Need advise on switching compact crank with subcompact

[biker.caliente]

My significant other likes to ride along but has trouble with the hill so I am thinking about changing out the compact front crank (50/34, Shimano 10-speed HollowTech II) to Shimano 11-speed FC-RX810-2 gravel crank, 48/31, which is also HollowTech II 24 mm spindle. By doing so, I hope the job is as simple as lowering the FD (FD-6700 10-speed) and maybe shortening the chain while keeping the entire drivetrain 10-speed. I have the following questions and like to borrow your experience/knowledge. Her bike is a roadie, not a gravel, nor a mountain. It currently has a 12-30 Ultegra cassette and that is the biggest we like to see.

1. Would the 11-speed crank work with 10-speed drivetrain? I think the answer is yes but like to make sure.
2. Would the chain line need to adjusted as a result of the switch due to the inner ring being closer to chainstay?
3. Do I need to add spacer(s) on the DS of the BB (BB-6700)? No spacer(s) is used presently.
4. Would the current FD-6700 work?
5. FD-6700 can handle 16t drop but the new crankset has 17t. Can the FD-6700 handle the extra tooth?

Thanks for the help in advance.

[Dave]

It's unlikely that the FD will be a problem, but will need to be lowered by 4mm. The chain length required is 1/2 inch less, but the minimum change is 1 inch. Try the chain length calculator at parktool.com.

The grx crank uses the same BB as other current Shimano 24mm spindle cranks. The chain line is +2.5mm, so spacing washers should be used to correct it. The Q factor is also wider. I bought McMaster-Carr 98370A015 washers, but they come in 10 packs and not every one may be the same thickness.

I use this crank with a sram force axs 12 speed drivetrain.

A shimano 11 speed chain may be worth trying.

[muz]

I hope this doesn't sound patronizing, but I don't think going from a 34t to a 31t is very meaningful for a casual cyclist. It is less than 10% reduction in effort.

For a casual cyclist, there is not much benefit in a close ratio cassette. So I would first try the largest cassette in the back. Mid cage 6700 will often manage a 36t cog. AS you have 10 speed. you can use a 9-sp Shimano MTB derailleur without other changes, and easily go to 40t or larger.

[bk12]

I recently tried installing a 46 large ring crank on a carbon Trek Domane with a braze on FD-9070, and couldn't get the vertical movement within the mount adjustment low enough. On that frame, there wasn't a lot of room to move much below a 50 tooth and have the FD spacing right.

[sjbraun]

You'll need a GRX front derailleur to shift a GRX crank. GRX cranks are offset by 2.5mm to accommodate spacing for wider tires. Hence, a regular FD may not work. You also might consider the 600 series crank as it comes in a 46-30 combination.

[biker.caliente]

Originally Posted by sjbraun

You'll need a GRX front derailleur to shift a GRX crank. GRX cranks are offset by 2.5mm to accommodate spacing for wider tires. Hence, a regular FD may not work. You also might consider the 600 series crank as it comes in a 46-30 combination.

This the very issue that I am most concerned about. Looks like the easiest which is probably also the safest and cheapest way to go is to drop in a bigger cassette and a new chain. The current RD is specified at max 30t but I will try playing with the B-screw to see if it'll clear a bigger cog. If not, there is always RoadLink. I just have to convince her to carry that 'pizza' around in the back.

Thanks for all the input.

[SDJ]

another option is to re-use the old crankset which I assume is a standard 110 BCD 5 hole?

I have been using these chainrings for some years now on my cyclocross/gravel/adventure bike and am very happy

https://www.ebay.com/itm/bikinGreen-...-/123706511747

only issue is that the construction of the chainrings put them a couple of millimetres inwards of the crankset towards the frame/bottom bracket which you have to compensate with the FD set-up.

good luck convincing the boss

[icepick_trotsky]

Originally Posted by biker.caliente

This the very issue that I am most concerned about. Looks like the easiest which is probably also the safest and cheapest way to go is to drop in a bigger cassette and a new chain. The current RD is specified at max 30t but I will try playing with the B-screw to see if it'll clear a bigger cog. If not, there is always RoadLink. I just have to convince her to accept the big 'plate' in the back of the bike.

Thanks for all the input.

I'll second the above recommendation to plunk in a 9 speed Deore (or LX, or XT, or XTR) rear derailleur. Will work perfectly with the existing shifters and easily get a max 34 or even 36t. Can be had cheaply, too.

[Wunder]

I would be worried about the wider chainline on the GRX crank. From what I understand it is built around a disc gravel bike with a 135mm rear end (142mm through axle) and won't play properly with a 130mm frame (which I presume you have now).

If you can find one a Sugino OX crank (901 or 601) in 46/30 is a drop in replacement for a Shimano crank and has the same chainline and Q Factor. All you would need to do is lower the FD a couple mm and probably take a link or three out of the chain. My wife's commuter bike has that with Ultegra 6700 shifters/rear derailleur (I'm using a CX-70 FD to closer track the 46t ring, but a 6700 should also be fine).

A new crank is pricey though. As others noted going 9 speed mountain bike rear derailleur and a 11-36 ten speed cassette is likely a cheaper option if you can stand the larger gaps in the cassette.