Saturday, April 05, 2014

Dual Drive earwax

One of the early repairs this year is a Cannondale hybrid with SRAM Dual Drive rear hub. The owner said the bike had been stored for a couple of years. He reported no functional problems, only requested that we figure out how to raise the handlebars.

Dual Drive uses internal hub gears combined with a freehub body to provide a set of external gears shifted with a derailleur. This particular bike had a three speed hub.
A single unit controls both internal and external gears
The control linkage for the internal gears has been disconnected in this picture. 

While I had the wheel out for truing I noticed that only one of the internal gears was engaging. I remounted the wheel to confirm this by reconnecting the shifter. Yep. It was totally air pedaling in low and medium gear ranges. This was not just an adjustment problem. 

With a little Internet mining I got to a tech manual on SRAM's site that showed me how to break into the thing, but I saw nothing in the schematics that suggested an obvious flaw that would lead to this kind of gear loss. The troubleshooting guide made no reference to it. Based on some residue on the end of the hollow axle I suspected congealed grease in the "lifetime lubricated" inner recesses. I flooded the hub with light oil and left it overnight. 

This morning it works.


Steve A said...

One concern with light oil is how long the fix might last.

cafiend said...

A concern I shared. But remember that Sturmey Archer hubs had an oil fitting built in.

SRAM appears to have used grease like the notorious earwax that Shimano used in Rapidfire shifters. That stuff stiffens up within a couple of years to create the impression the shifter is worn out.

I figure if the hub starts to act up again I can follow up with slightly heavier oil. The first treatment needed to break loose the clog. As with any experimental treatment, we have to follow up. I will report if there are further developments.

Steve A said...

Maybe have the guy come back and soak it in 90 weight gear oil. If it is good for Jaguars, it ought to be good for bikes...

greatpumpkin said...

I have a Sram Dual Drive on my Dahon Speed TR. I like how it works--the convenience of internal gears with the range of derailleur gears. I have never had any trouble with it, but I've also not left it sitting idle for long periods of time.

cafiend said...

Idle gears are earwax's playground. :-D

greatpumpkin said...

To say nothing of idle ears.