Would this be feasible and compatible with existing changers etc,I have a 11sp 11x36 cassette and at present all operates perfectly but I would like to lower the gearing by reducing chainwheel sizes.
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Hi, welcome to bicycles. Why do you need to change the cranks instead of just swapping in a smaller chainring?– DavidWCommented Sep 14 at 18:37
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1@DavidW You can't actually put a smaller chainring on a Shimano road crank.– Weiwen NgCommented Sep 14 at 18:43
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What's your cassette? R7000 should be able to take at least a cassette with a 32-tooth large cog, almost certainly a 34, and maybe even a 36.– Andrew HenleCommented Sep 14 at 22:46
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2 Answers
MTBs and drop bar bikes have different bottom bracket widths. For BSA shells, MTBs use a 73mm shell, and road bikes use a 68mm shell. Your first problem is that the Deore crankset's spindle will be longer than your shell, so you need 2.5mm spacers on each side.
For your front derailleur, MTB chainrings are a full 8.5mm to the right of where the FD is expecting. It won't shift. If you have a front derailleur braze on that you bolt the derailleur to, you also won't be able to get the FD, or any FD, low enough. If your FD uses a band mount, you probably can (if the seat tube is round), but a road FD will have the wrong curvature for MTB chainrings. You would basically need an MTB FD. I think but I am not sure that a road shifter would shift an MTB FD (do not try this with an MTB RD, the cable pull is different).
Also, if you could somehow pull this off, you'd need to shorten the chain.
This illustrates why Shimano should make sub-compact road cranks. That said, GRX offers an option for people seeking lower gearing in the front; they have 48/31 and 46/30 options. The GRX cranks may or may not work with road FDs; the GRX chainline is (I think) 2.5mm more than the road chainline. Road FDs often but not always have enough travel. If not, you'd need a GRX FD.
Be aware that 36/26 front gearing is very, very low for a road bike. Your top end speed will be compromised a lot.
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1Many thanks for your very comprehensive reply, I suspected it wouldn’t be that straightforward and I know I can’t fit smaller chainwheels on the 105 so I will have to think again. I also have a Trek with a triple chainset 26/36/48 so I know the gearing, I would prefer something like a 26/46 on my Orbea if there was such an option, now in my 80’s, I need lower gears !– PeterKACommented Sep 14 at 22:00
Not a direct answer but an answer to the idea of getting lower gears. An alternative answer would be to go have a larger cassette (11/42), there are hacks for the gravel groupsets, that should work with the R7000 as the GRX600 is a derivative from the R7000. With an 11/42 cassette, the range would be equivalent to a triple. Note that those are hacks, so not really supported by Shimano, but they work.
- using a goat link (adapter that allows the derailleur to work with larger cassettes). Cheapest option, but the shifting performance on the small sprockets is impacted.
- Using an RD-M8000-SGS mountain bike rear derailleur and a tanpan adapter, that allows to use an MTB derailleur with a road brifter. Compared to the next solution it is simpler to install, but more expensive if you need to buy the derailleur and the adapter.
- To modify a GRX812 rear derailleur with the cage of the RD-M8000-SGS rear derailleur. I detailed this setup in this question, that describes the process. I’ll answer the question soon, as I have tested the system in the meantime and it works well.
