Compact vs Triple

[TBird]

Thanks again everyone, this is like a crash course in bicycle mechanics. I have a question that may be a silly one. I understand that in the front, the big chain ring is for more speed/power and progressively smaller ones are more for climbing. I also understand that in the cassette in the back, the opposite is true—the bigger the cog (ie, greater number of teeth), the lower the gear.

So my question is this: how do I compare two different cofigurations? For example, how different would it feel to climb in 34 front /25 rear versus, say, 36 front/ 27 rear? (If those are even possible configurations.) You can’t just add the numbers for a comparison. So is it just be experience that you know? Would I even notice the difference? As I demo compacts vs triples, it’s hard to gauge these subtleties. I can only be on one bike at a time, after all, and even if I play with bikes back to back climbing hills, I’m obviously more fatigued the second time around. On the Trek I am considering, the lowest gear combination is 34/25 on the compact, 30/27 on the triple. Is that a huge difference--one gear? Two gears? They only have the compact for me to demo.

Maybe I’m overthinking this, but I do keep getting mixed reactions on the compact vs. triple debate. I visited a second LBS and they, too, thought the compact was the way to go, saying that it shifted much smoother. (And they had both compacts and triples on the floor, so no apparent bias.) I also want to make sure I consider some of the points some of you have made, such as making sure I don’t sacrifice gears in “my” typical spinning range.

And frankly, at this point, I’m just curious about how this gear business really works. I had never even contemplated the term “gear inches” until aicabsolut mentioned it. The big ring/little ring thing seems so counterintuitive to me!

[OakLeaf]

Here's Sheldon Brown's gear calculator, to compare various setups. That whole site has a wealth of technical information. RIP Sheldon.

It's really not that complicated. You want to know for every revolution of your legs, how far will your rear (drive) wheel travel? Let's say you're in a gear where your rear cog has the same number of teeth as your front one. Maybe a 30x30. (That's a very low gear, stock road bikes don't normally go that low, but mountain bikes do - anyway just for the sake of discussion.) For every revolution of your crank, your rear wheel will also make one revolution.*

Okay, now shift your imaginary derailleurs into a gear where your rear cog has exactly half the teeth of your front one. Say it's a 42x21. Now, every time you move your legs and crank one revolution, your rear wheel will make TWO revolutions. So you're going twice as far for every revolution of your legs (faster given a fixed cadence), but you also have less mechanical advantage (you have to exert more force on the pedals to go the same speed).

Bigger cog in front = more revolutions of the rear wheel per crank revolution ("more speed"); bigger cog in back = fewer revolutions of the rear wheel per crank revolution ("more power").

HTH to get you started. There's more to think about, but that's a beginning.


*"Gear inches" is based on the diameter of the wheel, not the circumference. I don't know why.

[TBird]

This is a belated "thank you" to everyone who offered advice on my compact vs. triple conundrum. After a lot of demo-ing and bike shopping, I ended up still back lusting after the original Trek Madone 5.1 with the compact that I had fallen in love with. But here's the best part: I convinced the LBS to swap out the compact for a triple that was on a men's Madone! All it took was a sixer of Jubilale (a great Deschutes seasonal brew).
I've only logged about 100 miles on my dreamy new bike, but I am SO glad I went with the compact. That lowest granny gear has already seen a lot of use.
Thanks again. This is such a great community of folks!

[tantrumbean]

This thread on another forum convinced me a triple would be a good idea:

http://www.cyclechat.co.uk/forums/sh...ad.php?t=25303

[jobob]

All it took was a sixer of Jubilale (a great Deschutes seasonal brew).

You have excellent taste in bribes.