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My gravel bike is set up with a stock Tiagra 2 x 10 drivetrain (4700 series). After beasting myself 80km through the tweed valley yesterday I think I need (I want) a bigger cassette (stock one is 32 teeth). Short of going full 1x I was wondering what I could bodge... seems the Tiagra 4700 shifters are compatible with GRX 11 speed RDs, so if I got one of those and a wider range 10 speed cassette e.g. Deore 11-42, should it not work? Someone on here must have tried something similar!
I have run a 36 rear cassette with an modified Tiagra derailleur. Do i think 42 might be possible with an extender
I also run a spa cycles 38 26 chainset up front
Plenty of good stuff in this video.
I think that the GRX400 rear mech. works with the Tiagra cable pull & handles a 36 tooth big cog & double chain rings.
nice one, that video is just what I was looking for!
The video reminds me of STW back in the day when people like Trouty etc were really innovative. The forums have changed, and taken over somewhat by mr OCD, (every thing must match)!
Hadn't thought of a smaller chainring 🙂
The urge to experiment is calling me. I found an old (pre-clutch) SLX rear mech in the spares bin, I wonder if that would work...
Your existing rear derailleur will most like work with a 11-36 cassette with the b tension screw at maximum adjustment, chainrings tend to not be as cheap as you think they should be and a new chainset is often cheaper, and whist a GRX 400Â 46/30 chainset is great you will need the GRX front mech to go with it, that will however work fine with the Tiagra shifters that you already have.
My road bike has a GRX 46/30 and a 11-36 cassette and it works great. It has made a big difference to me as I am old and struggle with big gears.
I've recently done this to my Gravel bike:
4700 F&R mech 4720 levers, Chainset is an R553 Triple with a 30t inner, 46t middle and no outer, Cassette is SRAM 11-36. All works a treat.
You could probably get your stock setup to work with an 11-36 cassette with no other bodgery or new mechs OP; worth a go at least before spunking more money on a GRX mech.
Edit:
 I found an old (pre-clutch) SLX rear mech in the spares bin, I wonder if that would work…
Nope, wrong cable pull.
I'm also doing what @cookeaa has done. Old tiagra triple with 28t on the inner position and 42t on the middle. Works well with 11-36 or 32.
For top level bodging you can also throw a Wolf Tooth roadlink into the mix.
Have used them in the past when using bigger cassettes.
Fine on gravel/easy trails,but it can drop the mech in to the crash zone,and presents a bigger lever if knocked.
I put a 40-28 MTB double chainset on to get lower gears. It worked acceptably if not slickly with the 10s 105 front mech- it was a band on so could slide it down to match the rings.