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Housing Friction: Hard Housing vs. Soft Housing

Posted Sunday, November 15, 1998

By Relative Workshop

The housings which route the cutaway cables from the cutaway handleto the 3-ring section of the right and left risers serve two purposes:

  1. The housings provide a clean, unobscured path from the cutaway handle to the risers.
  2. The housings protect the cables from damage.

Unfortunately, all cutaway cable housings have one inherentdrawback: FRICTION! This friction plays more of a role on the leftriser’s cable housing due to the length of the housing and the number of turns thatthe housing and cable must make before arriving at the 3-ring. This means that given anequally distributed load between the right and left risers, the pull force at the handlewill always be more for the left riser’s cable than for the right.

How much more? The amount of friction force that must be overcome tomove the cable at the handle is directly related to the force needed to move the cable atthe riser. Simply put, the pull force at the riser is magnified at the handle due tohousing friction.


Housing Type

The type of housing (i.e. soft or hard) has an affect on how muchthe pull force is increased due to housing friction. A hard housing does a very good jobof keeping outside forces away from the cable. Due to its lack of rigidity, a soft housingalone does not provide this same protection. This protection reduces the friction byreducing the amount of contact between the cable and the housing. Consider this example.Take two housings -- one hard, one soft -- and put a cutaway cable into each. Now, whilesqueezing the housing with your fingers, remove the cable. Which housing do you think willmake for an easier pull in this situation? The turns that the left riser’s housingmust make produces a similar "squeezing" effect. The rigidity of your housinghas a large affect on the amount of increased pull force due to friction.


Hard vs. Soft Housings: A Simulation

The two test setups on the left of this display are meant tosimulate the left riser’s cutaway housing. One setup simulates a soft housing (left),and the other, a hard housing (right). In both setups, a 34" long housing isrouted along a path which approximates the degrees of turn the left riser’s cutawaycable housing undergoes while in use. A 1 lb. weight suspended from the cable exiting theriser end of the housing simulates a 1 lb. pull force at the riser. Table 1 shows theforce needed to move the cutaway cable when pulling the handle with weights between 1& 5 lbs. suspended from the end of the cable. A graph of this data can be seen inFigure 1.

Table 1: Soft vs. Hard Housing Cutaway PullForce Comparison

Pull Force at Riser*

Pull Force at Handle (lbs.)

(lbs.)

Soft Housing

Hard Housing

1

8

4

2

14

9

3

21

12

4

27

15

5

30

19

*Pull Force at Riser simulated by suspending weightsfrom cable.

Friction Chart

A 200 lb. jumper in a 2G spin can generate a force on the right sideriser of 200 lb. or more. At this loading, a Reversed, Type-17 Mini 3-ring Riser cangenerate a cable pull force at the riser of about 5 lbs. ANon-reversed riser would generate a cable pull force at the riser of about 1 lbs.According to the data in Table 1, a 5 lb. pull force at the riser would mean a 19 lb. pullforce at the handle if using a hard housing and 30 lb. pull force at the handle using asoft housing. A 2 lb. pull force at the riser would mean a 4 lb. pull force at the handleusing a hard housing and an 8 lb. pull force at the handle using a soft housing. Accordingto these tests, the basic soft housing shown here produces a higher increased pull forcedue to friction than the hard housing. A system using reversed risers in conjuction with asoft housing such as this can produce a significantly higher cutaway pull force than asimilar system using non-reversed risers with hard housings.

Not all soft housing are built alike. The soft housing used here isjust one type, and even it was built to no manufacture’s exact specification. If youhave soft housings, ask the manufacturer of your system what types of pull forces you canexpect and what he or she recommends.

Send comments or questions to aimee@relativeworkshop.com
Copyright 1998 The Uninsured Relative Workshop Inc.
Reprinted with Permission




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