Custom Shimano GRX gravel bike configuration query.

flashcurdflashcurd Posts: 4
edited 11 October in MTB workshop & tech
First, apologies if this is the wrong forum, the MTB section seemed to be the most appropriate.

I'm thinking of building a gravel/bikepacking bike for myself using Shimano's new GRX groupset but would like some advice to a couple of tweaks I'm thinking of making to it, mainly the front chainring/crank setup to give me lower gear ratios. I've done some research into the gear ratios on both the 1x and 2x GRX groupset and it seems that the lowest gear ratio possible is a front 46/30T with a rear 11/34T giving a lowest possible gear ratio of 30:34 or 0.84 decimalised, this is with a 2x setup. With a 1x setup it's even worse with a minimum of front 40T and rear 11/42T for a low gear of 40/42T or 0.95.
I've been looking over the parts/component list and it appears that the GRX range uses parts from some of Shimano's other ranges, some of the rear cassettes from the Deare XT and SLX range. So I was wondering if the following is possible. Ideally I'd like a 1x setup but with a 30T or 32T front chainring/crank. The RX600 front chainring/crank (FC-RX600-1) only goes down to a 40T but could I substitute it for a Shimano mountain bike 11-speed crankset (e.g. the SLX 32T, FX-M7000-11-1) from the same range as the rear cassette?

If a normal GRX RX600 chainset configuration consists of (using documentation here https://bike.shimano.com/en-EU/product/ ... speed.html )
- 40T front chainring/cranks, FC-RX600-1
- Tiagra bottom bracket, BB-RS500
- GRX rear derailleur, 42T, RD-RX812
- SLX 11-42T cassette, CS-M7000-11

What I want to do is:
- Replace the bottom bracket with a shimano mountain bike bottom bracket, BB-MT800
- Replace the front chainring/cranks with a shimano 11 speed mountain bike chainring/crank, e.g. the M7000 SLX, (FC-M7000-11-1).

All of this would be built into a standard BSA threaded 68mm english BB frame.

I've done some research into the difference in the cranksets/chainrings and I can't see why it wouldn't work but this is why I'm asking here. What am I missing?
The GRX and SLX chainrings/cranks _appear_ to be almost completely compatible with each other using the information found at the following links.
https://bike.shimano.com/en-EU/product/ ... -11-1.html
https://productinfo.shimano.com/#/spec/ ... m7000-11-1

https://bike.shimano.com/en-EU/product/ ... 600-1.html
https://productinfo.shimano.com/#/spec/ ... fc-rx600-1

The only very slight difference is in the chainline and Q-factor. The chainline differs by 0.3mm (50mm vs 49.7mm), and the q-factor by 25mm (176 vs 151). Everything else appears to be the same including the type of chain. But I don't think those differences are anything that would cause this configuration not to work would it?

Finally, given the above frankensteins monster of a configuration, I can achieve a front chainring of 30T with a rear cassette of 42T giving me a low gear of 0.714. Small enough to get my up any challenging climb fully loaded with overnight packs.
I should also say that I'm not bothered about spinning out at top speed, I realize that this setup is sacrificing a lot of top end speed for low climbing gears.
Thanks for the help and apologies for all the newbie questions, I've not tinkered with a drivetrain to this degree before.

Posts

  • whyamiherewhyamihere Posts: 7,142
    It will work fine.
  • Thanks for the response. Now I just need to decide on the frame I'll be building all this into.
  • alexfthealexfthe Posts: 69
    If I were in your shoes I’d go a step further and see whether I could fit the SLX ring to the GRX crank - narrower Q factor will be much more comfortable as long as there’s clearance for everything. And unless you’re building onto a boost frame spacing or using full MTB tyres I don’t see why you’d need the extra clearance offered by SLX.
    2018 Vitus Vitesse Evo CR Disc
    2019 Cannondale Synapse AL 105 SE Disc
    2017 Dawes Galaxy Plus
    2014 Boardman MTB Pro 29er HT
    2007 Trek 7.3FX FrankenPubBike
  • I did originally think about doing this but encountered two potential problems.

    The GRX 1x cranks are 110BCD and the lowest chainring I could find for this was a 36T from WolfTooth. The SLX cranks are 96BCD which lets me go down to 30T (maybe 28T).

    The second problem might just be in my imagination. Whilst the GRX cranks are 110BCD they _appear_ to use a slightly different bolt angle for the chainring than other shimano cranks. I can't find any documentation on this anywhere but I noticed it when looking at the WolfTooth chainrings for Shimano ( https://www.wolftoothcomponents.com/col ... 8700609861 ) and for Shimano GRX ( https://www.wolftoothcomponents.com/col ... grx-cranks ). The WolfTooth GRX chainrings have the bolt circles slightly offset from centre of the tabs. I have no idea if this would actually cause a problem in practice and as soon as I can find some GRX cranks in the wild I'll check.
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