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Old 02-20-09, 05:40 PM   #1
iduhfs
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Max difference between chain ring sizes

I have a 2x6 drive train on an '85 Fuji Del Rey. I'd like to put on smaller chain rings for touring but am unsure what I can get away with while still using the short cage Suntour LePree RD and LePree FD.

As I read the archives here, I see that there are two issues at play: First, that you can only get away with about 28 teeth worth of difference with a short cage derailer. Second, that the FD can only make so big a jump between chainrings.

So, how far can the 28 tooth rule be pushed and how big a jump can a mid quality old school front derailer make happen. If you have suggestions for the optimal ring sizes for such a touring setup, I'm all ears.
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Old 02-20-09, 06:17 PM   #2
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How about telling us what exactly the sizes of chainrings and rear cogs you plan on running...?
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Old 02-20-09, 07:18 PM   #3
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I was just about to post this exact question, after looking through some of the posts the original poster must have looked through. (Hopefully will not feel hijacked, as it may be the same answer)

My bike, Schwinn Tempo, 1987, original except for wheels and hubs.

Shimano 105, Geared presently 52/38, 13/24.

Same question, what is my best option. I really hate to change too much, as this was my dads bike, and we both always thought it shifted and rode better than my Pearl White Trek 1000 all dura ace equiped dream bike.

Addendum: It has Index shifting on the Rear Derailer (6 speed)

Last edited by woodenidol; 02-20-09 at 07:22 PM. Reason: Added information
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Old 02-20-09, 08:32 PM   #4
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The rear is 14-30 and the front is currently 52/42. Hijack away Woodenidol, looks like we're trying to get to the same place.
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Old 02-20-09, 08:37 PM   #5
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Im trying to get up the hills for Cycle Oregon. In the old days I would have worked harder, now Im looking for shortcuts. lol.
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Old 02-20-09, 08:44 PM   #6
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You can safely violate any rear derailleur's published wrap capacity if you are careful to avoid using the small chainring(s) with one or more of the smallest cogs.

As to chainring gearing, the smallest ring is defined by the crank's bolt circle diameter. A 130 mm crank can fit a minimum of a 38T ring and a 110 mm can fit a 33T as the smallest.

I've heard of TA cranks being fitted as 52/28 doubles. I'm sure the shifting wasn't terrific but with friction shifters and by anticipating the shifts you can get away with a lot.
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Old 02-21-09, 05:04 AM   #7
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You can safely violate any rear derailleur's published wrap capacity if you are careful to avoid using the small chainring(s) with one or more of the smallest cogs.

As to chainring gearing, the smallest ring is defined by the crank's bolt circle diameter. A 130 mm crank can fit a minimum of a 38T ring and a 110 mm can fit a 33T as the smallest.

I've heard of TA cranks being fitted as 52/28 doubles. I'm sure the shifting wasn't terrific but with friction shifters and by anticipating the shifts you can get away with a lot.
Further detail clairification-that presuposes you set chain length to accept big-big and avoid pulling your rder off.
Then the small-small would skip.
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Old 02-21-09, 05:06 AM   #8
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The rear is 14-30 and the front is currently 52/42. Hijack away Woodenidol, looks like we're trying to get to the same place.
Pretty easy to unbolt the 42 and bolt in a 38.
Lot more ratio range.
If you want more, an 11-34 cassette, but you'll need change the cassette and rear der.
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Old 02-21-09, 07:32 AM   #9
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Further detail clairification-that presuposes you set chain length to accept big-big and avoid pulling your rder off.
Then the small-small would skip.
I believe you HAVE to set the chain length to accept big-big for mechanical safety reasons and therefore forgo using small-small and maybe even small-next to smallest. However, small-small is a useless combination as you can always find the same gear somewhere else in the range.
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Old 02-21-09, 07:49 AM   #10
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11-34 with a 50T or larger front ring might require a full 114 link chain on some bikes with longer chainstays.
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Old 02-21-09, 10:10 AM   #11
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The mountain bike RD's I have been looking at, for the most part say they are compatible with 7 and 8 cog casettes. Nothing about six. Doesnt seem that a six would make any differance either as far as I can see?

Looks like my cheapest option would be a mountain bike RD change, 34T large cog. Going to pencil out the gear ratio's, but Im not liking what I see with only a 6T on the back. Not sure there is enough differance in changing out the Crank to a compact 50/34, compared to my 52/38.

I use my bike as a dailey commuter, and then will use it on the supported tour.
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Old 02-21-09, 10:15 AM   #12
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Fujis of that era with Suntour shifting usually came with Sugino cranks that have 144mm BCD. The minimum chainring size for 144mm BCD is 41 so you may need a different crankset to get a smaller inner ring.

Most FDs for double can handle a 14-15 tooth difference. And you can exceed that if you can put up with sluggish shifts and the loss of the smallest 2 or 3 cogs on the small ring.

You can usually exceed the RD wrap spec by a few teeth. I've run 33 total difference with a RD rated for 29 and I could get all the gears without issues.
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Old 02-21-09, 11:22 AM   #13
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Thanks for the input, this is exactly what I'm after. I think I want to run 48/34 Which only gives a hi/hi to low/low difference of 30 teeth -seems like that should work just fine with a little carful shifting.

Gonzo Bob, The Seguno cranks on the del Rey have a 5 hole spacing of 65mm, and as I understand BCD, you take that measurement and multiply by 1.7 which gives 110mm -is that correct?
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Old 02-21-09, 11:34 AM   #14
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fwiw, modern compact doubles are 50-34.
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Old 02-21-09, 12:47 PM   #15
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Sounds like you guys need a compact... I mean why bother having a 52t chainring up front if you're going to slap a gigantic 30 on the rear...?
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Old 02-21-09, 12:52 PM   #16
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Gonzo Bob, The Seguno cranks on the del Rey have a 5 hole spacing of 65mm, and as I understand BCD, you take that measurement and multiply by 1.7 which gives 110mm -is that correct?
Correct. I just checked on my 110mm BCD Sugino crankset.
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Old 02-21-09, 06:34 PM   #17
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Sounds like you guys need a compact... I mean why bother having a 52t chainring up front if you're going to slap a gigantic 30 on the rear...?
I agree for my part. Im looking at a compact and then weighing my options for the rear. Guessing on what my old RD can handle, I think I could go 50/34-14/26, leaving me to only switch out the crank and redoing he rear cogs.

It looks like if I do a whole bunch more, changing RD, 6t to 9t rear, new or rebuilt rims, that as much as I love the frame, I might be better off looking at another bike. shruggs.
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Old 02-23-09, 02:00 AM   #18
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Sounds like you guys need a compact... I mean why bother having a 52t chainring up front if you're going to slap a gigantic 30 on the rear...?
The 52T ring is the top end used on flats and downhills with the smaller rear cogs. The 30T rear is used for climbing-it isn't going to be used with the 52T ring.
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Old 02-23-09, 08:06 PM   #19
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I agree for my part. Im looking at a compact and then weighing my options for the rear. Guessing on what my old RD can handle, I think I could go 50/34-14/26, leaving me to only switch out the crank and redoing he rear cogs.

It looks like if I do a whole bunch more, changing RD, 6t to 9t rear, new or rebuilt rims, that as much as I love the frame, I might be better off looking at another bike. shruggs.
Dooood... check out the Late Sheldon Brown's Hetchins:
http://sheldonbrown.org/hetchins/index.html
It's got a 50/28 double on the front!

Personally, I think shifting would be dicey with that big of a gap, but it's doable. If it were my bike, I'd probably go triple: 24/39/50 with a 13-28 9-speed on the rear. But then I've got too many parts as it is.
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Old 02-23-09, 08:20 PM   #20
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I have a bike with a half step plus granny. Thats 52/48/28. I admit shifting from the middle to the inner is a bit hairy, you don't really have to do it that often. You know when your going to need that bailout gear, so you can anticipate when to shift better
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Old 02-24-09, 02:29 AM   #21
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I have a bike with a half step plus granny. Thats 52/48/28. I admit shifting from the middle to the inner is a bit hairy, you don't really have to do it that often. You know when your going to need that bailout gear, so you can anticipate when to shift better
+1. I shift over a 45/42/24 using a FD-5500 double front derailleur with stated capacity of only 15t, but it works fine. The stated capacity of a double front derailleur relates to shifting between outer and middle; since the cage moves down as it moves toward the frame, that stated capacity can be exceeded a bit when a double FD shifts over a triple. With such a big jump you need a chain watcher to keep from dropping the chain when going to the granny. A ramped and pinned middle ring is a big help for the upshift.

A wide range double with a wide cassette is a good practical setup. You spend most of your time in the big ring, only using the granny for extreme climbs. It avoids the problem of a compact double where the flatland riding range straddles between the two gears and you have to shift the front all the time.
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Old 02-24-09, 04:03 AM   #22
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I've got a setup similar to Sheldon's, with 46-28 chainrings. Shifting isn't pretty, but it's good enough for me. On another bike, I have 26-42, which works even better. On both bikes, I'm using "triple" front derailleurs.
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