Why you want a compact crank
There's been a lot of discussion in the IM WI group about gearing, particularly after a lot of us went to the course for the rally.
Everyone, of course, always replies that "you can never have too many gears", but people throw a lot of numbers around and some of them aren't accurate. Also, there is the fact that cassettes can be had for something like $75, and to get into a compact crank, the minimum is about $150.
Unfortunately, if you are starting with a standard crank (53/39 in the front) and cassette (12-25 in the back), the better solution is to spend more money and get the compact crank (50/34 in the front). The reason for this is that what matters is the gear ratio and the difference between 25 and 27 isn't as large as you think...it's less than 10% when you look at it that way.
Below is a table showing the gear ratio for the standard small ring and the compact small ring up front, paired with various reasonable large cogs in the back.
Assume that you are going up a hill such that your cadence is 50 if you were to use the standard crank small ring (39) up front and 25 in the back. In the last column, you see the cadence that that same hill and same speed (and same watts) would be for the different gear combinations.
| Front Chain ring | Rear chain ring | Gear Ratio | Cadence |
| 39 | 25 | 1.56 | 50.0 |
| 39 | 26 | 1.50 | 52.0 |
| 39 | 27 | 1.44 | 54.0 |
| 39 | 28 | 1.39 | 56.0 |
| 34 | 25 | 1.36 | 57.4 |
| 34 | 26 | 1.31 | 59.6 |
| 34 | 27 | 1.26 | 61.9 |
| 34 | 28 | 1.21 | 64.2 |
Notice that the 34/25 gives you an easier gear than even the 39/28! Clearly enough, getting the compact crank is a better way to get the easy gear.
The other two issues are (1) how fast can you go in your "fastest/hardest" gear and (2) how bad are the cadence gaps. Below are three tables that address these. In each comparison, the top one has a standard crank on top and compacts on the bottom. The cassette is listed in each case. We assume 100 rpm as a typical fast cadence. (If you go 110 rpm, you would obviously go 10% faster!)
11-28 on the standard and 12-25 on the compact:
11-28 on the standard and 11-26 on the compact:
12-25 on both:
Obviously, with the 53, your high-end speed is several percent faster than with the 50, using the same cassette. However, consider that most of us spend most of our time between about 8 and 27 mph, and look for the gear gaps. They are considerably larger with the standard crank 11-28 cassette than they are with any other combination. This gives another reason to go with the compact cranks.
OK, so let's assume you are convinced you want the compact, but you are concerned about the high end gear. There is nothing to be done about that, except also buy a new cassette. Look at the middle graph and you'll see what happens when you pair an 11-26 cassette with a compact crank. Note that the 50/11 pairing has a higher gear ratio than the 53/12. Switching to compacts with a 50-34 and an 11-26 gives you a much easier gear on one side, a higher gear than you started with, and the gear gaps are only in the range where you are really starting to fly downhill.
Finally, it is a very minor matter, but a compact crank is going to be lighter than a standard crank.
Comments
Once again, nice post Dr Jenks. Another useful bit of information involves shifting from large to small chainring to minimize the gear gaps. For the case of 50/34, 12-25 the optimal time to shift from large to small chainring (from a gear ratio perspective) is when you are in the fourth cog from the top. This requires up-shifting four gears on the rear cassette.
As for myself, I run a 50/34 and plan on ridding it WITH either a 12-27 or an 11-28. My FTP is 260 and I weigh 82Kg on a good day... Happy with the Watts, but I'd prefer a few less Kgs for IMWI
Willaim - great post. I stumbled across a web site that had a great view of gearing and rpm and the resulting speed ranges. home.earthlink.net/~mike.sherman/shift.html
This one also includes the crank length. That comes into play in terms of how "hard" the gears feel, but doesn't change the gear ratio. I wonder what he's using that for.
(I have thought about going to short cranks to ease a steeper fit (e.g., 165 mm instead of 172.5) but have not done it. If you do that, it makes each gear "feel" about one gear harder because you have a shorter lever arm. You have to apply more force, but over a shorter distance, to get the same watts.
I should have included the URL from where I made the graphs - here it is:
http://public.tableausoftware.com/views/bikegears/CompareBicycleGearing
Wm
Posted to the Wiki here
I will buy the beers!!!