Understanding Gear Ratios
I was looking at a 2012 Trek this weekend with a compact. I currently have triple and live in Central Vermont with lots of hills.
The sales guy explained to me the new bike had 10 rings in the back and gave me a bunch of numbers to explain to me I would not miss my lowest granny gears (which he felt are not used enough to be worth the trouble). I wondered after leaving wouldn't I also be missing the top end of the gears for descents?
Any help in understanding the ratio numbers would be great!
here's a calculator so you can compare in detail: Sheldon Brown's Bicycle Gear Calculator
now for the fluff: the extreme gears for a compact and the triple will be very similar. if you always were in the smallest cog descending there might be a small issue with a compact otherwise that will be fine.
The main difference is the large gap between the small and large chainring that requires a larger change in the cogs to make it fit when you change. whereas on the triple you could change chainring and obtain a similar gear ratio by changing 1-2 gears in the back it will be 2-3 with a compact. most people are fine with it. I was not but that is probably just me
it is the number of teeth of the chain ring divided by the number of teeth of the cog on the cassette
34 teeth in front, 34 teeth in back, you turn the pedals 1 complete turn and the wheel will turn 1 complete turn (34/34 = 1.0)
34 teeth in front 25T in back, you turn the pedals 1 complete turn and the rear wheel will turn 1.36 times
(34/25 = 1.36)
The bigger the number the more resistance you will need to propel yourself up a slope.
If you have the usual 30t small ring on your current triple and the bike with the compact has the usual 34t inner ring, the gear ratio from any given cog on the same cassette will be about 13% higher. That will feel like being about two shifts higher, depending on the specific layout of your particular cassette. You'll (almost certainly) react by shifting down two gears. So, if you use the lowest two gears of your current setup, you'll miss them if you switch.
Bear in mind that I'm making a big assumption here - that you're sticking with the same cassette. Choosing a cassette with larger cogs for the lowest gears would give you back those ratios, probably at the cost of having slightly larger steps between gears. That drives some people nuts and others don't notice it.
Suppose you currently have a 12-25 cassette. Your current lowest ratio is 30/25, or 1.2. To get the same ratio, you'd need a cassette with a 28.3t largest cog. Obviously you can't have a third of a tooth, but 28 is close enough. 11-28 (and 12-28?) cassettes are readily available.
If you're already using a 27t or 28t largest cog, you need a mountain bike cassette to hold onto those low gears. While doable, IME the jumps on a MTB cassette feel pretty big on the road. Whether or not this is acceptable is pretty subjective. You need to decide that for yourself.
If you already have a MTB cassette, don't get a compact.
As far as the upper ratio is concerned - if you have an old-school triple with a 52t big ring and you actually use 52/11, you need to learn to spin. 50/11 is a higher gear than 52/12.
For me, triple vs. compact is about whether I get to have acceptably close gear spacing on my cassette and what the shift between the inner and outer ring is like. Is it f'ing huge? Do I have to do it constantly? I have a pretty rapid cadence, and can be relatively happy in the 34t ring for all flat and climbing efforts on my own at an aerobic pace. Riding with a fast group or doing intervals, I can be pretty happy in the 50t in the flats, but I tend to cross-chain if I'm solo and not trying to be fast. Some people with slower cadences find that their preferred flats gear ratio is right in the overlap between the 50t and 34t. Then they shift a lot and are very often cross-chained. Not so good. Even this is somewhat tunable by choosing a different cassette size that moves a preferred gear more fully into 50t or 34t chainring territory. I don't think that's a great way to choose a cassette, but I have the luxury of not having to.
My nicest road bike has a 50/39/30 crank and 12-27 cassette. I really like that setup. I recently acquired one with a 50/34 compact and a cassette with fewer teeth on the largest cog. For how I use it, it hasn't really bothered me, but I do run out of gears now and then. Finally, I have a 'cross bike that I just switched over to a 48/34 compact off another bike. It has a 11-32 9-speed MTB cassette. I don't have a lot of saddle time on it because of some other damage. It seems to have low enough gearing (it had better, it's a lower minimum ratio than my nice road bike) and if I'm on city streets I'm not bothered by the size of the jumps, but with the previous crank and on race courses, I noticed them some. I'm anticipating switching to an 11-28 cassette if I think I can get away with it. It wouldn't be as bad if it was 10-speed, though, so take my conclusion with a little bit of a grain of salt...
I think any crank that required me to use an MTB cassette to have gear ratios I wanted on the road is too big. So if you want my opinion about whether or not you'll be happy with this switch, here it is - if your favorite ratios are right in the middle of the overlap between chain rings or you'd need to put on a MTB cassette to have some gear ratios you like for climbing, don't do it. Otherwise, you'll be fine.
Jeeze... this is probably why people usually give the sheldonbrown link and tell posters to do their homework. Hope one of my points gives you the missing piece that makes the decision easier.
I'm going with old memory here but...
When I had a compact crank with a 12t on the back, I would spin out somewhere between 30-35 mph.
100 rpm is at 32.6mpg with 23mm tyres. (50tooth in the front)
Originally Posted by HikenBike
The OP was specifically talking about gear ratios. Gear ratio, by definition, is the speed ratio: the speed of the driving gear (chainring) divided by the speed of the driven gear (cog). In terms of the number of teeth that would be the number of teeth in the cog divided by the number of teeth in the chairing and not the other way around.
Originally Posted by PoorCyclist
For example, if one is currently riding a 54-tooth chainring with a 11-tooth cog, then one's current gear ratio (just for this pair of gears) is 0.2037, and not 4.9 as people often mistakenly believe. While in informal conversation people often use it the other way around (i.e. divide 54 by 11, get 4.9 and then incorrectly refer to this number as "gear ratio"), it should be kept in mind that in formal technical texts the proper definition is going to be used. So, again, for 54-tooth chainring with a 11-tooth cog the gear ratio (AKA speed ratio) is 0.2037 (while 4.9 is the reciprocal of the gear ratio).
The bigger the gear ratio the easier it is to propel yourself up a slope (and not the other way around). Remember that the higher-lower gear terminology is the opposite of the higher-lower gear ratio terminology: higher gear means lower gear ratio and vice versa (confusing, I know).
Last edited by AndreyT; 01-03-2012 at 02:42 PM.
Originally Posted by lcecere
Well, the first thing to understand is that the full gearing system of the bike depends on such parameters as: crank length, chainwheel diameter (tooth count), cog diameter (tooth count) and wheel diameter (assuming that there is no other gearing involved).
Normally, the concept of "Gear ratio" (which you mentioned in the title of this thread) is not used with bikes, since it is not sufficiently intuitive for most folks. Instead, "Gain ratio", "Gear Inches" and "Meters Development" are used.
You can read more about these concepts at the previously linked page to Sheldon Brown's Gear Calculator.
Last edited by AndreyT; 01-03-2012 at 04:45 PM.
Living in the Rockies and Sierra I used to use 9 speed triples with a 30 ring and 24 cog for big climbs. I've switched to 10 speed compact doubles with 34 ring and 28 cog which, though about a 3% higher gear, is close enough for my purposes.
I cannot give any specific details about gearing but I have a similar experience to relate to you.
I started road riding last year and the Trek I bought came with a triple crank and 11-27 (28 if not a 27 anyway) 9 speed cassette. I have since upgraded to a compact crankset and a 12-25 10 speed cassette and love it!
I live in Western PA (pretty hilly) and I can climb the hills better with this set up as well as finding a more comfortable gear on the flats also.
Even if the compact would work for you now, as you get older you will eventually need the triple again and lower and lower gears. It happens to everybody so take that into consideration.
On a typical compact 34 tooth setup you will miss the lowest gear and half the next lowest gear as compared to a 30 triple. On the top end you can pedal on the downhill until it gets more efficient to coast. If they are proposing the broader geared SRAM Apex design you might want to reconsider because there have been quality issues that may have not been corrected yet.
Just so you're clear, the 34/28 is 3% LOWER gear than the 30/24. Not much difference, but a couple of rpm faster at a typical 70 rpm climbing cadence.
Originally Posted by looigi
Here's another gear-calculator that is most useful one I've seen so far. It has real-time sliders for things like gears, cadence, etc:
Gear Ratios for Compact Crank w/ 11-28 cassette
Triple Crankset w/ 12-23 cassette
I've used bikes with both the above gearing setups. I prefer the finer gear spacing offered by the triple cranks, and the fact it's possible to ride while rarely leaving the middle ring, but prefer the Q-factor (narrower spacing between the pedals) of the compact.
If I were to buy a new bike today, I'd probably go with a compact, but switch the small chainring from 34 to 36T: 50/36 w/ 11-28
This would let me get up to as much as 24mph @ 110rpm (my usual cadence when riding a constant speed) without cross chaining from the small ring compared to just 22.5mph with a 34 tooth ring. There are two reason I'd go for a 50-36 compact and not a 53-39 standard crank:
1) I don't need a 53/11 high gear. I seldom use 53/12 (or equivalent 50/11) ratio as it is. A 36T ring with a 11 starting cog is almost equivalent to 39T ring with 12 starting cog at high speeds.
2) Getting an equivalent LOW gear as 36/28 with a standard crank would require me to run a MTB derailluer and cassette. A 12-32 cassette with standard crank would result in more annoying large gear jumps around my preferred cruising speeds compared to an 11-28 with compact.
Last edited by PhotonFreak; 01-05-2012 at 03:13 PM.
OK, maybe I'm reading too much into the OP's question, but to me the simple question he's asking is how does the gearing of a compact crank system compare to a road triple.
High gears: The compact will give you good enough high gears if you're happy with the high gears on your current road triple. A 50X11 is actually higher (harder to pedal, faster, etc.) than a 52X12, which is a typical road triple highest gear. If your road triple has a 50t large sprocket, the comparison is even easier.
Low gears: If your triple's lowest gear is 30X25 or 26, the compact will need a 28 tooth largest rear sprocket (e.g. 34X28 lowest gear) to approximately match the gear ratio, although it won't be quite as low.
If your triple's lowest gear is 30X27 or 28, the compact willl need a 32 or 34 tooth largest rear sprocket, which in turn will require a mountain bike rear derailleur to work properly (most likely). This is not a "bad" solution, but you'll need to do it. I have my wife's road bike set up with a MTB rear derailleur and cassette, and it works fine.
Of course, all this depends on the actual lowest gears that you use and need. If you never use the lowest gears you now have, adjust accordingly.
But even if you get the high and low gearing where you want with the compact, the jumps between the rear sprockets will be greater than what you have with the triple, and therefore there will be MUCH, MUCH more front shifting required.
And, in my experience with both a good road triple (Ultegra) and a good compact (Sram Red) the compact's front shifting will inevitably be rougher because there's a much larger jump between the rings. They work fine, but again, it's a fact.
There's nothing wrong with the compact, especially if you don't need really low gearing or understand that you might need to use a MTB cassette and RD to achieve what you need; I have both a road triple and a compact. My wife has a road bike with a MTB cassette and RD. All this stuff works just fine and we're very happy with it.
But there's a good reasons to stick with a triple if you need the low gears and have that option. You can make the compact work just fine, but it's a compromise, imho, and one that is unnecessary for those of us that need low gears - except for some riders' egos and the manufacturers not wanting to market the specialized parts for the triple drive train.
Last edited by Camilo; 01-05-2012 at 12:04 PM.
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