My road/gravel bike currently has 50/34 on the front with a 11-34 on the rear. When it is on the turbo for zwifting it has an 11-28 cassette on it. The rear mech works fine swapping between the two without any adjustment (wheel off trainer so no need to swap).

I am looking to replace the crankset to give me a lower gear as I ride up some fairly steep climbs (for me anyway) and I can be left grinding the cranks round at 50-60 rpm in the 34-34 lowest gear.

So I am looking at replacing the tiagra crankset with a Shimano GRX crankset so going to 46-30 on the front. This would lower my gear inches from 26.3 to 23.3 at the lowest gear but would also change my upper end from 119 to 109.

For Zwift I don’t think this matters – I rarely use the 11 speed gear and I can just increase the trainer difficulty if I find I start spinning out. I should add that my natural cadence has gone from mid 70 to mid 90s so I am able to generate power across a much wider range and on the road the difference in the upper end is easily made up by the increased natural cadence.

Can anyone see any issues with this change? Will a Tiagra 2 speed front mech work fine with a GRX crank? The bottom bracket is the same so no issues from this side and the lower gearing will open up the climbs a bit and allow me to use the cadence that I now am able to sustain.

The GRX has different spacing, so you need a GRX mech or do some fettling with mounts.

Shimano slightly changed the spacing between their chainrings on their latest Dura-Ace, Ultegra, 105 and GRX double chainsets.
Strictly speaking this means you need to use the corresponding new front mechs to match this spacing.
Personally I find this a non issue, it is only shifting between 2 positions and I’ve set-up/used R5600/R8000/R7800 double cranks with older 11/10 speed mechs/shifters with no problem.

They’ve also moved the rings outwards on the GRX by a couple of mm for tyre clearance – I suspect this is also within a road mechs adjustment range but I havn’t done it myselft

OK sounds like I should be good to go then. At least give it a try – I cant find any GRX front mechs and the only double chainset for a reasonable price will need to come from Germany!

I’d be willing to part with my GRX 810 front mech, with one minor disclaimer, I was fiddling with the limit adjusters on the trail but got confused and wound one of them right out, which caused the little black nylok washer to fall off. I think this just served to stop the limit adjusters rattling loose so I’ve put some blue loctite on them and all seems good 500 very rough km later…

Am going 1x in the next week or so so can have it removed and cleaned up. £15 posted?

@13thfloormonk Thanks for the offer but from what I have read it shouldn’t be an issue. I also don’t have a braze on mech mount so I would have to get another bracket.

From what I can see all the GRX mechs seem to be braze on?

Ah yeah, had forgotten, mine comes with a braze on adaptor if it makes any difference, but yeah, they’re all braze on, a reaction I guess to too many weird shaped carbon seat tubes! 😀

@13thfloormonk. I will take that front mech please.
I shall PM you

Regards
Ian

I have installed it and got the front mech to work. It is right on the limits but does shift enough to change gear smoothly. Biggest issue was getting enough tension into the cable to start with. Anyone got any tips? I ended up pulling the mech slightly, tightening the cable clamp bolt and then releasing so it started with tension.

Use the low limit screw to push the mech out a bit (GRX installation instructions actually tell you precisely how far to push the mech with the LL screw).

Attach cable etc. then release low limit screw, the mech returns to its lowest position and tensions cable (to a point, some fine tuning still required).

From what I can see all the GRX mechs seem to be braze on?

yes, but it will connect to a band on adapter, I removed a 105 and replaced it with a grx by doing just that.

In terms of fitting, the GRX front crank is 2.5mm wider on each side, hence the official need for a new front mech. I have done it successfully with a 105 mech and mix and match 10 speed GRX components with 11 speed 105 quite well.

That said, I eventually went for a GRX400 front mech and whilst it is a bit more of a faff to set up, it does work very well when fitted.

currently running

GRX400 10 speed front mech
GRX RX600 2×10 46/30 front crankset
ultegra hydraulic shifters
105 11speed rear mech
11-32 R8000 ultegra cassette