Reeses

I have road bike with a 30/39/50t FSA Vero Crankset, Shimano Tiagra Triple FD, Shimano Tiagra Triple Shifter, and Shimano 105 RD.

I recently rode a mountain path where I had to go in the easiest gear to get up at 10 mph, and the cranksets I'm looking at are mostly 34/50t, how much resistance does 4 more teeth add?

And is a Shimano 105 FD compatible with a Shimano Tiagra Triple shifter? Can I get 34/50t crankset suggestions for under $150?

ThermionicScott 

I can't think of a good reason to.

HillRider

The 34T chainring will be 13% harder to pedal than the 30t in any particular cog. If you are already struggeling a bit with your triple in the lowest gear you have, the last thing you need is a higher low gear.

ultraman6970

If you have problem with triple then with compact will be the same, maybe 40% of the triple gears are repeated or are too close, too many useless combinations IMO, compact is more efficient IMO. But based in your description you need to train more, is not a problem with the bike, the problem is you.

jyl

I don't know what your sprockets are, but supposing the biggest is 26, you've got a 2.33 gain ratio low. At 70 rpm you're moving about 6-7 mph. That's pretty low. If you're not riding loaded with luggage, it should be "low enough" and you probably should work on the legs. Otherwise, ask a bike shop about a smaller inner chainring, like a 26, which will give you a 2.0 gain ratio which is nearly mountain bike low gearing. I don't see what a compact double is supposed to get you - your low gear won't be as low, that's going in the wrong direction.

Drew Eckhardt 

No. If you have a smaller low gear than you need and want a performance improvement of some sort swap a cassette on with a smaller ending cog. For example, if you're running 12-25 10 speed now switch to a 12-23 which includes an 18 cog for tighter spacing at cruising speeds in a big ring.

About one gear worth. 34x23 is like 30x21.

The biggest difference are in how fast you can go on your second largest ring. Eschewing the small cog in that ring you have two more gears usable with the triple and if you don't skip it (the middle ring is farther outboard than the small ring on a double) you have three more gears. Skipping the smallest cog on the triple lets you put 50% more power into overcoming aerodynamic drag on flat ground and shallow down-hills until shifting to the big ring and three gears let you nearly 100% more.

Next is where you are on your cassette at comfortable cruising speeds. With a compact I run 13-14-15-16-17-18-19-21-23 in back and spend most of my time at a pleasant 17-19 MPH in 50x21 or 34x14 one cog in from the end while with a triple I could do that or 12-21 and ride 39x16 or 39x17 in the middle of the cassette where the chain runs more quietly.

Reeses

My sprockets are 12-25t
The reason why i was thinking of switching to compact is that I don't usually use the middle chainring, and when I do, I usually have to shift down on the cassette to get the amount of resistance I'm comfortable with. But I can get that same amount of resistance by going on the 50t and in the middle of my cassette. Basically this:

FastJake

I wouldn't make the change. What do you expect to gain?

Sometimes I wish all bikes came with triples, even though a double is enough for me on my road bike. I don't see any downside to a triple ever, since it could be geared xx/39/53 and be the same as "standard" road double plus a granny ring. Or assuming the right BCD it could be setup like a compact plus a granny, 24/34/50.

juls

10mph uphill??!! Spinning like crazy!!

HillRider

As I understand the OP, his current problem is that his low gear isn't really low enough, not that it's lower than he needs. Changing from a 30T granny triple to a 34T small chainring compact double will only make the situation worse.

ThermionicScott 

God, I hate it when the duplicate gearing combinations slow me down. Oh wait, they don't.

Reeses

Actually my low gear is just low enough but I'm curious how much harder I'd have to pedal if my lowest chainring gained 4 teeth. Which as you said was 13% more effort.

HillRider

OK, then I misread it. You have a 12x25 cassette right? Changing to a 12x27 will give you an 8% lower gear which will compensate about half way for the higher gear from the compact double.

Drew Eckhardt 

That's good.

Riding 50-34 x 13-14-15-16-17-18-19-21-23 and eschewing the fully cross-chained combinations the only overlap is 50x21 and 34x14. Going slower than 17 MPH in the big ring suggests shifting five cogs smaller to 34x14. Going faster than 19 MPH on the small ring suggests shifting five cogs larger to 50x21. If I rode shimano I'd probably have scrapped the compact crank set long ago (it's one big thumb shove to drop five cogs with Campagnolo).

Where professionals need a 39x23 low gear in the mountains with the generic potential for half the power to weight ratio a mortal cyclist would need a 34x40 to climb at the same non-fatiguing cadence or 24x28.

Some of those guys choose 34-36 small rings and 28-34 tooth large rings for the worst climbs in the Giro.

rdtompki

No problem. Just climb the same hill without using your largest cog. You probably have a 21t in your 12-25? If you're comfy in the 21t then go for it.

Cog_wild

change to compact with a 12-28 cassette and lose the extra weight and redundancy of a triple.

Cog_wild

FD you can tweak the limit screws or just remember to not drop all tension, or add a chain catch to protect your BB area from chain drop..., or just buy new shifters. on my commuter I use old triple shifters and a 53/39 crankset no issues but I pretty much leave it in the big ring anyway.

Drew Eckhardt 

The 70 grams separating quality triple cranks from the double versions will only net a 140 pound rider aboard a 15 pound bicycle 3.6 seconds an hour climbing in the mountains.

If you're a competitive racer, weigh the 2 pounds/inch (140 pounds at 5'10) which implies a body morphology conducive to the power to weight ratio which wins king-of-the-mountain jerseys, and are racing off the front to an up-hill finish that matters. Otherwise it does not.

Depending on your starting cog preference and cog count in back you could be better off with a triple if you can't spin (80-90 RPM) something between a 39x19 and 34x25 up any hill you'll encounter (otherwise you're compromising on gear spacing (one tooth jumps to the 19 cog only permit a 19 big cog starting with an 11 and 9 cogs in back; although with a 50x12 high gear and 11 cogs in back you can have a 25).

jyl

Climb that same hill in your 30 chainring and your second-to-largest or third-from-largest sprocket, whichever is closest to 23. Since 34 front 25 rear is same as 30 front 23 rear.

Reeses

I think I'll hold out a little longer with what I currently have but I'll have to try this:

How do you come to the conclusion that Gear combo 30x23 = Gear combo 34x25 though?

Six jours

I use a 30-46 with a 12-25 freewheel. This gives me an amply wide gear range even living in the mountains. Prior to that I was using a triple with the same rings as the OP. It was a constant, albeit low-level, annoyance for me because of the near-duplication in so many gears. I don't like having to think about my bike when I'm riding, but I was constantly trying to decide what combination of cog and chainring would give me the gear I wanted for the next stretch of road. The "ultra compact" double solved that for me.

The only real downside is that such doubles are rare, often expensive, and usually "retro" in appearance, which seems to turn off many "modern" cyclists - even though a pair of V/O "Gran Cru" cranks weigh less than most modern carbon cranks...

Drew Eckhardt 

Simple arithmetic.

23 cog / 30 ring * 34 ring = 26 cog and 25 is only 4% off.

or the other way

25 cog / 34 ring * 30 ring = 22 cog and 23 is about 4% off.

Drew Eckhardt 

It's easy.

Shift to the next smaller or larger cog. When you run out shift to the next ring and an appropriate number of cogs in the opposite direction. Also shift if the chain angle is getting excessive (which makes it noisy) if you have an alternate ring/cog combination.

This may be simpler with more rings with tighter cogs than fewer with wider cogs because for the same overall range you don't end up in situations where the cogs are as close as you like on one ring but not another.

Seb71

22 is a better match.

34/25 = 1.36

30/1.36 = 22.0588

jolly_ross

It you are a low horsepower spinner like me then (in general) a compact is not as enjoyable as a triple. You lose out on the spread of gears (not as high and/or not as low) and the really close ratios. Most of the disadvantages of a triple have been mentioned in earlier posts. However personally the triple is bad for me in one more way, on long climbs I very quickly gravitate toward the very low gears - which are slow. A compact would force me to be a bit faster, and the extra effort might slowly make me stronger.

Not gonna happen though. I'm staying with the triple for alpine climbs.