Compact cranks... Here we go again

Triathlon Forum
1

Let me start by saying that I think that a lot of triathletes would beneficiate from compact cranks. However, the article on Slowtwitch by Simon Butterworth is full of incorrect affirmations. Like all new cycling fads, this one comes with lots of misinformation.

Higher RPM’s can be maintained on climbs

Higher RPM’s can only be maintained with the use of a lower gear. This can also be achieved on standard cranks or using a triple.

Closer spacing of the gears makes it easier to maintain the optimal cadence**

This is the one that really gets to me. Basically when selecting appropriate gearing, you have two choices: highest gear and lower gear. If you want a lot of range, you have to accept a wider spacing. You want very tight spacing, you gotta give up range. Compact cranks do nothing for you when it comes to spacing. In fact, if your reason to get a compact is to get easier gears so you can spin uphills, your spacing will be wider !!! unless of course, you’re willing to sacrifice a higher gear. You want really close spacing ? There is only one way. You need more gears. And that means using a triple.

The combination of compact cranks and appropriate cassettes has less rotating mass

While probably true, this argument is moot since the impact of rotational weight is negligible, especially considering the short radius involved, and the fact that crank weight is essentially at the center and in the crank arms. If we want to argue minute details like that, I could point out that compact cranks will result in more drive train losses caused by the higher curvature of the chain profile around the smaller chain rings. I could also argue that a 16 teeth difference will make shifting less efficient.

Comparison of gearing between compact and standards

All of the comparisons are beside the point. The shown combinations indicate basically very similar high and low gears (and therefore spacing) and, as such, indicate that either choice is fine. We could get into double shifting advantages (or lack of) but this has been tackled in another thread a while back.

The only real advantage of compacts is that they give additional gearing options. Compact gives you more options when it comes to lower gears, whereas with standard cranks you are pretty much limited to 39-27. When selecting appropriate gearing, you want to get a combination that will let you ride most of the time with a straight chain. If you spend a lot of time riding on the four largest cogs of your cassette, a compact is likely a good idea. Compacts may be good for weaker riders on flats and for stronger cyclists on hillier terrain.

Finally, as mentioned in the article, having more than one cassette makes a lot of sense. I wondered numerous times why people are willing to pay in excess of 2000$ for a bicycle and frown at the thoughts of spending 70$ for an extra cassette and tool.

Francois in Montreal

2

Mon ami,

Now you’re really gonna confuse everybody. I like my compacts because I live in a hilly area. But I was under the impression that you get a closer ratio gear set with a 50/34 and 11-23 than say 53/39 and 12-25. I’m using 12-25 any ways with the compacts in my immediate area and would switch to the 11-23 cassette if riding in less hilly areas.

3

I DON"T CARE WHAT YOU SAY!!! compact cranks are great!!! My grandma uses them to tootle around town on her comfort bike and loves them. Of course she doesn’t use them on her race bike. But then grandma can CLIMB for an 80 year old ; )

4

OK denewone, now’s your chance to tell us you did Mountains of Misery last year with your 56/42 and 11-21.

5

You do get closer spacing with compact cranks. The reason is that you have fewer overlapping gears between the large and small chainrings. The compact crank knocks the small chainring from 39 to 34 or about 12%. It knocks the big chainring from 53 to 50 or only about 6%.

The closer spacing comes at a price. You will be shifting between chainrings more often because of less overlap. You will also have a near crossover gear more of the time. For example you will often have to compensate for the very small chain ring by shifting to a smaller cog.

I haven’t hooked up my compact crank yet, but I would think it would work poorly in areas with lots of rolling hills since it will require the rider to shift chainrings frequently. I bought mine for Lake Placid. It is not an issue for that course since the hills are long and steep (for me anyway).

My compact crank will shave about a half pound off my bike. No biggie, but it is better than a stick in the eye. You are right that shifting will probably be less smooth.

You obviously get more gears and lots of overlap with a triple. Mine works great, but it makes me look like a dork. Can’t have that.

6

No ------------------------------------------------

-----------But Grandma did
.

7

Your grandma sounds really hot.

Pictures?

8

The closer spacing comes at a price. You will be shifting between chainrings more often because of less overlap. You will also have a near crossover gear more of the time. For example you will often have to compensate for the very small chain ring by shifting to a smaller cog.

This brings to mind another disadvantage to compact cranks. Because of the near crossover situation, they may not work so well on a bike with short chainstays - like many steep angled tri bikes have. By comparison, my commuting bike has chainstay lengths of 440mm, my mtn bike has 430mm, my rode bike 400mm, and my 78-degree tri bike has 388mm. Of course, this disadvantage may not be a problem in actual riding. I don’t know because I’ve never tried it.

9

“Because of the near crossover situation, they may not work so well on a bike with short chainstays - like many steep angled tri bikes have.”

i got an email last night from a very respected tri shop owner who doesn’t like compact cranks because the 16-tooth difference between the chainrings is larger than shimano recommends, and will make for bad shifting. plus, the b/o FDs aren’t made to accept 50t chainrings.

before i let simon’s article be published monty and i spent quite a bit of time on compact cranks. neither of us have had any shifting problems, neither of our bikes had to be altered to accept the 50t rings. as engineers are fond of saying, one test is worth a thousand expert opinions.

monty rides a 650c cervelo which, from memory, has 368mm chainstays, certainly as short as the shortest stays currently on any production bike.

my advocacy of the 110mm bolt pattern is different than simon’s. i just think almost every triathlete’s bike is overgeared, at least among athletes that train or race in hilly terrain. i don’t like triples. hence, 110mm cranks.

10

Bravo Dan. My experience also,

11

Did up a quick gear chart to compare a 53/39 with a 12-26 to a 50/34 with a 11-23 cassette. The compact gives a wider range of gears, as well as tighter spacing since there are only 1 tooth jumps over 5 gears rather than 4, same as if you were switching to 10 speed from 9.

The other difference is that to get the same gear on the large vs small ring, you need to shift 4 gears in the back rather than 3, so you get 1 less crossover gear. Whether this is an advantage or not is debatable.

edit - took out the chart, since it didn’t display properly

12

The compact gives a wider range of gears, as well as tighter spacing

As stated before, this is a mathematical impossibility. Compacts are not magical. You cannot get a wider range AND tighter spacing. You need more gears for that to happen ! Get 10 speed campy or a triple.

From Slowman:

i just think almost every triathlete’s bike is overgeared, at least among athletes that train or race in hilly terrain. i don’t like triples. hence, 110mm cranks

Amen to that ! This is what compact cranks are about. Nothing less, nothing more.

Francois in Montreal

13

Now you’re really gonna confuse everybody. I like my compacts because I live in a hilly area. But I was under the impression that you get a closer ratio gear set with a 50/34 and 11-23 than say 53/39 and 12-25. I’m using 12-25 any ways with the compacts in my immediate area and would switch to the 11-23 cassette if riding in less hilly areas. Using the first two combinations, you get max and min gear ratios of 4.42 and 1.56 on the standard and 4.54 and 1.47 on the compact. So in this case, you get a tighter spacing on the standard (but less range then the compact). Switching to the 12-25 on the compact woulg give you similar range as with the standard (and similar spacing). Your range would however be shifted toward easier gears (4.17 and 1.36). Francois in Montreal

14

Who in the world started the 78 degree, 650 wheel bike coming with a 55/42 as standard gearing? Must have been some whacko ; )

15

Do the chart and you’ll see what I mean.

50/34 11-23 (compact) compared to a 53/39 12-26 (standard) The compact has a higher high gear and a slightly lower low gear. Ergo, wider range.

an 11-23 (9 speed) increments by 1 tooth for the first 5 cogs, and by 2 teeth thereafter. a 12-26 (9 speed) increments by 1 tooth for the first 4 cogs, 2 teeth for the next 4, and by 3 teeth to the 26. ergo tighter spacing.

That mathematical “impossibility” is achieved courtesy of fewer overlapping gears between the small and large chainrings. Whether that is a good thing or not is up to the individual.

16

I’ve been going back and forth on the compact-vs-triple choice for quite a while myself: while yes, the compact gives you -theoretically- a wider range, most of the time (due to the very small overlap) you’ll be in a cross over gear which is not really that great for power transfer and your drivetrain in general.

Personally I think the triple is the best choice, you can use the 42 all day on flats and very slight rollers, the 52 on downhills and the 30 on uphills: yeah, some people in North America think it’s dorky (not people in Italy, where if you ride in the alps you’ll see that nearly everybody has one) but hey, IMHO it works much better.

17

50/34 11-23 (compact) compared to a 53/39 12-26 (standard) The compact has a higher high gear and a slightly lower low gear. Ergo, wider range.

That’s correct**
an 11-23 (9 speed) increments by 1 tooth for the first 5 cogs, and by 2 teeth thereafter. a 12-26 (9 speed) increments by 1 tooth for the first 4 cogs, 2 teeth for the next 4, and by 3 teeth to the 26. ergo tighter spacing.

That’s incorrect and this is probably where the confusion arises. Tighter spacing is linked to the average gear ratio spacing ! The tooth increment in the cassette means nothing in itself. The following is taken from one of my previous post a while back:

Comparing a=; b=;
c=; d=;

Here are the gear ratios you get with both combinations:

Gear ratios in ascending orders for combination a-b 1.4444 1.6957 1.8571 1.9630 2.0526 2.2941 2.3043 2.5238 2.6000 2.7857 2.7895 3.0000 3.1176 3.2500 3.5333 3.7857 4.0769 4.4167

Gear ratios in ascending orders for combination c-d 1.4783 1.6190 1.7895 2.0000 2.1739 2.2667 2.3810 2.4286 2.6154 2.6316 2.8333 2.9412 3.0909 3.3333 3.5714 3.8462 4.1667 4.5455

We get a mean gear ratio spacing of 0.1748 versus 0.1804 for the compact. So you get wider range with the compact (with this combination), and obviously, tighter spacing on the standard.

In this case, both combinations are nearly identical.

Francois in Montreal

18

I have a set on my tri bike and have been happy with them. Only put a few hundred miles on them and racing WF with them this weekend. The bike actually shifters better than my RR bike (that damn thing shifts like crap), but I have had no issues going from small to big.

I had the local tri shop ask me why I went with the compact setup on my new bike. I gave him a pretty lengthly answer about gearing and how I believe it will benifit me. He said most folks just buy them because they are cool.

I can get a straighter chainline on the flats by going from the 15t to the 14t.

19

Damn, I had a nice long dissertation which went up in smoke when my browser crashed. Oh well.

The gist of it was that folks generally don’t double shift a lot; they tend to stay on either the big or the small ring. So a better way to figure out the tightness of gear ratios would be to take the average jump in the big ring, the average in the small ring, and then take the average of those 2 averages.

Doing that, we get (in gear inches) -

standard =7.0 inches per shift

compact = 6.6 inches per shift

Of course, that isn’t enough to make me switch, I’m perfectly happy with my standard set-up 53/39 12-23.

20


We get a mean gear ratio spacing of 0.1748 versus 0.1804 for the compact. So you get wider range with the compact (with this combination), and obviously, tighter spacing on the standard.

Francois in Montreal

I think what JasonK was getting at is that the gear ratio spacing for the compact is more uniform (i.e. smaller standard deviation) because of less overlap.