krokumr

I am looking to change the large chainring on a Shimano 105 triple crankset from a 50 tooth ring to a 52 or 53 tooth ring. Does anyone know if this would create problems shifting into the large chainring? This is on a Trek 7.7 FX flatbar road bike. (Shimano 105 components-shifter, derailleur and crankset.)

HillRider

A Shimano 52T or 53T chainring will indeed fit but it won't have exactly the same shifting enhansements as your 50t so shifting will be compromised slightly. Not a big deal but it will not shift quite as cleanly. You will have to position your front derailleur a bit higher on the seattube to properly clear the larger diameter chain ring.

Also, be sure your chain is long enough to allow big-big with the new chainring. If it is sized exactly right for the 50T you may have to add a couple of links to get an adequate length.

Finally, the longer chain may go slack in the new small-small combination if your rear derailleur chain wrap is marginal. That isn't a big deal either as there is no need for that combination and accidentally shifting into it won't cause any damage.

sch

Main consideration is whether you can raise the FD mount height the necessary
0.32-0.5 inch without any difficulty to clear the larger CW. If so it ought to
work without much of a problem. Chain length probably ok, unless it is already
at the short end of the recommended length. Shifting will be a bit less crisp.
Not sure what the point would be unless the 50t is worn and the other a
freebie.

CACycling

Mainly depends on the smal ring size. It is all about the tooth differential.

HillRider

Current 105 triples are geared 50/39/30. Why does the differential matter except for chain length and rd wrap?

Al1943

Exactly what I was thinking, why bother? It will not add any speed except down a steep hill.

Chris_W

It sounds lie this is a 50/39/30 triple crank. After raising the FD (and checking that the chain length is sufficient) then shifting up to the new 52 or 53 tooth ring should still be acceptable, although not ideal. What I would worry most about is the shift down from the 39 to the 30 tooth ring. The shaping on the outside of the FD cage will now be too high, which means that it will not make that shift very well. The simple solution if you had a drop-bar bike would be to get an Ultegra triple FD, which is designed for a 52/39/30 crank, but that won't work with your flat-bar shifters. I'm not sure if there is a FD designed for that combo and works with a flat-bar shifter. The other option would be to switch your middle ring to a 42 tooth. Shifting should then be fine in all directions because the middle ring will be in the correct place for your current FD.

Whether changing both middle and large rings is worth it for this small change might not be worth it. Instead, you could just work on using a higher pedaling cadence. Or, if your cassette has a smallest sprocket of 12 teeth, then switch it for one with an 11 tooth cog.

HillRider

Aren't "flat-bar" shifters intended to work with road components? The OP said he currently has a 105 front derailleur. I realize MTB shifters won't work well with road front derailleurs but these are designed to do just that.


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hybridbkrdr

https://cycle.shimano-eu.com/publish/...andlebar0.html

There are flat bar front derailleur used for flat bar shifters and road drivetrains. FD-R443 for example.

CACycling

105 triple FDs are rated for 20 tooth max between small and large rings. If the OP goes to a 53T big ring and leaves the 30T small ring it would be a 23 tooth difference which may cause issues.

HillRider

Those ratings are very concervative. I've replaced the 30T granny ring on numerous Shimano 8 and 9-speed cranks and a Campy 10-speed triple (OEM as 52/42/30 and 53/42/30) with a 26T which certainly exceeds the published rating and they still shift fine. You may have the chain drag on the tail of the fd's cage in small-small but that's easy to avoid.

BCRider

Coming back to why you want to change the rings... Are you often in the big front and small rear and spinning out so you can barely stay in the saddle? That implies a really serious turn of speed. Because the bigger ring won't make you go any faster in the rest of the gears. It's only useful for a small gain for the top end speed. And most folks this side of a major ranked pelloton don't ever see that gear combination. Or is it more for a looks issue? Either way it's a cost that you likely shouldn't bother with unless your current big ring is damaged or worn out.

Chris_W

The stated big to small ring difference of FD capacity is conservative, as mentioned before, and can easily be exceeded without many problems. Far more important is the middle to big ring tooth difference, as I emphasized - get that wrong and your shifting will suffer. I'm surprised that the manufacturers state the FD capacity as the big to small difference and not the functionally more important big to middle difference. To find out what the recommended big to middle difference is then you need to find out what chainrings the crankset from the same model series has, and go with that value.