inner and middle chainrings would do it; I'm looking around if there are doubles with that small BCD and road-ish chainline. I do not want to take an MTB double as nearly all of my riding will be on the larger chainring (I'll use the smaller one less than 1% of my riding time - but I do not want to dismount for that 1%).
  • The shifter is stated to work only with FD-4700. Really, cable pull ratio of Tiagra 4700 looks much bigger than the "Shimano standard". Does someone have experience successfully using any other FD with ST-RS405?
  • General comment: Shimano maintains compatibility charts. These indicate that the ST-RS405s aren't officially compatible with 11s rear derailleurs. However, 10s cassettes with 34t are available. productinfo.shimano.com/#/com – Weiwen Ng Sep 26 '19 at 19:59
  • Search for "46/30 road crank" and you'll find a couple (product recommendations are not allowed here), but most of them are either expensive or for square taper or proprietary bottom bracket standards. Also make sure that you can lower your front derailleur enough. – StefanS Sep 27 '19 at 7:11
  • @StefanS So my question reads "does anyone have practical experience with chainrings so different from the "native" 50/34?" – Konstantin Shemyak Sep 27 '19 at 7:37
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Shimano's most recent compatibility chart is available here. Note that these are official compatibility ratings. It is fine to exceed official specs by a bit.

At the rear, you can get a 10s Tiagra cassette that has a 34t cog, but it does technically require the long cage Tiagra RD. The issue was discussed here. The short cage Tiagra RD is rated for a maximum of a 28 tooth cog. If you have this, you could install a Wolf Tooth RoadLink to increase its maximum cog specification. You would need to be cautious not to exceed the rear derailleur capacity, or the maximum amount of chain that an RD can wrap.

You could also get an 11s GS RD, but be aware that they aren't officially compatible. In particular, if the 11s system is made with a different cable pull ratio (i.e. the length of shift cable pulled by each shift on the STI shifter), then an 11s RD won't shift correctly. You stated in comments that the cable pull ratio is the same, however.

At the front, you can't simply change the chainrings, and these are expensive in any case. Stock cranksets with Shimano's asymmetric 110mm bolt circle diameter can only handle a minimum of 34 or 33 tooth chainrings. Please don't construe any products mentioned below as recommendations; they are given as examples, and aren't an exhaustive list of options. You should Google to confirm.

Option 1: You could get a sub-compact crankset.

The Praxis Alba crankset can handle 48/32 chainrings. It doesn't appear to handle smaller ones. As stated in the dealer manual, this crankset maintains your 43.5mm chainline. However, this requires a new bottom bracket. For even smaller chainrings, you would definitely need a third party crankset, but many would also require you to change your bottom bracket. For example, the White Industries G30 crankset can accommodate an inner ring as small as 24t. This is a premium option. I'm uncertain about this crankset's chainline. Be aware that many sub-compact cranksets are designed for gravel bikes. Thus, most of them may have different chainlines.

The Shimano GRX cranksets are available in 48/32 or 46/30 ratios and they shouldn't require a new bottom bracket. However, they don't maintain your desired chainline. Nonetheless, I have heard anecdotal reports that people are running GRX cranksets with standard road front derailleurs.

Option 2: Get oval chainrings, such as the Absolute Black chainrings. They claim to have designed oval chainrings with average effective sizes of 48/32t and 46/30t that fit standard Shimano cranks.

The issue is that you may not like them, and they do need the front derailleur to be set up properly. I have heard some users comment that this was more difficult. Their purported advantage is that they decrease the length of the lever arm when your pedal stroke is in recovery phase, and lengthen it during the power phase of the stroke. These are claims only, and I don't believe studies have shown that they are generally effective.

The chainrings themselves are about as expensive as a new crankset, and they don't get you all the way to your desired gearing. However, you would be able to keep the rest of your current setup.

As discussed in the second link to a previous StackExchange post, you will need to be aware not to exceed your front derailleur capacity as well as the RD capacity.

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    +1 for a complete answer that has avoided becoming a product rec. – mattnz Sep 26 '19 at 21:07
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    Tiagra 4700 is different from all other 10 speed road Shimano groupsets. Actuation ratio of RD-4700 is identical to Shimano road 11sp derailers. – Konstantin Shemyak Sep 27 '19 at 6:09
  • GRX cranks feature 46.9mm chainline, which is 3.4mm more than that of FC-4700. I have indicated that I'd like to avoid increasing the chainline. – Konstantin Shemyak Sep 27 '19 at 6:11
  • GRX has a wider chainline, which I forgot. However, there are reports that unofficially, Shimano road FDs will work with that chainline. Also, I believe many sub-compact cranksets will have some sort of modified chainline. – Weiwen Ng Sep 27 '19 at 11:04
  • I'm unable to confirm if the 4700 Tiagra groupsets have the same cable pull as 11s RDs. This may well be true, but the 11s RDs aren't listed as compatible in the official guide I linked. You would be doing this at your own risk, although your statement is definitely plausible. You could simply get the GS version of the 4700 RD, though. – Weiwen Ng Sep 27 '19 at 14:16

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