Gear

Technical FAQ: Shifting Issues, gear combos, and more

Among this week's questions is one about a shifting problem, the solution to which ends up being easier than first thought.

Diagnosing a shifting problem

Dear Lennard,
I’m having problems with my rear derailleur shifting and was wondering if you know of any instructions for replacing the b-axle assembly on a Dura-Ace 7900 rear derailleur? It looks complicated and I’m not sure which tools I need.

I have tried almost everything: replacing the dropout, installing new inner and outer shifter cable, ensuring inner routing isn’t tangled/obstructed, switching the exit point of the cable on the shifter (fore/aft of handlebars), etc., and I still can’t get it 100 percent (it had worked fine with an Ultegra shifter but since switching to a new Dura-Ace shifter I’ve had the problem). This will be my last shot before replacing the derailleur.
— Phil

Dear Phil,
Why do you think the b-knuckle needs an overhaul? It sounds more likely that you have an incompatible shifter. You sure you don’t have a shifter for a different amount of speeds than your drivetrain?
― Lennard

Dear Lennard,
I think I misunderstood the use of the b-knuckle. I had read that the spring on Dura-Ace rear derailleurs was a weak point. My understanding was that replacing the b-knuckle spring would restore the indexing ability of the rear derailleur. However, there are 2 springs in it and the b-knuckle spring doesn’t seem to have anything to do with the indexing. Plus, I’m still not sure how to replace it.

My shifter should be compatible as it is also Dura-Ace 7900, so it’s also 10-speed compatible, and I’ve had a look to make sure it is pulling and releasing the cable at the required distances.

It seems the problem with my shifting is just when I’m in the big ring.

Using Dura-Ace 10-speed 7900. When I’m in the small ring (39-tooth) I have no problems shifting from the big cog to the small cog. However, when I shift to the big ring (53-tooth) it won’t shift all the way down to the 11-tooth small cog and the indexing is all over the place.

Have checked a few things like the dropout to make sure it is securely fitted. No problems with it. Cables all look good. They’re relatively new, 2 months old, Shimano. Front derailleur is correctly aligned. Can’t see to figure it out.
— Phil

Dear Phil,
Do you have internal shift cables? Sounds like the cables twisted around each other inside of the frame.
― Lennard

Dear Lennard,
Yes, that was exactly the problem. Very frustrating diagnosis having tried literally everything else. Thanks.
— Phil

Shifting performance

Dear Lennard,
I’m curious to get your take on the Santana modification at the bottom of this page. What do you think they might have done? I wondered if maybe they used an XTR triple derailleur set, but with a front derailleur cage from a road triple? Would such a thing be possible?
— Steve

Dear Steve,
That bike has XTR Di2 triple front and rear derailleurs. They will shift (by that I mean move laterally the correct distance) using R785 levers without any problem. That said, the chainring combination they have mounted is well outside Shimano’s recommendations, and I would be amazed if it works like we’re used to Di2 working.

MTB front derailleurs are meant to handle about 10-tooth jumps, whereas road front derailleurs are designed for 16-tooth jumps. But with a triple, as long as the chainring steps are fairly similar to an MTB triple’s 40/30/22 combination (such as 52/40/30), the front probably shifts OK.

I’d be very surprised if the rear shifts well, though; you might have a complaining stoker bake there when the derailleur fumbles around. That cassette does not look the XTR Di2’s proscribed 11-40. The single-spring Shadow Plus rear derailleur lacks the ability to track different cassette ratios the way a dual-spring Shimano derailleur accommodates so well. Note that the upper pulley of the M9050 rear derailleur is visibly not close to tracking the larger cogs. I’d be amazed if it shifts anywhere close to Shimano Di2 shifting standards for crisp shifting under power and breadth of adjustability range.
― Lennard

SRAM 1×11

Dear Lennard,
I saw your youtube on SRAM 1×11. Great info and exactly what I plan on doing. One question though: you did not address how the rear wheel needs to be converted. It is my understanding that I will need a different hub to accommodate the 11-speed cassette. Please advise.
— Dean

Dear Dean,
I discussed this last week.
― Lennard

Follow up to junior gearing

Last week I said that there were no available 11-speed junior cassettes starting with a 14-tooth cog. That was incorrect. I had for a long time searched QBP (Quality Bicycle Products) for a Miche like this (Miche often offers solutions for both Campagnolo and Shimano freehubs that are hard to find otherwise), but I never found one. A friend then sent me this link.

Furthermore, I had never found an 11-speed cassette starting with a 14-tooth cog on Shimano’s site, but it turns out that QBP carries an Ultegra 14-28.

So there are more options out there for juniors than I thought.
― Lennard

Disc brakes

Dear Lennard,
I have changed disc pads several times before on my Avid Elixir3 brakes. This time, one of the pistons on the rear brake would not set, or zero, to allow new pads and the rotor back in the caliper.

When I have pushed the piston in with a tire lever, the  piston pushes back out. Only one side. — Mark

Dear Mark,
It sounds like it’s just dirty and the square seal is stuck on the piston walls.

I talked to SRAM brake product manager Paul Kantor, and he agreed that it is most likely dirt causing it to be stuck.

He said, “Not sure when he bled the brake last, although typically if it was a fluid issue it would cause both pistons to hang up. He could try holding one piston in place and agitate the other by gently working the lever to force it free.”
― Lennard

Follow up to chain wax in the rain

Dear Lennard,
Great article on rain and wax, love to see the discussion!

As I was reading, I realized I had left out an important point about wax (credit to Velo’s first test): in the water and grime, a waxed chain can actually get faster! This is why we like wax so much for ’cross. If it looks like a mudder, we’ll start with a freshly waxed chain that has only been ridden 10 minutes or so after waxing, just enough break-in time so that shifting isn’t compromised. This way, the maximum amount of wax is on the chain for the onslaught of mud and water.
— John