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  1. #1
    Suffers From Binge Biking
    Reputation: marsh rider's Avatar
    Join Date
    May 2008

    Full Metal Jacket Flex

    After making the switch to BB7s and SD7 levers, I decided to finish off the setup by throwing on the full metal jacket brake cables. I took my time and got the whole system set up perfectly, but when I went to pull on the rear brake, I noticed the housing running from my lever to the TT was still flexing. I was hoping for (and expecting) virtually no flex at all, so for those of you who have the FMJ, is this normal? Do I need cable housing with straight strands of cable (like with shifter cables) to achieve flex-free performance? Thanks.
    If it ain't broke, fix it 'till it is.

  2. #2
    mtbr member
    Join Date
    Jul 2008
    I don't know that you'll ever get flex-free 100%, but the FMJ allows you to run your cables straighter, and keeps them pretty well sealed. It's very important that you make perfect cuts in the cable housing so the ferrules go on completely flat. Some people will even use a dremel with a cutting wheel instead of housing cutters.

    I have hydros now, but when I ran BB7's I was using the FMJ. I think the performance difference over high-quality standard housing & cables was not too much, but they kept dirt out and looked very cool!
    Rigid bikes FTW!

  3. #3
    mtbr member
    Join Date
    Jul 2003

    And DO NOT use....

    stranded housing or derailleur housing for your brakes! They are not designed to take the stress that is applied to the housing when the brakes are pulled. You'll end up pushing those strands right out of the outer sheath and eventually right through the ferrules on the ends of the housing or the outter sheath of the housing will split open. When that happens, no brakes!

    You will never find a housing that will not move or appear to flex when you apply the brakes with cable brakes of any type. But it is not the housing that is actually flexing. The key to good brake feel is compressionless housing. As you pull the brake lever you put stress on the ends of the housing between the cable stops, the idea is to keep the housing from compressing under that stress. What you are seeing and calling flex is simply a bit of movement as the cable tensions. Brake cables are actually quite slack inside the housing when the brake lever is at rest. Not so slack that the exposed cable flops around, but usually not as tight as derailleur cable. Also you have a looped run that is not straight so it tends to move a bit. So as you pull the lever the cable tightens and puts tension on the inside of the bends in the housing. This causes the housing to move a bit because that loop of cable is trying to straighten out, but once the pads make contact with the rotor there should be little or no further movement. A little side to side movement has little affect on brake performance, but housing that compresses (like stranded derailleur housing) will give a mushy lever feel and low brake power.

    So, what you are experiencing is normal even with the best brake housing money can buy. What counts is the feel at the lever and the performance of the brakes. The only way to get zero movement out of a run of brake cable housing would be if the housing where completely ridged and unbendable. Not something that would work on a bike for the most part.

    Good Dirt
    "I do whatever my Rice Cripsies tell me to!"

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