what is compact gearing?
trevtherev
Posts: 372
Question from a novice soon to get first road bike. What is compact gearing and is it an advantage on a first road bike?
"Cycling is like a church  many attend, but few understand."
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'standard' is to have either 5242 or 5339 chainrings on the front. 'compact' usually 5034 or 5036.
Bit of a jump between the two so not perfect but gives you some decent lower gears for the big hills. Def worth it for a beginner IMO0 
The "compact" in compact gearing refers to the slightly smaller size of the two chain rings at the front. They usually have 50 teeth and 34 teeth instead of 53 and 39. This makes the gears lower ("easier"), all else being equal. But the gearing you get when you pedal is also determined by the sprockets at the back (this time, the more teeth they have the lower the gearing), so if you fit smaller sprockets you can get much the same gearing with compact as with a standard chainset. A compact chainset gives you more leeway if you want to have lower gears for steep hills, and most complete bikes sold with compacts are likely to have lower gearing than those with standard.
Many people (like me!) think that compact chainsets give you the best of both worlds, but the disadvantages are very slightly faster chainwear and an uncomfortably big jump between the large 50 tooth ring at the front and the 34 tooth. The 36 / 50 combination is better in this respect, but for some reason much more difficult to get hold of.0 
neeb wrote:The "compact" in compact gearing refers to the slightly smaller size of the two chain rings at the front. They usually have 50 teeth and 34 teeth instead of 53 and 39. This makes the gears lower ("easier"), all else being equal. But the gearing you get when you pedal is also determined by the sprockets at the back (this time, the more teeth they have the lower the gearing), so if you fit smaller sprockets you can get much the same gearing with compact as with a standard chainset. A compact chainset gives you more leeway if you want to have lower gears for steep hills, and most complete bikes sold with compacts are likely to have lower gearing than those with standard.
Many people (like me!) think that compact chainsets give you the best of both worlds, but the disadvantages are very slightly faster chainwear and an uncomfortably big jump between the large 50 tooth ring at the front and the 34 tooth. The 36 / 50 combination is better in this respect, but for some reason much more difficult to get hold of.
I tried a compact and in the end took it off. Just couldn't get on with the massive jump between chain rings. Horses for courses! If you are buying a new bike I would strongly recommend working out exactly what you want in the way of gears. I look at my average speed, ride at this speed and note the gear I'm using, then go to the biggest hill you are likely to climb and see what ratios you need for that. Change them to gear inches (use Sheldon Brown) and you have the lowest gear you need and the middle ratio. In the middle ratio I want a nice straight chain line! If you need the bike for racing or TTs then you will have to have a high enough ratio to cope with a "gentle downhill with the wind behind you" type riding, say 3035 mph. Phew!0 
A question  should the jump between a 53 and a 39 feel the same as, or less extreme, than the jump between a 50 and a 36? (not a 34). The "tooth difference" is the same, but as the 14 teeth are a bigger percentage of the total tooth number on a compact, will that make the jump feel bigger? Or not, assuming you are in the same "gear" in terms of gear inches? (can't quite get my head around this ). If so, what size of small chainring would you pair with a 50T big ring to get the same subjective difference when you shift? A 37?0

Yes, you're right, it's the percentage difference that counts. you just need a gear table to work it out and I don't have one to hand. It's one thing working it out and quite another trying to find a 37 or 38 inner ring. I recently tried a compact for the first time and didn't really like it. It's clearly a good option if you really need the lower range of gears but if not i wouldn't bother.0

Yes, you're right, it's the percentage difference that counts. you just need a gear table to work it out and I don't have one to hand. It's one thing working it out and quite another trying to find a 37 or 38 inner ring. I recently tried a compact for the first time and didn't really like it. It's clearly a good option if you really need the lower range of gears but if not i wouldn't bother.
It strikes me that the 14T difference should feel the same whether you are going between 53 and 39 or 50 and 36, assuming the same gear (in gear inches). But of course with a compact, you will tend to be changing down at the front while in a lower gear (in gear inches), so in most reallife circumstances the 14T difference will be a greater percentage of your total gear...
Is that right? :?:0 
If you stay on the same sprocket at the rear then a 14 teath jump will feel bigger on a compact. For the same reason cassette have smaller jumps from the bigger gears i.e. 12,13,14,15,16,17,19,21,24,27 which starts with one tooth difference then 2 and 3.0

But the same sprocket at the rear is a differentsized gear in a compact and in a standard. So for the same gear (different sprocket), the 14T jump will feel the same.
But the point at which you change down will be determined by running out of sprockets at the back or trying to avoid excessive chaincrossing, which will happen at a lower gear for a compact chainset than a standard, assuming you have the same sprockets at the back.
But if the cassettes are chosen to make the total gear range equal for the compact and the standard, you will have smaller cogs at the back on the compact, so actually, you will change down while on a smallertoothed cog (larger gear), at least partially compensating for being in a smaller gear due to the chainring, and partially offsetting the magnitude of the 14T as a percentage...
So it's quite complicated?0 
Sorry, I've started so I'll have to finish, compulsive pedantry attack...
Let's say you are comparing a 53/39 with a 1225 cassette to a 50/36 with an 1123 cassette. The total gear ranges of these are pretty much the same.
If you are a real chaincrosser and change down at the front when you are on the largest sprocket at the back, then going from 53/25 to 39/25 is 57.2 gear inches to 42.1 gear inches. On the compact it is rather close to this  50/23 to 36/23 is 58.7 gear inches to 42.3 gear inches. So the 14T jump should feel much the same. Similarly near the middle of the cassette (e.g. the 6th gear from the bottom, 17T on a 1225, 16T on an 1123)  53/17 => 39/17 is 84.2 => 61.9, nearly identical to 50/16 => 36/16, which is 84.4 => 60.8. There will be a very slight difference, but perhaps not detectable?
So assuming the same total gear range, a 50/36 compact shouldn't feel much worse than a 53/39.
So compacts are just better, which is what I thought all along..0 
neeb wrote:But the same sprocket at the rear is a differentsized gear in a compact and in a standard. So for the same gear (different sprocket), the 14T jump will feel the same.
But the point at which you change down will be determined by running out of sprockets at the back or trying to avoid excessive chaincrossing, which will happen at a lower gear for a compact chainset than a standard, assuming you have the same sprockets at the back.
But if the cassettes are chosen to make the total gear range equal for the compact and the standard, you will have smaller cogs at the back on the compact, so actually, you will change down while on a smallertoothed cog (larger gear), at least partially compensating for being in a smaller gear due to the chainring, and partially offsetting the magnitude of the 14T as a percentage...
So it's quite complicated?
No. No matter what sprocket you are in, as long as you stay in that sprocket, changing from a 50t ring to a 34t will always give you a gear about 32% shorter (while changing from a 53 to a 39 will give you a gear about 26% shorter). The absolute change in gear ratio will differ depending on what sprocket you are in  change from the 50 to the 34 chainring whilst in the 25 sprocket and it's a difference of about 15 gear inches, change from the 50 to the 34 whilst in the 11 sprocket and the difference is about 38 inches, but as a percentage of the gear you were in before, it's always the same.
If I've understood you correctly, the first sentence in the post above is incorrect. Imagine you had a 100 tooth chainring and a 100 tooth sprocket  giving a gear of 26". Now move up to a 114 tooth chainring and you wouldn't notice much difference. If, though, you had an 11 tooth chainring and an 11 tooth sprocket; you'd have the same 26" gear to start with, but if you moved to a chainring 14 teeth bigger (i.e. a 25)  you'd notice a hell of a difference. I know these are totally unrealistic chainring/sprocket combos, but it illustrates the point that the amount of difference moving up/down x teeth on the chainring makes depends on the ratio between the two chainrings, rather than the ratio between the chainring and sprocket.0 
Sorry, just realised I'd compared a 5339 with a 5034, which obviously have a different number of teeth between them :oops: Same principle applies though  the jump from 36 to 50 will always be bigger, percentage wise, than from 39 to 53, but I agree that you'd probably not notice much difference!0

If I've understood you correctly
I meant for the same gear, as in gear inches gear. The point I was trying to make is that while you are right (for the same sprocket, the 14T change on a compact is always going to feel greater than on a standard) if you have a cassette that is chosen to provide the same total range of gears, you are not going to be in the same sprocket when you are changing, but rather a smaller one.
So it is complicated..0 
trev  try the compact (50/34) with a 1223 or 25 cassette and see what gears you mostly use, and, whether they fall mostly in the '50' ring, with only the top couple (the rear 23 or 25) with the '34' for serious hauling up a slope.
If you try the 52 or 53/39 option, I suspect you will be in the '39' most of the time and only rarely use the big ring for speed over a flat distance. When climbing with the '39' you may also find you need a bit more help at the other end and so maybe get a 1227 cassette.
I personally found that a 53/39  1225 was not enough to get me up local hills; a 53/39  1130 was, but meant a lot of doubleshifting around the 'middle' gears for most of my riding. I eventually found personal heaven with a 50/34  1225 setup.
Choice depends upon you, your age, your fitness and your hills.....and how much pain you want to give yourself.
Check out other threads on this subject from previous weeks  there is at least one a week, and all will tell you something extra to consider. Trying a setup out on the road is the only real answer.Spring!
Singlespeeds in town rule.0