FatBottomedGirl

Hi,

I currently run a 50/34 compact crankset.
I get the feeling that when not mountain riding (with long steep climbs), 34t is really too small a chainring.
On the other hand I like my 50t big ring (I feel like the typical 52-53 of racing cranksets would be too big), so I was thinking replacing the 34t by 38 or 39t ring.
Is that stupid?
What should I consider and be careful when getting this?
So far, the only critical point was the bolt circle diameter that has to be 110mm since I have a compact to begin with...

Anything else?

FBinNY 

No, it's not at all stupid. To get an idea of what size to go with, think about the largest rear sprocket you now use with the 34t. Then see what chainring would combine with your largest rear sprocket to produce the same ratio, ---- 34/sprocket used = X/largest sprocket. You might leave room for the occasional steeper hill by using a ring slightly smaller than calculated this way.

In case you're worried about it, going to a larger inner ring, does not affect the required chain length.

FatBottomedGirl

Thanks!

I ride a 12-27t, 10sp cassette so I ran the simulation and it ends like this.
So if only I watch the extreme low, I "lose" one speed (and there is only one spot I know when I really used the 34/27 combination, but I would have done it in 34/24 so I guess I would be fine).
Obviously climbing actual mountains would require the 34t ring, but I have no intention of throwing away the one I have.

The good thing about changing only the inner ring (instead of getting a 53/39 crankset) is that I don't need to move the derailleur up so less adjustments.

But my question was really whether I needed to worry about anything technically about compatibility: I have the 5 bolt thing and 110mm BCD, the chain length is fine since I actually narrow the whoe speed-range.
Just making sure I am missing nothing...

FBinNY 

There's only one or two details.

The first is that you lose the phasing between the rings, so there might be some loss of the glide in hyperglide, especially when down shifting. This isn't a big deal a all, and we all lived fine before chainrings were phased for shifting. OTOH, the smaller jump will probably make upshifting a bit easier.

the second is that 110mm 39t rings are fairly rare, with 38t being much more common. So you might have to settle for that, either because you can't find a 39t, or because it's pricier. Not a big deal, and might even make the range nicer.

jimc101

What about a 36t? these are easy to source in a compact (110mm BCD)

Looigi

What benefits do you feel you will get from a 38 small ring over the 34 you have now?

FBinNY 

I don't know what the OPs reasons are, but take a look at the two gear strips.

with 34/50 there's almost no overlap, and the most commonly used gears in the 70-85 range are crossed.

By enlarging the inner, he adds overlap in the center, increasing the options at the transition point. I don't know where or how he rides, but I long ago (decades before they came up with so-called compact drive) set my high gear at nearer to 100, and in the OPs situation would not only go to a 38 inner, but would shop for a 46 or 45 outer, to pack the gears into the range where he does most of the riding.

zacster

I just went from a 36 to a 34 to train for a long uphill ride, with the intent of switching back once I was done. But then I started thinking, I wouldn't gain anything by switching back, I'd just lose the lowest gear. Shifting is fine, the whole system works. You don't get a higher gear by going bigger on the small ring, all you do is shift the overlap with the big ring. So to me the trick is to get as wide a range as possible without causing problems with the shifting. Some may say large shifts are a problem, and in that case get a narrow range cassette, some think that a 16 tooth difference is too much for the derailleur, then don't use 50/34. But overall, a small change isn't going to really do much for you except at the very low end. That one extra click when you're struggling up a hill can really help.