I'm thinking of getting a GRX chainset for a gravel build, will I need a GRX front metch (double chainset) or will my current ultegra 8000 seriew one work ok with the extra 2.5 mm width on the chainset?
I had this very issue when I bought an Ultegra equipped Tempest in January. I fitted a GRX 46/30 chainset to lower the gearing. I lowered and adjusted the Ultegra front mech for the 2.5mm wider GRX chainset. It initially seemed to work as it does physically travel far enough out to shift the chain. On the 2nd ride out, the high adjuster screw fell out, presumably as it was operating out of range / been unscrewed too far. I've since fitted a GRX front mech, which works perfectly. Not that I'm getting much use out of the thing!
Pretty sure the GRX chainline is unique and only the GRX front mech works with the chainset
There was an almost identical thread a few days ago and I asked then - why do you want the GRX crankset? If it is for lower gearing then you may find it cheaper/easier to consider a larger cassette instead.
I'm currently running a compact (50-34) crankset and a 11-40T cassette with Ultegra mechs.
The design isn’t radically different to existing products but, critically, the chainline has been shifted outwards by 2.5mm to increase tyre clearance and be better optimised for wider rear spacing.
From bikeradar easy google
There was an almost identical thread a few days ago and I asked then – why do you want the GRX crankset? If it is for lower gearing then you may find it cheaper/easier to consider a larger cassette instead.
I’m currently running a compact (50-34) crankset and a 11-40T cassette with Ultegra mechs.
The issue there is the bigger jumps between ratios. My OH recently decided she wanted lower road gearing (Peak District so plenty of steep hills) than the 34/28 bottom she had on but 11/28 was already at her tolerance of jumps. Going to 11/36 or similar was discounted immediately. Also, by the time you factor a new chain and long cage rear mech in as well as the cassette it isn't much, if at all, cheaper.
We went for 30/46 GRX and she's very happy. Interestingly one bike needed the GRX mech to make it work, the other is still fine on its 6800 Ultegra. The only issue is the braze-on mounts don't quite allow the mech to drop as low as you'd really want it, but they work fine.
I have 30/46 GRX on my do-it-all bike and that runs fine on an old CX70 mech.
Scotroutes mainly as I have an 11-32 cassette and chain ready art present to go on and the grx chainset is slightly cheaper than a 105 compact, I may still just go compact yet not sure.
Compact chainset and 700 x 40 tyres gives a 50 x 11 top gear of 125". Even a 46 x 11 gives 115" Unless sprinting downhill, go for the shorter gears.