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(similar question: Front Derailleur Chainring Size Tolerance)

I am trying to understand the possibility of maximum difference between chainrings on road bikes. I currently have Shimano GRX 48/31 (difference 17) which works fine. How can I see what is the maximum difference? The link above mentions:

According to Shimano front derailleur specs here all road triple front derailleurs support max 50 tooth and have 20 tooth capacity

However I couldn't find anything about that in the linked document. Would it be possible to have Ultegra chaingring 50/34 with the 34 replaced with a smaller one? Like 50/32?

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The maximum rated difference is listed on the specifications page. For Shimano’s Ultegra R8000 FD and others of that era, the rated capacity is 16t. The GRX 810 FD is rated for 17t.

With the RD, exceeding the capacity by 2t is often fine. I have less experience with the FD, but it may also be doable. There are some caveats, however. You mention a GRX FD, but then you mention an Ultegra crank which you want to equip with 50-32 chainrings. First, the GRX Groups are designed for a slightly wider chainline than the road groups. The GRX FD should have enough lateral travel to shift an Ultegra crank, but there’s a chance it may not. I’d be more worried about a road FD with GRX cranks.

Second, a 32t ring won’t fit on road cranks, as the bolt circle diameter is too small. Sub compact road cranks use a smaller BCD, with some very limited exceptions. Third, if you got a 50t ring for a GRX crank, the shift ramps wouldn’t align, so your shifting might not be ideal.

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    A standard round chainring on the current Ultegra is limited to a 34 tooth as you mentioned. However there are some oval options that can offer a 48/32 (and a 46/30) on an Ultegra 8000 crankset. I picked up a 48/32 set, but ran into some interference with the frame on my Salsa Warbird, so I ended up going with the 48/31 GRX set (and have been happy with it). Here is the link to the AbsoluteBLACK smaller oval chainrings I mentioned above. absoluteblack.cc/…
    – Ted Hohl
    Aug 28, 2022 at 3:40
  • @TedHohl is correct that AB offers chainrings that can fit on a Shimano road crankset that have 48/32 or 46/30 effective gearing. However, they are oval. I'm very much not sold on the benefit of oval chainrings, and they do affect front shifting. Off road, though, they might help maintain rear wheel traction if they actually smooth your pedal stroke out.
    – Weiwen Ng
    Aug 31, 2022 at 19:50

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