Technical FAQ: Leaky forks, Di2 dropped chains, and more

Home » Bikes and Tech » Technical FAQ: Leaky forks, Di2 dropped chains, and moreLeaky suspension fork

Dear Lennard, 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?— Kosta

Dear Kosta, 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.― Lennard

Straightening j-bend spokes

Dear Lennard, 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?— Ivan

Dear Ivan, 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.― Lennard

Dropped chains with Ultegra Di2

Dear Lennard, 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 …

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