At the Bench: Stretching Cables

When I first went to work in a bike shop, other mechanics stressed to me how important it was to “stretch” the new cables on a bike during assembly. In the course of assembling a bike, I’d pull hard on shift levers without turning the pedals and I’d squeeze the brake levers as if I was trying to skid the length of Manhattan.

Invariably, the stretching resulted in a need to readjust the derailleurs, especially on mountain bikes, and the brakes would require a dialing in of a barrel adjuster to take up the newfound slack. My experience backed this up: The rear brake and rear derailleur always needed more adjustment than the front brake and derailleur. And why not? It stands to reason that a cable twice as long will stretch twice as much.

What I didn’t understand at the time was that steel cables don’t stretch. Think about it: What are the chances steel will stretch even a millimeter just from some college kid squeezing a brake lever? My guess is somewhere around none. The word asymptotic comes to mind.

But the fact remained that every time I assembled a bike, one hard downshift on the rear shifter would cause the indexed shifting to slip out of adjustment. Without fail, I’d need to turn the barrel adjuster one to two full turns to restore the shifting.

Years would pass before I would find out why I needed the squeeze.

A product manager for one of the mid-sized bike companies that prided itself on the quality of their bikes relative to the price point revealed just what I’d been doing every time I pushed a shifter with twice the force required to make a shift. He had been working with the company’s assembly factory in Taiwan to give the workers who hung all the parts on the bike a bit more time in brake and derailleur assembly. Why? He wanted them to give the shifters and levers a hard squeeze to seat the cable housing in the ferrules and the ferrules in the braze-ons. With that accomplished, the workers could actually adjust the brakes and derailleurs and have the cables trimmed to length.

The product manager’s goal was to make their bikes as ready to go as possible the moment they were pulled from the shipping carton. Poor factory assembly can reflect poorly on a bike company in a couple of ways: First, if a bike is a pain to assemble, the mechanics will hate it because assembling one can take up too much of their day. They’ll be less enthusiastic about selling those bikes in some shops. Second, if the shop’s mechanics are uneven in their work, some of those bikes may make it out the door without being perfectly assembled. Any problems with shop assembly ultimately reflect back on the bike company, much the way when David Millar broke his Dura-Ace chain in a sprint finish at the Tour de France and he threw his Felt off the road, well, everyone blamed Felt—not Shimano—on social media. Was it rational? No. But that’s how the world can work.

What I would go on to learn was that higher quality bikes used better braze-ons, better ferrules and even better cables, which made everything assemble more easily, and with less fuss and readjustment. That explained why I always had to do more “stretching” on less expensive bikes.

To this day I give brake levers a good squeeze and shifters a firm push to make sure everything is seated before I start dialing in the last of the shifting or lever free-stroke.

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