Bicycle Mechanics - Front Derailer H limit screw
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04-15-11, 03:50 PM
When I am in the smallest chain ring and the largest cog in the back, I have < 1 mm of clearance. Which I understand is the idea.
When I am on the largest chain ring, I have well over 1 mm of clearance. As I understand it, I am supposed to use the H limit screw to bring the cage in lower to the chain.
HOWEVER, and here is the problem, the H limit screw is all the way in there! It won't turn no more?! No what? I've had the chain jump off to the outside of the chain ring at least a few times now. Pretty annoying.
04-15-11, 04:32 PM
it also true for < 1 mm clearance when you are in the large chainring and the smallest in the back. are you sure the chainline is right ? maybe a longer BB axle is needed. when you move the chain in the back the space within the cage will change . or a different front derailer would work . just some ideas here ,need more info . or pictures.
04-15-11, 04:35 PM
What front derailleur are you using, what crankset is it and do you know how wide the bottom bracket is?
04-15-11, 04:39 PM
thanks for the quick replies. It is an all stock bike from bikes direct. (you can see how I am down this path)
So, a Sora FD no idea how wide the bottom bracket is.
going to try to take a pic.
04-15-11, 05:11 PM
I would take a look at how the FD is oriented in comparison to the chain-rings. It should be parallel to the chain-rings when viewed from the top and a penny should just barely fit between the large ring and the FD's cage:
04-15-11, 06:23 PM
Yeah, that is the site I started with. I've adjusted the height. (only a small clearance of 2mm with the teeth) the angle (lined up parallel) the cable tension (seems right). I mean it shifts well up and down down, but when there are bumps and I'm riding the big ring, it can skip off. I thought the cage was to prevent this?
And when I said earlier re the clearance on the outer ring. There is roughly 4 mm or more of clearance when I am on the big chain ring and the smallest cog. I think I am supposed to minimize this gap to reduce the chain coming off by the H limit screw. However, I am at the point now where it doesn't move. SO Frustrating.
I did notice one thing, despite only having two chain rings, I feel like I can get 3 positions out of the shifter. The derailer seems to move roughly the same, but I feel like I can get three distinctive "clicks" That make any sense what so ever?
I do like the bike though. The perfect urban assault vehicle/ commuter for the $$.
I did notice one thing, despite only having two chain rings, I feel like I can get 3 positions out of the shifter.
That's because it probably does have 3 positions. The current ST-3400 doesn't come in a double only configuration.
04-18-11, 09:19 AM
I redid the cable tension, and I feel confident that potential problem spot has been eliminated.
I completely removed and reinstalled the FD. I feel like it is vertically positioned correctly. (~1-2 mm clearance large ring) and is correcly parallel to the chain rings.
I re-adjusted the high and low limit screws to adjust the cage. When I am in the small chain ring, biggest cog, it is right on top of it. When I am in the large chain ring, smallest cog, it is again, right on top.
Although not always flawless, I can shift up and down pretty regularly. I've now discovered that the problem seems to happen independent of shifting. I threw a chain twice (to the outside) on the commute this AM while running the large chain ring and applying heavy torque well after a successful shift.
SOOO... is the problem a bad chain? I need more chain tension? Can I get more chain tension by adjust the RD? Remove a link?
04-18-11, 10:33 AM
Is the bottom bracket installed correctly? Is your chainline correct?
I am just guessing here but if you have a front derailleur designed for a triple crankset and you have a double crankset you may need to put a spacer on the drive side bottom bracket cup to optimize your chain line.
Find a good quality local bike shop and have them take a look at it. The problem with buying a bike mail order and learning how to put it together as you go is that one normally does not have the experience to effectively trouble shoot problems like this and it is very difficult for folks out here in the ethereal to troubleshoot this effectively without seeing the bike and to see what factors are at play...
...so sure your chain can be too long, not enough tension, your derailleur could be defective for all we know.
Do yourself a favor and ask your question at a good shop on one of their slow days and I would be surprised if they do not give you a good education on what the problem might be.
If you think it is a chainline issue you can buy different width bottom bracket spacers at good shops and online. You may need to purchase the tools to remove the crank and bottom bracket to install the spacer depending on what tools you have on your workbench.
Good luck with this.
04-18-11, 11:16 AM
Already did, guess I need to bring it back to the LBS tomorrow. I still like the idea of trying to teach myself, but this has gotten a little expensive for trying to be on the cheap :). Ah well, I'm still really enjoying the commute. 8 miles each way, adds up over the week. I'm feeling a lot more fit already.
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