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Old 02-13-12, 09:04 AM   #1
bobotech
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Can you use a 2 speed front derailleur on a 3 speed front crank? And crank issues.

I'm finishing up my Redline hybrid bike. Its all together except I have major problems at the front crankset. The bike was involved in an accident which damaged the front crankset. I replaced the 2 bigger chainrings with junkyard speicals but there is still strong warpage.

Anyone have a cheap 3 speed front crankset for a hybrid? LOL

I am also having problems with the derailleur. I am wondering though, is it possible to use a 2 speed front derailleur and make it work with a 3 speed crank? I don't think that the right derailleur was in the box of parts I received when I got the frame from the previous owner.
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Old 02-13-12, 09:45 AM   #2
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Most 2 speed FDs have enough travel to work on triple fronts, but that isn't the issue. The issue is chainring sizes. A typical 2x FD is designed around 39-53 rings and can probably work with something as small as a 36t inner, but the cage will be too short to work with a smaller granny. Even an FD made for compact drive and sized for a 44-48t outer, still won't reach small enough for a 24-28t granny.

So if you're building up a road triple set with something like 36-44-53 you'll probably be find with a toad 2x FD, but for a 26-36-46 triple you'll want an mtn 3x FD.
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Old 02-13-12, 10:27 AM   #3
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You need a new crankset either way, I think. If you go triple, yeah, you'll need a derailleur that can handle that. You should be able to grab one for less than $30. Actually $28 to be more precise.

The other option is (and not guaranteed to work) but something I'm an advocate of: Replace the triple with a compact double.

Went from this (on 700x28c tires):

Code:
	52	42	30	
12	116	93.7	66.9
	107	86.5	61.8
	99.4	80.3	57.3
	92.8	74.9	53.5
	81.9	66.1	47.2
	73.2	59.2	42.3
	66.3	53.5	38.2
23	60.5	48.9	34.9
to this:
Code:
	50	34	
12	111.5	75.8
	102.9	70
	95.6	65
	89.2	60.7
	78.7	53.5
	70.4	47.9
	63.7	43.3
23	58.2	39.6
Replacing the 12-23 with a 12-26 would give pretty much the exact same range. I actually have an 11-28 to replace it.

Last edited by dwellman; 02-13-12 at 10:32 AM.
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Old 02-13-12, 11:06 AM   #4
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Quote:
Originally Posted by FBinNY View Post
Most 2 speed FDs have enough travel to work on triple fronts, but that isn't the issue. The issue is chainring sizes. A typical 2x FD is designed around 39-53 rings and can probably work with something as small as a 36t inner, but the cage will be too short to work with a smaller granny. Even an FD made for compact drive and sized for a 44-48t outer, still won't reach small enough for a 24-28t granny.

So if you're building up a road triple set with something like 36-44-53 you'll probably be find with a toad 2x FD, but for a 26-36-46 triple you'll want an mtn 3x FD.
Ah ha! that probably explains why my shifting isn't working currently. I can upshift to the big ring but its hard when it does, but it does. I can then downshift to the middle ring but when I try to shift to the small ring, it just won't go. it appears to move over enough but I wasn't looking at the bottom of the cage, I'm betting that the chain is resting/rubbing on the bottom of the cage while not falling enough to actually transfer to the small cog. The derailleur that i'm using is a Shimano Deore FD-M750. Doh. So I definitely need a new derailleur. After reading your post, its all making sense now why things just aren't working right.
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Old 02-13-12, 11:18 AM   #5
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I encountered a bike with a double front mech and a triple crankset a few months back at the co-op I volunteer at. It was a late '80s MTB, and I never got a chance to see if it worked, because the little nub for the spring on the derailleur body had sheared off, so the derailleur didn't move back when the cable went slack. I can only assume it worked at some point, as all three chainrings were worn. The granny ring had about 30 teeth, as I recall. It also had friction shifters, which was probably a good thing, as I suspect it would have needed a lot of trimming to keep it running smoothly.

tl,dr A double front mech CAN be used with a triple crankset, but it'll probably shift like crap and may not work with indexed shifting.

Last edited by Airburst; 02-13-12 at 11:21 AM. Reason: clarity
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Old 02-13-12, 11:26 AM   #6
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Deore front derailleurs are always intended for triple chainring setups, IIRC.

What shifters are you using?
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Old 02-13-12, 11:51 AM   #7
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Deore front derailleurs are always intended for triple chainring setups, IIRC.

What shifters are you using?
+1 AFAIK, the Deore XT M750 is a triple FD. If that's so, the source of your difficulties isn't that you're using a double FD.
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Old 02-13-12, 12:35 PM   #8
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The Sram SX5 shifters that came with it. I know Sram vs Shimano, however from what I have read, the front derailleurs aren't nearly as brand critical as the rear derailleurs but its what I have.

I have an 8speed SX5 for the rear and a 3speed SX5 for the front shifter.
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Old 02-13-12, 12:39 PM   #9
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The thing is that even with the Deore XT M750 low screw completely backed out so that the derailleur is on its absolute rest position, it won't kick the chain off to the smallest cog. As i was testing, I even removed the front derailleur shift cable completely off and tried spinning the pedals, still won't downshift. Rubs like crazy on the cage but won't kick over. I have a 30T, 42T, and a 52T on there currently. Since the shift cable was completely removed, I would say that the shifters aren't an issue at this point.
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Old 02-13-12, 12:55 PM   #10
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Quote:
Originally Posted by dwellman View Post
If that's the case then the crank isn't on tight enough OR the bb "spindle" is too wide.
I'm going to take some pictures tonight when I get home, but if I look down at the derailleur from above, the chain is being pulled towards the seat tube quite a bit. Its just not engaging the smaller cog. I think that once i get some good detailed pictures on here, we can get a much better idea of what is going on.
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Old 02-13-12, 01:39 PM   #11
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Or perhaps the front derailleur is either too high or not parallel with the chainrings.
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Old 02-13-12, 02:24 PM   #12
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I was trying to be extremely careful with both of those measurements. When I pulled the cage over the big chainring, I had at most 2mm of space, probably closer to 1 mm. And I was trying my best to make it as parallel to the chainrings.
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Old 02-13-12, 02:56 PM   #13
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They made (or make) two different versions of the XT front derailleur - a 'top swing' and 'bottom swing.' IIRC, the top swing FD like the FD-M750 is designed for smaller chainrings and smaller jumps between chainrings. The bottom swing versio, such as FD-M751 or similar might be a better choice.

The other option is a 'road' front derailleur, although if you have an indexed front mtb shifter this will not work well - although some grip-shift style shifters are not indexed 1-click-to-1-gear, but have multiple clicks between gears. If your shifter is like this then you can likely use a road front derailleur.
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Old 02-13-12, 03:25 PM   #14
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Are you sure the FDER isn't hanging up on the middle ring?
I did an oddball combination on my rings (22-32-36 & 22-32-38 on my 2 bikes).

When I set the FDER the "proper amount" above the large ring, it hung up on the middle ring and thus wouldn't go to the granny.
My FDER's are actually set (now) about 1/2" above the large ring.
You might try raising the FDER some and see what happens.

Then again, you may have either insufficient travel or too long a BB.
Before replacing a BB with a shorter one, make sure the FDER has enough travel so that it can "span" all 3 rings.
You don't want to get into the same situation where you can't reach the top ring either.
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Old 02-13-12, 03:38 PM   #15
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Quote:
Originally Posted by Bill Kapaun View Post
Are you sure the FDER isn't hanging up on the middle ring?
I did an oddball combination on my rings (22-32-36 & 22-32-38 on my 2 bikes).

When I set the FDER the "proper amount" above the large ring, it hung up on the middle ring and thus wouldn't go to the granny.
My FDER's are actually set (now) about 1/2" above the large ring.
You might try raising the FDER some and see what happens.

Then again, you may have either insufficient travel or too long a BB.
Before replacing a BB with a shorter one, make sure the FDER has enough travel so that it can "span" all 3 rings.
You don't want to get into the same situation where you can't reach the top ring either.

I think you are looking at the probldem backwards... inability to reach the small ring can be caused by a BB spindle the is too short - not too long.
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Old 02-13-12, 05:00 PM   #16
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What size chainrings were on the bike new? The 52-42-30 rings are for a road triple. I wouldn't expect them to play well with your mountain bike shifters and FD.
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Old 02-13-12, 05:08 PM   #17
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Okay, I went to my local bike shop during lunch for some advice! They pointed some things out that I was totally unaware of.

First of all, the chainrings were damaged in a wreck before I got the bike. I went to my local bike coop and got 2 replacement rings for the crank after straightening it out as best as possible. I found a 42 and 53t rings and put them on. The crankrings still wobble from the straightened bends so that is a problem in itself.

However the LBS said that my problem could very well be that the rings don't have the upshifting/downshifting ramps on the chainrings. Sure enough both of the rings I found were flat rings, I remember noticing that but didn't think that was a big deal when i found the rings last week at the coop. Neither of the rings had any ramps or things that look like they would help shifting. The LBS said that the rings need the ramps to upshift and downshift properly otherwise I could be fighting an uphill battle the whole way.

The guy at the LBS said that he had a Shimano Ultega 52-42-30 crankset with the octadrive? bottom bracket and he would let me have it for 50 dollars (used of course). I am thinking I should bring my bike into him tomorrow and have him check it out and confirm that the crank/crank spacing might be the issue and have him replace the bracket/crankset to at least get that portion of it out of the way.
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Old 02-13-12, 05:10 PM   #18
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What size chainrings were on the bike new? The 52-42-30 rings are for a road triple. I wouldn't expect them to play well with your mountain bike shifters and FD.
According to http://www.bikepedia.com/quickbike/B...R540&Type=bike, the bike came with 30-39-50t FSA rings.
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Old 02-13-12, 06:49 PM   #19
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Quote:
Originally Posted by DCB0 View Post
I think you are looking at the probldem backwards... inability to reach the small ring can be caused by a BB spindle the is too short - not too long.
Yeah, I was thinking backwards.
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Old 02-13-12, 11:40 PM   #20
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I took a few pics and some video.

When I took the pictures, I had the Sram derailleur installed but after I took the pics, I swapped it out again with the Shimano Deore XT derailleur.

All of the pics and videos can be seen here:
http://s904.photobucket.com/albums/a...ng%20problems/

A few pics and a video at the end showing the warping in the chainrings:













A video of the warpage in the crank:

Last edited by bobotech; 02-13-12 at 11:44 PM.
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Old 02-14-12, 03:21 PM   #21
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Brought the bike to the shop. They said that the bike had a bent crank spider, wrong chainrings (single speed), and that the spacing was too far out for the bottom bracket meant that it would be a good idea to replace the bottom bracket and crank. So I'm going to have them swap out the crankset and bottom bracket for a Shimano Ultega Octalink setup with 52/40/30 chainrings. They thought that the problem with the derailleurs was due to the current crank being too far spaced out plus the incorrect chainrings.
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Old 02-14-12, 06:28 PM   #22
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bobotech, I don't quite agree about the crankset being too far outward, but they have the bike in front of them and I'm not a professional bike mechanic. I hope they're correct about that and the FD cage's curvature clears the large chainring and you're on the road again. Be sure to let us know.

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Old 02-14-12, 06:33 PM   #23
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If it swings out far enough, for the chainline that you need/.

NB My Touring triple the edge of the 3rd chainring sits over the Fixed cup,
just barely clears the chainstay, which gets dimpled, if needed.

looks like a shorter BB axle can be used.

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Old 02-14-12, 07:49 PM   #24
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Despite all the information above on why it won't work, sometimes it works anyway.
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Old 02-15-12, 12:16 AM   #25
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Well I got my bike back and took it for a quick ride. Was nasty out, snowed today and was about 33 degrees this evening when I got home and took it for a ride. Shifted SO much better but still might need to be dialed in. The guy at the bike shop said that the Deore derailleur was too short to work with the chain rings on my bike so he put the Sram derailleur back in place.

He also replaced my headset bearings because I had the wrong ones in there. I had used old style caged bearings and apparently that is totally wrong. I can't really afford it but he only charged me 75 for everything including the head bearings and the Shimano Ultegra Octalink 52-42-30 crankset and bottom bearing and the labor to put it all in. All told, I will frequent this shop in the future considering how well he took care of me.

Well at least I think I got a good deal.
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