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Tuesday, May 1, 2012

Post Prandial Pedaling and Cable Stretch

05/01/12 246# 26m
Had lunch with a few old friends in our periodic Geezers 2.0 luncheon, then rode 26 miles on the Montour Trail. Nothing unusual other than truly great weather.

I've put a few hundred miles on the bike since a new drivetrain was installed, including new cables and housings, and there's been a bit of gear chatter and uncommanded shifts on the rear cassette which I attribute to the new cables stretching after use. I am not a derailleur guru, but I was bold enough to make a one-half turn on the cable adjustment, and then another half-turn later on.

When I got off the bike and back to the web, I read Sheldon Brown (RIP) on the topic and then found this video and it was pretty straightforward, just FYI:

What eludes me about cable stretch are the implications for civil engineering. When they build a suspension bridge, there aren't any adjusters for cable stretch built into the mechanism. If cables stretch after break-in, shouldn't the bridges sag? Conversely, if the bridges aren't sagging then the cables aren't stretching, and then what is this craziness the bicyclists are putting up with?

1 comment:

  1. "post prandial" - one of my favorites!

    Also -

    When designing cable or wire rope systems, the amount of stretch that will occur when a force is applied is an important consideration. Keep the following in mind when you do your calculations:

    There are two forms of stretch in cable and wire rope: Structural Stretch and Elastic Stretch.

    Structural Stretch

    Structural Stretch is the lengthening of the lay in the construction of cable and wire rope as the individual wires adjust under load. Structural Stretch in Loos & Co., Inc. products is less than 1% of the total cable length. This form of stretch can be completely removed by applying a cable or wire rope prestretching operation prior to shipment.

    Elastic Stretch

    Elastic Stretch is the actual physical elongation of the individual wires under load. The elastic stretch can be calculated by using the following formula*:

    E = (W x G) / D2


    E = Elastic Stretch, as a % of Length**

    W = Weight of Load, in pounds

    D = Diameter of cable, in inches

    G = See Chart Below (omitted)

    *Elastic Stretch derived from this formula is an approximation.

    **Remember to keep your units of measure constant. The length of your cable must be calculated in inches to correspond with the diameter measurement, also in inches