Shimano 105/GRX Compatibility

[astanzio]

Hi Everyone,
thank you for welcoming me to this forum. I have an old Giant TCR equipped with a Shimano 105-5700 Groupset. Since I am passing the bike on to my father, he'd need a reduced gear ratio. Are the FD-5700 derailleur and the BB71 41B bottom bracket that I currently have compatible with Gthe RX FC-RX600-10 crankset?

 

[BikeGremlin]

Hi Everyone,
thank you for welcoming me to this forum. I have an old Giant TCR fully equipped with a Shimano 105-5700 Groupset. Since I am passing the bike on to my father, he'd need a reduced gear ratio. Are the FD-5700 derailleur and the BB71 41B that I currently have bottom bracket compatible with GRX FC-RX600-10 crankset?

Hi and welcome,

The current FD is probably designed for double cranks, with around 50-34 to 53-39 teeth chainrings.

To reduce the gear ratios (make "shorter" gearing), you may need to go with 46-30 cranks (GRX series has some cranks like that).

The existing FD will most probably not shift perfectly with such a difference in chainring size (and curve), but it is worth a try to see if it is good enough.

If it's not working, you would probably need to replace both the FD and the shifter (STI), since the modern road shifters use a different cable pull (why I love the good old down-tube friction shifters ).

Alternative could be to get a modern Tiagra or an older 105 triple cranks: 50-39-30. That would require a triple FD, and a triple shifter. 105 5700 triple shifters are not the most reliable or durable, even if you find one that is brand new.

There may be other options, depending on what parts you are willing to change (and what you have on hand), but these are the most reasonable so to speak.

Relja

 

[BikeGremlin]

Are the FD-5700 derailleur and the BB71 41B bottom bracket that I currently have compatible with Gthe RX FC-RX600-10 crankset?

Almost forgot to answer this!

The bottom bracket should be fine as far as I know, both GRX and 105 are 24 mm spindle diameter Hollowtech II cranks.

However, GRX cranks are designed to provide a 2.5 mm wider chainline. That is ~ 46 mm as opposed to ~43.5 mm for the old road double (like the 105 5700).

Hence, you might have problems with the FD outward movement. It might not be enough to shift onto the large chainring, and you might need to look for a different FD.

Again, it's worth a test first. You might even improvise by switching the spacers (switch one right hand side spacer to the left side). My video about spacers:
https://www.bikegremlin.com/1146/hollowtech-ii-bottom-bracket-spacers-explained/

Chainline article:
https://bike.bikegremlin.com/1755/bicycle-chainline/

 

[astanzio]

Hi Relja,
Thank you for your suggestions, very informative. My idea was exactly to install the 46-30 cranks (down from the 50-34 that are currently installed), and I figured that it might be a bit of a stretch on the FD, but I did not anticipate the chainline issue. I'll give it a try by keeping the same FD and switching the spacers, otherwise, I'll look for an alternative combination

 

[BikeGremlin]

Hi Relja,
Thank you for your suggestions, very informative. My idea was exactly to install the 46-30 cranks (down from the 50-34 that are currently installed), and I figured that it might be a bit of a stretch on the FD, but I did not anticipate the chainline issue. I'll give it a try by keeping the same FD and switching the spacers, otherwise, I'll look for an alternative combination

That sounds reasonable.

Let me know if you have any further questions - and I'm curious to hear how it went once you are happy with the shifting.

Wishing good health and many miles and fun for the both of you.

Relja

 

[AndrejC]

Hello,

Maybe I can add my case.
I have a road bike with FD Ultegra R8000 (FD is braze on version). My setup was crankset 52-36 and casette 11-32.
For me this combination was a simply "to hard". I hav never used gear ratio 52 /11 (not even during decend).

So I have decided to use a smaller crankset GRX 810. I have added a small plate (2,5mm thicknes) between bike frame and FD hanger to compensate a 2,5 mm wider chainline.

Result is as it shall be
There was no problem with FD set-up and shifting works flawless.

Andrej