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Old 12-04-07, 12:32 AM   #1
ancient
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How do you choose a F.D.


Hi - again - still struggling with acera-x f.d. - cant get it to go on the largest ring - so lets assume it is worn and needs replacing

I have an Altus 3 ring 24:32:38 teeth Hyperdrive-C chainset
The down tube is 33mm diameter
The distance from the outerface of the tube to the outerface of the largest ring is 47mm - the chainset being 17.5mm over the 3 rings

What do I look for in a repalcement that will fit these specis

regards

p.s. the rear is a 7 cog shimano alivio
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Old 12-04-07, 01:06 AM   #2
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Maybe you mean the seat tube is 33 mm? That's where the FD attaches. Front derailleur bands come in 28.6 mm (1-1/8"), 31.8 mm (1-1/4") and 34.9 mm (1-3/8"). 33 mm is right between the two larger sizes. It might be printed on the inside of the band on the old FD (or at least measure the band; it's more accurate).

Not really necessary to know right now, but rather than "distance from the outerface of the tube to the outerface of the largest ring", chainline is a more common measurement (center of seat tube to middle ring on a triple).

Not sure what "chainset being 17.5mm over the 3 rings" is referring to.

I think 7 and 8 speed chains are the same width, so any 7 or 8 speed Shimano MTB front derailleur will work (8-speed will give you better options). Be sure to match whether it is top pull or bottom pull (refers to whether the cable comes from the top or bottom of the seat tube).

Also be aware that using a 34.9 mm clamp FD, may require a slightly longer bottom bracket. My crank called for either a 110 or 113 square-taper BB. Tried the 110 mm for tighter chainline, but my 34.9 mm FD (w/shim for 31.8 ST), couldn't get to the inner ring.

I don't know if they make front derailleurs specifically for a 38T max ring, so you may have to get one for 42T or 44T max ring. (Common MTB chainrings are 22/24, 32, 42/44)

Generally, you should set up the FD so that it clears the teeth of the large ring by a mm or 2, but if your large ring is smaller than its rating, you may have to set it up a little higher, making room as if your large ring was that big. My new FD called for a 44T large ring, when I set it up with my 42T ring, the inside of the cage actually hit my inner ring when I tried shifting into the large ring.

I vaguely remember your old thread (or one similar), perhaps your problem is similar to mine (FD too low on a smaller outer ring? Like the FD is meant for a 42T.).

Last edited by JiveTurkey; 12-04-07 at 01:13 AM.
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Old 12-04-07, 10:59 AM   #3
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new fd required

Hi
Wow Many thanks for quick response :
Rechecked
The original fd is 31.8mm diameter fixing
The 17.5mm is the measure from the inner face small chain ring to the outer face of the large chainring (3 ring chainset) thought it may be relevant for travel across info
the chainline is appprox 55mm
The large cchainring is definitely 38 teeth

hope that clears matters a little
i assume from the info that I neeed a shimano top pull mtb 8 speed front derailleur with a 24:32:42(if 38 not available)

regards - once again many thanx for the help
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Old 12-04-07, 01:17 PM   #4
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ancient, not sure if the FD sent in the PM will work for you: http://www.chainreactioncycles.com/M...x?ModelID=1257

The only one available is "31.8 BP Black", which is for "Bottom Pull", meaning, the cable comes from around the bottom bracket below. Don't confuse bottom/top PULL w/ bottom/top SWING. The FD in the link, is top swing (TS). The clamp is relatively lower and the FD swings above the clamp. Bottom swing (BS) has a higher clamp and it swings below (this is the traditional design for road FDs). Both swing-designs would probably work, but if you match the same design to the old one, you'll eliminate any hassles (e.g., some frames bottom brackets can interfere w/TS, water bottle mounts can interfere with BS on smaller frames).

So....final check on your current FD: BP or TP, BS or TS? Then, we can offer replacements.

Do you live in the UK? If you're in the US, there are plenty of local options.
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Old 12-08-07, 09:56 AM   #5
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Yes I live in the uk
Well after all that I went away and did a lot of reading - I learnt a lot and ended up wondering about rear end width dropout
I need a fd that gives me 55mm-56mm chainline
the shimano saint m805 gives 57.5mm but states that you need 83mm bb width and 150mm rear end width dropout
Not knowing too much about thee factors would this work in my setup

I'm looking to replace my shimano acera-x fd (31.8mm diam)- no matter what adjustment , it will not go on to the largest front ring (3 ring) Iassume it is worn

it is a top swing, top pull on a shift grip
chainset - shimano hyperdrive-c 24:32:38 teeth
chainline approx 55mm
RD shimano alivio 7cog

would the
SHIMANO SAINT FRONT DERAILLEUR fit the bill
OR
Is there anoher shimano ts tp front derailleur with a 55mm chainline
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Old 12-08-07, 10:11 AM   #6
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Quote:
Originally Posted by ancient View Post

Hi - again - still struggling with acera-x f.d. - cant get it to go on the largest ring - so lets assume it is worn and needs replacing
"Wear" to the front derailer is not likely the problem.

Quote:
Originally Posted by ancient View Post
I have an Altus 3 ring 24:32:38 teeth Hyperdrive-C chainset
The down tube is 33mm diameter
The distance from the outerface of the tube to the outerface of the largest ring is 47mm - the chainset being 17.5mm over the 3 rings

What do I look for in a repalcement that will fit these specis
I have an article you might find useful on front derailer selection:

http://sheldonbrown.com/front-derailers

However, my guess is that no derailer made will work well with this setup. One of the main specs of a front derailer is the size of the big ring. I don't know of any that are made for a big ring as small as 38 teeth.

Most "triple" front derailers are designed for at least a 10 tooth difference between middle and large ring. That 32 middle will prevent you from mounting the derailer low enough to get good shifting to the 38.

Sheldon "No Way" Brown
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Old 12-08-07, 07:14 PM   #7
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Do I change the chainset

The BIKE - EMMELLE TROJAN was bought early - 1996 from a retail cycle store. I believe it as the 1995 model

I have made no changes and the 3 ring set is as original and again I have checked and it is deinitely 24:32:38 and has written on it
SHIMANO HYPERDRIVE-C

The set is spline attached and is approx 16mm thick (over th three rings)

From your info , should i be changing the set with a matching f.d.

If so what would i look for so as to keep my original rear setup alivio 7 cog

I was sking if the shimano saint would work BUT I assume that your recommended reading link - ecelllent - would infer i may not get the inner ring

Is that so

regards and thanx for the help
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Old 12-08-07, 09:52 PM   #8
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You might get the one you have to work if you bend the front of the inner plate slightly outward
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