How much lower does the front mech have to go (possible conflict with bottle cage)?
And who has GRX 11 speed cranks working with a 105 front mech? Just wondering if the wider chainline really will necessitate a GRX front mech.
TIA
Watching as I was wondering similar, but going from 46/34 Ultegra and a 2x R8000 Ultegra front mech.
An estimate because IDK exactly what the diam of a 50T or 46T ring is.
Maths to the rescue. As pitch is 1/2" on a chain, a 50T diam chainring will be give or take 25" circumference and hence (pi * D) = 7.96" = 3.98" radius
A 46T chainring will be 23" circ = 7.32" diam = 3.66"
So about 1/3 of an inch or 8mm
It'll be a bit off because the chain is actually a bit taller than the pitch centreline but I'd warrant it'll be off by similar for both calcs, and pretty well cancel out as you're only looking for the difference rather than an absolute.
I just measured mine (as it is sitting on the turbo next to my WFH desk) and I moved it down 10mm based on the marks of where the mark was and where the mech is now. This is with a Tiagra front mech but I assume it would be a similar amount for any mech (just the reference point might change).
Yes, it looks as though I dropped mine around 10mm when I switched from a 50-34 105 chainset to a 48-31 GRX one. More or less anyway. Let's call it 8mm 🙂
Can't help with the mech though as I switched to a GRX one.
I couldn't get a GRX 46/30 chainset to change properly with a 105 derailleur. I had to buy the GRX version.
I've got a 46/30 GRX chainset running fine with a 105 front derailleur on a Genesis Croix De Fer.I bought a GRX mech when I got the cranks as I'd read about the wider chainline but tried it with an existing 105 I already had and it worked so I left it on.This was a new build and I haven't used a 50/34 so no idea about lowering the mech band.I've got no issues with the bottle cage.
Something like this might help:
https://wickwerks.com/products/fit-link-adapter/
If you don't want to use GRX crankset, Middleburn do a RO2 crankset with 94bcd 5 arm double with 46/30 round rings with a more "normal" chainline. I have this working with a 105 R5600 FD and 415mm chainstays.
https://www.mountainbikecomponents.co.uk/product/94bcd-5-arm-11-speed-double-46-30-for-ro2-and-ro1/
But It's not just the drop in diameter to consider. The relative circumference of the chainring and cage are different on a "normal" road derailleur (I guess that's why GRX) and the shorter the chainstays the less room you have for adjustment. I found I needed a wedge between the derailleur and the mount to push the back of the cage down-over while maintaining an appropriate height at the front of the cage. This is with Absolute Black 46/30 rings and 105 R5700 FD, 405mm chainstays.
https://www.ebay.co.uk/itm/402895494005
Without it, in the 30T I couldn't use the smallest 5 sprockets as the chain rubbed on the horizontal cross piece at the back of the cage. If I lowered the derailleur enough to clear that part, there was insufficient adjustment to avoid rubbing at the front of the cage in the big ring.
I have an FSA Omega 46/30 chainset running with an R7000 derailleur. I had to drop the derailleur maybe 8 or mm.
I swapped from a Sora 50/46 to a GRX 48/31. FWIW the Sora front mech worked fine. I replaced with a GRX mech when I got hold of one. The shifting is a bit better but probably wasn't worth the expense TBH.