Compact Gear, Big ring vs Little ring

[mudmucker]

I ride a triple. I am mostly in my big ring. I drop to the middle ring once and a while and I am in the small ring very infrequently.

However, it is a bit different in March and April when I am just getting out when I use the middle and small ring more.

[radacrider]

Being rather OCD and nerdy at times, maybe seeing how much overlap there can be between two chain rings. The numbers are based on my road bike set up with 50/39 chain rings and a 7sp cog set of 12-22. The "ratio" numbers are based on Sheldon Brown's gain ratio formula (I have 175 cranks and 700x23 tires).

Maybe seeing that you do have overlap between the front chain rings will help ease the argument of which one you "should" use. Which the answer as others have said is what ever feels good to you for the terrain you are riding.

Happy riding.

[indysteel]

To the OP, do you have any idea what cadence you feel comfortable with? I agree in theory with your boyfriend. A higher cadence, say around 90 rpm, is generally better from an endurance standpoint. I disagree, however, that you necessarily have to be in the little ring of a compact crank to achieve that. I can easily do so in my big ring. Now, you may have to build up to that, but in time, you should be able to comfortably use a wider number of your available gears.

[Yelsel]

Sheldon Brown's Gear Calculator: http://www.sheldonbrown.com/gears/ - this is *great* to play around with to get the optimal setup in gearing for your bike. Right now I am playing around with how to change my commuter / errand bike to be able to haul more stuff uphill.

Unfortunately it doesn't save the output as a link - it is dynamic within the page.
For the 2010 Dolce Pro compact at least:
* Wheel Size: 700c x 23
* Crank Length: 170mm (I am assuming)
* Gear Units: Gain Ratios (for every circle the crank makes, the gain ratio is the multiplier to determine distance traveled -- you can use any units you like here, like light-years, since it is a ratio!)
* Chain Rings: 50, 34
* Stock Cassette: 10 speed, 12-27
**Calculate**

chrng ** 12 ** 13 ** 14 ** 15 ** 16 ** 17 ** 19 ** 21 ** 24 ** 27 cog

* 50 * = 8.2 * 7.6 ** 7.0 ** 6.5 ** 6.1 [5.8] [5.2] *[4.7] * 4.1 ** 3.6 GR

* 34 * = [5.6] [5.1] [4.8] * 4.5 ** 4.2 * 3.9 ** 3.5 * 3.2 * 2.8 ** 2.5 GR

You essentially have 15-16 unique gears; three that are nearly the same, and two that alternate.
50 x 17 [GR 5.8] ~= 34 x 12 [GR 5.6]
50 x 19 [GR 5.2] = 34 x 13 [GR 5.1]
50 x 21 [GR 4.7] = 34 x 14 [GR 4.8]

The "most ideal" shifting would be to have the least cross-chaining: shift from 50 x 21 (skipping the largest two cogs) to 34 x 14 (skipping the smallest two cogs) -- nearly identical gain ratios of 4.7 / 4.8. Practically speaking, it is easiest to stay on one chainring, hit the end of the cogs, and then shift, unless you can remember what gear you are in. I'm assuming with compact doubles cross-chaining is less of a problem (I don't have one). So 34 x 12 is similar to 50 x 17, gain ratios of 5.6 / 5.8. If you keep going down in the 50 ring, your cog sizes are much larger, and you will drop a lot in speed at the same cadence. Your drop in gain ratio / speed is less on the 34 ring. (You can also play with speed at various cadences.)

If you switch to a 12-25 cog, then the jump in cog sizes is less for the larger cogs, and there would be more "overlap" in gain ratios between the 50 chainring and 34 chainring. You would essentially have 14-15 unique gears (and the lowest gear, 34 x 25, would be higher than 34 x 27). But it would be just as effective to stay on the 50 ring, since the drop in gain ratios at the larger cog sizes will be less, and will overlap the gain ratios in 34 chainring and smallest cogs. I think it is more typical to have 12-25 cogs, they probably set up the women's bike with 12-27 to give a lower gear for climbing. Most women have a lower power/mass ratio to men, since on average women have a higher fat % / lower lean mass % than men. With 12-25 cogs, you have the option to stay in the larger chainring or the smaller chainring without a "gap" / higher drop in gain ratio in the 50 x larger chainrings.

The other option is shorter cranks. Since women are typically smaller and have shorter legs, shorter cranks are more effective. The smaller revolution puts less strain on the knees, important since this is the most common repetitive strain injury. Really, women's bikes should have shorter cranks standard -- unfortunately it is difficult to even find shorter cranks, and they are expensive (check out Peter White Cycles for one source). The best method is to measure the femur (in mm), from the top where the ball sits in the hip to the knee, and multiply by 18.5%. You can also use your inseam measurement in mm, multiply by 2% to 2.16%. If you take a 160 mm crank arm, for example, in the 12-27 cogs, your drop in gain ratio would be less on the larger cogs in the 50 chainring, and overlap the gain ratios on the 34 ring on the smaller cogs. Your speed in a gear at the same cadence will be the same in a 170 mm crank or a 160 mm crank. If your legs are shorter, the strain on your knees will be less (smaller circles), you would have all the gears as before, and your preferred shifting pattern of staying in the larger ring will work better.

I have a 52 / 42 / 32 triple x 8 cogs (24 speed), with 14 unique gears. I stay on the middle chainring, and then 52 x the three smallest cogs and 32 x the 3 largest cogs are the "extension" of the middle chainring, = 8+3+3 gain ratios. This makes shifting much easier, since I hit the end of the middle cog, switch to the larger / smaller one, and and then click up or down 2 cogs to get the next gear. With a compact double, you can hit the end of the cog, but to get to the next gain ratio up / down, you need to be in the middle of the other chainring. Now I understand why Campy went to having the 5 up / 3 down shift option on cogs!

[Yelsel]

Another note about switching crank sizes -- your gain ratios will not change much in lower gears, but will change a lot in higher gears. If your favorite gear right now is 50 x 17 for example (gain ratio of 6.1), on a 160 mm crank the equivalent gain ratio of 6.1 is 50 x 16. Your gain ratio will be pretty much the same in lower gears. At the high end, the gear ratio for 50 x 12 will be 8.7 on 160 mm cranks vs. 8.2 for 170 mm cranks. I don't think this is a bad thing -- you usually use the highest gears downhill, and you won't spin out as fast with higher gain ratios in your "downhill" gears.

[Kelly728]

I'm glad this was brought up because I'm always pondering if it would be more beneficial for me to stay in the small chainring most of the time with my lack of speed/strength. Typically on flats I find myself hanging out on my large chainring (50/34) and 21t cog in the back. I can easily hold about a 82 rpm cadence with this gearing. With some tinkering with gear calculators, I see that I can get the same gear ratio on the small chainring and the 14t cog. My worry would be running out of gears on the small chainring very quickly especially on rolling hills.

And now that I just rambled I highly doubt I made a point (or made any sense)...

[GreatPaws]

well, since you all are so nice about dumb questions...

a) The bike at the gym says my rpms are mostly 70-75 rpm when I'm comfortable.(my real world bike doesn't have rpms) I"m supposed to go faster than that?????? Why? I have aways to build up to then...

b) How do you know if you have a "compact crankset"?


A url/ link is fine if this is too newbie...

[tulip]

A better measure of effort at spin class is a heart rate monitor. Cadence is important, but you don't want to be bouncing in the saddle (cadence is too high in that case).

BTW, GreatPaws, glad to see another TEer in Richmond!

[OakLeaf]

Cadence is important, but you don't want to be bouncing in the saddle (cadence is too high in that case).

It doesn't mean your cadence is too high, it means your pedal stroke is uneven. If you're not smooth at a high cadence, then you're not smooth at a lower cadence either, it just isn't as obvious.

One way to build an even circular pedal stroke is with cadence drills, either downhill or downwind on an outdoor bike; on rollers; or on a stationary bike with very low resistance. Spin at the fastest cadence you can without bouncing (which can be kind of hard to tell on a stationary bike, but anyway), then increase your cadence by 10 rpm for one minute, and repeat several times.

[Andrea]

He's probably just trying to slow you down because you're wearing him out