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  1. #1
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    Any Strength Advantage to Post Mount vs IS disc brake mount?

    I have a steel rigid fork that I'm having problems with lately. I'm about 220 lbs so heavy but not super heavy. I'm not going off jumps or anything, its usually when falling off a skinny or something goes wrong on the back side of a logover - common denominator is a lot of weight ending up on the front wheel while it is in an awkward position. But whatever happens, it gets the wheel so crooked that I can't start. Undoing the QR, recentering the wheel and reclamping does not help. The only fix is to turn the bike upside down, stand on the handlebars, grab the front wheel by the tire and twist the wheel as hard as I can. Usually I need to do this in the direction that pushes the IS tab out, so I think that the problem occurs when the wheel is off center and I come down on it in a way that sort of folds the tab inward. At least I think that is where it deforms - I don't see any signs of stress on the fork blades or on the crown, so I assume its along the weld where I may not notice as easily.

    I'm currently going through various steps to eliminate the QR as the offender. Hoping I find something wrong with my setup that accounts for this problem but not terribly hopeful. So just wondering in case I find myself in the market for a new fork - is there any inherent strength advantage in the post mount system vs IS tabs?
    Yeah I only carry cans cause I'm a weight weenie.

  2. #2
    dru
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    I'd be super surprised if you were bending the IS tabs. You'd flex or bend your rotor long before that would happen. As for your front QR, is it shimano? If not, try one.

    I had a rigid fork on my Salsa and my Salsa QR couldn't bite hard enough to hold the wheel secure unless I had the QR super tight. The wheel would shift around just from hard braking.

    Now that I've got a suspension fork, the Salsa QR holds the wheel fine because the QR grabs the softer aluminum fine.

    As for your question re post mount vs IS, I suspect that the switch to post mount was some kind of industry BS that had to do with manufacturing costs or marketing/competition.

    IS type brake mounts are used on virtually every motorcycle made including MotoGP, World Superbike, and motocross.

    As for you, don't fight the move to post mount. Soon enough you'll find it hard to get IS calipers, or you'll need PM adapters and bigger rotors because pretty much no one makes IS suspension forks anymore.

    Drew
    occasional cyclist

  3. #3
    El CicloPath!!!!!!!
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    Most likely it us the inabilty of the qr to hold the wheel in place, therefore causing flex. juat a little fkex is enough to cause issues. One solution would be to go to a hub that uses a 9mm thru bolt like the rws. Its what i have and has eliminated all flex issues on my qr fork.

  4. #4
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    Re: Any Strength Advantage to Post Mount vs IS disc brake mount?

    Looking again I think I worded that poorly. I suspect it is bending at the weld between the blade and tab, but I suppose the fork blade could be bending also (or the crown/blade junction).

    I guess what I'm really wondering is whether the post mount system makes for a stronger connection to the fork blade.

    I am using a shimano QR. Gonna switch back to my carbon fork for a while, will examine closely while it's off the bike and maybe file the paint off the dropouts in case that is causing issues.

    Hadn't considered QR bending, though its been fine with other forks. maybe I will start saving my Pennies for RWS conversion.
    Yeah I only carry cans cause I'm a weight weenie.

  5. #5
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    Quote Originally Posted by AlexCuse View Post
    Looking again I think I worded that poorly. I suspect it is bending at the weld between the blade and tab, but I suppose the fork blade could be bending also (or the crown/blade junction).

    I guess what I'm really wondering is whether the post mount system makes for a stronger connection to the fork blade.

    I am using a shimano QR. Gonna switch back to my carbon fork for a while, will examine closely while it's off the bike and maybe file the paint off the dropouts in case that is causing issues.

    Hadn't considered QR bending, though its been fine with other forks. maybe I will start saving my Pennies for RWS conversion.
    I doubt it is an issue with the brake mount. You are not going to be "fixing" the mount in the manor you describe. More likely you are just pushing the pads out a bit.

    A new through axle fork may help, but i think part of the issue is the pad alignment unrelated to the tabs.

    You can test this next time by inserting the wheel in the fork (wheels on ground). Before closing the QR wiggle the wheel so the rotor pushes the pads out. Then close the QR.
    mtbtires.com
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  6. #6
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    Thanks Shiggy - since I was resetting the QR each time anyway may as well try this. Probably won't be riding this fork for a while but we'll see.
    Yeah I only carry cans cause I'm a weight weenie.

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