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Old 05-17-09, 10:38 AM   #1
vmaniqui
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HELP: Problem with my Double Crankset Upgrade on Dahon.

hi all,

i upgraded my dahon boardwalk crankset to a Shimano 105 54/39T chainring. it wasn't the easiest job as i have to resort to using my auto tools instead of the bike tools. the BB of my Dahon Boardwalk D7 is the cheap one that has unusual treading. i have to used my long nose plier coupled with adjustable wrench. while sticking the 2 ends of the plier to the notches of the bottom bracket, i have to crank it with an adjustable wrench. it took me more than an hour just to remove the BB bracket. once they're out it's pretty much an easy install of Hollowtech II BB and the 105 crankset (i attached a pic). now here's my problem. when on the 53T the gear shifting is ok but when i changed it to the 39T the chain slacks. it seems that the chain is too long. how can i fix this ?

thanks for the inputs,
vic
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Old 05-17-09, 11:10 AM   #2
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1) Try removing a link from the chain.
2) Try a chain tensioner.
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Old 05-17-09, 11:45 AM   #3
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1) Try removing a link from the chain.
2) Try a chain tensioner.
thanks. removing a link may not do it as it's really a big slack. see my pics. on the first pic that is 39T and 30cog (rear) and the chain stress seems ok but when i shifted to the highest gear (11T) and 39T crank the slack is so big that i should remove about 10 links (see pics). do you think i need to change the rear derailleur ? any other input ? appreciate it.

thanks,
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Old 05-17-09, 01:09 PM   #4
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A different rear derailleur should help. Be careful running too short a chain, as it has the possibility of ruining your rear derailleur. As always, Sheldon has something to say about this:

http://www.sheldonbrown.com/derailer-adjustment.html
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Old 05-17-09, 06:11 PM   #5
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Put the chain on the big cog front and back simultaneously, and shorten the chain to the minimum that won't over-stress the derailer.

The derailer has to be able to handle (53-39) + (30-11) = 33T. The derailer specs at Shimano will tell you what each derailer is capable of. You can exceed that a bit by a few teeth.
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Old 05-17-09, 09:21 PM   #6
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you'll need a MTB medium cage for that.
you will also need to get a derailer hanger for the dahon.

the stock suntour derailer was only designed to handle around 20T total capacity, which is perfect for it's 11-30 cassette with only one chainring.
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Old 05-17-09, 10:47 PM   #7
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Put the chain on the big cog front and back simultaneously, and shorten the chain to the minimum that won't over-stress the derailer.
thanks jur. i checked the chain just this pm and it has the exact length. in fact i tried putting it on the largest cog/chainring without going over the rear derailleur and so far the length are ok. i was hoping this is just a quick one but it's getting so frustrating now. any more possibility?

thanks again,
vic
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Old 05-17-09, 10:49 PM   #8
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you'll need a MTB medium cage for that.
you will also need to get a derailer hanger for the dahon.

the stock suntour derailer was only designed to handle around 20T total capacity, which is perfect for it's 11-30 cassette with only one chainring.
thanks AEO. you think i need to get a new rear derailleur ? does it need to be the one that has hanger and med. cage ? i will get a new rear derailleur. but do you think the one without hanger will work, too ? any more possibilities. appreciate it.

vic
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Old 05-18-09, 07:37 AM   #9
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Hi
your Dahon has very short chainstays and therfore a shorter than regular chain... now if you take chainlength or teeth calculation than you need to consider the specifics for a 20 inch folding bike and not a 700 c Race bike .... In the older days we used to say that a 10 teeth jump on the front makes good shifting possible ( 52/42 ) an additional tooth like a 53 wouldnt make a huge difference .. but than the rear was usually a 23 - 15 ish and not a 20 tooth difference by itself.....

get a smaller inner front chainring and you will be fine

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Old 05-18-09, 08:08 AM   #10
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Originally Posted by vmaniqui View Post
thanks AEO. you think i need to get a new rear derailleur ? does it need to be the one that has hanger and med. cage ? i will get a new rear derailleur. but do you think the one without hanger will work, too ? any more possibilities. appreciate it.

vic
What is the cassette on the rear? If it is especially wide -- in Shimano terms that would be wider than a 11-28 -- then a regular road derailer will have a hard time allowing you to access all of the rear cogs from both chainrings. In practice, this should not be an issue since one generally avoids cross-chaining; i.e., small to small and big to big.

One answer would be to get a derailer with a long tension pulley. The other answer would be to keep removing links until you get a good balance between how many cogs can be accessed by each of the front chainrings. So remove a few links such that the big-big combination is no longer feasible and see how close you can get to the small-small combination. The shorter your tension pulley the more you need to compromise.
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Old 05-18-09, 08:10 AM   #11
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get a smaller inner front chainring and you will be fine

thor
Do you mean a larger inner chainring?

Just for completeness, a more narrow cassette would do the same thing.
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Old 05-18-09, 08:32 AM   #12
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I don't know about your dahon, but all the recent ones are all built with the capability to attach a derailer hanger. The ones that come with the suntour derailer have an aluminum cover where the hanger would go, so you just need to get the dahon derailer hanger and replace it to use a regular derailer with it. it's only held in with one screw.

I don't know if it's a special part or just a regular part, but it's something thorusa doesn't stock so you'll probably have to go through dahon directly to get one.
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Old 05-18-09, 11:55 AM   #13
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my gear is 11-30T (standard for boardwalk D7). don't know if it's narrow but i am guessing should be standard width. i went to check my chain again and the length seems ok. no rear derailleur over stress and understress. i went for a spin yesterday to check how it is but unfortunately it has the big chain slack as what is on the picture i sent. since the chain is not the culprit, i will try and order the shimano tourney megarange derailleur and see if it will work.

thanks for the input and would appreciate if anyone has any ideas to solve this.

vic
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Old 05-18-09, 01:58 PM   #14
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Originally Posted by vmaniqui View Post
my gear is 11-30T (standard for boardwalk D7). don't know if it's narrow but i am guessing should be standard width. i went to check my chain again and the length seems ok. no rear derailleur over stress and understress. i went for a spin yesterday to check how it is but unfortunately it has the big chain slack as what is on the picture i sent. since the chain is not the culprit, i will try and order the shimano tourney megarange derailleur and see if it will work.

thanks for the input and would appreciate if anyone has any ideas to solve this.

vic
Vic,

I believe we already know what the problem is ...

http://www.sheldonbrown.com/deakins/lowgears.html

Scroll down to the section about cross chaining. From what I gather, you set up your chain such that -- given your present derailer's tension pulley length -- you can access all of the rear cogs from your big chainring but get chain slack in the small chain ring small cog position. If you want to access more rear cogs from the small chainring you have to do one of two things ...

(1) give up some of the cogs accessible from the big chainring or
(2) get a derailer with a longer tension pulley or
(3) get a larger inner chainring.

Thor can give you a definite answer, but I thought that the Dahon derailer has a very short tension pulley which is great for small wheel bikes with a single chainring since it reduces the possibility of damage. Consequently, it has a smaller range than a typical rear derailer. So with a normal short cage rear derailer you might be able to access most if not all of the cogs. You should be able to access all of the cogs with a medium cage (GS if my memory is correct) derailer typically made for road triples.
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Old 05-18-09, 02:50 PM   #15
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Vic,

(2) get a derailer with a longer tension pulley or
You should be able to access all of the cogs with a medium cage (GS if my memory is correct) derailer typically made for road triples.
thanks invisiblehand - i did the option 2 in your reply. i just ordered a Tourney TX51 megarange RD (long cage with hanger). i am not so sure if this is a good one. i was checking the ACERA M360 but i think this one is medium/short cage at it mentioned it is has a smart cage. i will let you all know what happens as soon as installed it.

thanks a lot guys for the inputs.....really appreciate it.
vic
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Old 05-18-09, 03:19 PM   #16
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One thing to keep in mind about switching to a mid- or long-cage derailleurs is that it may well hang even lower than your current RD, thus reducing your ground clearance -- possibly to the point where the RD will scrape the ground on some turns.

There are reasons why many folding bikes use the SRAM DualDrive system rather than derailleurs to get a wide gear range....
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Old 05-18-09, 03:41 PM   #17
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Originally Posted by Bacciagalupe View Post
One thing to keep in mind about switching to a mid- or long-cage derailleurs is that it may well hang even lower than your current RD, thus reducing your ground clearance -- possibly to the point where the RD will scrape the ground on some turns.

There are reasons why many folding bikes use the SRAM DualDrive system rather than derailleurs to get a wide gear range....
thanks bacciagalupe. this could be one of the risk. when i received and installed it i will all let you know. i really wanted to get the Dual Drive but just not for me as it is too costly. same thing with the Sturmey Archer. in any case i saw this in one of the forums regarding the long and short cage derailleur:

You've got a medium cage. Sram makes a short cage mountain derailleur in the X.0 line, but only goes as low as a medium in the X.9.

Quick answer: The medium cage will work, but you'll drop your chain if you accidentally shift to the small-small combo. Suspension *could* be a factor, depending on how much "chainstay growth" your frame experiences as your suspension cycles.

Long answer:

Derailleurs have a rated capacity. This is their ability to take up excess chain. After all, you need just about all of your chain to run in the big-big combo, whereas you have a bunch of extra links doing nothing when you run in your small-small combo.

Not that either of those cross-chain combos are normal to run in, but let me get to that in a minute.

Manufacturer stated derailleur capacities are as follows:
Shimano long = 45T; medium = 33T
SRAM long = 43T; medium = 37T; short = 30T

Speaking from experience, Shimano is a bit conservative in their capacity rating. I can only assume the same is true of SRAM (I'll get to that, too).

The easy capacity formula is to add your big ring & cog sizes, then subtract your small ring and cog sizes. It looks like this:

cap req'd (T) = (BIG ring - small ring) + (BIG cog - small cog)

...so for a typical 44-32-22 mountain crank & 11-34 cassette...

T = (44T - 22T) + (34T - 11T)
.. = (22T) + (23T)
.. = 45T

Using this simple forumla, you would need a derailleur with a 45T rated capacity to absorb all the possible extra links of a typical 27-speed drivetrain.

(I make the assumption SRAM stated capacity is conservative, since they list 43T as the long cage capacity -- 2T short of what is required by this forumla).

Where do shorter cage lengths come into play? Right here!

Even though the long cage will, in theory, take you down to the 22x11 gear combo and hold adequate chain tension, let's be logical: 22x11 is a combo you don't use!

Rather than use the generic formula, let's map out the capacity for each gear combination (based off of a Shimano cog pattern; SRAM will be slightly different):



44x34 starts off at zero because in that combo, all of the chain is being used up by the ring and cog, and the derailleur needs to take up none of it. As you shift through the cassette range (moving down the column), the amount of free chain increases as the cog size decreases.

Take a look at the useable gears, which I've outlined in green and yellow. Those fall near the stated capacity of the medium cage derailleurs. (I mentioned that Shimano's stated capacity is conservative, and in practice, I find their medium cage to be closer to 39T.)

For instance, in the middle ring (32) and the small cog (11), the table shows you've got to absorb 35T. This is near the stated capacity of either of the medium cage derailleurs. This gear combo remains useable, but you'd be better off shifting to your big ring for better chain tension.

You can also see that to use a SRAM short cage derailleur (30T capacity) on this drivetrain would leave you with two or three unusable gears while in the middle ring, and only about three useable gears from your granny ring. (Any number greater than 30T on the table would be near the limits of the short cage derailleur.)

Oops! Accidentally shifted into the unusable "red zone"? Nothing major: the derailleur cage folds back on itself, the chain droops, and you maybe drop the chain if you don't catch it in time.

In my opinion, it'd be stupid to size a chain any smaller than what is required to shift into big-big. If you accidentally force a shift into that combo, which is certainly possible when you're tired or "in the moment", you don't want to break anything. So chain length will be the same no matter what derailleur you choose.



Benefits of a shorter cage length?
- snappier shifts
- better chain tension
- less chain slap / greatly decreased drivetrain noise (!)
- better obstruction clearance / improved spoke clearance.
- slight weight loss -- but you gotta be a real weight weenie to appreciate this one.
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