I have a standard GRX 400 set up on my gravel bike & I'd just like to drop the lower end of the gear range slightly.
The chainset is 46/30 but TA do GRX specific rings and a 28T inner option (which is specced to run with a 44T outer). Would 46/28 be likely to cause major issues?
It would push the front mech. out of its max. difference capacity by 1 tooth, but there's room to drop the mech. slightly. It would also put the rear mech. (41T) out by 2 teeth so I'm assuming that there would be slack in the small ring/small cog combination. However, my chain is currently cut according to the GRX method of counting on 4 rivets. I cut a previous chain using the old MTB 2 rivet approach & it was fine, even in big/big so I main be able to gain back a bit.
Is it just a "try it and see?" set up & would a 2 tooth drop even be worth bothering with?
It *should* work. The only front mech i've had lately that won't support "weird"/slightly out of spec combos was an XT front which is limited to a 10 tooth difference. I've got 12 teeth *and* oval rings, and it catches, just, and only when i unload the suspension in certain gears.
Rear mech will be fine.
You probably won't even get slack in small/small with such a small amount out of spec.
First thing I'd do is pop it up on a stand drop it into the inner and just see how clear of the Front mech cage the chain sits with the rear mech at either end of the cassette.
When you do the sums there's basically a -4mm difference radially between a 30 and a 28t chainring.
What cassette are you running? Could you fit one with a slightly larger range instead of switching chainrings?
When you do the sums there's basically a -4mm difference radially between a 30 and a 28t chainring
I never was any good at sums, but that's something I hadn't thought about. It might be tight and I did notice that in small/small the chain is pretty close to the chainstay (I am aware that you shouldn't really be in the extreme gears).
I had thought about the cassette option and I think I could run an 11-38 without lengthening the chain or breaking anything if I accidently shift into big/big, but GRX 400 is 10 speed & when I looked around 11-38 10 speed cassettes didn't seem particularly plentiful although I could have a look when the cassette needs replacing.
-- Edit --
Hmm, Microshift do one.
11-40 will be more common (and a very slightly lower ratio than the other options). Pretty sure people have done 11-40 and ever 11-42 with a GRX 2x setup.
My bike runs 26 38 at the front with a standard 10 speed Tiagra derailleur. It works just fine. The back is 11-36 with a grx 400 derailleur, again no problems. My chain is long enough to run fully crossed.