Understanding gears & cranksets

The Sheldon Brown website has lots of good, easy-to-read information. 

Chainrings are the gears attached to the pedal cranks.  The smaller the number, the easier to pedal.  Road/tri cranks are often "53/39" (53 teeth on the large, outer ring, 39 on the small, inner ring).  Compact cranks are often 50/34 or 50/36 so would be a little easier to pedal (everything else being equal), but you don't move the chain quite as far with each revolution.  Your triple crankset may be a 52/42/30. 

Cogs/cassettes/freewheels are the gears (or set of gears) on the rear wheel.  The smaller the number, the harder to pedal.  They are often denoted with a dash, so an 11-21 has a range of gears between 11 and 21 teeth.  Older bikes and many mountain bikes have a freewheel and are usually 5, 6, 7, or sometimes 8 speed (times the number of chainrings on the front for total number of gears).  Newer road bikes usually use a freehub and cassette and these are often 8, 9, 10, or even 11 speed.  An 11-21 rear combination could have anywhere from 5 to 11 gears in it.  Mountain bikes and touring bikes may have 26, 27, or even 34-tooth gears in the rear. 

If they talk about a 39/15 or 53/15, they're indicating which chainring and cog they're using.  This provides a very specific mechanical advantage, but the overall relationship between pedal force and wheel force is complicated by the length of the cranks (170-175 mm are common) and the diameter of the rear wheel (700C versus 26") and tire combination.  It's possible to have almost identical mechanical advantage in some of the "middle gears" (39/14 and 53/19 are almost identical). 

The front deraileur must move the chain from one chainring to the next.  As you can imagine, the bigger the difference in number of teeth, the bigger the difference in diameter and there is usually a limit (often 14 teeth for a double) where the deraileur can't easily move the chain without it coming completely off.  The rear deraileur does the same thing on the rear, PLUS it tries to keep the chain tension tight enough that the chain doesn't jump by itself from gear to gear.  There's a size limit on the rear gears just like the front, plus an additional limit to the total number of teeth variation for both the front and back combined.   The longer the rear deraileur frame, the more teeth it can work over, but the longer it takes to shift from gear to gear. 

I've duplicated some other posts (it happens, since I type slow), but to answer the other part of your question:

Count the teeth (some chainrings have the number stamped on them) front and rear (it can make you dizzy) will tell you what you have. 

If you're spending a lot of time in the big chainring and the smallest cogs, you might benefit from either a bigger chainring or smaller small cog (this doesn't describe most of us).  If you're using the small chainring and the biggest cog and you still struggle getting up a hill (this does describe many of us), you might benefit from a smaller chainring (compact double or a triple) and/or a larger gear on the cassette.  Swapping these components can be tricky, so you most likely can't just put a triple crankset on in place of the double, nor can you just throw a 13-34 cassette on in place of a 12-23. 

You'll also get better by riding a lot, so don't assume that a new cassette is going to make all of the work go away.   

tkd.teacher - 2009-08-25 4:11 PM

Now, to understand how all this relates to speed, you have to do a bit of math.  Sorry.

Gear inches = drive wheel diameter * front chainring / rear chainring.

So, if you have a 27" wheel (Which is basically what a 700c wheel is, that's pretty much standard on any road/tri bikes), and you in a 50 tooth chainring up front, and a 16 tooth ring in the back, you get:
27 * 50 / 16 = 84.375.  This means for that every time you turn the pedals one full time, you travel 84.375 inches.

Now you get to play with numbers to determine your speed if you want, and thats what the actionscript does is it translates those inches * cadence to get your speed. And, you can also start thinking about how the length of the crank arms affect all of the above if you want to. :D :D

John

Just as an FYI, gear inches is not (exactly) how far you move for a single rotation of the crank (its related to it). Gear Inches is the diameter of the wheel if you were using a direct drive highwheeler (aka penny farthing) .. so the distance you travel is actually Gear Inches * pi  .. so for the 50/16 combination you would travel 84.375 * 3.14159 = 265.07 inches for a single rotation of the crank

Even going to a 12-27 may require a longer chain, and some of the short road deraileurs won't handle a 29 tooth range.  Some will...

Point is, it isn't always as simple as it seems. 

With the road-triple, you're lowest gear works out to about 8.4 mph (700Cx23 tires and a 90 rpm pedal cadence).  Currently, your tri-bike low gear works out to 11.0 mph (31% harder).  The compact crank with a 34-tooth inner ring will give you a low gear of about 9.6 mph (14% harder).  Changing from a 13-25 to a 12-25 cassette won't help the low speed.   

To switch to a compact crankset, you'll need a new crankset (chainrings & crankarms) compatible with your current bottom bracket.  (The bolt spacing on compact cranks is 110 mm Bolt-Circle-Diameter while standard cranks are usually 130 or 135 mm BCD)  Are the crankarm lengths the same between the tri bike and the road bike?  Longer crankarms give you more leverage (pedal to bottom bracket), but require greater range-of-motion in your knees and hips and may give you less biomechanical  (hip to foot) leverage.  You will most likely want to move the front derailuer down on the seat tube since the chainrings are a little smaller.  Not sure how easy this will be on your bike.  Your Local Bike Shop should be able to help you with all of this. 

The top right set of numbers shows what you have right now.  The top left show what you could get if you switch to a compact.  Notice that by going to a compact, you lose basically one high end gear (108.0) and one low end gear (32.4).  The larger the number, the faster you go, but the harder it is to pedal at a given speed.

That's why compacts are so SWEET, IMO!!!  You lose a whole front ring, but still get just about all the gears.

Also notice that on the triple, you get a lot of gear duplications which are not needed.  The 30/14 = 42/19 = 52/25 more or less.

Since you seem to be having an issue with hills, you would want to pay attention to the lowest/easiest gear possiblities.  If you switch to a compact crank and keep the same cassette, then you atually lose a gear which is bad in your case.  And like McFuzz said, you have to buy the crank, bottom bracket, etc.  Big $$$$.

However, if you can get a 12-27 rear cassette, which is about $80 bucks, and keep the triple, you gain a lower gear.  But it's not much better than what you have.

To make a long story short, I think what you have is just about as good as you are going to get and it's not worth the money to get that one extra half/full gear (going from the 32.4 with the 30/25 combo to the 30.0 with the 30/27 combo).  You may just need to get a bit stronger for the routes you ride.

I hope I didn't make this more confusing.

Akidoman,

I think Erincs' issue is she has a road bike with the triple, but her tri-bike has a standard double and she doesn't have the low-end gears she wants on the tri-bike. 

Depending on what crankset you already have, you can probably get a compact crankset for ~100.  Figure on another $50 for the LBS to re-tune the bike (and move front deraileur if it's easy).  That's probably easier than trying to switch to a triple crankset. 

Easy enough, I have 650's on my tri-bike.  39/25 = 10.2 mph, 34/25 = 8.9, 30/25 = 7.9 (you can work the percentages). 

The biomechanics get complicated.  While the lever arm between the pedal and bottom bracket is easy to analyze, the biomechanical part of the equation isn't so easy.  Moving the pedal through a bigger circle for the same femur/tibia results in subtle angle differences that can dramatically change the force vectors through your leg.  While you get 3% more leverage with the 170 mm cranks, you may lose 5% because you're leg is now at a "funny angle." 

You may be suffering as much from the 5 mm difference in crankarm length as the gearing (maybe not).  If you go with a compact square taper crank, you can use "Bullet" crankarms which are available in 165 mm lengths (I got them for my wife who is also short).  Otherwise, most of your options start at 170 mm.  Square taper is the "old school" bottom bracket that uses a square end to the spindle and a square hole in the crank.  Still common, but incompatible with ISIS designs that use small splines instead of a square.