Technical FAQ: Leaky forks, Di2 dropped chains, and more
This week, Lennard Zinn addresses questions regarding straightening j-bend spokes, dropped chains on Di2, and more.
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Leaky suspension fork
I have a little scratch on my suspension fork, which was causing some oil to leak. I tried to polish it and the leak seems to be reduced a bit, but the polish didn’t solve the problem.
Should I replace my seals also, or maybe there’s something else I can do?
I assume you’re talking about a scratch in the upper tube of your suspension fork. In that case, every time it slid in and out of the upper seal, it would have tended to tear the seal. So just sanding and polishing the scratch smooth will not fix the leak; you do indeed need to replace the seal as well.
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Straightening j-bend spokes
Have you ever heard of straightening j-bend spokes to a straight-pull due to lack of original replacement spoke? I have some old wheels that need a few new spokes and guys at my LBS proposed straightening some high quality stainless spokes as they do not have any suitable straight-pull replacements in stock. How much can I trust this solution?
I wouldn’t trust it at all. I’ve never heard of such a thing, and I can imagine that it would make the spoke very weak and likely to fail right at that straightened bend. And I doubt he could even get it very straight anyway. I’d say it’s worth the wait to get the right spokes.
Dropped chains with Ultegra Di2
I’m a mechanic at a shop and I’ve recently noticed an issue on a few bikes equipped with Ultegra Di2. I have had several customers report chains dropping inboard (between the frame and crank) while shifting in the rear. In order to replicate this while the bike is in the stand I need only shift to the big chainring and then shift the rear derailleur rapidly between the upper part of the cassette. The chain is cross-chained, although not necessarily to the most extreme degree. For example, in quick succession the chain is shifted from the 25 to the 23 to 25 to 23 to 21. In the moment of the chain dropping from the larger diameter cog to a smaller diameter cog, there is a brief window of slack in the system and that bounce seems to cause the chain to pull inboard off the big chainring, and rather than continue riding that gear it drops inside. It may only drop or nearly skip off once every 30 times, but I can observe the chain threatening to jump pretty easily. I’ve attempted any number of both clever and silly corrections to the front derailleur to little avail. I have, however, noticed a trend on the bikes to which this happens:
1. They are time trial or triathlon bikes. I am thinking the short chainstays may be affecting the chainline and this plays a role.
2. It has been only on Ultegra Di2 and not any mechanical group. Perhaps the rear derailleur lower pulley isn’t as strong? Although this slack is occurring between the cassette and cranks ABOVE the chainstay and should, therefore, be under tension from the cranks.
3. The issue is happening with third-party cranks such as Rotor or FSA. Perhaps the machining of the chainring is not retaining the chain as well as a Shimano crank?
4. The frames are either press-fit 30 or BBRight. I have not personally seen this issue on frames with bottom bracket shells that are a full 86 or 90 millimeters wide, only frames between 68mm-79mm wide. I’m not convinced this plays a role.
5. All bikes have been 11-speed.
Can you expound on how some of the trends or characteristics I have mentioned may or may not be playing a role in this issue? Is there any advice you can offer for front derailleur adjustments that may go beyond the obvious? Is this problem known to other mechanics and is there a trick I don’t know about?
1. I think the most critical common characteristic may be that these are all aero bikes with short chainstays, and presumably a recessed or curved seat tube to fit tightly around the rear wheel. Short chainstays will make the chain angles sharper at the chainring and will certainly make it more likely to jump off to the inside when cross-chained.
2. I suspect it is also important that these are all non-Shimano cranks. Even though other brands are widely used and generally deemed good enough, Shimano says all other crank brands are officially incompatible with Shimano drivetrains. I’d love to hear if these same bikes also do this with a Shimano crank installed.
3. There is no spring or cage difference between Shimano mechanical and electronic derailleurs, so I don’t think there is extra chain slack happening because these are Di2 bikes. It’s conceivable that the Di2 auto-trim feature moves the front derailleur inboard of where it would be with a mechanical derailleur in order to not rub in the big-big combinations. Then perhaps the inner cage plate will not be in a position to push the chain back on the chainring when it starts to derail.
4. You could try rotating the tail of the front derailleur out slightly and then adjust it so that the chain just barely rubs the inner cage plate when standing up and stomping the pedals in those cross gears. The cage might then prevent the chain from dropping to the inside. While in the Di2 adjustment mode and tapping the left shifter buttons, experiment with trim positions of the front derailleur to keep it closer to the chain in cross gears.
5. When a chainring is worn out, it will produce intermittent chain suck when shifting to the inner chainring, and I suppose it could also cause occasional inward chain drop from the big ring. Look at the teeth to see if they are hook-shaped and check to see whether you can pull the chain straight forward off of the front of the chainring and create a gap between it and the ring through which you can see light. If you can, replace the ring (and, of course, replace the chain if it’s worn out, too).
SRAM 10- and 11-speed compatibility
Can a 10-speed SRAM Red double tap shifter work with an 11-speed SRAM front yaw derailleur? I don’t know if there are any differences between the 10- and 11-speed front shifters.
I need to replace my front 11-speed lever and the 10-speeds are a lot less expensive. The rest of my drivetrain is SRAM 11.
I’ve tried this in the past temporarily on a cyclocross bike on which the left lever had gotten broken, and I had no replacement 11-speed lever to put on. To the best of my memory, it worked fine.
Gear combos for storing bikes
Is there an ideal gear combination for storing a bike when it’s not being used? Even if you ride a lot, the vast majority of a bike’s life is spent in the garage, bike rack, etc., so I thought it was worth asking.
I imagine there are potentially two considerations (which are at odds with each other):
1. Align front and rear to make the chain line as straight as possible.
2. Run small front and small back to minimize tension on derailleur springs.
What are your thoughts — any other considerations?
For short-term storage, I doubt it would make a measurable difference. But for long-term storage, leaving the chain on small-to-small would probably add some life to the rear derailleur’s jockey-cage return spring.
I see no reason to worry about cross-chaining during storage because problems associated with that have to do with wear on the chain and gear teeth, and if the chain is not moving and there is no load on the drivetrain, wear is not a factor.
Feedback from last week’s column regarding notched freehub splines
You could have mentioned in your response to this question that Shimano uses titanium for the freehub body on its Dura-Ace wheels and hubs, as does White Industries. In my experience, this doesn’t suffer the notching problems of aluminum freehub bodies, while still offering a weight advantage over steel.
Good point! I indeed should have mentioned those. Thanks.
I too had the same problem. But once I followed torque recommendations the damage has been minimal. It might help to clean your bike once in a while also.
Greasing the splines on a freehub could help reduce sprocket notching. It helps on motorcycle wheels that use splined drives.
Then there is the freehub used by American Classic wheels. The freehub has steel splines added to the aluminum freehub body. I don’t have enough miles on one yet to know how it wears.
I’ve destroyed every aluminum freehub I’ve ever ridden.
I’ve noticed that all the cassettes I get have three small holes around the inside that used to be used for locking pins so that the cogs would share the load. The pins no longer seem common from manufacturers, and some spacers, but not all are compatible. I’ve had very good luck using a cotter pin in these holes and through the spacers if arranged properly (usually metal) or compatible (usually plastic). There is a certain size cotter pin that is a slight friction fit that I will then file or grind off the protruding end after installation. This will lock together groups of cogs, leaving the bottom 11 and 12 free. A bit of a cobbled fix, but it works very well to save the freehubs.