Perp

I'm going to change my S-works to compact rings (for climbing steep grades). My cassettes top out @ 26. I prefer climbing seated with high cadence.

Looking at getting Praxis Works chainrings, just not sure if I should get a 36 or 34 as my little ring.

Carbon Unit

If you need a compact for climbing, a 34 will give lower gears.

kv501

This is a good reference to use when making gear comparisons.

You are looking at 2 gear inches difference between the two. Not a huge difference (to me, at least), and not enough to determine whether you are sitting or standing.

Werkin

The cadence difference between dropping from a 50 to 34 while on the same rear cog is dramatic; less so when dropping to a 36T ring. Cadence difference between a 34 and 36 when on the same rear cog is not so different. The choice depends on how much you value the transition between the rings, and/or the leverage while climbing. A 34T requires extra planning when making that front ring shift so your legs aren't ballistic jerked or forced to mash.

Be aware the crank spider must have a 110BCD for compact rings. If your current crankset has standard tooth range and a 110BCD it would be unusual.

Bob Ross

I got ballistic jerked just the other night...

AdelaaR

ColinL

Just use an online gear calculator and look at the speed vs cadence plot for 34T vs 36T with your favored cassettes. Then decided if you can get up hills at those speeds.

It sounds like you are sure that climbing in your existing 39T isn't working. That alone makes me think that if you don't want to change cassettes you probably do want the 34T.

ColinL

Overlooked point.

You probably can't go lower than 36T on the current 130BCD crank.

grwoolf

I'd go with the 34 if you want a better climbing gear.

A related question, has anyone tried running a 52/34? I know the jump would be extreme, but is shift performance reasonable?

I have a bike with a 53/39 (usually with a 12/27 cassette) and one a 50/34 (usually with an 11/28). The bike with the 50/34 is a Roubaix that I use for heavy climbing or recovery days where I want to keep the power low on the hills. I'd like to put a 52 tooth big ring on my Roubaix, keep the 34 as a bail-out/climbing gear, and standardize on a 12/27 cassette (to get the 16). My only concern is that the shifting performance might really suck. Anybody try it?

ThermionicScott 

46/34.

Perp

It's an S-works crank and I plan on buying a 110 BCD spider. My 3rd bike has a 34 and that seems low enough for me for any climb that I've done so far. I just like a closer gearing that a 50/36 would give me.

Just looked a gear calc and the diff between 34 and 36 is minimal plus my S-works is a good 4lbs lighter. I think I'll get the 50/36 for smoother front shifting.

Thanks

Nick Bain

this is something I have put a lot of thought into myself and I bought a 36 tooth chainring the moment I saw one I ebay for cheap.

The only thing that I can attribute to why they put a 34 on is too mimmick a triple and AND because it is compact "deferred success"(1) gearing you will be going slower naturally and the jump will be (feel) less than what the math indicates relative to a 53 to 39 jump.

If that doens't make sense, a simple way to put it would be to look at most cog sets and notice the gear spacing increases as the tooth count goes up.

deyendznyr

38T is the lowest you can go on a 130BCD

deyendznyr

Check the specs on your front derailleur. 16t is usually the max difference between Big ring and little ring. So if you have a 52T big ring, a 36T little ring is the smallest you can go.

WarLordZ

50/34

Nick Bain

36/26/36 OIS's 5'3"

gregf83 

You need to provide more information if you want a useful answer. At the very least:
1. what kind of hills will you be climbing?
2. power/weight estimate for a few time periods, i.e. 20 min, 60 min?
3. preferred cadence?

Without that info any answers you receive are meaningless.

Perp

Locally, we only have tiny hills here (but steep). Just under 1km, but 20%+ peak grades. I prefer climbs that are over 8% grades. I will be bringing this bike to travel / race in hillier terrain. Close hills that I'll travel to somewhat frequent are "Cat 4/3" in strava.

Haven't tested in awhile, but last spring after 3 weeks off a bike (vacation) I was 3.5W/kg for 30min on a friend's computrainer. Didn't test other time intervals but for being slow, I have a decent jump - I can create gaps quickly (on other slow riders), but get caught pretty quickly on the flats.

Low 90s is where I usually avg for cadence, I prefer spinning up a hill than mashing (unless I'm out of gears)

bbattle

Pros will use a 52-34 on difficult climbing days. Cassette will often have a 32T cog, too. Shifting is probably less than ideal but doable.

icyclist 

" I think I'll get the 50/36 for smoother front shifting."

I don't think you'd notice a significantly smoother shift with the 36 or the 34.

Also, definitely check a gear chart - you might be duplicating a few gears one way or the other, which, if you plan on racing, you might want back.

Finally, I think someone above mentioned that there isn't much of a difference between a couple of gear inches. Well, you can say that about 36 and 38 get inches. At some point, though, you're going to run out of gears.

Werkin

I'll assume what he meant by "smoother front shifting" was less cadence variance. The original post was framed in the context of cadence. With a 36T I only needed to shift one higher gear in the rear before making the drop up front to keep spinning at a similar rate. I usually shift two higher gears with the 34T before the drop if only a gradual grade. The ratio is near the same when shifting up. I can also hold out until the grade increases a bit more before dropping down from the big ring for a similar effect. With a 26 big cog there probably isn't a two tooth gap in the midrange like some cassettes without a 16. If the big ring shift point is across the 15-17 gap, the difference is greater from where you are and where the target rate is. On my 52-40 crankset multiple tooth cassette gaps make little difference when shifting rings either way.

I also changed the spider on one of my cranksets; I understand how that works.

At my fitness level a 34/30 combo is very welcome on 20% grade peaks.

dstrong 

<insert gratuitous Andy Schleck reference here>

Drew Eckhardt 

How much do you weigh, what's your mean maximal power curve look like, what's you're preferred cadence range, what happens to your fatigue when you're forced to pedal slower, and what are the lengths and grades of the climbs near you? Without that we can't provide any sort of meaningful answer.

I found that a 34x23 low gear (and 30x21 or 42x28 before that which are all the same) was enough to spin up almost any grade (obviously not the short 12% segments of Magnolia Road). A little arithmetic suggests that at 145 pounds I'd have been doing 90 RPM up 6% grades for climbs under an hour.

With a 36 small ring you need another 1-2 teeth to gain an equivalent low gear which may not be significant.

OTOH, at the small end of your cassette a 36 gives you another cog's worth of low end before you need to shift (ex - 36x13 is close to 34x12) and being able to put out over 15% more power before you shift is significant.

You have a much better idea than we do about whether or not you'll be happy with a low gear equivalent to you current 34 ring with about a tooth less.

Perp

I'm 150-155 @ 5'9. I usually hover around 8% bodyfat, lean athletic build.

I don't have easy access to a power meter, so I don't know what my power curve would be. When climbing I don't like to get under 70RPM sitting. I prefer high 80s on sustained climbs. My last big ride with 6.5k' of climbing, near the end, I was struggling with my 39/26 (legs were all cramped up).

Climbs I'll do on a regular basis are pretty short - just under 1km to ~1.5km. My favorite short hill is 0.6km, 2 sections of 20+% (garmin shows 23-25% as a max) and nothing under 10%. I'm usually fine on these climbs with my 39 ring, until I'm doing heavy repeats (10+). About once a month in the summer, I'll drive to get better climbs in, but no big mountains near by. These climbs are ~10km in length.

When I said earlier another smoother front shifting, I meant less variance in cadence.

If it matters, my bike is on DA 7900.

gregf83 

At your weight and power I would suggest the 34 rather than 36. Climbing an 8% hill at 240W with a 34-26 will drop your cadence to around 76RPM. If you've already been riding for a couple of hours before a long climb you might be struggling. You probably also want a 28 which would raise your cadence to 82.

You can play around with the numbers at www.kreuzotter.de