ishaankarnik

Hello,
I am considering changing 50/34 tiagra 4600 crank with GRX 46/30 crank, This wold enable me in using my road bike for touring purposes.
My current drive train is tiagra 4600(shifters, FD, RD) 10speed.

Issue is, my current crankset chain line is 43.5, where as the crank I want to fit (FC-RX600-10) has chain line of 46.9.

Total capacity wise I think I am in clear, as difference between 50/34 is same as 46/30, 16. I am not changing cassette so "Total capacity" of the system wont change.
GRX RX400 groupset is said to be on tiagra level, so i am guessing tiagra FD will work with "FC-RX600-10".

Pop N Wood

I don't know but I am considering a similar swap. I have "read" the GRX crank requires the matching GRX front derailleur since the old one won't have enough reach. I'm hoping that is just Shimano talking. Have yet to read a swap of someone who has actually done it.

If you do this let us know if it works.

dsaul

The chainline is not an issue. MTB drivetrains use an even wider chain line than the GRX and they use the same cassettes as road and gravel bikes. As for the front derailleur, you may need the GRX if your current one doesn't have enough adjustment.

BoraxKid

Just a couple of thoughts come to mind:

• will pushing your chainrings farther out cause cross-chain issues when you are on the biggest cog? Since you are doing this for touring, I would make sure that your chain line isn't going to be running at a bad angle when you hit your lowest gears
• does your current front derailleur have the ability to be re-mounted lower on the seat tube to better line up with your new chainrings? If your front mech mounts via braze-on, this might be problematic, but it depends on your setup. You are correct that your total chain capacity won't be affected, but you should plan on sliding your derailer down the seat tube so the cage is the usual 1-2 mm above the big ring. If you don't do this step, your front derailleur may not line up properly with that smaller 30t chainring.

Let us know what you decide to do and whether or not it works!

jp911

I've read that the different chainline has created some compatibility issues with cranksets and derailleurs. Chainline can usually be adjusted using spacers to a certain extent. If the chainline is longer/wider on the GRX cranks, you would probably have challenges spacing the chrain rings inboard 3.4mm. You could try the GRX front derailleur to solve the issue OR go with one of the direct-mount cranksets that allow for a smaller small chain ring.

70sSanO

You can always disconnect the FD cable, back out the limit screw and manually move the derailleur out to see how far it goes. It would be good to measure from the seat tube to the inside of the cage before you do this so you’ll have real numbers to compare.

If it works and you can successfully move the crank in a couple of millimeters, it will probably be close enough.

The other question would be the 30t to 46t with a double road FD. Not sure if it would be better to use a Tiagra 4603 triple FD.

John

ishaankarnik

My current FD is mounted on seat tube by clamp, so will be able to slide it down. But yes, not sure about the reach of FD. I will check that before buying the crank.

I plan on using initial 3 cogs with big ring, so i am hoping cross chaining wont be a issue, as long as i am able to to use rest of the cogs with small ring.

Not considering tiagra triple crankset because it would involve many more changes, like my shifter only has two clicks, i am guessing FD also would be different for triple crank.

Any chance I can adjust chain line by adding both bottom-bracket spacers to drive side.

dsaul

Don't worry about the chainline. As I said before, MTB's use the same cassettes in the exact same position and the current chainline for MTB double cranksets is 51.8mm. 3.4mm is less than the center to center spacing of the cogs on the cassette. The GRX cranksets are made to work with the same cassettes as the one on your bike. They don't have any special GRX cassettes that are offset by 3.4mm.

Seriously, don't go messing around with bottom bracket spacers or chainring spacing. Shimano has already done the math and it works fine.

70sSanO

if road dropouts measure 130mm and mtb measure 135mm, it is impossible for the cassette to be in same position. think about it. The 1st cog sits the same distance from the DS dropout and the overall dropout width is greater. It might be in the same position to the the DS dropout, but not in relation the the centerline of the frame.

John
.

dsaul

Most current frames don't use 130mm any more and I haven't built a frame with anything other than 142 thru axle in a couple of years, so I don't think about frames with 130mm spacing. In any case, another 2.5mm won't matter.

70sSanO

Tiagra 4600.

John

70sSanO

If by initial 3 cogs, you mean the 3 smallest cogs, positions 1, 2, 3, then adding the 3.4mm is actually a benefit. Cross chaining is nothing more than a more severe angle of the chain to reach a first or last position cog.

Chainline is merely centering the chainrings to the middle cog position of a cassette. If you move the crankset out by 3.4mm you are shading it to center closer on the inner chainring, 30t, and not middle of the 2 chainrings. This will allow you to run all or at least 9 of 10 cogs with the 30t chainring. You can probably run the 46t easily over 5 or 6 cogs.

If you plan on using the 30t for most riding and the 46t only for higher speeds ,it is better setup than the recommended chainline.

John

Edit Added: Using a triple FD, 4603, is not for 3 chainrings, but for the 30t chainring. You set the limit screws so it only shifts as a double.

You may not need it, some doubles will shift from a 30t chainring.

And a triple FD may have a bit more reach.

squirtdad

need to make sure no matter what that you can do big/big. The result if you can't and accidentally do so is not good.

jp911

As I understand, the chainline issue with the new GRX gear is with the front chainline and the fact that the front derailleur is set 3.4mm further outboard to work with the new GRX crankset-chainring position which is also 3.4 mm further outboard. The potential issue being that the older road front derailleur placement may not actually be able to push to the distance required to move into the larger chainring. On a 2x setup, this might mean a greater angle when in the small chainring-largest cassette cog, but it's probably within acceptable tolerances.

Iride01 

Can't you find something that doesn't change up your chain line? If you want the same shifting performance you need to keep the back and front gears in the same orientation. As well as consider all the other stuff about DR height and ability to move out that far.

Perhaps you need a different bike for your purpose. I think GRX was aimed at the cross and gravel guys running frames with clearance for wider tires. Is that your frame?

Pop N Wood

I want the GRX for the 30 tooth sprocket. compact doubles have 110 mm BCD. The smallest ring for those is effectively 34 tooth. I want just a little more for a gravel bike I use for touring.

FSA makes a crank that allows a smaller little ring but rumor is it requires some odd bottom bracket. A GRX uses the same BB I have now.

Iride01 

I wouldn't let the cost of a BB limit my choice of cranksets. Why someone called it "odd" is beyond me. What is odd would be expecting a crank with different spindle design to work with another's BB.

squirtdad

+1 pick your crank and then use the matching bb. Shimano ultegra level BB are like 30 dollars so it is not a budget buster