standard and compact cranks, the long and the short of it

[Ypsylon]

"What's better, a compact or a standard crank?" is one of the questions that pops up every once in a while and often lights a lively debate about the details that confuses the novice cyclist rather than help with his decision, so I figured it would be nice to have the basics, that are the same for everybody, covered, so that everybody can then discuss on the same page.

So, what's the difference?

That's pretty easy, actually. The only difference is the bolt circle diameter (bcd), sometimes called pitch circle diameter.

A standard crank has a bcd of 130mm, a compact crank has a bcd of 110mm. That means two different things. First is, that you cannot mount a 130mm chainring to a compact crank, and vice versa. The second is that you can not mount a chainring smaller than 38t to a standard crank, as the bolts would have to be higher than the teeth.

On the other hand you can mount a 53t chainring to a compact crank, if the chainring has a 110mm bcd. So you can say that a compact crank is more versatile.

The possible downside is that the distance from the bolt to the teeth that the chain has to climb to while shifting to the big ring, is greater, so that the lever is longer and more area to flex over, which, in theory, would make shifting of a 53/39t chainring combination on a compact crank "worse" than on a standard crank. If that turns out to be noticeable in the real world depends on what you compare to, what your shifting technique is like and how you perceive your shifting.

Any major problems would most likely be due to a factor other than a difference in bcd, like comparing a very stiff Dura Ace 7900 chainring to a dremeled down super light chainring, or a mal-adjusted derailleur.

In practice the only true "problem" is rather that the choice of chainrings with 53 teeth and a 110mm bcd is limited and you might not find one that you like the looks of or feel like paying the price that's being asked for it, as it's usually a specialty product and not massed produced.

Now that you know the difference, which one is better for you?

This answer is definetly more complicated, as it's not just a matter of what will fit with what, but it actually depends on you, your power output and your self selected cadence and is something that the nice people on this forum are able to help you with, once you know the basics.

Right now, this is pretty much just a draft and I invite people to point out mistakes, help make things even clearer and whatnot and I'll edit this post.

[Juanmoretime]

While I do believe everything you've pointed out is correct ultimately I think the general conception when using both terms are the configuration of each. When I think compact I think of a 50, 34 or 36 combination. Standard cranks equal 53, 39 or 52, 39 or 38 just like a triple is three rings. I ride a Zipp Vumaquad's so that one breaks out of the norm since you can go compact or standard on the same crank with 110mm circle with only 4 mounts. My preference is the standard. (53,39)

[Gumgardner]

Thanks Ypsylon. Nice little summary. I've always riden a standard setup, but with my new build im going to try a compact setup. I knew I wanted to do the Fiberlyte carbon chainrings so I figured with the compact there will be less big to little shifts up front. I'm going to try 50/36.

[Jeffr]

Some things I would point out based on my exploration of this topic:

1) FDs seem to be more finicky with compact. Most are shaped more or less for standard chainrings.
2) Regardless of the number of teeth the step between them will affect the ratio change on front shift. In the case of 50/34 in particular it can be extreme.
3) Make sure the ratios you commonly use will not cause extreme chainlines or frequent front shifts
4) Spider is stiffer than chainring. I once rode 53/39 110bcd and it was relatively flexy, not just bad for shifting but the big ring could brush the FD when properly adjusted.

[slisk]

I'm a complete compact rider. I represent the old fat guys who would never find an occasion to use 52-11...but 34-29 is a lifesaver in Colorado. All my bikes are Record 10, with 50/34 Fibre-lytes baxk is Campy 13-29. I used the CT QS FD and medium cage in the rear...KMC chain and shifting is perfect for me.

[Boonen]

If you're concerned about having a gear big enough it might be good to know that 50x11 is actually a bigger gear then 54x12.

If you aren't going to use smaller chainrings I don't see any reason to go for a compact crank. As Jeffr points out a compact crank with larger chainrings is less stiff then a similar setup with a regular crank. I don't think the problem lays in the shifting as much as in just powering along in the big ring. If there's to much play between the rings and the frame you can get rubbing of the front derailleur and even 'ghost shifting' which is very annoying. Note this can happen with standard cranks as well, especially in combination with a soft frame.

[Montana]

I have found that a 50-36 shifts much better than a 50-34 / Riding a 11-25 with a 50-36 gives me a very wide range of options -

[tilburs]

I run a SRM Pro Square taper compact crank with 50/34 and 11/25 in South Spain, ratios work a treat (50/11 is a bigger gear inch than 53/12) and never had a problem getting upto as much as 103kph in 50/11

Nice thing is with it's 110/130 BCD's I can simply swap in 39/53 for really flat courses/races

Seems to the best compromise of weight and gearing IMO.

David

[JOWUL]

Although its a while since someone posted to this topic, I like to add my experience.
I ride in Southern Ontario where we have small elevation gains but frequent, short, steep climbs (Niagara Escarpment). Once a year I ride a couple of weeks in France; the usual stuff, Mt Ventoux, Alpe d'Huez, and the last trip in the Vercors.
I ride a FSA Light compact on my Treck 5200 and my Parle Z4. My set-up was always 52/38 with 12-27 when riding in Ontario and 50/34 with 12-27 when riding in France. I use a KMC-X-10SL chains. I do not need to adjust the Front Deraileur when changing from a 50 to a 52 large chain ring and I have no issues with chain drop or chain rubbbing.
After reading reports of last years Giro, where many of the top riders used compacts in the tough climbing stages with 52/34 rings and 11-25 or 11-27 cassettes, I decided to do the same, i.e. 52/34 rings and 12-27 cassette. I get the 52-12 combination for the flat portions and I have the 34-27 for the +18% climbs.
My opinion, based on my riding style, power, climbing abilities and chosen riding terrain, is that a compact crank set is more vesatile, probably for the majority of competitively recreational riders.

[slisk]

I'm curious how they managed more than a 16 tooth difference in rings? Did that include medium cages or something else?

[JOWUL]

The front derailleur has no problems handling the jump from the 52 to the 34 chain ring. The front and rear derailleurs are standard DA 7900 units. I do not need to change chains either when going from 50/34 to 52/38 or 52/34. The cassette is always 12-27.

[slisk]

I remember having issues running 50-34 and a short cage so that is why I asked the question. I run a 13/29 though