A week after a tip related to using a cable tie to solve a badly frayed cable, the doom scroll of Instagram strikes again. A video posted last week showed a SRAM Force DUB crank being undone by progressively wrapping and tightening an inner tube around a hex key and an attached pedal before the stuck crank bolt broke free. It was a proper “oh wow” moment, and I soon felt compelled to try it myself.
Such crank bolts often undo without much of a fight, but there have been a few types, namely on SRAM’s higher-end DUB cranks, that have been known to break hex keys and pop forehead veins in how tight they can be. I very much intend to do an edition of Threaded covering all the tricks and tools for saving your body and bike from such pain, but for now, I’ll focus on this impressively clever approach shared by Korean bicycle mechanic BikeRoid.
The video
The video is simple. It shows a modern SRAM Force DUB crank with an 8 mm hex bolt that doesn’t easily budge, a symptom of what I believe is related to a mechanical outcome known as galling (which explains why swapping the aluminium bolt for a steel one helps). The video then transitions to the gloved hands wrapping a road inner tube tightly around the hex key and pedal until the satisfying sound of the bolt moving is heard. It all looks too easy.
“One day, I watched a video of a watermelon exploding with multiple rubber bands,” BikeRoid explains of how they came to the idea.
Other popular methods among mechanics typically involve securing the opposite crank arm to the frame or with a long pipe, both of which the Korean mechanic said made them anxious about the twisting exhibited on the frame. Likewise, they shy away from my usual method of using an impact wrench due to the potential strain on the bolt. By contrast, “this method slowly increases torque and does not strain the frame at all.”
They’ve been using this method for two years without a single failure. I like those odds, so I pulled out a bike with an SRAM DUB crank that I couldn’t undo with a 12-inch wrench and tried it myself.
Does it work?
For this technique to work, you ideally need a hex key with at least enough length to sit inline with the pedal axle. Meanwhile, any butyl rubber inner tube should work, although I suspect the narrower it is, the more pressure it’ll effectively create for a given amount of loaded stretch.
My attempt was done with a thicker gravel-type tube, and after two attempts, I was getting ready to trash-talk the method. On the third try I decided to focus my effort into my tube stretching and looping, I pulled it tight, I ran out of length in the tube, it didn’t work. I stared at it for a second, then just before giving up, I gently squeezed the hex key toward the pedal and BANG. The bolt had moved, not enough to bite my finger off between the hex key and crank, but enough that I could then undo the rest by hand. Success.
According to BikeRoid, sometimes a second tube is needed to add further load progression, while other times, a single tube makes quick work of it.
Risks and thoughts
Overall, I’m rather impressed with this method. It uses two things almost everyone should already own. The load is wholly concentrated on the crank, with nothing stressing the rest of the bike. And it’s obviously low impact and risk on the body.
That said, and while unlikely, it is possible for that inner tube to yield and whip you, so wear eye protection. Also, I had a Favero Pro MX power meter pedal installed in my test example, which I felt compelled to swap out before attempting this method. I suspect any steel-spindled pedal will be up to the task, but removing ultra-fancy pedals with titanium spindles or strain gauges does feel smart.
The big question is, after seeing and trying this hack, will I change my previous approaches? The answer is: it depends.
In the workshop, I’ve become quite fond of the efficiency of an impact wrench, and it’s very rare that a few quick ugga-duggas don’t break the crank bolt free. Meanwhile, I’ve only had positive results through the controlled use of long levers (with the bike on the ground!). By contrast, I found this inner tube method a little slow and still a tad tiring to get the desired result, and that was with a bolt I know the history of (it wasn’t seized). Certainly, expect me to return to the specifics of my methods, alternative options, and tips for prevention, in a future edition of Threaded.
However, if I’m out of the workshop and away from being surrounded by dedicated tools, I will be thankful for this tip. Thanks to BikeRoid, I don’t doubt this will become a popular solution to a common problem.
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