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Any Problems With FD Shifting Between 40/38/28 Chainrings?

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Any Problems With FD Shifting Between 40/38/28 Chainrings?

Old 03-26-14, 07:09 PM
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JBHoren
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Any Problems With FD Shifting Between 40/38/28 Chainrings?

My 1990 Bianchi Volpe has the stock 48/38/28 triple chainrings, with the Suntour XC series top-pull front derailleur -- the rest of the drive-train is also the stock Suntour XCE components. Along with changing the 7-speed freewheel to a two-tooth interval 13-26 unit, I want to replace the 48-tooth chainring with a 40-tooth chainring -- the combination will give me the uniformly bite-sized steps I seek.



Would/will there be any problem with the FD shifting between the 38- and 40-tooth chainrings?
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Old 03-26-14, 07:30 PM
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Originally Posted by JBHoren View Post
.... I want to replace the 48-tooth chainring with a 40-tooth chainring -- the combination will give me the uniformly bite-sized steps I seek.....

Would/will there be any problem with the FD shifting between the 38- and 40-tooth chainrings?
The general rule that shifting over smaller steps is easier will work to your favor. However, resist the temptation to lower the FD to the usual 2mm over the ring. That will put the granny-middle ramp of the cage too low.
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Old 03-26-14, 07:32 PM
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Probably some.
You have to Keep the FDER about where it is currently in order to clear the middle ring on your way to the small ring.
Pic below is my 22-32-36. I have to "help" it shift to the large ring. IF I set the stroke to shift to the large ring OK, it won't drop to the small ring. A friction shifter would "fix" that, but I don't want mismatched shifters/brakes.
My other bike has the same set up except a 38T large ring. No shifting issues.
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Old 03-27-14, 05:53 AM
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There will probably be problems with the chain getting stuck between the rings. The teeth of chain rings are narrower than the body of the rings and when the rings are really close in size, the teeth of the inner ring overlap with the start of the taper of the outer ring teeth and the gap might be wider than the width of the chain. I think you need at least a 4-5 tooth difference in rings to work well.
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Old 03-27-14, 07:32 AM
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Looking at your gear chart and your location, may I suggest alternative gearing. Since you live in FL (one hill of note in the entire state!) and already have a triple consider a 13x21 7-speed freewheel/cassette (13/14/15/16/17/19/21) and keep your current chainrings or change the 38T to a 42T. The one tooth steps are very close and still allow a 99 gear-inch high gear which can be useful with a tailwind and a 36 gear-inch low which should get you up anything FL has to offer.
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Old 03-27-14, 09:00 AM
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I agree that a narrower range freewheel is the better alternative. I find the OP's proposed solution one of those that looks great on a chart but is not so pleasant in real life. To get those bite-sized steps requires a lot of double shifting, which is a pain, and there likely could be problems as noted above with the 42/40 combo on a triple.
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Old 03-27-14, 09:06 AM
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I use friction shifting for all my FD , rear, If you want indexing thats a separate choice.

Shallow differences in chainring, sizes dictate using a road , less deep back plate, FD.

Would/will there be any problem with the FD shifting between the 38- and 40-tooth chainrings?
but why not just put a 40t in the middle. and eliminate the 38.. entirely .. ?
simplify .. its 1 link of chain , 2t difference.

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Old 03-27-14, 09:28 AM
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Originally Posted by cny-bikeman View Post
I agree that a narrower range freewheel is the better alternative. I find the OP's proposed solution one of those that looks great on a chart but is not so pleasant in real life. To get those bite-sized steps requires a lot of double shifting, which is a pain, and there likely could be problems as noted above with the 42/40 combo on a triple.
+1
It sounds good "on paper" but get's to be annoying in real life.
The 1/2 step concept made more sense when you only had 5 cogs in back.
For medical reasons (emphysema & bad knee), I like lots of closely spaced gears, so I simply upgraded to 9 speed.
Even then, when just "poking around", I often shift 2 gears at once.
When on the open road logging miles, it's wonderful to shift 1T as needed for changing head winds, fatigue etc.
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Old 03-27-14, 10:13 AM
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get a NuVinci 360 CVR hub , you just turn the handle grip until the ratio feels right.

dont sweat the hub's weight, its Florida renowned for it's lack of mountains..
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Old 03-27-14, 10:59 AM
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Thanks for all of your replies. I appreciate the thought, time, and courtesy you've extended to me.

I'm not (yet) such a strong cyclist, and all of my riding is on the 28/38/48 chainring and 13-15-17-20-23-26-30 rear sprockets (and mainly those in larger type). My goal is to add more gear choices, and swapping the /48 for a /40 chainring does that, within the 40"-70" gear-inch range where I'm comfortable. While a "corncob" freewheel and a 28/38/42 triple would give me even smaller steps between gears, they're in the upper portion of my comfort zone (and North of it, as well). For now, I'll stick with my plan: "losing" the 30- and 20-tooth sprockets, and replacing them with 19- and 21-tooth cogs, will give me an additional gear + better spacing; and, if the 38/40 chainrings don't present a problem, I'll have three more gears available to me, where I need/use them. OTOH, if there is a problem with it, I can always move to a 38/42, or simply stay with the stock 28/38/48 rings.

Don't think I haven't considered upgrading to a 9-speed rear; but the cost of parts, alone, is a show-stopper. The NuVinci 360 CVR, while appealing, is, similarly, not an option (but I enjoyed reading about it).
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Old 03-27-14, 11:40 AM
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Originally Posted by JBHoren View Post
Thanks for all of your replies. I appreciate the thought, time, and courtesy you've extended to me.

I'm not (yet) such a strong cyclist, and all of my riding is on the 28/38/48 chainring and 13-15-17-20-23-26-30 rear sprockets (and mainly those in larger type). My goal is to add more gear choices, and swapping the /48 for a /40 chainring does that, within the 40"-70" gear-inch range where I'm comfortable. ....
You've given this some thought, and analyzed it the right way by looking at the gears you use, and trying to get more in that range. Going to the 40 will give you some utility out of that ring, but I think you should also consider that dropping the middle to 34 or 36t. You might also flip the existing 37 to the outside (requires non-shiftgate chainring -- ie all uniform symmetrical teeth), and going 38/34/28.

I agree that a narrower cassette might be a nicer way to shift, except all the available narrow ranges compress to the small end, which is what you don't need.

IMO- you've come up with the right arrangement for your needs, and should go with it. After all it doesn't matter what's "better" if it isn't suited to your needs.
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Old 03-27-14, 12:36 PM
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JBHoren, My daughter has a '90 Volpe and the OEM Suntour FD's cage is fairly sculpted to shift the 10T differences, yet not interfere with the middle chain ring. My guess is that shifting to the 40T chain ring from the 38T chain ring will be okay, but the reverse maybe a little rough. I could be backwards.

You might consider replacing the crank set with this and a 111-113 mm UN-26 bottom bracket. This is the least expensive replacement option and I used the 24-34-42 crank set on a recent touring bike build.

Brad
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Old 03-27-14, 09:46 PM
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This is certainly an education! And defining a search and search terms reminds me of Donald Rumsfeld's "known knowns, known unknowns, and unknown unknowns." So, it's an iterative process, and I've learned more about the problems of shifting between closely-spaced chainrings with a made-for-mountain-triple derailleur that has a low-hanging inner cage plate. And still, I won't really know until the 40-tooth chainring arrives and is installed; but the chances are good that I'll need to swap my front derailleur for one which is either a road triple, or a road double -- it'll only have to handle a 12-tooth difference across the three chainrings.
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Old 03-27-14, 10:09 PM
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I would have gone for a 40t middle ring and a chainguard disc for the outer ..

80's when the small-high freewheel cogs were 13t the middle ring was a 52t, inner , 36t.
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Old 03-27-14, 10:12 PM
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Originally Posted by FBinNY View Post
You've given this some thought, and analyzed it the right way by looking at the gears you use, and trying to get more in that range.

IMO- you've come up with the right arrangement for your needs, and should go with it. After all it doesn't matter what's "better" if it isn't suited to your needs.
+1

This is something few people on here ever do. I see so many threads that start with "how do I put a 10-speed cassette on my bike, I need more gears" or something similar. No one seems to bother to find out WHICH gears they are missing.

You seem to have a pretty good handle on this so I'm not even going to offer any gearing advice. Try what you like and modify as desired.
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