# Bicycle Mechanics - gear ratio question

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njlonghorn
06-19-09, 02:15 PM
My bike has a 48/38/28 crankset and an 11-32 casette. Why is there so much overlap between the cranset steps? For example, the tallest gear (11) when riding in the 38 gives me a mechanical advantage of .289, which would fall between the second (13 -- .271) and third (15 -- .313) tallest gears riding in the 48.

If you are going to have a triple crankset, why not space them out more? If they went with 53/38/23, for example, the range would be 10 teeth larger and you would still cover the entire range, with considerable overlap.

If the answer is, you can do it that way, what should it cost to have my LBS make the conversion? I don't need high-performance parts, but I don't want total junk, either.

Thanks for any input.

hernick
06-19-09, 02:31 PM
Well, closer steps between chainrings make for a better-shifting crankset. Also, front derailleurs have a limited range - and so do rear derailleurs. Even a super-long-cage SGS rear-derailleur would have trouble with a 53-38-23 x 11-32 - many gear combinations would result in a slack chain that slaps the chainstay and can get caught in stuff and tear off your rear derailleur.

Increasing cage length of the rear derailleur to increase capacity makes shifting performance more sluggish and the derailleur more exposed as it becomes closer to the ground.

Increasing the step between chainrings makes for more sluggish and difficult shifting, as well as a greater possibility of throwing the chain when downshifting. It also makes for shifts more complicated, as to maintain a gradual increase gain in shifting from gear to gear you now need to downshift two gears on the RD while upshifting the FD... Closer-spaced cranksets are more convenient for the cyclist who doesn't want complicated shifting patterns.

That said, I have a bike with a 52-42-26 x 11-32 and friction shifters. It works well, but upshifting from the small chainring to the middle one is slow, downshifting is only safe because I have added a chain watcher device to prevent derailment, and the chain is slack in many gear combinations that I must remember never to use.

dbc
06-19-09, 02:35 PM
Yes, if you spread out the front some more, there will be less overlap. But we've been living with this overlap for ages now. Why this is so is likely for 2 reasons.

1. The front is more difficult to shift. I don't recall seeing a front derailleur rated for a 30 tooth jump.

2. Users are not complaining about the overlap because the total range of gears on a 20 tooth jump triple is acceptable to them.

We're a long past days where every gear combination was precious (Alpine gearing). With 7-10 cogs in the back, there's plenty of room for waste.

njlonghorn
06-19-09, 03:45 PM
I had thought about the smoothness of the shift on the crankset, but figured I don't usually change the crankset while climbing so I should be able to make that work.

I hadn't, however, considered the slack problem. Adding 10 more teeth of range adds ten chain links of slack.... I think I'll stick with what I have!

Thanks for the quick response. Happy cycling!

johnknappcc
06-19-09, 03:49 PM
It also comes down to optimum chain position. You shouldn't really be in the middle, highest combo. You should move over to the biggest chaining, and find the similar gear in back to reach you optimum cadence. Ideally you want the straightest chainline, at any given time, and try to avoid cross chaining. With a triple, on the middle, its a little different, but I still keep my middle chainring around my middle cogs in back. You might have a 21 or 24 or 27 speed, but you only have a much smaller functional number of gears. Just get used to shifting both front and rear, look down at your chainline, and listen for chainrub, eventually you wil get used to the best combs for you bike, is you wanted though, I might go for a 50 chainring, and you should be able to get one, and I doubt you would want any smaller than a 28 in the front with the range of your cassette.

Garthr
06-19-09, 04:41 PM
My bike has a 48/38/28 crankset and an 11-32 casette. Why is there so much overlap between the cranset steps? For example, the tallest gear (11) when riding in the 38 gives me a mechanical advantage of .289, which would fall between the second (13 -- .271) and third (15 -- .313) tallest gears riding in the 48.

If you are going to have a triple crankset, why not space them out more? If they went with 53/38/23, for example, the range would be 10 teeth larger and you would still cover the entire range, with considerable overlap.

If the answer is, you can do it that way, what should it cost to have my LBS make the conversion? I don't need high-performance parts, but I don't want total junk, either.

Thanks for any input.

I'm not quite sure what you're after. If want no overlaps ...... Go the other way for your answer though. In a gear chart, try a 26/44/48. http://home.earthlink.net/~mike.sherman/shift.html

This give you a nice easy progression if you need it, or more traditional full steps if you stay in one ring at a time.

DaveSSS
06-20-09, 08:41 AM
The answer to this is simple. If the amount of overlap is reduced by using large jumps between the chainrings, it requires a lot of cog shifting to produce a uniform gearing progression. The most common 53/39/30 triple has a jump of 36 percent from the 53T to the 39T and only 30 percent to the 30T. Both require a 2-3 cog shift after every chainring shift, to produce a uniform gearing progression. If the percentage difference is increased 47%, like you get with a 50/34, is requires one more cog to be shifted after every chainring shift. Some people find this to be annoying, partly because they don't understand what's needed and partly due to the nature of the shifter's operation. With a Shimano shifter that can only shift one cog smaller with each push of the finger lever. You need 3-4 taps of the right finger lever after dropping from the 50T ring to the 34T. With a Campy shifter, it's one push of the right thumb button (through 3-4 clicks). Going the opposite direction, both brands can shift 3-cogs larger with one sweep of the Shimano brake lever or Campy finger lever.

For '09, the Shimano 7900 shifters have reduced function, so the brake lever can only shift two cogs larger with one sweep. Compact users will be needing a two-sweep shift fairly often.

The other issue is the jumps between cogs. Most road bike riders want close cog spacing, not an 11-32 that has no 1T jumps. That affects the number of cogs to be shifted for a uniform progression. Road bike triples are not setup with the assumption that a wide-spaced MTB cassette will be used.

AEO
06-20-09, 08:54 AM
the 'ideal' triple has chainrings spaced 10T apart.

on paper, there is a lot of overlap, but in use, there aren't as many as you think.

on a 10sp cassette with the small ring, you should only use the first 7 cogs.
in the middle ring, you should use the middle 8 cogs.
in the big ring you should use the last 7 cogs.

it's possible to use the other 8 combinations but this results in an extreme chain angle, increased wear and also results in a lot of excess noise.

put in the way of a car, sure you can hit 32mph in 1st gear, but should you? nope, you should use 3rd/4 or 4th/5.

DaveSSS
06-20-09, 10:32 AM
If you want a uniform progression toward lower gears, 10T jumps will not do the trick. The old 52/42 spacing is really quite close, requiring only a 1 or 2 cog shift after shifting to the next smaller ring. It's only a 24% difference. If a 32T little ring was used, that would be larger 31% drop.

I prefer a larger percentage drop, like the most common 53/39 that requires an easy 2-3 cog shift after the chainring shift. To get close to the same percentage drop, a 28T little ring works well, since it requires about the same 2-3 cog shift after the chainring shift. I found a 53/39/28 to work quite well when riding in the mountains.