Front Derailleur getting stuck in the middle

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BertYardbrush
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Front Derailleur getting stuck in the middle

Postby BertYardbrush » 20 Dec 2012, 8:05pm

I am building a road bike based on a steel frame with a very short wheelbase, 15" from rear axle to bottom bracket, (is that the chain line?). I am having trouble setting up a front derailleur. I am using the inner 2 rings of a triple crank from Spa with chainrings 44/32 and 10sp at the back. I am using a Shimano CX70 front mech with 105 STI's.
The chain isn't shifting all the way across, it struggles to shift from small to big and when going from big to small, gets stuck mid way between the rings (chain suck?).
My thoughts are, 1) Is the bottom bracket too short, & the derailleur is at the limit of its capacity? 2) Is the distance between the chainrings too great & would using spacers to bring them closer together work? 3) Could the short distance between the rear axle & bottom bracket leading to a lot of angle on the chain make the idea of a double front impractical? It was originally a 7 speed bike.

Brucey
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Re: Front Derailleur getting stuck in the middle

Postby Brucey » 20 Dec 2012, 8:26pm

the chainline normally refers to the distance from the bike centreline to the chainring centreline. If the chainline is too small, the front mech can run out of travel; check for ineffective limit screws, and check the chainline vs the specifications.

I suspect that your 44T ring is meant to be an outer ring, in which case it isn't meant to be shifted over in the way you are doing at present. It may well be that you can improve matters by respacing; the limiting condition is often the chain scraping on the big ring when you are running small-small combinations. If you have good clearance at present, you can try spacers, I'd say.

Dedicated middle rings often have a 'shoulder' that persuades the chain towards the small chainring. A simple experiment would be to turn the 44T round the other way, and allow the bolts to poke out slightly (I assume they are in the countersinks at present). The bolts might help the chain not to fall into the gap. Spacers, like you say, are not difficult to try, either.

hth

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

Ayesha
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Re: Front Derailleur getting stuck in the middle

Postby Ayesha » 20 Dec 2012, 9:48pm

Usual ‘Chainline’ on a road double is 43.5mm. That’s the distance from bike centerline to midway between the two chainrings. The rings are 5 mm apart, so the inner is at 41 mm and the outer is at 46 mm.
Chainline on a MTB triple is 50 mm. That’s from bike centre line to the middle chainring.
To get the middle of a triple to be where the outer of a double is, you’ve got to take 4 mm off the BB on the drive side.
If the BB is a 115 mm, fit a 107 mm.

You are proposing a 12 tooth difference in rings, which is OK.

The configuration of some triple chainsets is that the inside diameter of the outer ring is smaller, so will not immediately fit on the seating shoulder for the middle ring. A big file will cure this.

The Ultegra front mech should be OK. Make sure the mech’s cage is 1 mm from the teeth extremities.
When changing up, slacken the torque on the pedals to allow the chain to climb. Likewise changing down. The chain must climb off the teeth before dropping onto the small ring.

AndyA
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Re: Front Derailleur getting stuck in the middle

Postby AndyA » 20 Dec 2012, 11:43pm

You're right with 2. I think that the cranks are for 8 speed or possibly 9. This means that the chainrings are a very small amount too far apart. 10 speed chain is very narrow and can get stuck between the chainrings. Similarly, the mech is designed to move a smaller amount as it's designed around shifting between chainrings that are closer together. If there are spacers between the chainrings and spider remove them and things should improve. If there are no spacers then I reckon your best option is to change the chainset for a 10 speed one, like this http://www.chainreactioncycles.com/Mode ... elID=67234

I've had the same problem with the chain getting stuck between chainrings when upgrading a bike to 10sp from 9. Changing the cranks solved the problem, but was quite expensive!

BertYardbrush
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Re: Front Derailleur getting stuck in the middle

Postby BertYardbrush » 21 Dec 2012, 9:20am

Brucey wrote:I suspect that your 44T ring is meant to be an outer ring, in which case it isn't meant to be shifted over in the way you are doing at present.

Dedicated middle rings often have a 'shoulder' that persuades the chain towards the small chainring. A simple experiment would be to turn the 44T round the other way, and allow the bolts to poke out slightly (I assume they are in the countersinks at present). The bolts might help the chain not to fall into the gap. Spacers, like you say, are not difficult to try, either.


The 44T is an outer which I have mounted as an outer but in the middle ring position - with the bolts standing proud. The BB is 110mm.
The limit adjuster screws are such that the outer one is a fair way down and the inner one is a fair way up, giving me reason to question BB dimension.

It looks like experimenting with spacers is the first thing to try - and the least expensive. Thanks for the replies.

Brucey
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Re: Front Derailleur getting stuck in the middle

Postby Brucey » 21 Dec 2012, 9:30am

another thought was that you could use the outer two positions with your present cranks if you were happy to settle for a 33T inside chainring; IIRC you can get a 110BCD (which is what I assume you have rather than 104BCD) 33T chainring.

cheers
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531colin
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Re: Front Derailleur getting stuck in the middle

Postby 531colin » 21 Dec 2012, 9:35am

I wonder if this chap ever got sorted out......http://forum.ctc.org.uk/viewtopic.php?f=5&t=59701
On that thread, I demonstrated that I don't understand all this....... :oops:
However, at least some Stronglight Impact, and Spa's own chainset are Sugino XD2, and if my memory serves they need a spacer between the crank spider and the "middle" ring...ie the ring BertYardbrush is using as the outer ring of his Alpine double.
110 is the usual BB for triple, which should give the "middle" ring a 45mm chainline vs 43.5mm between the 2 rings for a road double chainline.......now my head hurts again... :(
If the front mech. is a triple, you won't use all the throw on a double?

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The Computer Monkey
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Re: Front Derailleur getting stuck in the middle

Postby The Computer Monkey » 21 Dec 2012, 10:34am

Ok here is what I know based on our experiences at Spa currently.

Not sure if you are using a 'new' Stronglight Impact (5 visible bolts) or the 'old' Impact/Spa XD2 (4 visible bolts) so will give information based on both.

We did have an issue on one bike of the XD2 not working on a 10 speed system and to be honest couldn't identify the reason, we have spoken to Stronglight and they say that the Impact is 10 speed compatible.

Yesterday we tested both the XD2 and the 'new' impact triple on a 10 speed Shimano 105 drivetrain, in both instances with the correct bottom bracket length (XD2 = 110, 'new' impact = 115) they shifted and worked. Please be aware this was an 'in the stand' test, we didn't ride them it up and down the road due to the weather.

So in theory these chainsets with the correct axle length 'should' work, hand on heart I personally don't think they will for everybody but I don't have an answer for what the limiting factors are currently.

Your setup is far from 'standard' though as you are using a double front mech and if I was setting it up I would have adjusted the axle length to allow for this. I would suggest trying a 107 bottom bracket (as you can't really get much shorter), I don't have a good 'engineering' reason for this beyond improving chainline.

Questions that spring to mind are;
Do you currently have spacers between crank and 'middle' (outer) ring as you would if it was built as a triple?
What chain and what rear mech are you using (and cassette for that matter)?
How have you determined appropriate chain length?

Colin: CX70 front mechs are all doubles I believe (Shimano assumes you wouldn't want to ride cyclocross with a triple ;) )
I'll raise my hands and say I work at Spa, however my views are my own. Just because I've said it doesn't mean anyone else will agree.

BertYardbrush
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Re: Front Derailleur getting stuck in the middle

Postby BertYardbrush » 21 Dec 2012, 11:52am

The Computer Monkey wrote:Not sure if you are using a 'new' Stronglight Impact (5 visible bolts) or the 'old' Impact/Spa XD2 (4 visible bolts) so will give information based on both.

We did have an issue on one bike of the XD2 not working on a 10 speed system and to be honest couldn't identify the reason, we have spoken to Stronglight and they say that the Impact is 10 speed compatible.

Yesterday we tested both the XD2 and the 'new' impact triple on a 10 speed Shimano 105 drivetrain, in both instances with the correct bottom bracket length (XD2 = 110, 'new' impact = 115) they shifted and worked. Please be aware this was an 'in the stand' test, we didn't ride them it up and down the road due to the weather.

So in theory these chainsets with the correct axle length 'should' work, hand on heart I personally don't think they will for everybody but I don't have an answer for what the limiting factors are currently.

I would suggest trying a 107 bottom bracket.

Do you currently have spacers between crank and 'middle' (outer) ring as you would if it was built as a triple?
What chain and what rear mech are you using (and cassette for that matter)?
How have you determined appropriate chain length?


There are 4 visible bolts on the crank, so it's an "old" XD2.
107 BB, I think the chainring would interfere with the chainstay. It's very short wheelbase which accentuates the chainline. If anything I was thinking a slightly longer BB might do.
"Do you currently have spacers between crank and 'middle' ?" Spacers, what spacers? The outer chainring is directly attached to the crank in the middle position with the outside facing out. Do middle rings have something outer rings don't have which would mean in this situation a spacer would be necessary?
The chain is quite short but in the highest gear, 11/44, the rear derailleur bottom sprocket is slightly behind the rear axle so I think it is ok.

I too had the setup working ok on the stand when I try it on rollers it's not working under load.

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The Computer Monkey
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Re: Front Derailleur getting stuck in the middle

Postby The Computer Monkey » 21 Dec 2012, 12:56pm

When we or Stronglight build an XD2, there are a set of chainring washers which fit between the middle ring and the crank arm thus pushing that middle ring (or your outer) slightly towards the granny.
I'll raise my hands and say I work at Spa, however my views are my own. Just because I've said it doesn't mean anyone else will agree.

BertYardbrush
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Re: Front Derailleur getting stuck in the middle

Postby BertYardbrush » 21 Dec 2012, 12:57pm

Just found this http://www.peterwhitecycles.com/chainrings.asp
"TA rings with Shimano compatible bolt circles are designed to work with Shimano cranks. The offset of TA chainrings are the same as Shimano's offset. So if you try using a TA ring with a Sugino crank, the inner ring on a triple ends up spaced further from the middle ring than it should. If you're running six speeds in back with a wide chain, you may be able to make it work by adding a spacer between the middle ring and the spider; I don't know for sure though. I intend to someday see how wide a chain has to be in order to allow TA rings to work well on these Sugino cranks. But for sure a 9 speed chain isn't recommended. Either the middle and outer ring are too far apart or the middle and inner ring are too far apart, depending on whether or not you place a spacer between the spider and the middle ring."
I've got a TA outer and a Stronglight inner. Looks like spacers.

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The Computer Monkey
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Re: Front Derailleur getting stuck in the middle

Postby The Computer Monkey » 21 Dec 2012, 1:15pm

Let us know if adding the spacers works.
I'll raise my hands and say I work at Spa, however my views are my own. Just because I've said it doesn't mean anyone else will agree.

BertYardbrush
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Re: Front Derailleur getting stuck in the middle

Postby BertYardbrush » 25 Dec 2012, 8:56am

I've put some 1.2mm spacers on and it has definitely improved. It now shifts ok under load - on the rollers. I can't use any wider spacers as I'm using an outer ring on the wrong side so the nuts stand proud and a wider spacer would prevent the inner ring from sitting properly flush. Maybe I should have used a middle ring as the outer.
Next thing is the bottom bracket. I am certain that the 110 is too short. It would be ok if I was using a triple crank as a triple, but using it as a double needs that extra 3mm. The deailleur adjustments are at their extreme, outer right down, inner right up, the extra 3mm will sort that out.
I'll be in touch will computer monkey and his pals for a JP400 113mm as soon as life gets back to normal.

Thanks for the help, I think a quick whizz out tomorrow might be in order on the other bike to try out my new Conti 4 seasons.

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531colin
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Re: Front Derailleur getting stuck in the middle

Postby 531colin » 25 Dec 2012, 9:08am

I can never remember if the extra 3mm from 110 to 113 is all on the right side, or if its 1.5mm each side.
However, it would be worth trying a spacer under the R. side BB cup to see if it made any difference.....or even just setting the R. cup say 2mm proud of the frame to try it on the workstand.

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The Computer Monkey
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Re: Front Derailleur getting stuck in the middle

Postby The Computer Monkey » 27 Dec 2012, 12:36pm

99% sure it's 1.5mm each side (just eyed them up and that seems to be the case).
I'll raise my hands and say I work at Spa, however my views are my own. Just because I've said it doesn't mean anyone else will agree.